1294 Steel Protection 0309

STEEL PROTECTIONBY HOT DIP GALVANIZING AND DUPLEX COATING SYSTEMS

GGEENNEERRAALL HHOOTT DDIIPP GGAALLVVAANNIIZZIINNGGGGEENNEERRAALL HHOOTT DDIIPP GGAALLVVAANNIIZZIINNGG

The Hot Dip Galvanizers Association of Southern Africa

HDGASA © 2009

II nntt rroodduucctt ii oonn ::The Hot Dip Galvanizers Association of Southern Africa wasfounded in 1965 and its membership represents the majority ofthe available hot dip galvanizing capacity in Southern Africa.

TThhee VViiss ii oonn ::To position the Hot Dip Galvanizers Association of SouthernAfrica, comprising all its Members and other interested parties,as a professional organization serving the interests of all partiesdependant upon the hot dip galvanizing industry.

MMiissss ii oonn SSttaatteemmeenntt ::To develop and expand the demand for hot dip galvanizing,identify and develop new market opportunities for the benefitof Members and other stakeholders.

SStt rraatteegg ii cc OObb jj eecctt iivvee ::To convince users and specifiers to use hot dip galvanized steelin preference to other coatings and alternative materials,where suitable. This is carried out in three ways:

1. Through general promotional activities.

2. Through focused technical marketing support.

3. Through training and education programmes.

DDee ll ii vveerryy AAcctt iivv ii tt ii eess ::� Promoting the use of hot dip galvanizing for cost effective

corrosion control in applications where its use isappropriate.

� Providing technical assistance and advice for specifiers,fabricators and end users while also recommendingalternative protective methods where appropriate.

� Identifying and investigating potential new applicationsfor hot dip galvanizing.

� Participating in development projects on behalf of industryby providing assistance in the form of technicalconsulting, practical recommendations and assistancewith the preparation of design specifications.

� Providing assistance with quality control during fabricationand hot dip galvanizing.

� Disseminating technical knowledge by providing aconsulting and training service as well as the publicationof technical literature.

� Providing training and education for member companiesto ensure a high standard of quality and servicethroughout the hot dip galvanizing industry.

TThhee HHoott DDiipp GGaallvvaanniizzeerrss AAssssoocciiaattiioonn ooff SSoouutthheerrnn AAffrriiccaa

HOT DIP GALVANIZERS ASSOCIATION

SOUTHERN AFRICAJOHANNESBURG OFFICE

Quality House, Unit U4, St. Christopher Road, St. Andrews, Bedfordview

P.O. Box 2212 Edenvale 1610

Telephone: (011) 456 7960

Fax: (011) 454 6304

Email: [email protected]

www.hdgasa.org.za

CAPE TOWN OFFICE

P.O. Box 2001 Clareinch 7740

Telephone: (021) 797 4735

Fax: 086 612 7284

Email: [email protected]

P R O M O T I N G T H E B E N E F I T S O F H O T D I P G A L V A N I Z I N G

1HDGASA © 2009

Steel Protection by Hot Dip Galvanizing and Duplex CoatingSystems has been revised and updated to include theSANS/ISO specifications for hot dip galvanizing. This includesthe specifications applicable to general galvanizing as well astubes by the semi-automatic process. In addition, morecomprehensive information has been added to the sectionscovering continuously galvanized wire and sheet. The latterprovides the latest available information on pre-coated sheetproducts available in South Africa.

The design and inspection of hot dip galvanized articles and theirexpected service life performance in a range of environments iscritical to the successful application of hot dip galvanizing forcorrosion control. Bolting and welding as well as comprehensivecoating repair of hot dip galvanizing is also discussed. This guideprovides ample support for the specifier, designer and user toutilize the unique properties of hot dip galvanizing when appliedto steel. As in other editions, information in this guide is basedon scientific literature supported by the invaluable experience ofvarious authorities, both local and overseas.

This edition is the 6th available in South Africa and the 4th

written specifically for the local market. Based on earlieroverseas editions, the contribution is acknowledged andgreatly appreciated.

Members of staff of The Hot Dip Galvanizers Association ofSouthern Africa are available to provide support and adviceon the design, application and performance of hot dipgalvanizing. Please feel free to contact us.

FFoorreewwoorrdd

It is estimated that this pressed steel panel water storage tank, known as a “Braithwaite” water tank has been in service for about 70 years and the hot dip galvanized coating is still in a serviceablecondition. The coating on the fasteners has now failed and must bereplaced or overcoated to ensure further service life. The original

“Braitwaite” tank was imported but several reputable local companiesproduce similar product of equal quality.

1 About Corrosion and Rust Prevention ...................... 3

2 Choice of Rust Prevention Method .......................... 4

3 Corrosion Protection Methods .................................. 7

3.1 Hot Dip Galvanizing

3.2 Electroplating

3.3 Zinc Metal Spraying

3.4 Sherardizing

3.5 Mechanical Plating

3.6 Coating with Zinc-Rich Epoxy or Paint

4 Hot Dip Galvanizing.................................................... 9

4.1 The Advantages of Hot Dip Galvanizing

4.2 The Disadvantages of Hot Dip Galvanizing

4.3 The Hot Dip Galvanizing Process

5 Hot Dip Galvanizing of Sheet Metal .......................... 12

5.1 Zinc Coating Surface Finish

5.2 Surface Treatment

5.3 Cut Edge Corrosion Resistance

5.4 Strain Ageing

5.5 Painting

5.6 Primer Coated Galvanized Steel Sheet Produced in a Continuous Coating Line (CHROMAPREP®)

5.7 Painted Cold Rolled Galvanized Steel Sheet Produced in a Continuous Coating Line (CHROMADEK® or CHROMADEK® PLUS)

5.8 Fastening Methods

5.9 The Handling and Protection of Galvanized and Prepainted Steel Sheet During Storage

6 Hot Dip Galvanizing of Wire ...................................... 17

6.1 The Process

6.2 Practical Aspects

7 The Reactions Between Iron and Zinc ...................... 19

7.1 Composition and the Metallurgy of the Steel

7.2 Zinc Temperature

7.3 Immersion Time

7.4 Alloying Additions to the Molten Zinc

7.5 The Withdrawal Rate of the Article from the Molten Zinc

7.6 Surface Condition

7.7 Thickness of the Steel

7.8 The Iron/Zinc Reaction in Continuous Galvanizing

8 Mechanical Properties of Hot Dip Galvanized Steels ...................................................... 25

8.1 Strength and Ductility

8.2 Embrittlement

8.3 Fatigue Strength

9 Design for Hot Dip Galvanizing.................................. 26

9.1 Introduction

9.2 Venting, Filling and Drainage

9.3 Masking, Welding, Handling, Clearance for Moving Parts and Identification

9.4 Preventing Distortion

9.5 Packaging and Transporting of Hot Dip Galvanized Steel

10 Specifying Hot Dip Galvanizing .................................. 35

10.1 Hot Dip Galvanizing Specifications

10.2 Lead Times

11 Quality - Inspection Before and After ...................... 36

11.1 Inspection Before Hot Dip Galvanizing

11.2 Inspection After Hot Dip Galvanizing

11.3 Thickness Testing

11.4 Appearance

11.5 Adhesion of the Coating

11.6 Testing for Adhesion

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TTaabbllee ooff CCoonntteennttss

12 Corrosion Resistance of Hot Dip GalvanizedCoatings ................................................................38

12.1 The Corrosion Test

12.2 Corrosion Resistance in the Atmosphere

12.3 Wet Storage Stain

12.4 Galvanic, Bimetallic and Crevice Corrosion

12.5 Corrosion Resistance of Hot Dip Galvanized Coatings in Aqueous Conditions

12.6 Corrosion Resistance of Hot Dip GalvanizedCoatings in Soil Conditions

12.7 Hot Dip Galvanized Steel in Contact withBuilding Materials

12.8 Abrasion Resistance of Hot Dip GalvanizedCoatings

12.9 Hot Dip Galvanized Coatings Exposed toElevated Temperatures

13 Bolted Connections.................................................... 45

13.1 Type of Structural Bolts and Fastening Devices

13.2 Corrosion Prevention

13.3 Corrosion Resistant Metals

13.4 Protective Coatings

13.5 Hot Dip Galvanizing of Fasteners

13.6 Bolt and Nut Assemblies

13.7 Washers

13.8 High Strength Fasteners – Class 10.9

13.9 Bolt Tensioning Procedures

13.10 The Effect of Hot Dip Galvanizing on Strength Properties of Fasteners

14 Welding of Zinc-Coated Steel .................................. 49

14.1 Shielded Metal Arc Welding (SMAW)

14.2 Gas Metal Arc Welding (GMAW)

14.3 Gas Tungsten Arc Welding (GTAW)

14.4 Flux Cored Arc Welding (FCAW)

14.5 Submerged Arc Welding (SAW)

14.6 Oxyfuel Gas Welding (OGW)

14.7 Brazing and Braze Welding

14.8 Soldering

14.9 Embrittlement of Steel by Liquid Zinc During Arc and Oxyfuel Gas Welding

14.10 Resistance Welding

14.11 Safe Health Practices

15 Reconditioning Damaged Coatings or Site Modified Hot Dip Galvanized Components ...................................... 51

15.1 Coating Repair Procedure by the Galvanizer

15.2 Site Repairs

16 The Cost Effectiveness of Hot Dip Galvanizing ........ 52

17 Painting of Hot Dip Galvanized Steel - Duplex Coating Systems ........................................................ 53

17.1 When to Paint Hot Dip Galvanized Steel Structures

17.2 Surface Preparation for Duplex Coating

17.3 System Selection

18 Proven coating Performance – Case Histories .......... 55

18.1 Pentrich Sub-Station, Mkondeni – Pietermaritzburg

18.2 Blouwater Sub-Station – Suldanha Bay

18.3 Electrical Transmission Towers

3HDGASA © 2009

AAbboouutt CCoorrrroossiioonn aanndd RRuusstt PPrreevveennttiioonn

11

Figure 1

Figure 3

Figure 2

Figure 4

Corrosion can be defined as thedestruction or deterioration of a mater-ial by reaction with its environment.Corrosion usually results in compro-mising the function of a metal, damageto its surroundings, or damage to thetechnical system in which they are bothincluded (figures 1 to 4). Broadlyspeaking, all metals, with the possibleexception of precious metals, are cor-roded and destroyed with time.

For steel to corrode – or rust – in nor-mal environments, it must have accessto both oxygen and water. In mostenvironments both oxygen and waterare available in sufficient quantitiesthrough most of the year to enable thecorrosion process to take place.

The engineering properties of steelhave made it the most widely usedmetal. However, its tendency to cor-rode readily means that corrosion pro-tection is of great economic impor-tance. The rusting process of steel canbe impeded by any of the following:

- By alloying the steel with elementssuch as chromium, nickel, molybde-num, etc. For ordinary steel struc-tures, however, these steels are tooexpensive.

- Changing of the corrosive environ-ment by reducing the access of waterand oxygen through techniques suchas dehumidification, inert atmos-pheric blankets, etc. In totallyimmersed aqueous environmentsinhibitors may be added to reducethe aggresivity of the solution.

- Cathodic protection through theutilization of sacrificial anodes or

impressed direct current. Themethod using sacrificial anodes canbe said to be a form of controlledgalvanic corrosion, since the metalsare arranged so that one of them isallowed to corrode while the other is

protected. Cathodic protection canonly be used in the presence of anelectrolyte, such as water or moistsoil. The method is used for the pro-tection of ships, small boats, quays,of fshore oil platforms, tanks,pipelines, etc..

- Coating with inorganic or organicmaterial, for the purpose of exclud-ing water and oxygen from the steelsurface. This is the most widely usedmethod of protection against corro-sion. The inorganic materials can bemetals and vitreous enamels. Theorganic materials can be paints, bitu-men products or plastics.

Metal coating of steel will provide pro-tection against corrosion, give wearresistance, and sometimes a decorativeeffect.

Only a few of the metals that can bedeposited on steel are cost-effectiveand cathodic to steel. In fact, only zincand aluminium can really be consid-ered. Cadmium is used to some extentbut environmental concerns limit itsuse.

Aluminium has good durability in mostenvironments, although it is difficult toapply. Thin sheet is aluminized on asmall scale. Thermal spraying is used toa certain extent.

A more detailed analysis of the differ-ent aspects of corrosion and corrosioncontrol would go beyond the scope ofthis publication. For those who areinterested, further information can beobtained from the Hot Dip GalvanizersAssociation Southern Africa.

CCHHAAPPTTEERR 11

change the environmental classification,and therefore the choice of rust preven-tion method.

Figure 6 serves as a guide for comparingthe technical characteristics of differenttypes of coating.

It should be noted that, even if zinc andpaint are applied with the same objec-tive - to prevent corrosion - they act incompletely different ways. The zinc coat-ing corrodes from the surface in towardsthe steel, and gives cathodic protectionin the event of damage to the coating.Corrosion does not occur between thezinc coating and the steel.

Conversely, paint coatings are oftendestroyed through the development of alayer of rust between the paint and thesteel. Since the paint coating gives nocathodic protection, rust is able to pene-trate further beneath the paint film oncethe coating has been damaged. Paintscontaining zinc are produced in order toprovide a degree of cathodic protection.

4 HDGASA © 200922

Figure 6. Comparison between the properties of different surface coatings.

When choosing a rust preventionmethod for a steel component or struc-ture, there are many technical issues tobe addressed. The environment in whichthe steel component or structure is towork must be analysed carefully. Theneed for handling, transport, fabricationand final erection require careful consid-eration.

There are numerous paint systems forsteel and a wide range of possible spec-ification and application variables. Thesevariables can substantially influence theperformance of a given system andtherefore its cost effectiveness. By con-trast, the hot dip galvanizing process issimple, standardised and virtually self-controlling, governed mainly by the lawsof metallurgy. As a result it is inherentlyreliable and predictable.

The reliability factor of a coating may bedefined as the extent to which its physi-cal, chemical and mechanical character-istics can be consistently realised duringand after application.

The reliability factor determines theoverall cost-effectiveness of a coating ina given environment.

Table 1 summarises factors determiningthe reliability of typical paint systems forsteel, and for hot dip galvanizing. The reli-ability factor for hot dip galvanizing isshown to be superior, mainly because it isnot influenced by most of the variableswhich can reduce the ultimate perfor-mance of most heavy duty paint systems.

Paints are available in countless varia-tions, with different properties.

Figure 5. Abrasion resistance of hot dip galvanized vastrap stairs (coating thickness - 49μm,taken 10 years after installation).

CChhooiiccee ooff RRuusstt PPrreevveennttiioonn MMeetthhoodd

Conditions and demands are variable inpractice, hence a comparison with actualparameters is often advisable.

An economic study of different controlmethods should be undertaken. It isimportant that the choice of method bebased not only on initial costs but also onpacking costs for transportation, touch-up painting after erection and futuremaintenance costs.

A good guide to the selection of corro-sion control methods in different envi-ronments can be found in SANS14713/ISO 14713 – Protection AgainstCorrosion of Iron and Steel in structures– Zinc and Aluminium Coatings –Guidelines and in SANS 12944/ISO12944 Parts 1 - 8 – Corrosion Protectionof Steel Structures by Protective PaintSystems.

The environmental classifications definedin these standards concern only theenvironment in which the structure willfunction. However, transportation, stor-age and erection environments can

CCHHAAPPTTEERR 22

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Paint System

ISO 8501-1:1988 prescribes abrasive blasting toSa 21/2. Unsatisfactory cleaning can reduce the ser-vice life of the paint system by 60-80%. Preparationcontrol is of decisive significance.

Careful formulation, mixing, agitation and correctthinning are factors of great significance.

The composition and uniformity of the coating varieswith the method of application. Inspection of eachstage of application is important. Abrasively-blastedsurfaces are reactive and must be painted very soonafter blasting.

Good results are difficult to obtain if the air temper-ature is below +10°C. Surfaces exposed to directsunlight can easily become too hot.

Dew and surface condensation delay painting,which should not be carried out if relative humidityexceeds 80%.

Steam, fumes, gases, dust and other pollutants havean adverse effect on the quality of the paint coating.

No influence.

Of great significance to service life. Varies with thenumber of layers and method of application.Inspection of thickness important for each layer.

Depends on preparation, type of paint system, inter-val between preparation and priming and harden-ing interval between layers.

The paint coating is thinner over corners and sharpedges. Holes and narrow crevices normally remainuncoated. “Shaded” sections can be subject to thin-ner layers.

Can vary, depending on type of paint and applica-tion conditions, from a few hours to several days forgood handling characteristics, and up to severalweeks for ultimate hardness.

None.

Various hold points to allow for interim inspectionsto be conducted i.e. after preparation and aftereach stage in the treatment to ensure good quality.Inspection of layer thickness upon application andon finished goods.

Great. Can necessitate repair to primer coating andcomplete overcoating.

Table 1. Comparison of the properties between a paint system and hot dip galvanizing.

Hot Dip Galvanizing

Pickling in acid is an essential part of the process. Ifthe surface is not clean, no coating will be formed.Preparation control is not essential.

The small variations that are possible have little or noinfluence on the quality of the zinc coating.

The zinc coating is formed through a reactionbetween iron and zinc. The reaction is controlled byphysical and chemical laws.

Not affected by the air temperature or normal varia-tions in the process temperature.

Not affected.

Not affected.

The content of, primarily, silicon and phosphorous in thesteel affects the thickness and appearance of the coating.

The reaction between molten zinc and iron gives acertain standard minimum thickness. Silicon and phos-pherous content at certain levels in steel, increasedmass, material thickness and surface roughness giveincreased coating thickness.

The zinc coating is bonded to the steel metallurgically.

Total uniform coverage through dipping in moltenzinc. Coating generally 50% thicker over sharpedges.

The coating hardens completely within a few secondsof withdrawal from the zinc bath.

Residual stresses caused by rolling, cold-working orwelding can, in certain cases, be released so thatsome deformation may occur. These, however, can toa greater degree be minimized by correct design,good fabrication and best practice galvanizing.

Visual inspection and measuring of layer thicknessafter hot dip galvanizing is all that is required.

Coating withstands mechanical impact. Minor damagedoes not need to be repaired. More serious damage mustbe repaired by means of zinc metal spraying or coatingwith zinc-rich paint, preferably containing an epoxy.

Factor

Preparation

Process

Application

Application Conditions

1. Temperature

2. Humidity

3. Air pollution

Type of steel

Properties of the Coating

1. Thickness

2. Adhesion

3. Uniformity

Hardening time

Dimensional Stability

Inspection

Risk of damage duringtransportation andhandling

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Table 3. Compatibility of hot dip galvanized coatings with variousmedia.

{Recommended MaximumCoating Thickness.Heavy duty coatingsnot possible withfasteners.

Hot Dip Galvanized Zinc Coated Fencing Wire to SANS 675 (1,2mm - 5,0mm Diameter)Hot Dip Galvanized Zinc Coatings on Steel Wire to SANS 935:2007 (1,2mm - 5,0mm Diameter) Grade Fully (Heavy) Galvanized

Hot Dip Galvanized Zinc Coatings on Steel Wire to SANS 935:2007 (1,2mm - 5,0mm Diameter) Lightly Galvanized

COMPATIBILITY OF GALVANIZED COATINGS WITH VARIOUS MEDIA

Compatibility of hot dip galvanized coatings with various media issummarised in the table below. Further specific information is

available from Hot Dip Galvanizers Association Southern Africa.

Aerosol propellants excellent

Acid solutions down to pH 6.0 fairstrong not recommended

Alcohols anhydrous goodwater mixtures not recommendedbeverages not recommended

Alkaline solutions up to pH 12.5 fairstrong not recommended

Detergents inhibited good

Diesel oil sulphur free excellent

Fuel oil sulphur free excellent

Gas towns, natural, propane, butane excellent

Glycerine excellent

Inks printing excellentaqueous writing not recommended

Insecticides dry excellentin solution not recommended

Lubricants mineral, acid free excellentorganic not recommended

Paraffin excellent

Refrigerants excellent

Sewage excellent*Fertilizers dry good

aqueous use with care

Timber preservatives:Copper-chromium-arsenic, freshly treated poorAfter drying is completed excellentBoron excellent

Trichlorethylene excellent

* Anaerobic conditions to be avoided.

Sewage Treatment

Hot dip galvanized coatings perform extremely well by comparison withother protective coatings for steel in the severely corrosive conditions pre-vailing in most sewage treatment operations. As a result hot dip galva-nized steel is used extensively in sewage treatment plants throughout theworld.

Table 2. Zinc coatings compared in terms of coating thickness and relative life expectancy.

GENERAL INFORMATION ABOUT ZINCAtomic Weight 65.37Density- rolled – 25°C 7192 kg/m3

- cast – 25°C 6804 kg/m3

- liquid 6620 kg/m3

Melting Point 419.5°CBoiling Point 907°CAppearance bluish-white metalAtomic number 30Modulus of elasticity 7 x 104 MN/m2

Specific heat 0.382 kJ/kg.KLatent heat of fusion (419.5°C) 100.9kJ/kgLatent heat of vapourisation (906°C) 1.782 MJ/kgHeat capacity- Solid 22.40 + 10.5 x 10-3 TJ/mol- Liquid 31.40 J/mol- Gas 20.80 J/molLinear coefficient of thermal expansion (20–400°C) 39.7µm/m.KVolume coefficient of thermal expansion (20–400°C) 0.89 x 106/KThermal conductivity: solid (18°C) 113W/m.KElectrical resistivity (20°C) 5.9uΩmStandard electrode potential (H2 electrode) -0.762VEnthalpy of Vapourisation 114.2 kJ/mol

Table 4. Properties of zinc.

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Figure 7. Section through electrolytically applied zinc layer.

Figure 8. Zinc metal spraying.

CCoorrrroossiioonn PPrrootteeccttiioonn MMeetthhooddssSee also SANS 4042/ISO 4042 for fas-teners and SANS 2081/ISO 2081 forother components.

3.3 ZINC METAL SPRAYING

The steel is cleaned by means of abrasiveblasting - at least to Sa 21/2, according toISO 8501-1. Zinc is fed into the spraygun in the form of wire or powder andmelted by a gas flame or electric arc (fig-ure 8). The molten droplets are thensprayed on to the steel surface with theaid of compressed air.

The zinc layer can exhibit porosity andthe surface may be coarse (figure 9). Thethickness of the coating can be variedfrom about 30µm to (in practice) about300 µm. Adhesion to the steel surface ispurely mechanical.

The method is suitable for larger objectsof relatively simple shape. It is also wellsuited to the repair of zinc coatings onhot dip galvanized components thathave been damaged by mechanical im-pact or welding.

See also SANS 2063/ISO 2063.

3.4 SHERARDIZING

Steel components, cleaned throughpickling, are packed together in a drumwith zinc powder and sand. The drum isrotated and heated to just below themelting temperature of the zinc. Duringa period at this temperature, and withcontinued rotation, iron and zinc reactwith each other to form an iron/zincalloy on the steel surfaces.

Sherardizing gives relatively thin coat-ings (15-40 µm) with dark grey surfaces.The coatings have good adhesion prop-erties and a very uniform thickness, evenon objects of complex shape. Themethod has about the same range of ap-plication as for electroplating.

See also SANS 53811:2006 / EN 13811:2003 Sherardizing – Zinc diffusion coat-ings on ferrous products.

3.5 MECHANICAL PLATING

Degreased objects are placed in adrum, together with glass balls. Theyare first tumbled in an acidic cleaningagent and then in a copper-platingagent. The objects are then tumbledwith zinc powder and certain activatingchemicals.

CCHHAAPPTTEERR 33

3.1 HOT DIP GALVANIZING

Steel components, cleaned of rust, millscale and other contaminants, aredipped into molten zinc, producing acoating of iron/zinc alloys with pure zincon the surface. Chapter 4 refers.

3.2 ELECTROPLATING

The steel surfaces are degreased andpickled to remove rust and mill scale.The component is then submerged in azinc salt solution and connected as acathode to a direct current source. Rodsor balls of pure zinc are connected as an-odes. The solution (electrolyte) can beacidic, neutral or alkaline, which deter-mines the type of zinc salt. When thecurrent flows, zinc from the electrolyte isdeposited on the steel surfaces. At thesame time, the anode dissolves and sup-plies new zinc to the electrolyte.

Items can either be supported in jigs orbaskets or they can be placed in drumsfor movement between the necessarybaths.

The deposited layer has a very fine crys-talline structure with a distinct boundarybetween the plated zinc and the metalsubstrate (figure 7). Thicknesses varyfrom 5 to 25 µm. However, layers thin-ner than 5 µm can often be found onitems such as fittings, small bolts, etc..Layers thicker than 25 µm can only beobtained on components or structures ofsimple smooth geometry, e.g. wire.

The surface of the zinc coating is veryeven, with a silvery, metallic sheen.Through the addition of special additivesto the bath, very shiny coatings can beobtained (bright zinc). Electroplatedcomponents are usually dipped in chro-mate to prevent corrosion during stor-age and transportation. The chromatelayer is often colourless but can, in thecase of thicker layers, be yellow-brownor green in colour.

Because of the thinness of the zinc layer,electroplated components should be fin-ished with a layer of paint or other or-ganic coating prior to outdoor exposurein order to increase the service life.

8 HDGASA © 200933

Figure 9. Section through zinc metal sprayed coating.

Figure 10. Section through zinc rich paint layer.

Figure 11. Comparison between the properties of different zinc coatings.

Zinc is usually deposited in layers withthicknesses between 12 and 15 µm, al-though thicker layers of about 75 µm aresaid to be obtainable. When coatingsthicker than 30µm are applied, low tem-perature heat treatment is necessary afterplating, to avoid flaking. The coatings arevery uniform, even on objects of complexgeometry. The surface is matt. Theiron/zinc alloys produced by the hot dipgalvanizing process are absent in mechan-ically plated zinc coatings and unlike hotdip galvanizing, the coating on edges andcorners is thinner than that on flat surfaces.This is due to impact during the tumblingprocess and for this reason, products witha mass of more than 0,25kg are not rec-ommended for coating by this method.When thicker coatings are applied,>20µm, oversizing of internal threads orundercutting of external threads, is neces-sary. Since there is little risk of hydrogenembrittlement even hardened steels canbe treated in this way.

ASTM-B695 may be applied.

3.6 COATING WITH ZINC-RICH EPOXY OR PAINT

As with zinc metal spraying, steel com-ponents should be cleaned by means ofcareful abrasive blasting - at least to Sa21/2, according to ISO 8501-1. Scrapingor wire-brushing alone does not givesatisfactory results when coating an en-tire component. However, when recon-ditioning a coating on site, proper abra-sive paper cleaning or wire brushing canbe quite successful.

Zinc-rich paint consists of fine grainedzinc powder in an organic or inorganicbonding agent. Both one and two-com-ponent paints are available. The zinccontent in the dry paint film should be atleast 80% by mass, which correspondsto 54% by volume. Whilst the zinc in thezinc rich paint does provide an elementof initial cathodic protection due to inter-spersed resins and binders, which are re-

Zivilsenats des Bundesgerichthof, saidin a verdict dated 12th March 1969that "cold galvanizing" was an illegalproduct description.

Coating with zinc-rich paint is a paint-ing procedure and not a method ofmetal coating.

The properties of zinc coatings appliedby these various methods are given infigure 11.

Refer to Chapter 15 - “ReconditioningCoatings Damaged or On-site ModifiedHot Dip Galvanized Components”.

quired to allow the paint to adhere tothe substrate, proper cathodic protectionis short lived to about 80 days (21/2

months). The zinc rich paint at this timebecomes a normal barrier coating. Thepaint is applied by brush or spray gun,depending on paint formulation.

Painting with zinc-rich paint is some timescalled "cold galvanizing", gives the impres-sion that zinc-rich paints provide zinc coat-ings with similar properties to those ob-tained by hot dip galvanizing. This is notso, compare figure 10 to figure 23.

The designation "cold galvanizing" hasbeen legally tested in Germany.

The French chemist, Melouin, discoveredas long ago as 1741 that zinc was capableof protecting steel from corrosion.However, the method was not used muchuntil another Frenchman, Sorel, intro-duced pickling in sulphuric acid as apreparatory measure. He subsequentlyapplied for his first patent on hot dip gal-vanizing on 10th May 1837. The main partof the procedure that Sorel sought topatent is still used today.

In an appendix to his patent applicationin July 1837, Sorel called the method"galvanizing", referring to the galvaniccell that is created if the zinc coating isdamaged. The steel in the damaged areabecomes a cathode in the cell, and isprotected from corrosion. The name hassubsequently been adopted by othermethods for coating steel with zinc andis sometimes used for electrolytic metaldeposition in general. To avoid confu-sion, hot dipping in zinc should bereferred to as hot dip galvanizing.

4.1 THE ADVANTAGES OF HOT DIP GALVANIZING

� Lower first cost. Hot dip galvanizinggenerally has the lowest first costwhen compared to other commonlyspecified protective coatings forsteel. The application cost of labourintensive coatings such as paintinghas risen far more than the cost of fac-tory applied hot dip galvanizing.

� Lower maintenance / lower longterm cost. Even in cases where theinitial cost of hot dip galvanizing ishigher than alternative coatings, gal-vanizing is invariably more cost effec-tive, due to lower maintenance costsduring a longer service life.Maintenance is even more costlywhen structures are located inremote areas. Maintenance pro-grammes also invariably have a neg-ative impact on productivity.

� Long life. The life expectancy of hotdip galvanized coatings on structuralmembers is in excess of 50 years inmost rural environments, andbetween 10 to 30 years in most cor-rosive urban and coastal environ-ments.

� Surface preparation. Immersion inacid ensures uniform cleaning of thesteel surfaces. In contrast heavy dutyorganic coatings must be applied onabrasive blast cleaned surfaces (gen-erally to ISO 8501 - 1 to SA21/2) andverified by third party inspection.

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HHoott DDiipp GGaallvvaanniizziinngg

44

Figure 12. Examples of profiles and structures that are difficult to access for mechanical cleaning. In hot dip galvanizing, all surfaces receive equally good coatings.

Figure 13. Micrograph showing the slightly thicker hot dip galvanized coating at corners.

zinc coating

steel

Figure 14. Paint coatings are usually thinner over corners and sharp edges. Hot dip

galvanized coatings, on the other hand, are at least as thick or greater at these locations.

Additionally, the application oforganic coatings is limited in terms ofprevailing ambient temperature andrelative humidity. This adds to thecost of applying a heavy duty paintsystem.

� Adhesion. The hot dip galvanizedcoating is metallurgically bonded tothe steel surface.

� Environmentally friendly. The coat-ing is not toxic, and it does not con-tain volatile substances.

� Speed of coating application. A fullprotective coating can be applied inminutes. A comparable multicoatpaint system, may require severaldays. The effective application of a

CCHHAAPPTTEERR 44

10 HDGASA © 200944

Figure 15. The principle of hot dip galvanizing.

� Reliability. Hot dip galvanizing isrequired to conform to the SANS121/ISO 1461 specification. Thecoating thicknesses specified arerelated to steel thickness. Coating lifeis reliable and predictable.

� Faster erection time. Once steel ishot dip galvanized it can immedi-ately be inspected, transported anderected. When assembly of struc-tures is complete, they are immedi-ately ready for use. No time is loston-site for surface preparation,painting, drying, curing and finalinspection.

� Ease of inspection. Hot dip galva-nized coatings are readily assessedvisually. Simple non-destructive test-ing methods are used to determinecoating thickness. Inspection oforganic coatings is necessary aftersurface preparation and each stage ofcoating application thereafter. The hotdip galvanizing process is such that ifcoatings appear sound and continu-ous, they are sound and continuous.

� Over coating with paint, (duplexprotection). If correctly applied aduplex system will provide durablecolour, chemical resistance and a syn-ergistically extended service life.

� Unsightly graffiti is easily re-

moved. Painted graffiti can be easilyremoved by solvents with no dam-age to the hot dip galvanized coat-ing. This is not easily achieved with apaint coating.

4.2 THE DISADVANTAGES OF HOT DIP GALVANIZING

� Hot dip galvanizing can only be donein a galvanizing plant. Site applicationis not possible.

� The colour of the zinc coating can bechanged only by painting.

� The dimensions of the componentor structure are limited by the sizeof the zinc bath. Innovative meth-ods of accommodating larger com-ponents have been achieved, dis-cuss with the Association or hot dipgalvanizer. For member bath sizes,see www.hdgasa.org.za

� Residual stresses in metals due torolling, bending and welding mayresult in unexpected distortion.However, careful design, good fabri-cation following the design criteriaoutlined in this booklet and con-trolled galvanizing, will eliminate themajor causes of distortion. Removalor redistribution of suspected residualstress by heat or other methods in

hot dip galvanized coating is notinfluenced by weather conditions.

� Uniform protection. All surfaces of ahot dip galvanized article are protect-ed both internally and externally,including recesses, sharp corners andareas which are inaccessible for theapplication of other coating methods(figure 12). The coating is thicker oversharp corners and edges than on flatsurfaces (figures 13 and 14).Thickness, coating adhesion and uni-formity are features of the process.No other coating applied onto astructure or fabrication can providesimilar uniform protection.

� Sacrificial protection at damagedareas. A hot dip galvanized coatingcorrodes preferentially to steel, pro-viding cathodic or sacrificial protec-tion to small areas of steel exposedthrough damage. Unlike organiccoatings, small damaged areas needno touch up while corrosion creepunder the coating cannot occur (fig-ures 89 and 90).

� Toughness. A hot dip galvanizedcoating has a unique metallurgicalstructure, which gives outstandingresistance to mechanical damageduring transport, erection and ser-vice.

critical components can be achieved.

� The welding of zinc-coated steel candemand a somewhat different pro-cedure compared to uncoated steel.The welding of hot dip galvanizedsteel results in a degree of coatingloss through the 1st and 2nd HeatAffected Zones although a portionof the original coating remains intactright up to the edge of the weld. Itis necessary to recondition the coat-ing over the weld and surroundingcoating.

4.3 THE HOT DIP GALVANIZING PROCESS

General Hot Dip GalvanizingThe metallurgical reaction between steeland molten zinc, which produces a hotdip galvanized coating, can only takeplace if surfaces are free from contami-nants. If steel surfaces are contaminatedwith marking paint, weld slag and othersubstances not readily removed by acid,these must first be removed by mechan-ical means, such as abrasive blasting orgrinding. Moulding sand on the surfacesof castings is removed by means of abra-sive blasting.

Grease and oil is removed by the galva-nizer with degreasing chemicals, eithercaustic or acid based. Rust and millscaleare removed from steel surfaces by pick-ling in diluted hydrochloric or sulphuricacid. After pickling and rinsing, a fluxingagent is applied. The purpose of fluxing isto dissolve surface oxides on both the

steel and the molten zinc surfaces thusenabling steel and zinc to make metalliccontact with each other. Fluxing can beapplied in two different ways, designatedwet and dry galvanizing respectively. Asfar as coating quality is concerned, bothmethods give equally good results.

In wet galvanizing the surface of the zincbath is divided into two sections by aweir. The fluxing agent - ammoniumchloride, is deposited on the zinc surfacein one section of the bath. The steel com-ponents, still wet from pickling and rins-ing are dipped through the molten fluxinto the zinc. The components are thenmoved into the flux-free section of thezinc bath. The flux residue and oxidesare skimmed from the surface of thebath, whereupon the components canbe lifted up through a pure, smooth zincsurface. Wet galvanizing is largely con-fined to small components and semi-automatic tube galvanizing.

Dry galvanizing is the preferred methodfor coating batch galvanized components.After pickling and water rinsing, the com-ponents are dipped in a flux solution ofammonium chloride and zinc chloride. Inthis way a thin layer of flux salts is deposit-ed on the surfaces of components. Beforecomponents are dipped into and with-drawn from the bath, the surface of themolten zinc is skimmed to remove zincoxide and flux residues. After withdrawalfrom the zinc bath, components arequenched either in a sodium dichromaterinse or plain water. Alternatively, theymay be aircooled. Components are then

11HDGASA © 200944

LIGHT WHITE DISCOLOURATION - THIN,WHITE POWDERY DEPOSIT

HEAVY WHITE DISCOLOURATION- THICK,CRUSTY DEPOSITS

BLACK STAINING AND WHITE DISCOLOURA-TION WITH POWDERY DEPOSITS

RED RUST

None required. The protective properties of zinc are not impaired by the presence ofsuperficial white discolouration. Existing white discolouration deposits will slowly con-vert to protective basic zinc carbonate. Not suitable for post painting before removingloosely adhering deposits.

Before painting, remove all traces of loosely adhering deposits with stiff bristle brush(not a wire brush). Check residual zinc coating thickness with an electromagnetic thick-ness gauge. (On continuously galvanized sheet, the electromagnetic thickness gauge isused merely as an indicator of the zinc coating thickness. The method cannot be usedto fail the coating in terms of thickness.) If the coating thickness is within specificationand if the sheet or component is to be used in reasonably dry or freely exposed con-ditions, no action is required.

Check zinc coating thickness using an electromagnetic thickness gauge. (The electro-magnetic thickness gauge is used merely as an indicator of the approximate zinc coat-ing thickness on continuously galvanized sheeting. The method cannot be used to failthe coating in terms of thickness.) If in doubt contact the HDGASA before painting, dueto the complex nature of stains.

In general, sheet or components showing red rust should be repaired or not used at all.

EVALUATION OF WET STORAGE STAIN(Refer to Chapters 5 and 12)

VISIBLE EFFECT CAUSE REMEDIAL ACTION

Table 5. Evaluation of wet storage stain.

Caused by moisture trapped between sheetsor components during transportation or stor-age, or by condensation in the absence ofadequate ventilation.

Prolonged adverse storage or inadequateprotection during transport, allowing consid-erable water ingress between closely stackedsheets or components.

Usually very early stage of superficial zinccorrosion normally due to the formation ofcomplex surface zinc corrosion product.Black staining does not imply that the zinccoating has been destroyed.

Corrosion of steel substrate where zinc coat-ing has broken down completely. Should notbe confused with superficial staining.

ready for fettling (if necessary), inspectionand dispatch (figure 15).

Centrifuge Hot Dip Galvanizing Small components such as nails, nuts,bolts, washers and fittings are cleaned asdescribed above and placed in perforatedbaskets, which are then dipped into themolten zinc. Upon withdrawal from thezinc bath, the basket is placed in a cen-trifuge. Rotation has the effect of throwingexcess zinc off the coated surfaces, leav-ing the components free from unevendeposits of zinc. The zinc layer on cen-trifuged articles is somewhat thinner, thanthat obtained by the general process.Centrifuging is essential for threaded arti-cles, where thread clearance and coatingthickness tolerance are critical (figure 15).

Tube Hot Dip Galvanizing Tubes are hot dip galvanized either bythe dry or wet methods in semi-auto-matic production lines. Immediatelyafter withdrawal from the zinc bath,excess zinc is wiped off external surfacesto provide a smooth and uniform coat-ing. The thickness of the zinc coating canbe controlled to some extent by adjust-ing the air pressure in air wiping equip-ment. Internal surfaces are cleaned ofexcess zinc with the aid of steam, whichis forced down the bore of the pipe. Thetube hot dip galvanizing process is nor-mally only applied to flangeless tubeswith a maximum nominal bore up to114mm OD. Larger diameters and tubeswith flanges are galvanized by way ofthe general process.

55

Hot dip galvanized sheet is produced oncontinuous zinc coating lines, (figure 16),from either cold rolled (thickness range0.27 to 2.0mm) or hot rolled (thicknessrange 2.1 to 3.0mm) steel coil to therequirements of SANS 4998 and SANS3575 or ASTM A653. Specification SABS934 should no longer be referred to as ithas been replaced by SANS 4998/ISO4998 and SANS 3575/ISO 3575.

Steel coils are welded end on end to form acontinuous strip. After degreasing the strip ispickled or oxidized. Oxides are thenremoved from the surfaces by reduction at950°C. At the same time the strip is soft-annealed. The surfaces of the strip, nowchemically clean, are moved through a pro-tective gas atmosphere and directly downinto the zinc bath.

The strip is withdrawn from the bath vertical-ly and passed through "air knives". Controlledjets of air or steam are blown through theknives, wiping the zinc coating to the desiredthickness.

The galvanizing process yields an even zinccoated sheet with a bright smooth metallicfinish. The zinc coating can be supplied witha regular or flattened minimised spangle fin-ish. (Refer to 7.8 The Iron/Zinc Reaction inContinuous Galvanizing).

After cooling, straightening and treatmentagainst wet storage stain, the strip is cut intosuitably sized sheets or rolled into coils fordelivery or subsequent painting and/or pro-filing (figure 16).

5.1 ZINC COATING SURFACE FINISH

The following surface finishes may beordered to suit specific end-use require-ments:

Regular spangle (also known as normalspangle)This is the unaltered, large, multifaceted crys-tal structure that occurs during normal solid-ification of a hot dip zinc coating on a steelsheet.

Variations in the size and brightness of thespangles are possible, depending on thegalvanizing process and conditions, butthis has no effect on the quality and corro-sion resistance of the coating. Regularspangle is supplied for a wide range ofapplications where overpainting for main-tenance purposes can be undertaken at alater stage.

Flattened minimised spangleThis is a zinc coating that is obtained byrestricting the normal zinc crystal growthfollowed by the application of a skin passprocess. The zinc coating thus obtained has

12 HDGASA © 2009

HHoott DDiipp GGaallvvaanniizziinngg ooff SShheeeett MMeettaall

Figure 16. Schematic diagram showing the continuous hot dip galvanizing process for the coating of sheet.

improved formability and the zinc surfaceserves as an excellent base for pre-paint-ing, post-painting and powder coatingapplications.

This finish is recommended for applicationswhere a high gloss paint finish is required. Itis available for zinc coatings of mass up toZ275, and a maximum steel thickness of1.20mm if passivation is required, or a max-imum steel thickness of 1.60mm if passiva-tion is not required.

Zinc coatings of different thicknesses inaccordance with SANS 4998/ISO 4998 orSANS 3575/ISO 3575 may be ordered tosuit specific end use requirements. Certaincoating grades are more readily available(tables 6 and 7 respectively).

The thickness and type of steel substrate isselected on the grounds of mechanical andstructural consideration, whereas the thick-ness of the zinc coating is selected accordingto the corrosion-resistant life expectancyrequired.

Corrosion resistanceThe protection afforded by a hot dip galva-nized coating under normal conditions ofexposure is directly related to its thickness.The coating on sheet, normally stocked byretailers, is Z 275, which is suitable for a mildenvironment.

It is recommended that galvanized sheet-ing be overpainted timeously, preferablybefore the first appearance of red corrosionproducts. Where conditions require greatercorrosion protection, a thicker class of coat-ing ie. Z 600 or the addition of a paint coat-ing should be considered. In the case of the

heavier coating, the sheet is not suitable forsevere forming other than normal corrugat-ing or curving.

Bend tests to evaluate the adhesion of thezinc coating are carried out and evaluated inaccordance with relevant specifications (table8). In addition to this, impact adherence cup-ping tests are performed on all products,irrespective of specification, to ensure goodadhesion of the zinc coating.

Wet storage stain (white rust)When galvanized sheet in coil or sheetpacks is stored under wet conditions, thegalvanising may be damaged by wet stor-age staining.

Rainwater or water vapour can easily bedrawn in between tightly profiled or flatsheets, or between laps of coils by capil-lary action. Due to the absence of freelycirculating air, this moisture cannot evap-orate, causing unfavourable conditionsthat may result in white rust on galva-nized sheeting.

Normally, light white staining on galva-nized sheet is not serious. The wet storagecorrosion process will stop when the affect-ed areas are dried and exposed to theatmosphere. The discoloration will disap-pear within a few months during the nor-mal weathering of the material. Whereaffected surfaces will form part of unex-posed overlaps or other concealed areasthat may be subject to extended periods ofdampness, such areas should be cleanedand additionally protected.

Galvanized material must under no circum-stances be stacked directly on a floor. See

CCHHAAPPTTEERR 55

55

5.6 PRIMER COATED GALVANIZED STEELSHEET PRODUCED IN ACONTINUOUS COATING LINE(CHROMAPREP®)

Coating ProcessCHROMAPREP® is a registered trade namefor cold rolled or hot dip galvanized steelsheet, coated with a high quality, flexibleand corrosion inhibiting chrome freeprimer. The substrate is chemically cleanedand treated to ensure good adhesion ofthe chromate free primer.

The coating has a nominal thickness of 4-6micrometres applied by a sophisticatedcontinuous roller coating process, permit-ting control of coating uniformity and filmthickness within narrow limits. The primercoat is finally oven cured and is suitable forovercoating with most locally available fin-ishing paint systems. (Refer to table 9).

CHROMAPREP® is supplied with a chromefree primer on both sides of the steelsheet. CHROMAPREP® with a cold rolled

13HDGASA © 2009

Table 7. Readily available zinc coating grades in Southern Africa.

Table 8. Ratio of the inside bend diameter to the thickness of the specimen.

Coating Commercial Steel (CS), Forming Steel Structural Steel (SS) Designation (FS) and Deep Drawing Steel (DDS) ASTM A653M-97�

ASTM A653M-97

Galvanized Sheet Thickness t(mm) Grade Grade Grade230 255 275

0.4 ≤ t ≤ 1.0 1.0 < t ≤ 2.0 t > 2

Z275 0 0 1 1.5 2 2.5Z600 2 2 2 2 2 2.5

� Note: Grades 340 and 550 do not have specified requirements for this property

CoatingDesignation

Mass of coatings*(both sides inclusive)

g/m2, min.

Average Individual Average Individual

Equivalent thicknessper side **

μm, min

Table 6. Mass per unit area of zinc coating.

Z 100† 100 85 7 6 (4,8)Z 180† 180 150 13 11 (8.5)Z 200† 200 170 14 12 (9,7)Z 275 275 235 20 17 (13,4)Z 350 350 300 25 21 (17,1)Z 450 450 385 32 28 (22)Z 600 600 510 43 36 (29)Z 700 700 595 50 43 (34)

CoatingDesignation

Mass of coatings*(both sides inclusive)

g/m2, min.

Average Individual Average Individual

Equivalent thicknessper side **

μm, min

ZINC COATING MASS IN ACCORDANCE WITH SANS 4998/ISO 4998 AND SANS 3575/ISO 3575

NOTES

* Not less than 40% of the individual value should normally be found on each surface, indicated in brackets.

† Although coating Classes Z 100, Z160, Z 180 and Z 200 are included in this table, these classes arenot recommended for bare external applications but have been included for products which would sub-sequently be further protected by suitable paint systems.

** For information only. The equivalent thickness is calculated from the following formula:

Thickness; μm = Mass per unit area, g/m2

2 x 7

(7 is the approximate specific gravity of zinc)

The letter Z in the coating designation indicates a pure zinc coating and the number denotes the totalmass of the coating on both faces of the sheet (g/m2)

1. Only available in 0.27 to 0.30mm full-hard material, except Z160

2. Iscor specification only

3. Not recommended for forming grades

4. Not available on full hard material

Z1601,2 † 160 135 11 9 (7.7)Z275 275 235 20 17 (13.4)Z6003,4 600 510 43 36 (29)

figures 19, 20 and 21, table 5 and Chapter12 and also Removal of Wet Storage Stain,page 16.

5.2 SURFACE TREATMENT

The following surface treatments are normal-ly used to reduce the possibility of wet stor-age stain during transport and storage:

PassivationPassivation by potassium dichromate is nor-mally applied to all galvanized material. Incases where this treatment may interferewith subsequent processing, the galvanizedsteel may be ordered without passivation, inwhich case oiling of the zinc surface is rec-ommended.

OilingA special corrosion-preventive oil is used tocoat galvanized sheet as an additional pro-tection against wet storage staining duringhandling and storage. Oil is only used ifrequested.

If unoiled unpassivated galvanized steelsheet is ordered, proper protective packingshould be requested to protect the materi-al against the ingress of moisture duringtransport and storage. (Refer to SafeStorage, page 16).

5.3 CUT EDGE CORROSION RESISTANCE

The introduction of continuously galva-nized coil that is subsequently cut intosheet lengths, has tended to focus atten-tion on the behaviour of cut edges whichare exposed to atmospheric corrosion.Sheet, thinner than 1.6mm is usually ade-quately protected at cut edges by thecathodic action of the zinc coating.Similarly, side trimmed edges seldom pre-sent a corrosion problem.

Thicker coatings provide superior cathodicprotection.

5.4 STRAIN AGEING

Galvanized steel sheet tends to strain ageand this may lead to the following:

1. Surface markings from stretcher strain(Lüder’s lines) or fluting when the sheetis formed.

2. Deterioration in ductility.

It is recommended that the period betweenfinal processing at the mill and fabrication bekept as short as possible, preferably notexceeding six weeks.

5.5 PAINTING

Chemical conversion coatings and primershave been developed to provide good adhe-sion of subsequent paint films on zinc coatedsurfaces. To obtain optimum results it isessential to adhere to the instructions of thepaint manufacturers.

Dry film thickness 4-6 micrometres

Heat resistance Max 120°C

Resistance to common water based detergents Excellent

Resistance to mild solvents(1) Fair

Flexibility(2) 1T

UV - resistance(3) Fair

(1) Sensitive to common lacquer thinners, i.e. chlorinated or aro-matic hydro-carbons and ketones but resistant to mineral tur-pentine, solvent naphta, methylated spirits and paraffins.

(2) No coating failure or loss of adhesion when bent around amandrel with a diameter as indicated (T is the thickness of thesheet in mm)

(3) As is the case with most chrome free primer coatings,CHROMAPREP® is sensitive to ultra-violet radiation andshould not be exposed to direct sunlight for prolonged peri-ods before application of the final coating system. Whendirectly exposed to sun-light (ultra-violet radiation) the finalcoat must be applied within seven days of being exposed.

Table 9.

coat, as well as a higher quality paint sur-face, may be obtained by application of aprimer or intermediate coat for theselected paint systems.

Amongst current industrial products, thefollowing paint systems can be appliedto CHROMAPREP®: alkyds, vinyls,acrylics, polyesters, powdercoatings,stoving enamels, epoxies and poly-ure-thanes.

5.7 PAINTED COLD ROLLEDGALVANIZED STEEL SHEETPRODUCED IN A CONTINUOUSCOATING LINE (CHROMADEK® ORCHROMADEK® PLUS)

CHROMADEK® is the trade name for thispre-painted galvanized steel sheet.CHROMADEK® is a colour coat compris-ing a Z200 hot dip galvanized substratewith a 4 to 6 micron DFT primer under-neath the top coat and an 8 micron DFTsingle coat paint on the reverse side.

CHROMADEK® paint is then applied at 20microns DFT to the top surface (figure 17).

The colour coated products are coated ona sophisticated continuous roller coatingline. The modern coating process permitsgood control of the important paintingparameters and rigid quality control oneach finished coil ensures that every batchconforms to specification. Excellent paintadhesion is achieved and corrosion resis-tance enhanced by careful preparation ofthe steel sheet under factory conditionsprior to paint application. The paint sys-tems are oven cured. The aestheticappearance and durability of CHRO-MADEK® cannot easily be achieved byconventional hand painted systems.

The coating is highly formable and pro-vides additional protection under condi-tions where the corrosion resistance of

Table 10. CHROMADEK® paint system properties.

PROPERTY TEST CONDITIONS METHOD SPECIFICATION TYPICAL

Resistance to colour QUV (1000 hours) ASTM G154 ΔE<5, e.g.change Gemsbok Sand

Resistance to chalking QUV (1000 hours) ASTM G154 Rating Range: 1-2ASTM D4214

Resistance to Salt spray ASTM B117corrosion: (1000 hours)

- Edge creep After 1000 hours ≤ 3mm < 2mm- Blister size After 1000 hours ASTM D714 ≤ 8F < 8F

Flexibility: ASTM D4145 3T. 2T.bend test No adhesion loss No adhesion loss

Flexibility: ASTM D2794 No cracks No cracksreverse impact No adhesion loss No adhesion loss

Film hardness ASTM D3363 F - H F - H

Dry film thickness NCCA 4.2.2 22µm minimum 22µm minimuminclusive of primer inclusive of primer

Gloss at 60° At time of coating ASTM D523 25 - 35% 25 - 35%

5514 HDGASA © 2009

unpainted galvanized sheeting may proveinadequate.

Corrosion resistanceCHROMADEK® is intended for exposureto rural, mildly chemically polluted ormoderate marine conditions. Best resultscan be obtained through the correct appli-cation, good workmanship and mainte-nance procedures.

NOTE: CHROMADEK® is not recom-mended for application in marine envi-ronments (area approximately 5km fromthe sea) or exposure to industrial envi-ronments where there is an accumula-tion of strong acid vapours. CHRO-MADEK® PLUS is recommended forthese areas between 1 and 5km fromthe sea.

CHROMADEK® PLUS is a colour coatcomprising a Z275 hot dip galvanizedsteel substrate, pre-primed on one or bothsurfaces with 20 - 25 micron DFT chromefree universal primer. Alternatively, onlyone surface is coated in accordance withthe above and the other surface as per thestandard CHROMADEK® (4 - 6 micronDFT). CHROMADEK® paint is then appliedto both surfaces, both to 20 micron DFT(figure 17).

The Plus system has excellent physicalproperties, excellent flexibility, excellentcorrosion resistance with excellent resis-tance to ultraviolet radiation (UV perfor-mance).

CHROMADEK® PLUS is recommendedfor exterior building profiles in applica-tions requiring high formability, goodgloss retention, high colour stability andexcellent corrosion resistance. It is suit-able for corrosive environments such asindustrial and marine environments.Marine environments can generally bedefined as areas within 1km of the sea(table 10).

steel substrate may be used for indoorapplications while CHROMAPREP® witha hot dip galvanized substrate can beused for both internal and external appli-cations. However, for external uses it isintended that it is used after applicationof a final paint coating.

Typical primer coat properties

Corrosion resistanceCHROMAPREP® serves as a good corro-sion inhibiting primer coat for subse-quent painting. Resistance to corrosioncreep is improved by using a galvanizedsteel substrate, which is strongly recom-mended for exterior applications.

Cleaning of primer coat before finalpaintingSurfaces should be cleaned by removingsurface contaminants by wiping withnatural mineral turpentine, solvent naph-ta or methylated spirits, followed by awarm water detergent wash and a cleanwater rinse. Users are advised to ensurethat thinners or adhesives used, arecompatible with CHROMAPREP®. TheCHROMAPREP® primer coat is slightlyundercured to ensure good bonding ofsubsequent top coats. The liberal use ofstrong solvents can and will detach theprimer coat, which may lead to prema-ture peeling of the paint.

Common lacquer thinners such as chlori-nated hydrocarbons or ketones (MEK)should not be used for cleaning purpos-es as these may affect the adhesion ofthe epoxy primer-coat.

Application of paint coatingsThe required paint finish can be appliedby normal spray, airless spray or brush-ing techniques. Usually an additionalprimer coat will not be necessary, but formost paints a better bond between theCHROMAPREP® surface and the top

55

include coastal locations (and thereforethe risk of saline spray and deposits col-lecting on the exposed reverse sides ofoverhangs), extremely polluted industri-al environments, and very low pitchedroofs. In these or similar conditions,extra protection may be necessary. Thiscan be achieved by specifying CHRO-MADEK® PLUS to both surfaces.

CompatibilityMost materials used in contact with tra-ditional galvanized steel can be safelyused with CHROMADEK®. Run-off waterfrom Cor-Ten, lead or copper products,however, may cause staining and shouldnot be allowed to come into contact withthe painted surface.

Edge protectionGenerally cut edges on CHROMADEK®

sheets do not present a corrosion prob-lem even in coastal areas as the galva-nized coating will sacrificially protect theexposed steel. Small traces of whitedeposits on cut edges should therefore,not be a reason for concern.

15HDGASA © 2009

5.8 FASTENING METHODS

Mechanical fastening systems such asrivets, self-tapping screws, bolts andnuts, spring clips and wire staples can beused, as well as various seaming meth-ods including lock- and box seaming.

Where protection is needed, fastenersshould, where possible, be:

� hot dip galvanized; or

� manufactured from a corrosion resis-tant material; or

� electroplated and overcoated with asuitable top coat.

Further information can be found in thelatest copy of SANS 1273.

Cutting, touch-up and maintenanceAbrasive cutting or trimming of CHRO-MADEK® sheeting on roof tops shouldbe avoided. Should cutting be necessary,remove all iron particles by vigorousbrushing with a broom or bristle brushafter cutting, to avoid tarnishing theCHROMADEK® paint surface.

In order to site cut a sheet with cleanedges and no paint damage, a sheet nib-bler is recommended.

Specially formulated air-drying touch-uppaints are available. Care should be exer-cised to minimise overpainting as thismight accentuate the defect. The ultra-vio-let resistance of air-drying touch-up paintsis generally less than the oven-curedCHROMADEK® finishes. Accordingly,touching-up of scratches should be donewith a thin paint brush to minimise unnec-essary overpainting. If aestheticallyacceptable, it is recommended that minorscratches resulting from erection andrough handling be left uncoated as thegalvanized substrate will offer adequatesacrificial protection against corrosion.

The life of a CHROMADEK® painted sur-face can be extended and the appear-ance maintained by washing down peri-odically with water and a mild detergentto prevent any build-up of corrosivedeposits, especially in marine or industri-ally polluted environments.

The extent of the damage toCHROMADEK® paint coatings is ratherdifficult to assess. In cases where theoriginal gloss and colour have beenretained, there should be no cause forconcern. On proper drying of the mois-ture contained between closely nestledsheets, no further deterioration willoccur. Where discolouration and/or signsof white corrosion products (except cutedges) are evident, such sheets shouldbe substituted with new material.

Certain situations can create unusuallyaggressive conditions for the exposed,reverse sides of roof sheets. These

Galvanized coating (14 microns)

Single backing coat (8 microns)

CHROMADEK® Paint (20 ± 2 microns)


STEEL BASE

Pre-treatment(Conversion layer)

Primer (4 - 6 microns)


Reverseside}

Topside}

CHROMADEK® PAINT

}Galvanized Coat (20 microns)




STEEL BASE


Primer (4 - 6 microns)alternatively primer(20 - 25 microns)

Primer (20 - 25 microns)


Reverseside

Topside}

CHROMADEK® PLUS

Figure 17.

5.9 THE HANDLING AND PROTECTION OF GALVANIZEDAND PREPAINTED STEEL SHEET DURING STORAGE

Galvanized and prepainted galvanizedsheet is known to perform exceptional-ly well when exposed to the elements.Under normal wet-and-dry conditions,e.g. when galvanized sheet is used asroofing and for cladding of buildings, aprotective zinc oxide/zinc carbonatelayer naturally forms on the exposedsurfaces of the material, whichimproves the resistance against corro-sion. In the case of pre-painted sheet-ing, the protective paint coating offersan additional physical barrier againstthe elements.

However, the protective nature ofthese coatings may be seriouslyimpaired when exposed to wet condi-tions for extended periods in theabsence of air. The material is at itsmost vulnerable during prolonged stor-age without the necessary precautions.

Figure 20.

Figure 19.

Rain water or water vapour can easily bedrawn in between tightly nested profiledor flat sheets, or between laps of coils, bycapillary action (figure 19).

Due to the absence of freely circulating air,this moisture cannot evaporate, causingunfavourable conditions which mayresult in wet storage stain, often referredto as “white rust” on galvanized sheet-ing. See Evaluation of Wet Storage Stain– table 5. In the case of prepaintedsheeting these conditions may cause dis-colouration of the paint film and inextreme cases wet storage staining, sim-ilar to galvanized sheeting.

Wet storage stain may start soon afternested packs or coils of sheet are exposedto wet conditions and may affect theexpected maintenance-free life of thesheeting unless arrested at an early stage.The material has to be thoroughly driedand exposed to freely circulating air tostop this corrosion process (figure 20).

Steps taken to protect galvanized sheetagainst damage by wet storage stainIt is standard practice to passivate thesurfaces of galvanized sheet by chemicaltreatment during processing, in order toinhibit the occurrence of wet storagestain. Furthermore, galvanized sheet canbe ordered with a special protective oil,which is supplementary to the normalpassivation and is intended to provideadditional protection during handlingand storage.

In spite of these precautions, galva-nized sheet cannot be entirely safe-guarded against wet storage stain,especially when stored incorrectlyunder adverse conditions.

A special type of packaging is providedfor flat sheets and coils. Users, who donot have the necessary facilities to tem-porarily prevent the ingress of moistureare advised to specify such protectivepackaging.

Every endeavour is taken by manufac-turers to ensure that coated sheetproducts leave the works dry and inprime condition. Such products,whether despatched in coils or cutlengths, are packed, handled andloaded, under cover, onto vehicleswhere they are covered with tarpaulinsor canopies.

Safe storageTo prevent unnecessary damage to gal-vanized or colour-coated sheets, prop-er measures should be taken to preventcontamination by moisture while thematerial is still bundled or nested instacks (figure 20).

If not required for immediate use, coilsor packs of sheets must be stacked onsite under properly designed cover,clear off the ground and protectedfrom wind-driven rain (figure 21).

Figure 18. S-Rib galvanized steel sheeting – Z600 coating classification used for architectural applications.

Figure 21.

5516 HDGASA © 2009

Plastic tarpaulins which completely envel-op packs of sheets or coils should not beused, as a sudden drop in ambient tem-perature may cause condensation of watervapour, which can easily be drawn inbetween nested sheeting by capillaryaction.

Ideally, deliveries of galvanized andcolour-coated steel sheet to the buildingsite should be scheduled for a storageperiod of not longer than two weeks priorto installation. Inspect the storage site reg-ularly to ensure that moisture does notpenetrate the stock.

Removal of wet storage stain Wet storage stain should rather be pre-vented than cured.

Although in extreme cases the protectivevalue of the coating may be impaired, wetstorage stain attack is often superficialdespite the relative bulkiness of the corro-sion product. Where surface staining islight and smooth without growth of thezinc oxide layer as judged by lightly rub-bing fingertips across the surface, thestaining will gradually disappear and blendin with the surrounding zinc surface as aresult of normal weathering in service.

When the affected area will not be fullyexposed in service or when it will be sub-jected to a humid environment, wet stor-age staining must be removed, even if it issuperficial. This is essential for the basiczinc carbonate film to form. The formationof this zinc carbonate film is necessary toensure long term service life.

Light deposits can be removed by clean-ing with a stiff bristle (not wire) brush.Heavier deposits can be removed bybrushing with a 5% solution of sodium orpotassium dichromate with the addition of0.1% by volume of concentrated sulphuricacid. Alternatively, a 10% solution ofacetic acid can be used. These solutionsare applied with a stiff brush and left forabout 30 seconds before thoroughly rins-ing and drying.

Unless present prior to shipment fromthe galvanizer, the development of wetstorage stain is not the responsibility ofthe galvanizer. The customer must exer-cise proper caution during transporta-tion and storage to protect against wetstorage staining.

Hot dip galvanized fencing wire is pro-duced from mild, high tensile or veryhigh tensile steel wire, on a continuouscoating line which includes annealing,acid cleaning, fluxing, galvanizing,wiping to remove excess zinc andrecoiling of the finished wire.

6.1 THE PROCESS

The process is similar in arrangement tothe continuous hot dip galvanizingprocess for the coating of coil.

Zinc coatings on wire are made bypassing wire beneath a skid immersedin a zinc bath (figure 22). The skid hasmultiple contact areas which enablemolten zinc and the alloy layers to actas lubricants to ease the passage of thewire.

Between 20 and 40 individual strandspass through the plant in parallel.

An even coating is obtained by wipingthe wire after galvanizing and thishelps to control the coating thickness.The wires are generally drawn througha bed of charcoal, gas, gravel or nitro-gen and for thinner coatings, syntheticfibre is used.

For heavy zinc coatings the intervaltaken for the wire to pass through themolten zinc is extremely short, therebylimiting the iron/zinc alloy growth. Thisis essential so that the galvanized wirecan readily be bent to make chain-linkfencing or even products such asbarbed wire. In other aspects, the gal-vanized coating on wire has propertiessimilar to those of batch hot dip galva-nized products.

Once the wire exits the wiping stage,unless specifically excluded on theorder, it is passed through a passiva-tion stage. This is usually sodiumdichromate, which is necessary to pre-vent the incidence of wet storage stainon the galvanized wire.

The coating thickness is related to thethickness of steel being processed. Thethicker the coating the longer it will lastin a given environment.

Two specifications cover wire galvaniz-ing in South Africa. They are SANS 675and SANS 935:2007, the former speci-fication was amended in 1993 to

17HDGASA © 2009

HHoott DDiipp GGaallvvaanniizziinngg ooff WWiirree

66

Figure 22. Section through the plant showingthe mounting and positioning of the skid.

CCHHAAPPTTEERR 66

Diameter of zinc-coated wireExcept in the case of oval wire thecross section of the wire shall be circu-lar. The nominal diameter(s) of the zinc-coated wire shall be in the range givenin column 1 of table 13, as required.The actual (measured) value(s) of thediameter(s) shall equal the nominalvalue(s), subject to the appropriate tol-erance given in column 2 of table 13.

6.2 PRACTICAL ASPECTS

Types of wireApproximately 50% of the materialcost of a fence is in the wire compo-nent. Consequently, it is important toselect the correct type of wire for agiven application, at the most econom-ical cost.

There are two basic types of wire avail-able in South Africa, namely:

1. Soft or plain wire

2. High strain steel wire

These wires differ in that they have dif-ferent chemical composition and differ-ent physical properties and perfor-mance in a fence.

Breaking loadThe breaking load is the maximum loadthat a wire can sustain before breaking.Breaking load is expressed in kN (kilo-newton) one kN is equal to a force of101,793kg.

ElasticityA fence wire behaves elastically up to acertain load. It can stretch when a loadis applied, then return to it’s originallength when the load is relaxed.

Elastic limitAfter a certain load has been applied tothe wire, the wire will reach a pointwhere it will not return to it’s originallength. (i.e. it has been stretched)

This load limit is referred to as the yieldpoint or elastic limit. The yield point ofany wire can be regarded as approxi-mately 75% of the breaking load.

The amount of elongation produced bythe same load will depend on the diam-eter of the wire. As such, a thinner wirewill elongate more than a thicker oneand is said to have a higher elasticity.

include only one class of coating. Thelatter specification includes three class-es of which only the class 1 is equiva-lent in coating thickness to SANS 675(table 11).

Fencing material failures are not alwaysdue to the failure of the zinc coatingand frequently occur when wire ofunsuitable tensile strength is selected(table 14). Damage to the coating mayalso arise during erection and result inlocalised corrosion and rust staining ifunsuitable tools are used.

Wire complying with these standardswill in time exhibit changes in mechan-ical properties if it is compared withnewly zinc-coated wire. The changesdue to strain aging or strain-age hard-ening generally result in an increase intensile strength and a decrease in elon-gation (ductility).

Adhesion of zinc coatingTest the adhesion of the zinc coating bywrapping a suitable length of wire atleast six close turns round a cylindricalmandrel. Choose the ratio of mandreldiameter to wire diameter in accor-dance with table 12.

When tested in accordance with theabove, the coating shall remain firmlyadhered to the underlying steel wireand shall not crack or flake to such anextent that any flakes of coating can beremoved by rubbing with the bare fin-gers. Loosening or detachment ofsuperficial, small particles of zinc dur-ing the test, formed by mechanical pol-ishing of the surface of the zinc-coatedwire, shall not be considered cause forrejection. Small particles of zinc,formed as globules on the surface dur-ing zinc coating, may loosen orbecome detached during the test.These shall not be considered cause forrejection either, provided that no barespots (exposed steel) are present.

exceed 1.3kN (132kg) in high firerisk areas.

- High strain steel wire, being finer,requires less heat to raise it's tem-perature to critical levels.

To reduce the risk of fire damage tofences, keep vegetation off the fenceand grade or clear tracks along eachside of the fence. This also makesfences more accessible for mainte-nance and checking.

18 HDGASA © 200966

Table 12 - Mandrel diameter.

1 2

Grade of SteelTensile Strength

MPa

Table 14 - Tensile strength of galvanized wire.

Mild (M) 350 - 575

High Tensile (HT) 1 050 min.

Very High Tensile (VHT) 1 400 min.

1 2

Diameter Range Tolerance

Table 13 - Tolerance on diameter.

up to 1,80 ± 0,05

1,81 - 3,00 ± 0,08

3,01 - 5,00 ± 0,10

Dimensions in millimetres

1 2 3

Wire diameter d

Over Up to andincluding

Mandrel diameter

Dimensions in millimetres

Table 11 - Mass per unit area of the zinc coatingfor SANS 675 and SANS 935 class 1. (Heavy

galvanized wire.

This also means that a thinner wire willlose less tension than a thicker one.

Length of strainThe length of strain has a direct effecton the amount of tension that will beretained in a wire once it is strained.The longer the strain the less tensionwill be lost. As a guide for fencesstrained to a similar tension under sim-ilar conditions, if one is twice as longas the other, the loss of tension will behalved. Similarly for a fence half thelength, the loss of tension will be dou-bled.

Effect of temperature on fence wireWire is affected by temperature varia-tions. As the temperature drops, wirewill contract, increasing the tension inthe wire, and as temperature rises, thewire will expand, decreasing the ten-sion. The change in length is similar forall types and thickness of wire, howev-er, the resultant change in tensiondepends on the wire's elongation andwill therefore differ with wires of dif-ferent diameters.

It is the increase in tension, due to coldweather, that causes major problems ina fence.

During cold temperatures the fencewill contract and this will increase ten-sion in the wire and also on the strain-ing posts. This could result in strainerpost movement and when tempera-tures increase the wires will slackenfurther.

If these factors are taken into account,then allowance can be made for tem-perature variations if necessary. As thethinner wires have a higher elongationrate, they will not be effected to thesame degree as a thicker wire.

For each 5 degrees C above or below15 degrees C, subtract or add the fol-lowing tensions when straining afence.

4,00mm - 200 Newtons

3,15mm - 100 Newtons

2,50mm - 50 Newtons

Protective coatingsAll fencing wires are hot dip galva-nized. Zinc withstands corrosion betterthan steel, and in fact corrodes in pref-erence to the steel under natural con-ditions. This process is known as sacri-ficial corrosion.

In this process, the zinc corrodes com-pletely before steel corrosion com-mences; thus the life of the wire can be

divided into two separate components,the life of the zinc coating and that ofsteel.

Corrosion rates vary considerably.Coastal areas can be much more corro-sive than inland areas, in turn theatmosphere in industrial areas can bemore aggressive than coastal areas.

The service life of the zinc coating isdirectly proportional to the thickness ofthe coating, irrespective of the thick-ness of the wire. Refer to Chapter 12.

Most wire galvanizers supply twotypes of galvanized coatings to pre-vent corrosion:

- Lightly Galvanized

- Heavy Galvanized

The heavy galvanized wire has morethan three times the weight of zinccompared with lightly galvanizedproducts. Therefore, heavy galvanizedproducts will have a much longer lifethan lightly galvanized products.Heavy galvanized coatings are fre-quently specified for high strain steelwire, as the wire is finer and there is asmaller mass of steel.

Heavy galvanized coatings shouldalways be specified for areas wherecorrosion is known to be a problem inabnormally corrosive situations such asmarine conditions or in areas whereground salts are prevalent, such asgabions, etc., even heavy galvanizedwire may have a relatively short life.

Expected life spanThe expected life span of galvanizedwire is affected by many factors, one ofthem being coating thickness. See alsoChapter 12.

Fire damage to wireWhen comparing the performance ofdifferent wires in the field, it is impor-tant that circumstances are similar inevery respect.

International studies carried out inthese conditions indicate that:

- Temperature, tension and wirediameters are the main factorsinvolved.

- Fire temperatures less than 400degrees C do not affect the perfor-mance of any wire.

- Failure in thicker soft wire could beexpected to be fewer, because ten-sions will probably be lower.

- Tension of any wire should not

1 2 3Nominal Minimum Approximatediameter mass per unit equivalent

of zinc coated area of zinc average wire coating thickness

mm g/m2 µm

1,20 - 1,50 215 301,51 - 1,80 230 321,81 - 2,20 245 342,21 - 2,50 260 362,51 - 3,50 275 383,51 - 5,00 290 40

- 3,8 4d3,8 5,0 5d

19HDGASA © 200977

Figure 23. Cross-section of the zinc layer formed by hot dip galvanizing on a relatively reactive steel.Eta layer with 0.03% Fe; Zeta layer with 5.8 - 6.7% Fe; Delta layer with 7 - 11.5% Fe;

Gamma layer with 21 - 28% Fe.

Eta - (η) - layer

Zeta - (ζ) - layer

Delta - (δ) - layerGamma - (Γ) - layer

Steel

TThhee RReeaaccttiioonnss BBeettwweeeenn IIrroonn && ZZiinncc

Zin

c C

oatin

g μ

m

400

300

200

100

0

0 0.1 0.2 0.3 0.4 0.5

Si Content of Steel %

3 min

9 min

460°C

Figure 24. Relationship between silicon content of steel and thickness of zinc coating for a dippingtime of 3 and 9 minutes at 460°C. The curve is an average curve. Significant variations can occurbetween steels with the same Si content, but from different charges. The high reactivity is between

0.05 and 0.15% Si. This called the Sandelin Effect.

and takes on a slightly bluish metalliclustre. In some cases, especially that ofthin sheet, the zinc can solidify in theform of randomly pointed crystals,which give the surface a distinct “span-gle” finish.

The spangle finish, is just a particularform of crystal formation, whichdepends on factors such as the solidifi-cation rate. It gives no indication ofgood or bad quality hot dip galvaniz-ing. Further, the spangle finish is of no

significance to the corrosion resistanceof the zinc coating.

In continuous hot dip galvanizing ofsheet, the size of the spangle can becontrolled (Chapter 5). This is not pos-sible in general hot dip galvanizing.

Silicon-killed steelsThe constituent of steel, which has themost powerful influence on the reac-tion between iron and zinc is silicon(Si). In the making of steel, silicon is

CCHHAAPPTTEERR 77

A hot dip galvanized coating is formedby interaction between iron and moltenzinc with the formation of a series ofiron/zinc alloys which bond the coatingmetallurgically to the substrate. Thesealloys are normally over coated with alayer of relatively pure zinc which dis-plays the silver appearance associatedwith a hot dip galvanized coating.Although in most instances, suitablycleaned steel dipped into molten zincwill display this silver appearance,there are instances when reactivesteels produce coatings that are thickerthan normal and aesthetically lessappealing. Figure 23 shows a micro-graph of the typical structure of a thickhot dip galvanized coating.

Factors which influence the thicknessand metallurgical structure of a hotdip galvanized coatingThe factors which determine the overallthickness and metallurgical propertiesof a hot dip galvanized coating are; thecomposition and metallurgy of thesteel, zinc temperature, immersiontime, alloying additions to the zinc,withdrawal rate of article from themolten zinc, surface condition andthickness of the steel.

7.1 COMPOSITION AND THE METALLURGY OF THE STEEL

High reactivity during galvanizing ofcarbon steels has been observed formore than half a century. Due tochanges in steel making practice andparticularly with the introduction ofcontinuous casting, this phenomenonnow occurs more frequently. With thecontinuous casting process, either sili-con or aluminium is added to the steelas de-oxidising agents. These steels arerespectively known as aluminium-killedand silicon-killed steels. While alumini-um additions to steel have no effect onthe structure and thickness of a galva-nized coating, the same cannot be saidfor silicon which has for many yearsbeen well documented as a majorcause of increased alloy layer growthduring hot dip galvanizing.

Aluminium-killed steelsWhen aluminium killed steel isimmersed in molten zinc, the initialiron/zinc alloy produced is such as toimpede growth of further alloy layers.Thinner coatings are therefore pro-duced (figure 26).

When the zinc in the outermost layersolidifies, the surface becomes smooth

Figure 26. Cross-section through zinc coating on aluminium-killed steel.

Zinc

Iron/zinc alloy

Steel

20 HDGASA © 2009

Figure 25. Relationship between dipping timeand thickness of zinc coating in steels with different silicon contents. The curves are

average curves, based on experiments andpractical experience. Significant variations can

occur between steels with the same silicon contents, but from different charges.

77

Figure 27. Cross-section through zinc coating on silicon-killed steel with 0.06% Si. Hot dip galvanizing carried out at 460°C.

Iron/zinc alloy

Steel

Figure 28. Cross-section of a coating on a silicon-killed steel with 0.26% Si.Hot dip galvanizing carried out at 460°C.

Iron/zinc alloy

Steel

nised either in combination with siliconor alone. It has been said that the influ-ence of phosphorus as an accelerator, isof equal importance to silicon in theiron zinc reaction.

It would seem that phosphorus sup-presses delta layer formation but

The Sandelin curve has been misinter-preted by some to indicate that highreactivity in galvanizing results from thepresence of silicon alone with a reactivepeak between 0.05 - 0.15% Si.

More recently, the important roleplayed by phosphorus has been recog-

added during the process to removeoxygen.

Silicon influences the reaction betweenzinc and iron in such a way that thecrystals in the outermost alloy layer(the zeta phase) are formed either assmall grains (figure 27) or as long stem-like crystals (figure 28).

Zinc from the bath is able to penetratenearly all the way down to the steelsurface. The reaction is not retarded,but remains rapid throughout the peri-od during which the object isimmersed in the zinc. The thickness ofthe coating therefore increases consid-erably with increased immersion time(see Relationship between dippingtime and thickness of zinc coating insteels with different silicon contents –figure 25) and the coating generallybecomes relatively thick.

It should be noted that the structure ofthe alloy layer described above doesnot mean that the coating will be“porous”, or full of cavities. The spacebetween the alloy crystals is alwaysfilled with pure zinc. With silicon-killedsteels, therefore, the same compactmetallic coating is obtained throughoutas with aluminium-killed steels.

However, the influence of silicon doesnot increase linearly with increasingconcentration, but follows the curvesshown in figure 24 which gives onlytypical values.

encourages zeta phase growth whilethe gamma layer becomes discontinu-ous. This observation is confirmed bypractical studies, which have shownthat an excessively thick and brittlecoating caused by a high phosphoruscontent in the steel (>0.02%), is proneto delamination in its entirety from thesteel substrate. In contrast, coatingswhich are prone to flaking, due mainlyto reactive silicon content of steel, arepartially detached in the vicinity of thezeta/delta interface with the result thatthe steel substrate is not exposed. Theremaining adherent coating can vary inthickness from about 15µm to as muchas 40µm.

Upon withdrawal of the article from thezinc bath, a layer of zinc adheres to thealloy layer, even on silicon-killed steels.However, the reaction speed in thesesteels can be so high that the pure zinclayer is transformed completely toiron/zinc alloys before the hot articlehas had time to cool down. The reac-tion does not cease until the tempera-ture of the article has dropped below300°C.

It is for this reason that galvanizerswho are processing thick reactivesteel, can to a degree, avoid the pos-sibility of a total iron/zinc alloy coat-ing forming, by immediate waterquenching. It must, however, beborne in mind that immediatequenching can increase distortion inarticles that have a propensity forthat condition.

The iron/zinc alloy formation can there-fore extend to the surface of the coat-ing, which would then be matt, roughand light to dark grey in colour. Thecolour is determined by the proportionof iron/zinc crystals that are mixed withpure zinc on the outer surface of thecoating - the more pure zinc, thelighter the surface; the higher theiron/zinc content, the darker the sur-face.

Welding of non-reactive steel to reac-tive steel, can result in two differentcoating thicknesses, when the article ishot dip galvanized (figure 29). Forcoating uniformity, both in appearanceand in coating thickness and hencecorrosion resistance, similar steelsshould be selected for the same fabri-cation.

Weathered hot dip galvanized coatings Where iron/zinc alloy crystals areexposed, the outer surface of the coat-ing sometimes shows signs of ruststaining after a few years in service.This is not necessarily an indication thatthe coating has corroded away.Invariably adequate protection of the

21HDGASA © 200977

underlying steel exists, (see figures 30and 31, Reddish-Brown Discoloration).

Often a hot dip galvanized surface isnot uniformly grey, but has a blotchyappearance with a mixture of matt greyand shiny areas. The reasons for thiscan be many - the concentration of sili-con (primarily), phosphorous and sul-phur or other elements in the steel sur-face; stresses in the steel surface; theheat treatment and structure of thesteel - all such factors influence thesequence of reactions. Even the cool-ing process of the steel after galvaniz-ing influences its appearance.

Zeta crystals have a tendency to growout at right angles from the steel sur-face. On flat and convex surfaces,therefore, the crystals grow withoutdisturbing each other. The moltenmetal is able to penetrate between thecrystals and promote growth. On con-cave surfaces, dips and depressions,however, the crystals block each otherand inhibit growth.

It is important to emphasise that continu-ous sheet and wire processes differ radi-cally from the general galvanizing process,particularly with respect to immersiontime. Immersion time plays a significantrole in determining the ultimate structureand thickness of the coating.

Arising from research carried out by theInternational Lead Zinc ResearchOrganisation (ILZRO), reactivity classifi-cations shown in table 15 have been

CLASSIFICATION

1(figure 36)

2

(figures 37 and 38)

3(figure 39)

4a(low phosphorus)

(figure 40)

4b(high phosphorus)

(figure 41)

5a(low phosphorus)

5b(high phosphorus)

6

(figure 42)

SILICONCONTENT(mass %)

0 – 0.035

0 – 0.04

0 – 0.04

0.04 – 0.135

0.04 – 0.135

0.135 – 0.35

0.135 – 0.35

>0.35

PHOSPHORUSCONTENT (mass %)

0 – 0.025

0.025 – 0.035

>0.035

<0.01

0.01 to 0,03

<0.03

>0.03

>0

STEEL REACTIVITY

Generally normal butoccasionally low

Generally normal.

High, especially with highphosphorus content

Moderate, increasing withsilicon content

High

High, but generallythinner coatings than

on class 5b

High

High, and increasing withsilicon content

COATING APPEARANCE

Few defects. Occasional thin coatings thatare below specification.

Localised defects due to outbursts of zetaalloy. (eg ‘pimples’ or ‘tree bark’ effect,

particularly on tubular and curved sections)

Pronounced surface defects high tendency to flake

May appear normal with few defects

Generally few defects

May appear normal with few defects

Tendency to flake, especially with highphosphorus content

Tendency to flake, increasing with phosphorus content

Table 15. Reactivity classification for steels. Coating appearance can be misleading. When specifying steelfor specific applications eg. architectural features, the information under the heading “Steel Reactivity” must

be taken into consideration, ie. high reactivity may be regarded as aesthetically less acceptable.

established. The classes in the tabledemonstrate the separate and combinedinfluences of both silicon and phospho-rus in the mechanism by which a hot dipgalvanized coating is formed. Six classeshave been identified. The conclusionsreached are based on varying immersiontimes at a zinc temperature of 455°C.

Class 1. This class is the recommendedsteel for hot dip galvanizing when aes-thetic appearance is important, eg.Architectural applications and highly vis-ible structures such as lighting masts andstreet furniture. These steels are also themost suitable for structures, which ofnecessity, require long immersion peri-ods in the zinc (figure 36).

Class 2. This class will provide coatingswith a reasonable appearance providedthe immersion periods are not extend-ed. The class may show frequent localoutbursts of reactive coating - eitherpimples, striation or tree bark effect,giving localised coating thickness up to500µm thick. Tubular or curved sec-tions show these effects at lower Si andP contents (figures 37 and 38).

Class 3. This class will provide thickand rough coatings with little or no eta(pure zinc) layer. Poor adhesion willresult when extended immersion timespertain. Frequent surface defects; poorappearance and easily damaged. Atphosphorus levels greater than 0,02%,an acceptable coating is not possible toachieve at normal immersion times(figure 39).

Figure 29. Micrograph showing the hot dip galvanized coating thickness of two different steels welded together.

Figures 30 and 31. The galvanized coating is sometimes not uniformly grey but has a blotchyappearance with a mixture of matt, grey and shiny areas. Signs of red rust staining after several

years in service is not necessarily an indication that the coating has corroded away.

� Neither prolonged pickling, strip-ping and regalvanizing or abrasiveblast cleaning will alter coatingstructure but these factors mayincrease coating thickness.

7.2 ZINC TEMPERATURE

The reaction between iron and moltenzinc is influenced by zinc temperature.Iron is dissolved by diffusion and thisresults in the growth of alloy layers onthe steel surface. The formation of thealloys creates a barrier between thezinc and iron and this has the effect ofretarding the diffusion rate.

With increasing temperature up to about485°C, diffusion accelerates slowly, caus-ing a slow but constant increase in coatingthickness, following a parabolic time law.Above 485°C and up to about 530°C,coating growth is more or less linear withtime (regardless of steel composition)after which the reaction reverts to a para-bolic time law (figure 32).

At a zinc temperature in the vicinity of510°C, the reaction between liquid zincand steel is so severe that a steel gal-vanizing bath manufactured from50mm thick plate will perforate withinthe space of about sixty days. The nor-mal life of a bath at temperatures below460°C is six or seven years.

Normal galvanizing is carried out attemperatures below 460°C. Hot dipgalvanizing at temperatures in excessof 470°C is not recommended.

At the normal galvanizing temperaturerange (440°C to 460°C), a reduction inalloy layer growth can be achieved for agiven immersion time by galvanizing atthe lowest possible temperature whenreactive steels are encountered. Figure33 illustrates the effect of temperatureon coating thickness in relation to theSandelin reactivity curve. Galvanizing attemperatures below about 438°C is notpractical since this is too close to themelting point of zinc (419.5°C).

7.3 IMMERSION TIME

The degree to which immersion timeinfluences coating growth at normalgalvanizing temperatures, is deter-mined by steel composition. At highertemperatures (>485°C) all steels reactmore or less in a similar manner. In thecase of aluminium killed steels with lowSilicon and Phosphorus contents,extended periods of immersion at nor-mal galvanizing temperatures result inonly a slight increase in ultimate coat-ing thickness, eg. If a coating thicknessof 85µm is achieved at a given zinctemperature in 5 minutes, doublingimmersion time to 10 minutes is

22 HDGASA © 200977

Classes 4a, 4b and 5a. These classesare suitable for heavy-duty coatings(coating thickness greater than105µm). Coatings in these classes maydevelop a tendency to be brittle andflake when damaged, if steel contactwith the molten zinc exceeds aboutfive minutes (figures 40 and 41).

Classes 5b and 6. These classes arenot recommended for hot dip galvaniz-ing except where immersion periodscan be kept down to two or three min-utes. This is frequently not practical in aproduction line (figure 42).

Some other conclusions reached fromthis research which dispel previous

misconceptions are as follows:

� Silicon and phosphorus contentsare jointly the most important fac-tors in influencing high reactivity inhot dip galvanizing.

� Other elements in commercialgrade steels have a lesser influenceon the formation of a hot dip gal-vanized coating.

� The bulk analysis of steel can be usedreliably to predict the type and thick-ness of a hot dip galvanized coating.

� There is no evidence that highreactivity is caused by segregationof elements at the steel surface.

unlikely to increase the coating thick-ness by more than about I0µm. In con-trast, with steels prone to high reactiv-ity galvanizing (reactive levels of Siand/or P), the ultimate coating thick-ness can well increase by 50 to 100%over similar dipping periods.

When feasible, reducing immersiontime remains the most practicalmethod available to galvanizers foravoiding excessive coating growth.The main difficulty arises with struc-tures where the configuration necessi-tates manipulation while under themolten zinc, particularly in the case oftubular components where zinc isrequired to penetrate and coat internalsurfaces and then be allowed to drainout of the product on withdrawal. It isrecommended that optimum sized filland drainage holes be used when fabri-cating a tubular structure. Failure tocomply with this requirement frequent-ly renders extended immersion periodsto be unavoidable.

7.4 ALLOYING ADDITIONS TO THE MOLTEN ZINC

AluminiumThe presence of aluminium in themolten zinc retards the initial formationof Fe/Zn alloys, even at low concentra-

increase in dross formation in the gal-vanizing bath.

Vanadium and titaniumRecent research has shown that addi-tions of vanadium and titanium to themolten zinc in the galvanizing bath canovercome the problem of reactive steelgalvanizing. The coatings producedconsist of uniform layered microstruc-tures similar to those found in coatingson non-reactive steels.

Negative aspects of this developmentare an increase of approximately 25%in metal cost (which could be offset toa degree by lower overall zinc con-sumption) and a substantial increase inoxide formation on the surface of themolten zinc in the galvanizing bath.

7.5 THE WITHDRAWAL RATE OF THE ARTICLE FROM THE MOLTEN ZINC

The principle of good galvanizing is rapidimmersion and slow withdrawal. Forinstance, if an article is withdrawn at3,0m/minute as opposed to1,0m/minute the resultant coating thick-ness will be greater by about 30%, asmolten zinc is dragged out of the bath.

7.6 SURFACE CONDITION

Varying surface roughness of the steelleads to variations in thickness of thecoating. The rougher the surface of thesteel, the thicker the coating.Depending on the type of steel and thesurface profile, preparation treatmentsuch as abrasive blasting can result in a15 to 25% thicker coating. Steel thathas been severely attacked by rust, orpickled without inhibitors, also resultsin increased coating thickness.

7.7 THICKNESS OF THE STEEL

The thickness of the steel influences thecoating thickness - the thinner thesteel, the thinner the coating. Thisapplies especially to silicon-killedsteels. One reason for this is that arti-cles fabricated from thinner steels, gen-erally require shorter immersion times.It is for this reason that when reactivethinner steels are welded to non-reac-tive thicker steels, inordinately thickercoatings may result on the thinnersteel. This thicker coating may be aes-thetically less acceptable and prone tobrittleness and therefore potentialdamage, particularly on edges.Working, rolling and heat treatment ofthe steel can vary, leading to differentreactions in the zinc bath.

The composition of the zinc bath can-not be varied in ordinary hot dip galva-

23HDGASA © 200977

tions (0.007%). When extendedimmersion cycles are unavoidable, theinfluence of aluminium on coatinggrowth is not effective, although it mayimprove surface appearance.

Aluminium additives have impactedpositively in the galvanizing of continu-ous strip. Thin sheet with aluminiumalloyed coatings have been commer-cially available under various tradenames for a number of years. They con-tain different levels of aluminium andother additives. Similar coatingsapplied by the general galvanizingprocess, require special fluxing agentsand have to date had limited success.

NickelAdditions of 0.06% nickel can retardexcessive alloy formation but nickel isonly a partial solution. While it controlscoating structure and thickness forsteels containing less than 0.2% Si, itfails to control alloy growth for steelswith a higher silicon content (figure34). The nickel-zinc concept can alsoresult in thicknesses below the speci-fied minimum in the case of coatingsapplied to less reactive steels.

It would seem that nickel does notretard the zinc iron reaction but ratherthat alloy is released into the moltenzinc as it forms. This results in an

Figure 32. The influence of temperature on diffusion of iron in molten zinc.

Figure 33. Bath temperature effect on the Traditional Sandelin Curve.

2477

Figure 35. Micrograph showing zinc coating oncontinuously coated sheet.

HDGASA © 2009

nizing. Zinc used for hot dip galvaniz-ing usually contains a minimum of98,5% zinc. Zn2 with a purity of99,95% is also often used. When highpurity zinc is used, a little lead (max1%) and aluminium (max 0,007%) isadded to the bath for technical reasons.Refer to SANS 20 and ISO 752.

Of all the precautions that a galvaniz-er can implement to avoid excessivealloy layer growth on high reactivitysteels, the shortest possible expo-sure to the liquid zinc, coupled witha low zinc temperature (440°C), arethe most effective.

7.8 THE IRON / ZINC REACTION IN CONTINUOUS GALVANIZING

In continuous hot dip galvanizing ofsheet, the stock material consists ofcold-rolled steel strip of a compositionsuited to the process. The immersiontime is very short, and the temperatureis kept within narrow limits. The zincbath is alloyed with a small amount ofaluminium (approx. 0.2%), which hasthe effect of retarding the iron/zincreaction at short immersion times. Thealloy layer will be thin, approx. 1 -2µm, with the remainder of the coatingconsisting of pure zinc (figure 35).

The iron/zinc alloys are relatively hardand brittle. However, since they haveto a large extent been replaced by softzinc, continuously hot dip galvanized

Figure 34. Relationship between the silicon content of steel and thickness of zinc coating

when hot dip galvanizing in alloyed zinc (0.1% Ni) (Traditional Sandelin Curve).

Iron/zinc alloy

Steel

Figure 36. Reactivity classification 1.


Figure 38. Surface appearance of a steel showing the outbursts illustrated in figure 37.


Figure 40. Reactivity classification 4a.

Figure 41. Reactivity classification 4b.


Zinc

sheet can be bent, curved, folded,press-formed and even deep-drawnwithout the coating cracking or flaking.

Thin sheet can even be coated with alu-minium-alloyed zinc, which givessomewhat better protection againstcorrosion in severe environments.Some common brand names includeGalfan (5% aluminium), and Aluzink,Galvalume or Zincalume (55% alumini-um).

The hot dip galvanizing process has noeffect on the mechanical properties ofstructural steel.

8.1 STRENGTH AND DUCTILITY

The published BNF report ‘Galvanizingof structural steels and their weld-ments’ ILZRO, 1975, concludes that‘...the galvanizing process has noeffect on the tensile, bend or impactproperties of any of the structuralsteels investigated when these arehot dip galvanized in the “as manu-factured” condition. Nor do even thehighest strength versions exhibithydrogen embrittlement following atypical pretreatment in inhibited HCIor H2SO4.’

Changes in mechanical propertiesattributable to the hot dip galvanizingprocess were detected only when thesteel had been cold worked prior togalvanizing but then only certain prop-erties were affected. Thus the tensilestrength, proof strength and tensileelongation of cold rolled steel wasunaf fected, except that the tensileelongation of 40% cold rolled steeltended to be increased by hot dip gal-vanizing. 1T bends in many of thesteels were embrittled by galvanizing,but galvanized 2T and 3T bends in allsteels could be completely straight-ened without cracking.

8.2 EMBRITTLEMENT

For steel to be in an embrittled condi-tion after hot dip galvanizing is rare.The occurrence of embrittlementdepends on a combination of factors.Under certain conditions, some steelscan lose their ductile properties andbecome embrittled. Several types ofembrittlement may occur but of theseonly strain-age embrittlement is aggra-vated by hot dip galvanizing and simi-lar processes. The following informa-tion is given as guidance in criticalapplications.

Critical applications.It is better to avoid cold work such aspunching, shearing and bending ofstructural steels over 6mm thick whenthe item will be galvanized and subse-quently subjected to critical tensilestress. If cold working cannot be avoid-ed a practical embrittlement test in

Recommendations to minimise embrittlementWhere possible, use a steel with lowsusceptibility to strain-age embrittle-ment. Where cold working is necessarylimitations of punching, shearing andflame cutting, bending, edge distancesand critical applications must beobserved. Refer to Chapter 9.

8.3 FATIGUE STRENGTH

Research and practical experienceshows that the fatigue strength of thesteels most commonly galvanized isnot significantly affected by galvaniz-ing. The fatigue strength of certainsteels, particularly silicon-killed steelsmay be reduced, but any reduction issmall when compared with the reduc-tions which can occur from pitting cor-rosion attack on ungalvanized steelsand with the effects of welds.

For practical purposes, where designlife is based on the fatigue strength ofwelds, the effects of galvanizing can beignored.

Fatigue strength is reduced by thepresence of notches and weld beads,regardless of the effects of processesinvolving a heating cycle such as galva-nizing. Rapid cooling of hot work mayinduce microcracking, particularly inweld zones, producing a notch effectwith consequent reductions in fatiguestrength.

In critical applications, specifications forthe galvanizing of welded steel fabrica-tions should call for air cooling ratherthan water quenching after galvanizingto avoid the possibility of microcrack-ing and reductions in fatigue strength.

25HDGASA © 2009

MMeecchhaanniiccaall PPrrooppeerrttiieess ooff HHoott DDiippGGaallvvaanniizzeedd SStteeeellss

88

accordance with ASTM A143 should becarried out.

Where the consequences of failure aresevere and cold work cannot be avoided,stress relieve at a minimum temperatureof 650°C before hot dip galvanizing.

Ideally, in critical applications structuralsteel should be hot worked above650°C in accordance with the steel-maker’s recommendations.

Susceptibility to strain-age embrittlementStrain-age embrittlement is caused bycold working of certain steels, mainlylow carbon, followed by ageing at tem-peratures less than 600°C, or by warmworking steels below 600°C.

All structural steels may becomeembrittled to some extent. The extentof embrittlement depends on theamount of strain, time at ageing tem-perature and steel composition, partic-ularly nitrogen content. Elements thatare known to tie up nitrogen in theform of nitrides are useful in limitingthe effects of strain ageing. These ele-ments include aluminium, vanadium,titanium, niobium, and boron.

Cold workingCold working such as punching ofholes, shearing and bending beforegalvanizing may lead to embrittlementof susceptible steels. Steels in thick-nesses less than 3mm are unlikely to besignificantly affected.

Hydrogen embrittlementHydrogen can be absorbed into steelduring acid pickling but is expelledrapidly at galvanizing temperaturesand is not a problem with componentsfree from internal stresses. Certainsteels which have been cold workedand/or stressed during pickling can beaffected by hydrogen embrittlement tothe extent that cracking may occurbefore galvanizing (see also chapter13, point 13.8).

The galvanizing processThe galvanizing process involvesimmersion in a bath of molten zinc atabout 450°C. The heat treatment effectof galvanizing can accelerate the onsetof strain-age embrittlement in suscepti-ble steels which have been coldworked. No other aspect of the galva-nizing process is significant.

CCHHAAPPTTEERR 88

Figure 46.

Figure 44.

Figure 45.

9.1 INTRODUCTION

When designing a structure which is tobe hot dip galvanized, it must be bornein mind that articles are immersed intoand withdrawn from a bath of moltenzinc heated to a temperature of 450°C.Design and fabrication is required toconform to acceptable standards whichapply, regardless of whether a galva-nized or a painted coating is to beapplied. In the case of hot dip galva-nizing, some additional requirementswhich aid access and drainage ofmolten zinc, will improve the quality ofthe coating and also reduce costs.

With certain fabrications, holes whichare present for other purposes may ful-fil the requirements of venting of airand draining of zinc; in other cases itmay be necessary to provide extraholes for this purpose.

For complete protection, molten zincmust be able to flow freely to all parts ofthe surfaces of a fabrication. With hollowsections or where there are internalcompartments, the galvanizing of theinternal surfaces eliminates any dangerof hidden corrosion occurring in service.

In addition to using the correct specifi-cations in terms of coating require-ments, the steel chemistry should be ofa quality suitable for galvanizing(Chapter 7).

Some general principles for guidanceare:

� Holes both for venting and drain-ing should be as large as possible.The absolute minimum hole sizesare given in table 16.

� Holes for venting and drainingshould be diagonally opposite oneanother at the high point and lowpoint of the fabrication as it is sus-pended for galvanizing (figure 43).

� With hollow sections sealed at theends, holes should be provided,again diagonally opposite oneanother, as near as possible to theends of the hollow member (figure44 and photos 1 & 2). In somecases it may be more economicalto provide “V” or “U” shapednotches (figure 45) in the ends ofthe tubes, or to grind corners off

26 HDGASA © 200999

the member to which the endplate is connected (figure 46).

� Internal and external stif feners,baffles, diaphragms, gussets etc.,should have the corners croppedand angle bracings should if possi-ble be stopped short of the mainboom flange to aid the flow of

DDeessiiggnn ffoorr HHoott DDiipp GGaallvvaanniizziinngg(For convenience purposes the content of this chapter is also available on a wall chart)

Figure 43.

rectangular hollow sections. Theseprocedures will provide idealmeans for venting and draining.

� Where holes are provided in endplates or capping pieces, theyshould be placed diagonally oppo-site to one another, off centre andas near as possible to the wall of

CCHHAAPPTTEERR 99

27HDGASA © 200999

Table 16.

Figure 47.

Table 17. Schematic indication of double-end dipping capacity of a galvanizing bath 8 metres long x 2 metres deep.

Figure 48. Progressive dipping.

molten zinc and to prevent airentrapment (figures 49, 50, 51 and 52).

� Bolted joints are best made afterhot dip galvanizing.

Hot dip galvanizing oversize objectsFacilities exist to hot dip galvanize arti-cles of virtually any size and shape.(See list of members with bath sizes -refer to www.hdgasa.org.za. When anarticle is too big for single immersion inthe largest bath available it may be

possible to galvanize it by double-enddipping (figure 47 and table 17),depending on the handling facilitiesand layout of the galvanizing plant(check with the galvanizer). Note: Thecost of double end dipping can behigher than the standard cost of hot dipgalvanizing. Large cylindrical objectscan often be galvanized by progressiveimmersion (figure 48).

These processes increase the potentialfor distortion as they introduce uneven

heating into the object. The areaimmersed in the bath is raised to thefull galvanizing temperature andtherefore expands more than theportion remaining outside of thekettle. This is more pronounced duringthe first dip when the object is raisedfrom room temperature. It is thedifferential heating and the resultingdifference in expansion that may causethe product to distort. Dipping thesecond part of the fabrication will notremove any distortion that has alreadyoccurred.

This problem will be aggravated if ventand drain holes are undersized as thiswill require longer galvanizing timeswhile the object fills with zinc anddrains while removing. This increasedPhoto 1. Photo 2.

time exaggerates the dif ferentialexpansion along the steel and hencethe possibility of distortion.

These problems can be overcome orreduced by:

� Large structures are also hot dipgalvanized by designing in mod-ules for later assembly by boltingor welding. Modular design tech-niques often produce economics inmanufacture and assemblythrough simplified handling andtransport.

� Ensuring that vent and drain holesare adequately sized to enablerapid immersion and withdrawal ofthe object (table 16).

� Allowing for linear expansion inthe design so that any distortion isplastic and not constrained by

28 HDGASA © 200999

Figure 49.

Figure 50.

Figure 53.

cross bracing.

� Utilise the longest bath availablefor the galvanizing.

These problems are rarely experiencedin simple pipes, poles or thin spiralsections because of their symmetry andsimple design.

Steel gradeIt is possible to hot dip galvanize allstructural steels and the ultimate coat-ing thickness achieved is determinedby steel analysis, immersion time andto a lesser degree, zinc temperature. Itis for this reason that hot dip galvaniz-ing specifications provide for minimumcoating thickness and no maximumlimit is set (see NOTE 1 in Chapter 10).Reactive levels of silicon in steel andexcessively high phosphorus even atrelatively low silicon levels can resultin thicker coatings. Thicker coatingsprovide extended corrosion protectionbut can occasionally be prone to brit-tleness. The resultant coating could beaesthetically less pleasing sometimesdisplaying dull grey to black surfacepatches. (Chapter 7).

FabricationBendingSteels that are susceptible to embrittle-ment and fatigue failure should be bentover a smooth mandrel with a mini-mum radius 2 to 3 times material thick-ness. Where possible hot work at redheat. Cold bending is unlikely to affectsteels less than 3mm thick. Beforebending, edges should be radiusedover the full arc of the bend.

Bending and forming after hot dipgalvanizingComponents which have been hot dipgalvanized should not be bent or

Figure 51.

Figure 52.

formed by applying heat above themelting temperature of zinc as this cancause embrittlement due to intergran-ular liquid zinc penetration betweensteel crystal boundaries.

BurrsUnlike a paint coating, burrs will be

Photos 3 & 4 above.

Figure 57.

Vent holes (pipes) diagonallyopposite manhole

Figure 59.

overcoated by hot dip galvanizing butthe removal of a burr after galvanizingmay result in the presence of a smalluncoated surface and for this reason,burrs must be removed prior to galva-nizing.

EdgesBecause a hot dip galvanized coating isformed by metallurgical reactionbetween molten zinc and steel, thecoating thickness on edges and cornersis thicker than that on flat surfaces.Thus the rounding of sharp edges, asrequired for paint coatings, is not nec-essary. If subsequent painting isrequired, sharp edges should berounded during fabrication to a radiusof 3mm or 50% of steel thickness.

Edge distances. In accordance withSANS 10162 Clause 22.3.2, whichdefines edge distance as “the mini-mum distance from the centre of a boltto any edge shall be in accordance withtable 8”.

Punching. Full size punching of holes ispermitted when (amongst otherrequirements such as distortion free,burr free, not subject to fatigue),according to Clause 4.3.6.3.c of SANS2001-CS1, “the thickness of the mater-ial is not greater than the hole diame-ter plus 3mm; nor greater than12mm”.

Clause 4.3.6.4 Punching and reamingeads: “Punching is permitted withoutthe conditions of 4.3.6.3 provided theholes are punched at least 2mm less indiameter than the required size and thehole is subsequently reamed to the fulldiameter.”

Material of any thickness may bepunched at least 3mm undersize andthen reamed, or be drilled. Good shoppractice in relation to ratios of punchedhole diameter to plate thickness, andpunch/die diametral clearance to platethickness should be observed.

For static loading, holes may bepunched full size in material up to4500 mm thick where Fy is material

Fy

yield stress up to 360MPa.

In order that the weld seals and continuesat the end of a double sided fillet weld,consider chamfering the long edges anddo a full penetration weldment along bothsides with runouts on each end to ensurefull seal welds (figure 16).

Weld spatterWeld spatter does not reduce the pro-tective properties of a hot dip galva-

29HDGASA © 200999

Figure 58.

Figure 60.

Figure 56.

Figure 55.

Figure 54.

Shearing and flame cuttingEdges of steel sections greater than16mm thick subject to tensile loadsshould be machined or machine flamecut. Edges of sections up to 16mmthick may be cut by shearing.

Sheared edges to be bent during fabri-cation should have stress raising fea-tures such as burrs and flame gougesremoved to a depth of at least 1.5mm.

Temperatures associated with flamecutting alter the surface properties ofsteel and if such surfaces are not thor-oughly ground, a thinner galvanizedcoating will be formed (usually belowthe specified minimum).

Welding and weld slagWelds should be continuous and free fromexcessive pin-holing and porosity. Weldslag, normally associated with stick weld-ing, is not readily removed by acid clean-ing and such slag must be removed byabrasive blast cleaning, chipping, grind-ing, flame cleaning or a pneumatic needlegun, prior to hot dip galvanizing. Shieldedarc welding is preferred since this methoddoes not result in the presence of tightlyadhering slag (figure 53 and Chapter 14).

Figure 65.

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Chamferthe longedges

Full penetrationweldments alongboth sides withrunouts on the endto ensure full sealwelds

DOUBLE SIDED FILLET WELDS

Weld Preparation Weld Detail

Figure 61.

nized coating to the same extent aswith a paint coating, but it is recom-mended practice to remove spatterprior to hot dip galvanizing.

9.2 VENTING, FILLING AND DRAINAGE

External stiffeners, welded gussets andwebs on columns and beams and gussetsin channel sections should have croppedcorners. The gaps created should be aslarge as possible without compromisingstructural strength. If welding is requiredaround the edge created, a radiused cor-ner is desirable to facilitate continuity ofthe weld around the cut end to the otherside. Circular holes are less effective: ifused, they should be as close to cornersand edges as practical. Where more con-venient, the cropped corners or holes maybe in the main beam. Consultation withthe galvanizer, regarding the appropriatevent and drainage hole sizes is recom-mended (figure 49 and table 16).

Welded pipe sectionsClosed sections must never be incorpo-rated in a fabrication. Sections shouldbe interconnected using open mitredjoints as illustrated in figure 54, orinterconnecting holes should be drilledbefore fabrication as in figure 55.

Alternatively external holes may be posi-tioned as in figure 56, a method which isoften preferred by the galvanizer, sincequick visual inspection shows that thework is safe to hot dip galvanize.

Pipe ends can be left open, or providedwith removable plugs. (See unwantedvent holes).

Unwanted vent holesThese may be closed by hammering inlead or aluminium plugs after galvaniz-ing and filing off flush with surroundingsurfaces.

Small tubular fabricationsSmall tubular fabrications must be vent-ed, preferably with holes not less than10mm diameter (table 16).

Figure 62.

Figure 63.

Figure 64.

Figure 66.

Shaft or Minimum radial spindle size clearance

Up to 30mm diameter 2,0mm

Over 30mm diameter 2,0 - 2,5mm

Table 18.

Tubular fabrications / hollow structuralsDrain/vent hole sizes should preferablybe 25% of internal diameter or diagonaldimension for components with a max-imum cross sectional area of 180cm2.This percentage can be influenced bythe shape of the fabrication. Consul-tation with the galvanizer at the designstage is recommended.

Tubular fabrication after hot dip galvanizingThe requirement for bending tubesafter hot dip galvanizing, ie. for the fab-rication of gates etc. must be carriedout according to the method set out inthe Bend Test (galvanized tube). See11.6 Testing for Adhesion and Note 2,regarding coating thickness, page 37.

Tanks and closed vesselsWhen both internal and external sur-faces are to be hot dip galvanized atleast one filling and draining hole mustbe provided, with a vent hole diago-nally opposite to allow the exit of airduring immersion (figure 57). For each0,5 cubic metres of volume, provide atleast one fill/drain hole of minimumsize ø60mm and vent hole of minimumsize ø40mm or both at ø60mm (figure58).

Internal baffles should be cropped asillustrated (figure 51 and 58). Man-holesor pipes should finish flush inside to pre-vent trapping excess zinc (figure 59).

Lifting lugs should be provided oppo-site the biggest and most accessiblefilling / draining holes and adjacent tothe vent hole on the opposite end (fig-ure 43). The lugs must be designed toaccommodate the excess mass ofmolten zinc within the cylinder / pipeon withdrawal.

Figure 68.

Figure 69.

Figure 67.

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Large vessels require an appropriatesize manhole in the baffle.

When vessels and heat exchangersetc., are not to be galvanized internal,‘snorkels’ or extended vent pipes mustbe fitted to allow air or steam to exitabove the level of molten zinc in thegalvanizing bath (figure 60).

9.3 MASKING, WELDING, HANDLING, CLEARANCE FOR MOVING PARTS AND IDENTIFICATION

MaskingMasking materials have been devel-oped, which if applied prior to hot dipgalvanizing, will prevent the formationof the galvanized coating on surfaceswhere it is not required.

Combinations of ferrous surfacesFabrications containing a combinationof castings and steels, or rusted andmill scaled surfaces must be abrasiveblast cleaned before hot dip galvaniz-ing.

Provision for handlingWork not suitable for handling withchains, baskets, hooks or jigs must beprovided with suspension holes or lift-ing lugs (figure 43). If in doubt, consultthe galvanizer.

Materials suitable for hot dip galvanizingAll ferrous materials are suitable,including sound stress-free castings.

Brazed assemblies may be hot dip gal-vanized but first consult the galvanizer.Assemblies soft soldered or aluminiumrivetted cannot be hot dip galvanized.

Overlapping surfacesA minimum gap of at least 2mmbetween overlapping surfaces and back-to-back angles and channels, must beprovided (figures 62, 63 and 64).

When small overlaps are unavoidable,seal edges by welding.

In circumstances where seal welding isnot practical, a degree of temporarysurface staining at crevices may beapparent after hot dip galvanizing andquenching. This is often incorrectlydescribed as acid staining. Clean with abristle brush and mild detergent if nec-essary. If necessary crevices of thisnature can be sealed after hot dip gal-vanizing with an appropriate sealant.

Larger overlapping surfacesIf contacting surfaces cannot be avoid-ed, one 10mm diameter hole should beprovided in one of the members forevery 100cm2 of overlap surface. Theperimeter of the contacting surface canbe continuously welded. This require-ment is of particular importance whenusing thin sections. Vent hole sizes forthicker steels >10mm thick and overlapareas > 300cm2 should be agreed uponby the galvanizer prior to fabrication(figures 65 and 66). A vent hole in onemember will ensure the safety of galva-nizing personnel and prevent damageto the article. Alternatively provide atleast a 2mm gap between members.

Strengthening gussets and websWelded strengthening gussets andwebs on columns and beams, andstrengthening gussets in members fab-ricated from channel or I-beam sectionsshould have corners cropped or holed(figures 49, 52 and photos 3 & 4),

Figure 72.Figure 71.

Figure 70.

BRACES

� to prevent the entrapment of air inpockets and corners allowing com-plete access of pickle acids andmolten zinc to the entire surface ofthe product, and

� to facilitate drainage during with-drawal from degreaser, acid solu-tions, rinsewater, flux and moltenzinc.

Clearance for moving partsDrop handles, hinges, shackles, shafts andspindles require a radial clearance, toallow for the thickness of the hot dip gal-vanized coating (figure 67 and table 18).

Identification markingsFor permanent identification use heavi-ly embossed, punched or welded let-tering (figure 68). For temporary identi-fication use heavily embossed metaltags wired to the work, water solublepaint or an appropriate marking pen.

Do not use paints, adhesive labels orany other product that cannot be read-ily removed by degreasing or pickling(figure 69). If present, these coatingsrequire to be removed by paint stripperor abrasive blasting prior to picklingand hot dip galvanizing.

Hot dip galvanized fastenersHot dip galvanized fasteners are recom-mended for use with hot dip galvanized orpainted structures, but if SANS 121/ISO1461 is not specified, there is every likeli-hood that thinner zinc electro plated coat-ings will be supplied. (Chapter 13).

9.4 PREVENTING DISTORTION

DistortionDistortion can be minimised by:

� Use of symmetrical designs (figure 70).

� Use of sections of a similar thickness(figure 71).

� Use of stiffened sections, particularlywhen steel is unsupported and of lessthan 3 - 4mm thick (figure 72 &photo 5).

� Use of preformed members with thecorrect minimum bend radius to min-imise stress.

� Use of balanced or sequence weldingtechniques to minimise stresses.

� Large open fabrications, thin walledtrough sections and rectangular tanksmay require temporary cross stays toprevent distortion during hot dip galva-nizing (figure 75 and photos 6 & 7).

� Maximise fill, drain and vent hole sizesand optimize their relative positions(table 16).

32 HDGASA © 200999

Use flat washer between brace and article to be hot dip galvanized

Figure 73 Figure 74.

Figure 75.

Photo 5.

Photo 6. Photo 7.

� Complete and rapid immersion of theitem in the galvanizing bath i.e. avoiddouble end dipping if possible.

� Air cooling after hot dip galvanizing inpreference to water quenching.

Use of symmetrical sections minimisesdistortion during hot dip galvanizing.

Products shaped by bending

Many items are formed by bending themto the correct shape at the fabricatingstage. This process induces stress into theproduct, which may be relieved duringthe hot dip galvanizing operation. Thisoccurs as the molten zinc temperature ofaround 450°C is at the lower end of thestress relieving temperature for steel.Consequently the stresses used to shapethe product may be released giving aresultant change in shape or dimension ofthe product.

Consider the case of a plate rolled to formpart of a circle. During hot dip galvanizing,the release of stress will cause the radiusof the circle to increase, and so the finalfabricated circle pieces may not meet up

tural integrity of the fabrication.

� Welding should be as symmetrical aspossible in order to ensure the stressesare balanced. This can be done by plac-ing welds near the neutral axis or bybalancing them around this axis.

� Use a well planned, balanced weldingsequence. With large structures extracare should be taken that stresses areminimised by preparing and workingto a welding plan.

� Weld seams which significantly rein-force the structural strength should asfar as possible be welded last so thatdo not hinder the contraction of otherwelds.

� Use as few weld passes as possible andreduce the welding time to control theheat input.

� Make weld shrinkage forces work inthe desired direction or balance shrink-age forces with opposing forces.

� Use backstep welding or staggeredwelding to minimise stresses.

If a steel fabrication distorts either afterwelding and before or after hot dipgalvanizing due to these stresses, it ispossible to restraighten the item. Bestresults are obtained by hot straighteningeither before or after hot dip galvanizing.Preference should be given to hotstraightening before as the time requiredis less and the possibility of damage to thezinc coating is avoided. Tests confirmthat hot straightened componentswhich were within tolerance before hot

33HDGASA © 200999

Figure 79.

Figure 76.

Figure 77. Figure 78.

(figure 73).

These difficulties can be overcome byinstalling temporary braces across thesection to ensure that the object retains itsdesired shape. The braces would be eitherwelded or bolted in position, with a sizeproportional to the size and thickness ofthe plate they are retaining. If bolted, a flatwasher may be used as a spacer betweenthe brace and article to be hot dipgalvanized, see figure 74. The smaller thespacer the smaller the final repair area.

The braces should be located at least atquarter points of the structure. Similarresults can be obtained with bent troughs,angle frames or with channels (figure 75and photos 6 & 7)).

It will be necessary to repair the areawhere the braces have been removedusing an approved repair material.

Welding or fabrication induced stressIt has been said that the internal stressesdue to welding play the greatest part increating distortion. Because the steel isheated to 450°C during galvanizing, thestresses introduced by welding arereleased and this may occasionally giverise to distortion. Welding, however, playsan essential part in creating thefabrications which are to be hot dipgalvanized. It is therefore important tounderstand how these forces aregenerated and to minimise them duringthe fabrication to obtain a satisfactoryproduct after hot dip galvanizing.

Fortunately, by following a few simplerules it is possible to get much improvedresults. These basic rules are:

� Avoid overwelding, welds should beno larger than is essential for the struc-

Photo 8.

Photo 9.

Figure 81.

dip galvanizing do not distort again duringthe galvanizing process as the stresseshave already been relieved.

Fabrications that lack symmetryWhen fabrications are substantiallysymmetrical in both the horizontal andvertical planes, they have a much lowerpotential to distort at galvanizingtemperatures. Under these conditions, theexpansion forces are balanced and theproduct does not suffer any distortion. Thiscondition exists with tubes, I-beams, RHSand other similar sections. When thesesections are combined in a fabrication, it ispossible to remove this symmetry.

Consider the case where a piece of thinwalled RHS is welded to the top of an I-beam section. In this situation, thegeometric shape is no longer symmetrical,even though the two individualcomponents are.

The thinner walled tube will reach thegalvanizing temperature sooner than thethicker flange at the bottom. As a result,the RHS will expand faster than thebottom flange, causing the section toexperience an upwards bow (figure 76).

Sections which are not symmetrical, suchas Channels and Angles will experiencesimilar problems because of their inbuiltasymmetry. In the case of channels, thesection will bow with the toes pointingoutwards.

There are three recommended ways toovercome this type of problem.

� Redesign the fabrication to make thedesign symmetrical. This will enablethe forces to balance each other andprevent distortion.

� Fabricate and galvanize the individualcomponents as separate pieces, then

weld them together after hot dip gal-vanizing. The welds can be touched upwith a suitable galvanizing repair mate-rial.

� When multiple pieces are availablethey can be hot dip galvanized back toback by using bolts with pipe spacersto separate the pieces.The assemblywould be separated after cooling com-pletely and the spacer contact arearepaired with a suitable galvanizingrepair material (figures 77 and 78).

Using thick and thin material in anassemblyWhen thin material is heated duringgalvanizing, it expands faster than anythick material heated at the same time.this is because the thinner material takesless time to be fully heated to thegalvanizing temperature. The thinnermaterial will therefore distort if itsexpansion is restrained by thicker material.

Consider the common case where a thinsteel sheet is welded to the frame of atrailer to form a tray. This sheet is generallysecurely attached by welds around itsperimeter. If, for example, the sheet is onlyhalf as thick as the material used in theframe, it quickly reaches the galvanizingtemperature of around 450°C and so hasreached the point where maximumexpansion will occur.

The frame being made of thicker materialwill not yet have reached the sametemperature and so will not haveexpanded as much as the thinner sheet.Because of the restraints from the weldsaround the perimeter, the sheet cannotpush its growth outwards at the edges,and so the increase in size causes bucklesto occur in the sheet surface (figures 71,79 and photo 9).

There are two recommended methods ofovercoming this problem:

� Hot dip galvanize the sheet and frameseparately and then join them after gal-vanizing. This may be done usingmechanical fasteners such as screws orbolts. If welding is used then the weldswill need to be touched up with galva-nizing repair material.

� Use the same thickness of material forboth the frame and the sheet.

In some cases this buckling of the surfacemay be acceptable (photo 8), as thematerial is fully protected againstcorrosion, however once this type ofdistortion occurs, it cannot be readilycorrected after galvanizing.

Long thin objectsLong thin objects include poles, tubes andlarger RHS sections. Generally these

34 HDGASA © 200999

Figure 80.

objects will not distort due to theirsymmetrical nature, however if they arelifted at both ends, they may take on acharacteristically bowed shape followingthe galvanizing process (figure 80).

This bowing is caused when the steel isheated to the galvanizing temperature of450°C. When withdrawing from thegalvanizing kettle, the products ownweight may exceed the yield strength ofthe steel at this temperature, causing theobject to bow. This bowing becomespermanent as the steel cools.

If the product has not been designed withsufficiently large vent and drain holes, theproblem can be aggravated by additionalzinc being trapped inside the object whenit is lifted. Further problems are created bythis as the time taken for the zinc to drainallows the deformation of the steel tocontinue for a longer period and thebowing to become worse.

There are two recommended ways toreduce this problem:

� Lifting lugs or holes should be provid-ed at the quarter points of these prod-ucts so that they do not need to be lift-ed at the ends (figure 81).

� Vent and drain holes should be placedand sized to maximise the rate ofdrainage and minimise the retention ofzinc inside the section (figure 43 andtable 16).

9.5 PACKAGING AND TRANSPORTING OF HOT DIP GALVANIZED STEEL

Even though the hot dip galvanizedcoating is capable of withstanding fairlyrough treatment it should be handledwith care during storage and transporta-tion. In the case of long sections, simplepackaging and binding into bundles notonly prevents handling damage but itoften facilitates transportation itself.Packaging and binding should be donein such a way as to avoid the risk of wetstorage stain. Spacers should be used tofacilitate air circulation between compo-nents (see photo 10).

Photo 10.

sedes SABS 0214.

SANS 4998/ISO 4998 Continuous hot dip zinc coated carbonsteel sheet of structural quality.

SANS 3575/ISO 3575 Continuous hot dip zinc coated carbonsteel sheet of commercial, lock formingand drawing grades.

Note: The above two specificationssupersede SABS 934.

SANS 675:1997 Zinc coated fencing wire.

SANS 935:1993 Hot dip (galvanized) zinc coatings onsteel wire.

General hot dip galvanizing specifica-tions state the local (minimum) and the(mean) coating thicknesses. The thick-ness actually achieved, varies with steelcomposition and this can range from theminimum up to at least 50% greater. Aslife expectancy predictions are normallybased on the minimum coating thick-ness, they are usually conservative.

NOTE 1: The specification does not stip-ulate a maximum upper coating thick-ness limitation, however, excessivelythick coatings on threaded articles areundesirable. In order to ensure effectivetensioning, the coating thickness on fas-teners should not exceed a maximum of65µm, this applies particularly to highstrength bolts and nuts.

In South Africa, the South African Bureauof Standards (SABS) has adapted ISO1461, EN 10240 and ISO 14713. Thespecifications are therefore published bythe SABS as SANS 121/ISO 1461, SANS32/EN 10240 and SANS ISO 14713/ISO14713.

10.2 LEAD TIMES

As a general guide, most articles can behot dip galvanized and returned to thefabricator within 3 to 7 days after receipt.

In the case of large contracts, thegalvanizer should be involved at theprogramming stage with the fabri-cator and the end user. Hot dip gal-vanizing is normally the finalprocess after fabrication prior todelivery and erection. If insufficienttime for hot dip galvanizing andinspection is provided in the over-all programme, costly delays mayoccur at the erection stage.

NOTES

* Local coating thickness is defined as the mean of themeasurements taken within a specified reference area.Mean coating thickness is the control sample numberaverage of the local coating thickness values from eachreference area.

� Where only one reference area is required according tosize of the article, the mean coating thickness withinthat reference area shall be equal to the mean coatingthickness given in the above tables.

� Deviation from standard coating thickness. Arequirement for a thicker coating (25% greater thanthe standard in table 19 can be requested forcomponents not centrifuged, without affectingspecification conformity).

Where steel composition does not induce moderate tohigh reactivity, thicker coatings are not always easilyachieved.

� Refer to Chapter 13.8 “Bolted Connections; HighStrength Fasteners” for information on the hot dipgalvanizing of High Strength Fasteners

The galvanizer acts as a sub-contrac-tor to a steel fabricator and as such,his contractual relationship is nor-mally with the fabricator, not withthe ultimate user or specifier. It isimportant, therefore, that the users'or specifiers’ requirements for hotdip galvanizing are made clear to thefabricator and that all instructionsare channelled via the fabricator tothe galvanizer.

When specifying hot dip galvanizingit is essential to demand a coatingapplied in accordance with therequirements of SANS 121/ISO 1461or SANS 32/EN 10240 whereapplicable. This will avoid confusionwith zinc rich painting often referredto as cold galvanizing and zincelectro-plating referred to aselectrogalvanizing.

To ensure the best quality and tech-nical support, a galvanizer who is amember of The Hot Dip GalvanizersAssociation Southern Africa is rec-ommended.

When hot dip galvanizing is applied,the steel substrate is completely cov-ered with a relatively uniform coatingof zinc. The minimum coating thick-ness required is related to the thick-ness of the steel being hot dip galva-nized, as shown in table 19 & 20.

10.1 HOT DIP GALVANIZING SPECIFICATIONS

SANS 121/ ISO 1461 Hot dip galvanized coatings on fabricat-ed iron and steel articles - Specificationsand test methods.

SANS 32/EN 10240Internal and/or external protective coat-ings for steel tubes - Specification for hotdip galvanized coatings applied in auto-matic plants (table 23).

Note: The above specifications supercedeSABS 763.

SANS 763:1997Hot dip (galvanized) zinc coatings (otherthan on continuously zinc coated sheetand wire).

SANS 14713/ISO 14713Protection against corrosion of iron andsteel in structures - Zinc and aluminiumcoatings - Guidelines.

Note: The above specification super-

35HDGASA © 2009

SSppeecciiffyyiinngg HHoott DDiipp GGaallvvaanniizziinngg

1100

MINIMUM COATING THICKNESS ON ARTICLESTHAT ARE NOT CENTRIFUGED

SANS 121/ISO 1461:1999

SANS 121/ISO 1461:2009

Category Category Local Mean and and coating coating

thickness thickness thickness thickness(t) mm (t) mm (minimum) (minimum)

µm* µm*t ≥ 6 t > 6 70 85

t ≥ 3 to < 6 t > 3 to ≤ 6 55 70

t ≥ 1.5 to < 3 t ≥ 1.5 to ≤ 3 45 55

t < 1.5 t < 1.5 35 45

t ≥ 6 70 80

t < 6 60 70

PROF

ILES

CASTI

NGS

Table 19.

� Thickness legend - 3mm > t ≤ 6mm - thicknessgreater than 3mm but equal to and less than 6mm.

MINIMUM COATING THICKNESS ON ARTICLESTHAT ARE CENTRIFUGED

SANS 121/ISO 1461:1999

SANS 121/ISO 1461:2009

Category Article Local Mean and thickness/ and its coating coating

diameter diameter (ø) thickness thickness(t) or (ø) mm or thickness (minimum) (minimum)

(t) mm µm* µm*ø ≥ 20 ø > 6 45 40 55 50

6 ≤ ø < 20 ø ≤ 6 35 20 45 25

ø < 6 20 25

t ≥ 3 t ≥ 3 45 55

t < 3 t < 3 35 45

FASTEN

ERS

OTHE

R ART

ICLES

(INCLU

DING C

ASTIN

GS)

Table 20.

� Thickness legend - ø > 6 = diameter greater than 6mm.

CCHHAAPPTTEERR 1100

11.1 INSPECTION BEFORE HOT DIP GALVANIZING

Good quality hot dip galvanized coatings onfabricated articles are more likely to beachieved if correct fabrication techniqueshave been adhered to. Inspection of fabricat-ed assemblies, castings and other compo-nents for hot dip galvanizing, should be car-ried out before despatch to the galvanizer(table 21) in order to ensure conformity to thedesign requirements detailed in Chapter 9.This may avoid costly rectification and unnec-essary delays at the galvanizers premises.

11.2 INSPECTION AFTER HOT DIP GALVANIZING

As a final step in the process, the hot dip gal-vanized coating is inspected for compliancewith relevant specifications. Interpretation ofinspection results should be made with aclear knowledge of the causes of various con-ditions which may be encountered and theirpotential influence on the ultimate objectiveof providing long term corrosion protection.

Inspectors should remember that the pur-pose of hot dip galvanizing is to protectsteel from corrosion. The length of time thatthis protection can be expected to last, iscalled its “service life or time to first mainte-nance”. This is defined as the time taken forthe appearance on an article of 5% surfacerust. The service life of a hot dip galvanizedcoating is directly related to the thicknessof the protective zinc coating. Corrosionprotection is greatest when the coating isthickest. Thus coating thickness is the sin-gle most important quality check.

Coating thickness is only one inspectionaspect. Other checks must include conti-nuity, coating adhesion and appearance.Embrittlement and defects, which arise

11.4 APPEARANCE

The ability of a hot dip galvanized coating tomeet its primary objective which is to providecorrosion protection, should be the chief cri-terion when evaluating coating acceptability.

The specified requirements for a hot dip gal-vanized coating are that it be:� continuous,� relatively smooth, � free from gross imperfections,� free from sharp points (that can cause

injury), and� free from uncoated areas

To be essentially free from uncoated areaswas best described in SABS 763 4.3.2 b.This reads as follows: “The area of an individual bare spot orthin area shall not exceed 5mm2. Thecombined area of bare spots or thin areasshall not exceed 25mm2 per metre oflength or per square metre of surface ofan article.”.

Note: SABS 763 is obsolete but for practicalpurposes the above clause has been retainedfor galvanizing inspectors.

The above requirements are of particularimportance when a subsequent organic paintcoating is to be applied onto a galvanizedsurface. Smoothness and absence of rough-ness achieved on mechanically wiped prod-ucts, such as continuously galvanized sheet-ing or wire, are not to be used as the criteriafor accessing surface finish on general hot dipgalvanized products. Roughness andsmoothness are relative terms. The end useof the product must be the determining fac-tor in setting standards.

In order to provide optimum corrosion pro-tection, the hot dip galvanized coatingshould be continuous. Handling techniquesfor hot dip galvanized articles may entail theuse of chain slings or other holding devices ifsuitable lifting fixtures are not attached to theitem. In exceptional circumstances, chainsand special jigs may leave a contact touchmark on the hot dip galvanized item. Thesemarks are not always detrimental and a rea-son for rejection. Should these marks, begreater than 5mm2 with bare steel exposed,suitable repairs should be carried out usingthe method described in SANS 121/ISO1461. Refer to Chapter 15 - ReconditioningDamaged or Site Modified Hot DipGalvanized Coatings.

Differences in the lustre and colour of hot dipgalvanized coatings do not affect corrosionresistance and the presence or absence ofspangle has no effect on coating perfor-

36 HDGASA © 2009

Figure 82. Using a digital instrument to measure zinc coating thickness.

from specific materials, design and fabri-cation, must also be considered wheninspecting susceptible items.

While minimum standards must be satisfiedin all these considerations, their relativeimportance varies according to the end useof the finished product. For example, the aes-thetic appearance of hot dip galvanizedstructural steel in an industrial application isless important than when a structure is des-tined for use in a decorative application.Understanding of the specific requirementsas well as the limits to what can be achievedby hot dip galvanizing is essential for effec-tive inspection.

11.3 THICKNESS TESTING

Several methods are used to determine thethickness of the zinc coating on a hot dip gal-vanized article. The size, shape and numberof pieces to be tested, will dictate themethod to be used. Specified test methodsare either destructive or non-destructive.These are detailed in SANS 121/ISO 1461and in SANS 32/EN 10240. The most practi-cal test is the non-destructive method utilis-ing the electromagnetic principle for deter-mining coating thickness (figure 82).

Threaded articles must fit their mating partsand, in the case of assemblies that containboth externally and internally threaded arti-cles, it shall be possible to screw matingparts together by hand.

For small items, particularly those with com-plex geometries, ISO 1460 provides forgravimetric measurements aimed at deter-mining mass of coating per unit area asopposed to thickness. This is a destructivetest method.

1111

CCHHAAPPTTEERR 1111

QQuuaalliittyy -- IInnssppeeccttiioonn BBeeffoorree && AAfftteerrHHoott DDiipp GGaallvvaanniizziinngg

mance. The well-known spangle effect foundon some hot dip galvanized surfaces is a fac-tor of primary crystallisation. It is chieflydependant upon the zinc bath chemistry, therate of cooling, the method of pickling, thesteel chemistry, and the thickness of thecoating. In fact, dull grey or patchy mattegrey hot dip galvanized coatings give servicelives equal to or greater than bright or span-gled coatings. Variations in coating appear-ance or finish are important only to theextent that they will affect corrosion perfor-mance or the intended use of the article. Theprimary function of a hot dip galvanized coat-ing is corrosion protection. Specific require-ments beyond the standard set out in SANS121/ISO 1461, (eg. aesthetic appearance)should be communicated to the galvanizer inwriting or negotiated at the contract reviewstage.

In order to comply with additional require-ments, the following information may berequested by the galvanizer:-a) Steel composition.b) Identification of significant surfaces

which require special care.

A significant surface can be definedas a surface which impacts on theperformance of that article..

c) A visual standard should be establishedif a special finish is required.

d) Any particular special treatments that arerequired before or after hot dip galvaniz-ing. Special treatment can include paint-ing after hot dip galvanizing, where anyrenovation requirement and materialshould be discussed before galvanizing.Refer Chapter 15.

e) Deviation from standard coating thick-ness. See information below table 20.

f) The presence of flame, laser or plasmacut surfaces.

g) Where fabrications include internal cavi-ties, provide written evidence that ade-quate venting has been provided.

h) Coating inspection arrangements.i) Whether a certificate of conformance is

required.

For ensuring optimum appearance of the hotdip galvanized coating after installation, referto Installation Do’s and Don’ts, figure 106 onpage 48.

11.5 ADHESION OF THE COATING

Acceptable adhesion is related to the practi-cal conditions pertaining during transporta-tion, erection and service. Hot dip galva-nized coatings should be sufficiently adher-ent to withstand normal handling withoutpeeling or flaking regardless of the natureand thickness of the coating. Bending orforming, other than straightening by the gal-vanizer after hot dip galvanizing, is not con-sidered to be normal handling.

When reactive grades of steel or very thicksections are hot dip galvanized, coatingswhich are thicker than usual may occur. Thegalvanizer has limited control over the for-mation of thicker coatings, since this is afunction of the chemical composition of thesteel. Extended immersion time also plays arole. Heavy hot dip galvanized coatingsgreater than 250µm thick, may have brittletendencies. Interpretation of the standard

37HDGASA © 20091111

DEGREE OF FLATTENING FOR TESTING COATING ADHERENCE FOR TUBES

Tube type Distance betweenplatens

Square 75% of side

Rectangular tube 75% of shorter side

Round ≤ 21.3mm 85% of outside diameter

Round > 21.3 ≤ 48.3mm 80% of outside diameter



Round > 114.3mm 65% of outside diameter

Table 22.

CHECK LIST PRIOR TO SENDING GOODSFOR HOT DIP GALVANIZING

Size and Shape Check that work is suitably sized and if necessary, lugs havebeen provided for the handling and galvanizing facilities of theselected galvanizer. It may be too late to make changes to thedesign, but it is costly to despatch work which the galvanizer can-not process.

Structural SteelCheck that bending, punching and shearing have been carried outin conformity with the recommendations in Chapter 9.

Satisfactory Hot Dip GalvanizingObservance of the points listed below and described in moredetail in Chapter 9 will ensure optimum galvanized product qual-ity and minimise extra costs or delays.1. Check that closed vessels and hollow structures are ade-

quately vented for safety and optimal fill and drain holeshave been provided to ensure satisfactory hot dip galva-nizing.

2. Check that all welding slag has been adequately removed.3. Check that assemblies comprising castings and steels of wide-

ly differing surface conditions have been abrasive blastcleaned to minimise differences in galvanized finish.

4. Check that castings are abrasive blast cleaned beforedespatch unless otherwise arranged. Check that large greyiron castings have been normalised.

5. Check that appropriate temporary or permanent markingsare provided.

Table 21.

Table 23.

MINIMUM COATING THICKNESS ON STEEL TUBES TO SANS 32/EN 10240 COATING QUALITY A1 A2 A3

Mandatory Minimum local coating thickness on the inside surface except at the weld bead 55µm 55µm 45µm

Minimum local coating thickness on the inside surface at the weld bead 28µm 1) 1)

Options Minimum local coating thickness on the outside surface 2) 2) 2)

COATING QUALITY B1 B2 B3 Mandatory Minimum local coating thickness on the outside surface 55µm 3) 40µm 25µm

1) This requirement does not apply

2) This requirement applies when the purchaser specifies Option 1

3) Option 3 specified (if >55µm required, purchaser to specify according to SANS121/ISO 1461)

Coating qualities ‘A’ and ‘B’ refer to end application with quality ‘A’ being for gas and water installations and ‘B’ for otherapplications. The number following the quality letter refers to specific requirements in terms of coating thickness.

NOTE: In South Africa, SANS 32/EN 10240 to quality A1 replaces the previous SABS 763, B4 coating.

adhesion tests must take this into considera-tion. The requirements for careful transporta-tion, handling and erection should be evalu-ated against the additional corrosion protec-tion afforded by these thicker coatings.

11.6 TESTING FOR ADHESION

Testing for adhesion is not necessarily a truemeasure of the adhesive strength of the met-allurgical bond between the hot dip galva-nized coating and the base steel, but it doesserve as an indicator of the adhesion proper-ties of the coating.

Paring testThis simple but effective test is conducted bycutting or prying the hot dip galvanized coat-ing with a sharp knife. Considerable pressureis exerted in a manner tending to remove aportion of the coating. Adherence is consid-ered satisfactory when it is possible toremove only small particles of the coating. Itshould not be possible to peel any portion ofthe coating in the form of a layer so as toexpose the underlying iron or steel inadvance of the knife. Although not men-tioned in SANS 121/ISO 1461, this test hasshown practical significance as a test foradhesion.

Cold flattening test (galvanized tube)For compliance with SANS 32/EN 10240, themost popular test is cold flattening in accor-dance with SANS 8492/ISO 8492. Testpieces not less than 40mm in length are flat-tened between parallel flat platens as shownin table 22. No cracking or flaking of the coat-ing shall occur on the surface away from thecut surface.

Bend test (galvanized tube)The bend test shall be carried out using a tubebending machine, and the test piece shall bebent through 90° round a former having aradius at the bottom of the groove equal toeight times the outside diameter of the tube.

Note 2: Should the above requirement ofbending be implemented for the fabricationof gates, etc. after hot dip galvanizing, themaximum coating thickness should be nogreater than 40% more than the minimumrequired in table 23.

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The life of a hot dip galvanized coatingis more or less proportional to itsthickness in a given environment.(Table 2).

Hot dip galvanized coatings on steel pro-tect against corrosion in two ways:

1 - Barrier protection is provided by avirtually non-porous film which iso-lates the steel substrate from corro-sion inducing substances in the sur-rounding environment.

2 - Cathodic or sacrificial protection isprovided at small uncoated surfaceswhile corrosion creep under the sur-rounding coating cannot occur.

The corrosion rate of zinc is low in mostenvironments. This is due to the naturalformation of a stable protective film of zincconversion products which develops onthe surface of the coating.

12.1 THE CORROSION TEST

SummaryThe selection of coatings for corrosionresistance is a process which normallycombines practical experience and scien-tific knowhow. One aid in the process isthe corrosion test.

Testing the corrosion resistance of materi-als is necessary in order to identify materi-als, coatings, and designs that will helpprevent corrosion damage. However,these tests can be confusing and even

in-house tests designed to simulate actualfield conditions.

Standardised tests have the advantages ofreproducibility and general acceptance.These tests have usually been evaluated ina number of locations (round-robin test-ing) to verify that results are not affectedby local conditions.

misleading if they are not understood andconducted properly.

Standardised testsMany types of corrosion tests have beendeveloped. Some are standardised testsspelled out by associations such as ASTMor the National Association of CorrosionEngineers (NACE). Others are

Figure 83. Exposed surface of a hot dip galvanized coating with outer layer of pure zinc. The

shiny surface disappears to be replaced by grey corrosion products (sometimes called zinc patina).

1122HDGASA © 200938

CCHHAAPPTTEERR 1122

Table 24: Description of categories of atmospheric corrosivity.

Corr

osiv

ityCa

tego

ry

Table 25: Estimated Service Life for Hot Dip Galvanized Steel complying with SANS 121 (ISO 1461:2009) and subjected to Atmospheric Environments Classified in terms of ISO 9223:1992.

USED BY THE HDGASA TO DETERMINE SERVICE LIFE OF HOT DIP GALVANIZED STEELEXPOSED TO VARIOUS ATMOSPHERIC ENVIRONMENTS

Reference Source: ISO 9223:1992

Category Corrosivity Description of Typical Corrosive Atmosphere

C 1 Very Low Interior: dry

C 2 Low Interior: occasional condensationExterior: exposed rural inland

C 3 Medium Interior: high humidity, some air pollutionExterior: urban inland or mild coastal

C 4 High Interior: swimming pools, chemical plant, etc.Exterior: industrial inland or urban coastal

C 5 Very High Exterior: industrial with high humidity or high salinity coastal

The typical descriptions detailed in Table 24 are intended as a general guide and it is recommended that a reviewof actual site conditions should be undertaken before finalising the applicable corrosive category. A generalreview of existing hot dip galvanized structures is an ideal method used to establish the corrosive conditions inthe general area of a particular site.

Corrosion Rates (rcorr) of Hot Dip Galvanized Coated Steel (Ref ISO 1461:2009)

Units Zinc 55µm for steel 70µm for steel 85µm for steel≥ 1.5mm to ≤ 3mm > 3mm to ≤ 6mm > 6mm

C 1 µm/a rcorr ≤ 0.1 > 100 years > 100 years > 100 years

C 2 µm/a 0.1 < rcorr ≤ 0.7 ≤ 78.5 years > 100 years > 100 years

C 3 µm/a 0.7 < rcorr ≤ 2.1 26 to ≤ 78.5 years 33 to ≤ 100 years 40 to > 100 years

C 4 µm/a 2.1 < rcorr ≤ 4.2 13 to ≤ 26 years 16 to ≤ 33 years 20 to ≤ 40 years

C 5 µm/a 4.2 < rcorr ≤ 8.4 6.5 to ≤ 13 years 8.3 to ≤ 16 years 10 to ≤ 20 years

The figures for service life estimated in table 25 are intended as a general guide and it is recommended that amore detailed assessment of the actual site environmental conditions should be conducted in order to definelongevity expectations for hot dip galvanized coated steel.

It is worthwhile noting that general hot dip galvanizing specifications state the local (minimum) and the meancoating thicknesses (see tables 19 and 20). The coating thickness actually achieved in practice, varies with steelcomposition and this can range from the minimum up to at least 50% greater. As life expectancy predictions intable 25 are based on the minimum coating thickness, they are conservative.

39HDGASA © 20091122

Figure 84. Discoloured surface on lightingcolumn. Coating consists mainly of an iron/zincalloy that grows out to the surface. Iron is freedduring corrosion, which leads to rust formation.

It is only surface rust and is of aestheticsignificance only. The bracket for the traffic sign

has a coating of pure zinc as an outer layer.

So many factors affect corrosion, however,that these standardised tests may not ade-quately simulate field conditions. Forexample, one of the most commonly usedtests for corrosion resistance is the ASTM-B117 salt spray test. Results of this test arestill frequently quoted in automotiveproduct specifications, although the testhas virtually been dismissed by auto man-ufacturers, primarily because it shows thatzinc coated steel does not perform as wellin an automotive environment as plaincold-rolled steel. Field experience hasproven the opposite to be true.

Electrochemical tests are appealingbecause of their precision. By immers-ing electrodes made of the materialsbeing evaluated in the electrolyte thatthe materials will be exposed to in ser-vice, a galvanic cell is created similar tothat in a battery. Because corrosion isan electrochemical process, the mea-sured potential and current flowbetween electrodes can be correlatedwith corrosion rates. However, thesetests do not necessarily reflect actualservice conditions, such as when thematerials are alternately wetted anddried. Without valid parameters,results from electrochemical tests canbe misleading as any other corrosiontest.

Another common mistake in corrosiontesting is to try to extrapolate long-termdata from short-term tests, or to rely ondata from a single sample. If the corrosionprocess does not reach steady state dur-ing the test, then results can be mislead- Figure 85. Wet storage stain that has formed between tightly packed angles.

trols are resulting in lower pollution levelsand hot dip galvanizing offers good pro-tection in locations where previously lim-ited coating life was experienced.

In marine environments the corrosion ofzinc is influenced by the salt content of theair. However, marine air contains smallquantities of magnesium salts, with goodpassivating influences. Corrosion is there-fore not as great as may be expected. Thesalt content of the air usually diminishesrapidly away from the coastline ie. by 80%over the first 800m from the high watermark.

The colour of zinc corrosion productsvaries according to the environment inwhich they are formed. Marine environ-ments give somewhat whiter corrosionproducts compared with rural and urbanenvironments. Corrosion products areusually darkest in urban environments.

The corrosion of zinc is influenced by manyfactors. This means that a generally applic-able formula for corrosion rates can not begiven.

The ubiquitous nature of hot dip galvaniz-ing means that there is always a productsuch as a lamp post or fence near a pro-posed future site that can be used to pre-dict future performance.

The Hot Dip Galvanizers Association havefrequently been involved in the assess-ment of the corrosive conditions prevail-ing at a particular site, prior to the selec-tion of the final coating specifications.Knowledge about the corrosion of zinc,and corrosion rates in different environ-ments, is therefore extensive.

Reddish-brown discolourationSome hot dip galvanized steel can adopt areddish-brown colour after a period ofexposure. After prolonged exposure, par-ticularly in sulphur-rich atmospheres, thisdiscolouration can gradually turn black.The discolouration occurs mainly on coat-

ing. Use of several specimens is also rec-ommended to get a good statistical sam-pling.

12.2 CORROSION RESISTANCE IN THE ATMOSPHERE

When a hot dip galvanized article is with-drawn from the molten zinc, the coatingsurface immediately reacts with oxygenand moisture to form combinations ofboth zinc oxide and zinc hydroxide.Carbon dioxide in the atmosphere rapidlyconverts these surface conversion prod-ucts into a stable, tightly adhering, basiczinc carbonate film with very low solubili-ty. This ensures that further attack of theunderlying zinc is prevented. The initialshiny surface with a metallic lustre disap-pears to be replaced by a matt, light greyappearance (figure 83).

The atmosphere contains greater or lessercorrosive substances such as chlorides, inmarine environments and sulphur dioxideassociated with industrial pollution.Humidity levels, rain patterns and conden-sation all influence the degree of corro-sion. The different factors can occur infavourable or unfavourable sequences,one after another, alternately, or in combi-nation with each other.

It is normal to differentiate between corro-sion rates in:

1. rural environments

2. marine (coastal) environments

3. urban environments

4. industrial environments

(See tables 24 and 25).

The atmosphere in cities and industrialareas contains various pollutants. Theseare able to attack the stable zinc carbonatefilm producing more soluble productswhich can be washed away. Consequentlythe corrosion rate of galvanized steel willaccelerate. Modern environmental con-

Table 26. Electrochemical potential scale in sea water at +25°C.

Gold

Silver

Stainless steel (304)

Nickel

Monel

Aluminium bronze

(95% Cu, 5% Al)

Copper

Brass

Tin

Lead

CAST IRON, unalloyed

CARBON STEEL

Cadmium

Aluminium

ZINC

MagnesiumElec

tro-

nega

tive

end

–m

ore

reac

tive

met

als

Elec

tro-

posi

tive

end

–m

ore

nobl

e m

etal

s

Figure 86. In order to avoid the formation of wet storage stain on newly galvanized surfaces, profiled steel, beams and structures should be packed at an angle and turned to prevent the

accumulation of water. Spacers are placed so as to avoid narrow crevices between the zinc surfaces.

Figure 88. Galvanic corrosion of zinc in contact with steel in water.

12.3 WET STORAGE STAIN

Sometimes a white, floury and volumi-nous coating called wet storage stain, orwhite rust, appears on galvanized surfaces(figure 85).

The deposit forms on freshly galvanized,shiny surfaces and particularly betweenclosely packed sheets, angles and similarproducts. A pre-requisite is that the mate-rial is exposed to condensate or rain waterin conditions where the moisture cannotevaporate quickly. Zinc surfaces that havealready formed a normal protective layerof conversion products are seldomattacked.

When freshly galvanized surfaces areexposed to the atmosphere, soluble zincoxide and zinc hydroxide are formed.

40 HDGASA © 20091122

ings consisting largely of iron/zinc alloy onsilicon-killed steels.

The source of discolouration is the corro-sion of Fe/Zn alloy to form rust in the pres-ence of humid air or rain water. Rust has agreat ability to stain, and even smallamounts can cause considerable dis-colouration.

Sometimes when discolouration is severeit is natural to conclude that rust protec-tion has been greatly reduced, or com-pletely destroyed. However, this is sel-dom the case. The iron/zinc alloys givebetter protection (in most environmentsup to 30-40% greater) to the underlyingsteel than pure zinc.

If appearance is important, discolouredsurfaces can be painted (figures 30, 31and 84).

Under the influence of carbon dioxide inthe air basic zinc carbonate is formed. If airaccess to the zinc surface is restricted, asin narrow crevices, then the area receivesinsufficient carbon dioxide to enable thenormal layer of zinc carbonate to form.

The wet storage stain deposit is volumi-nous and porous, and attached only loose-ly to the zinc surface. As a result, protec-tion against continued attack does notexist. Corrosion can therefore continue aslong as moisture remains on the surfaces.When wet storage stain has occurred theobject should be stacked to enable the sur-faces to dry quickly. This will stop theattack and, with free access to air, the nor-mal protective layer will be formed. Thewet storage stain is gradually washedaway and the coating acquires an appear-

Figure 87. Galvanized bolt in contact with3CR12 plate after 10 cycle SO2 test.

Note the cathodic protection provided by thegalvanized bolt head to the surrounding steel.

Damage

Figure 90. Schematic diagram to illustrate the consequences of damage to different types of coatings offering corrosion protection.

Zinc CoatingElectro-negative to steel,

Zinc preferentially sacrificed to protect steelNo corrosion undercreep

Paint CoatingOnly barrier protection

providedCorrosion undercreep

can occur

Nickel, Chromiumor Copper

Electro-positive to steelCorrosion acceleratedat exposed surfaces

Figure 89. After 20 years of marine exposure, this site cut unrepaired hot dip galvanized steel grating still offers cathodic protection at the cut ends.

ance that is normal for exposed, hot dipgalvanized steel.

Since the product of wet storage stainis very bulky (about 4.5 times the solidvolume of zinc from which it isformed), an attack can appear to beserious. However, wet storage stainoften has little or no significant impacton the service life of the coating but inthe case of very thin coatings a severeattack of wet storage stain can be sig-nificant.

Wet storage stain is best avoided by pre-venting closely packed galvanized sur-faces from coming into contact with rain orcondensate. Freshly galvanized materialwhich is exposed to the elements shouldbe stacked in a manner that ensures freeair circulation (figure 86). Temporary pro-tection against wet-storage stain is

41HDGASA © 20091122

Figure 92. Brass bolt in hot dip galvanized steelon a parking deck.

Figure 91. Stainless steel fasteners attached tohot dip galvanized plate in immersed

conditions, note the sacrificial attack of the zinccoating surrounding uninsulated fasteners

compared with the insulated fastener where noattack of the surrounding zinc has taken place.

obtained through chromating or phos-phating.

Wet storage stain which has alreadyformed can be removed completely orpartially by moderate mechanical orchemical treatment. See “Removal of WetStorage Stain” page 16.

12.4 GALVANIC, BIMETALLIC AND CREVICE CORROSION

Corrosion can be defined as an electro-chemical process. Galvanic or bimetalliccorrosion occurs when two different met-als or alloys in the presence of an elec-trolyte, are in direct electrical contactwith each other. Basis corrosion theorystates that for corrosion to take place,there are four essential requirements, i.e.an anode, a cathode, an electrolyte and

an electrical circuit. If one of these isabsent, corrosion ceases (figure 88).Different metals possess different elec-trochemical potentials as shown in table26. The electronegative and more reac-tive metals will corrode in preference toa more electropositive metal when thetwo are in direct electrical contact, i.e.the anode is attacked whereas the cath-ode is protected. The electrical potentialscale of some metals may vary, depend-ing on the electrolyte but the informationcontained in table 26 which relates to seawater is typical for most liquids.

If hot dip galvanizing is in direct contactwith 3CR12, stainless steel or brass, it con-stitutes the anode and it will be preferen-tially attacked (figures 87, 91 and 92). Onthe other hand, if steel is coupled to cad-mium, aluminium, zinc or magnesium, itwill constitute the cathode and be protect-ed, while the anodic material is con-sumed.

A hot dip galvanized coating primarilyprovides barrier protection since in mostenvironments it corrodes at a substan-tially slower rate than steel. The secondline of defense is however the cathodicor sacrificial protection at small uncoated

42 HDGASA © 20091122

Figure 93. The influence of pH on the corrosionrate of zinc in aerated (CO2 free) solutions.

(Dilute HCI and NaOH at 30°C).Note: The curve only applies for continuous

exposure under the specific conditions. For otherconditions it can be used as a guide. In

hard/scale forming waters protective layers areformed which greatly alter the curve.

12.5 CORROSION RESISTANCE OF HOT DIP GALVANIZED COATINGS IN AQUEOUS CONDITIONS

GeneralZinc carbonate, the protective film formedover a hot dip galvanized coating, is rela-tively insoluble in water. However, this sta-bility is restricted to an acid/alkali pHrange of 6 to 12,5. Zinc is amphoteric innature; that is, it forms soluble salts at lowand high pH values. This is clearly shownin figure 93.

Notwithstanding the above, water con-tains numerous dissolved salts as well ascarbon dioxide and oxygen in solution.Organic matter can be picked up by wateras it passes over vegetation. This can alsobe a major contributor to corrosion insome instances. The effects of water qual-ity on the corrosion rate are summarizedin figure 94.

In soft waters, zinc corrosion is accelerat-ed. Also, the tolerance for chloride salts isreduced. A reserve alkalinity level isrequired to stabilize the zinc carbonatefilm. This is generally assumed to be of theorder of 50 - 75mg/l (as CaCO3). In hardwaters, high chloride levels (>2000mg/l)can be tolerated. Sulphates, nitrates andphosphates are generally considered tobe protective towards hot dip galvaniz-ing. However, when combined withammonia compounds (such as with fertil-izers) soluble zinc compounds may beformed and acid conditions can arisecausing attack of hot dip galvanized steel.Organic compounds such as tannins willarrest the corrosion of hot dip galvanizedsteel but the settling of solids can createconditions for crevice corrosion. Similarly,slime build-up should be avoided asmicrobially induced corrosion (MIC) canoccur, leading to rapid attack.

Figure 94. Effects of water quality on the corrosion rate of a hot dip galvanized coating.

Flow rates should be maintained at suffi-ciently high levels to ensure that all debrisis held in suspension rather than allowedto settle. It should be considered “goodpractice” to flush systems on a regularbasis. This should be carried out on all fireprotection systems although, as the waterentering these systems is generally ofgood quality, corrosion rates tend to below provided that MIC does not occur. Inall instances, the corrosion performance ofgalvanized piping in fire protection sys-tems is far superior to that of bare steel.Crevice or under deposit corrosion is like-ly to occur where sediment becomesdense and compacted. This may result inthe provision of anaerobic sites suitable forthe start of MIC.

Under normal circumstances the amountof dissolved oxygen in a water would besufficient to ensure that no deleteriouseffects occur. However, anaerobic orseptic conditions can affect hot dip gal-vanized piping adversely as is the casewith other metals. For drinking waterpurposes some form of chlorination isgenerally applied. Therefore, in normaldistribution systems anaerobic condi-tions giving rise to MIC, should notoccur. It is important when testing waterlines that clean water be used and thesystem drained if it is to be left unusedfor some time. Chlorination has noeffect upon the protective properties ofgalvanizing. High oxygen levels acceler-ate the corrosion rate of zinc. Similarly,high carbon dioxide levels tend to pro-

surfaces which is provided by the elec-tronegative potential of zinc in relationto carbon steel.

Zinc coatings on steel are unusual, since afairly large area of damage to the coatingdoes not cause catastrophic corrosion (fig-ures 89 and 90). The range of cathodicprotection is dependent on coating thick-ness and the nature of the electrolyte thatcreates the cell. For structures in normalatmospheres it is usual to expect protec-tive action over several millimetres.However, in sea water significantly greaterdistances can be expected.

The impact of bimetallic corrosion can beprevented by the provision of a paint orother insulating material between thedissimilar metals.

The concept of sacrificial protection isharnessed to provide cathodic protec-tion to structures subjected to severecorrosive conditions such as immersionin aggressive water or corrosive soils.Zinc or magnesium anodes are attachedto steel components to provide protec-tion to the steel. These sacrificial anodesare replaced once they have been con-sumed.

Hot dip galvanized components in con-tact with aluminium conductors mayrequire the use of an electrical conduct-ing compound at joint faces to repelmoisture and inhibit corrosion.

Crevice corrosion can occur in conditionsof high humidity at overlapping hot dipgalvanized surfaces. This can be prevent-ed by the application of an inhibitivejointing compound in accordance withSANS 1305. Alternatively a suitablepaint may be used. Hot dip galvanizedsurfaces in contact with other materialsalso require insulation.

Table 27. Soil aggressiveness at different resistivity levels with hot dip galvanized coatings.

Table 28. Corrosivity of different soil types.

Soil type Aggressiveness

Lime, calcareous marl,moraine, sand marl Low

Sand, gravel Moderate

Clay, peat, bog,humus-rich soils High

No. Aggressiveness Soil Resistivity Method of protectionCondition in ohm

1 low dry >100 Hot dip galvanizing > 200μm

2 low moist >450 Hot dip galvanizing > 200μm

3 moderate dry <100 Hot dip galvanizing > 200μm plus a rust allowance on the basis material of 0.5mm on each side.

4 moderate moist 150-450 Same as for 3

5 high moist 50-150 Hot dip galvanizing > 200μm and rust allowance of 1mm on each side.

6 very high moist <50-100 Same as for 5 but rust allowance(In certain of 1.5mm on each sidecases H2SO4

can form)

duce acid conditions, which can acceler-ate corrosion in flowing systems.

Effect of water temperatureHot dip galvanized piping has been usedfor hot water supplies with no deleteriouseffects in many applications. However,when used above 65°C the zinc is nolonger protective to exposed steel. It istherefore recommended that hot dip gal-vanized systems not be used above 65°C.

The electricity supply commission(Eskom), advise that with proper pipeinsulation, the maximum temperature forhot water cylinders be 60°C. For practicalpurposes therefore, hot dip galvanizedpiping is acceptable for use in both hotand cold water systems.

In domestic systems copper should onlybe used downstream of hot dip galva-nized piping. This will avoid the possibilityof pitting corrosion.

Effect of sea waterHot dip galvanized coatings perform rela-tively well in submerged seawater condi-tions which are severely corrosive to mostprotective systems. Dissolved salts pre-sent in seawater react with zinc to form aprotective layer minimizing corrosiveaction. The pH of seawater tends to beconstant worldwide as a result of thebuffering action of the hydrogen-carbon-ate salts present. The presence of pollu-tants is equally not detrimental providedthat levels are within internationallyacceptable norms.

A simple nomogram (table 29) has beenproduced to allow the specifier to deter-mine the suitability of hot dip galvaniz-ing for the protection of steel piping inwater. This provides guidance basedupon the water quality and general oper-ating conditions likely to be encoun-tered. More detailed information is con-tained in ARP 060: Guidance on the useand application of hot dip galvanizedsteel piping for the transportation ofpotable water in South Africa.

43HDGASA © 20091122

12.6 CORROSION RESISTANCE OF HOT DIP GALVANIZED COATINGS IN SOIL CONDITIONS

Soil can contain weathered products, freeor bound salts, acids and alkalis, mixturesof organic substances, oxidizing or reduc-ing fungi, micro-organisms, etc..Depending on its structure, soil has differ-ent degrees of permeability to air andmoisture. Normally, the oxygen content isless than in the air, while the carbon diox-ide content is higher. The corrosion condi-tions in soil are therefore very complicatedand variations can be great between dif-ferent locations, even those in close prox-imity to each other.

Southern African soils vary from highlycorrosive in some regions to moderatelycorrosive in others.

One method of determining the corrosivi-ty of a soil is to measure its resistivity.Recommendations are given in table 27.

If the resistivity of the soil cannot be deter-mined, the rule-of-thumb method listed intable 28 can give a measure of guidance.Where the exposure of metals to soil isconcerned, it is advisable to seek expertadvice from suitably qualified sources.

12.7 HOT DIP GALVANIZED STEEL IN CONTACT WITH BUILDING MATERIALS

Mortar, plaster and woodDamp mortar and plaster attack zinc. Theattack ceases when the material dries out.Dry or moderately damp wood, bothimpregnated and unimpregnated, can benailed with hot dip galvanized nails togood effect. However, in the case of nailsor threaded unions that are constantlyexposed to water an acid-resistant mater-ial is preferred. Other dry building materi-als, such as mineral wool, do not attackzinc.

Wood with acidic properties should notcome into contact with galvanized steel.

Table 29. Probability of performance.

VALUE PARAMETER UNIT RATINGCONDITION OF WATERA Flowing 2

Standing 1

Anaerobic -5

CORROSIVITY INDEX *B <1 0

≥1, <2 -1

≥2, <5 -2

≥5 -4

TOTAL ALKALINITYC <50 ppm as (CaCo3) -1

≥50, <200 1

≥200, ≤300 0

>300 -1

CALCIUM HARDNESSD <50 ppm as (CaCo3) -1

≥50, <200 2

≥200 3

pHE <5.5 -6

≥5.5, <6.5 -4

≥6.5, ≤7 -1

>7 1

CALCIUM CARBONATE PRECIPITATION INDEXF <-2 -2

≥-2, <0 -1

0 0

>0, ≤6 1

>6 0

Probability = Sum (A to F)Result PerformanceGreater than 1 Satisfactory (+25 years)1 to -1 Fair-3 to -5 Unsatisfactory* Corrosivity index (B) can be calculated by -

(C1 x 0,03) + (SO4 x 0,04)

ConcreteUnprotected reinforcement can corrode incertain environments when moisture pen-etrates the concrete through cracks andpores. Since rust has a greater volumethan the steel from which it was formed,the covering layer over the reinforcementcan crack and spall (figure 96).

Steel components such as bolts and edgeguards that have been partly grouted inare often poorly protected against rust.Apart from crack formation and scaling, aproblem occurs with unsightly rust stain-ing on the concrete surfaces below.

This kind of damage can be avoided ifthe reinforcing steel is hot dip galva-nized (figure 95). Hot dip galvanizedreinforcing steel or mesh can thereforebe used in grouted facade sections. Oneof the advantages of this is that there isno risk of rust runs discolouring thefacade.

According to the Building ResearchEstablishment in the UK, the averageadhesion for smooth reinforcement steelin concrete is as follows:

hot dip galvanized steel 3.3-3.6 MPa

black steel 1.3-4.8 MPa

Local pull out tests confirm these results.

The large range for black steel stems fromdifferent degrees of rust and compositionsof oxide scale.

According to work done in Finland, thestress for 0,1 mm of slip in reinforcementbars in concrete is approximately as fol-lows:

black steel 150 MPa

hot dip galvanized steel 160 MPa

hot dip galvanized andchromated steel 190 MPa

When concrete is cast its pH value isaround 13. At this high pH, fresh zinc isattacked and hydrogen is produced,which could give rise to poor adhesion.However, the attack ceases as soon as theconcrete has hardened and any residualpores are not harmful.

In order to avoid fresh zinc surfacescoming into direct contact with wet con-crete it is advisable to allow the galva-nized steel to age for several weeks. Thecover layer of basic carbonates whichthen appears will minimize both attackand the production of gas, and will alsopromote adhesion. Another common

44 HDGASA © 20091122

Figure 96. Spalling of the concrete layer on reinforcing steel on the underside of a concrete bridge.

Figure 95. Hot dip galvanized reinforcing barsprior to casting concrete, marine conditions.

Figure 97. Microsection of a hot dip galvanized coating showing variations in hardness through the coating.

50 Microns

Eta-Layer

Zeta-Layer

Delta-Layer

Steel Base

70 kg/mm2

179 kg/mm2

224 kg/mm2

159 kg/mm2

MICROHARDNESS METER LOAD 25 GRAMS

way of limiting a reaction from fresh con-crete is to chromate the galvanizedsteel. A further alternative is to addabout 40ppm (by mass) of chromates, tothe water when concrete is mixed.

12.8 ABRASION RESISTANCE OF HOT DIP GALVANIZED COATINGS

Pure zinc is a soft metal, even though it isharder than most organic coating materi-als. The iron/zinc alloys produced in hotdip galvanized coatings are, however,very hard. In fact, they are harder thanordinary structural steel (figure 97).

The alloys are therefore more resistant toabrasion than pure zinc and experimentshave shown that the alloy layer has a resis-tance to abrasion 4-5 times that of purezinc.

Hot dip galvanized articles are often usedwhen the surface is to be subjected toabrasion. Examples of this include stairs,

floor hatches, hand railings, grid flooringand walkways (figure 5).

12.9 HOT DIP GALVANIZED COATINGS EXPOSED TO ELEVATED TEMPERATURES

Conventional zinc coatings can beexposed continuously to temperatures upto about 200°C and non-continuously totemperatures of up to 350°C.

At sustained temperatures in excess of200°C a diffusion reaction begins insidethe coating and causes the outer layer tosplit-off from the underlying iron/zinclayer. However, the iron/zinc layer has avery good resistance to corrosion and can,depending on its thickness, protect thesteel from rust for a very long time.

Aluminium-alloyed zinc layers on thinsheet can resist even higher temperatures.Aluzinc and galvalume for instance, canwithstand sustained temperatures up to315°C.

Bolted connections are one of the mostwidely used, versatile and reliablemethods for joining structural steelmembers. Some of the advantages ofbolting over methods such as weldingand rivetting are:

� Economy, speed and ease of erec-tion;

� Reliability of service;

� Ease of inspection

� Fewer, and less highly skilled oper-ators required;

� Reliable performance under fluctu-ating stresses;

� No pre-heating of high strengthsteels;

� No weld cracking or induced inter-nal stresses;

� No lamellor tearing of plates;

� No heat damage to the coating onhot dip galvanized or paintedstructures.

13.1 TYPE OF STRUCTURAL BOLTS AND FASTENING DEVICES

Low carbon steel bolts, generallyknown as class 4.8, have been in use formany years. Continuing developmenthas produced high strength structuralbolts for use in high strength bearingtype joints and high strength frictiontype joints, which are referred to asclass 8.8 and 10.9. These newerstrength bolting methods have greatlyincreased the scope of structural bolt-ing.

In terms of the SANS 1700-7-7; SANS1700-7-8; SANS 1700-14-8; SANS1700-14-9; SANS 1700-14-10,strength of structural bolts is specifiedin terms of the tensile strength of thethreaded fasteners. Two numbers sepa-rated by a full stop are stamped on thebolt head. The first number representsone hundredth of the nominal tensilestrength and the second number repre-sents one tenth of the ratio betweennominal yield stress and nominal tensilestrength expressed as a percentage. Forexample, a grade 4.8 bolt has:

� Tensile strength of 4 x 100 =400MPa;

� Yield strength of 0,8 x 400 =320MPa.

A large variety of fastening devices, otherthan bolts and nuts, are used throughoutindustry and these include componentssuch as spring clips where permanentretention of clamping force is essential.

13.2 CORROSION PREVENTION

While the mechanical properties of fas-tener assemblies are structurallydependable and cost ef fective, thedurability of such connections will beinfluenced by the degree of corrosionencountered in service. Deteriorationbrought about by rusting can lead to theseizure of fasteners and premature fail-ure, in the form of corrosion fatigue.Adequate corrosion protection of fas-teners is, therefore of paramountimportance if the overall integrity of astructure is to be retained throughoutits life (figures 98, 99 and 100).

In bolted steel structures the bolts andnuts are critical items on which theintegrity of the entire structure depends.Protection from corrosion is provided byusing corrosion resistant materials or byproviding a protective coating, eitherbefore or after installation.

13.3 CORROSION RESISTANT METALS

The use of fasteners, manufactured fromcorrosion resistant metal alloys, frequent-ly provides the most cost effectivemethod of avoiding degradation by cor-rosion in very aggressive environments.Contact between dissimilar metals canresult in galvanic corrosion, particularlywhere a large cathode is in electrolyticcontact with a small anode. Austeniticstainless steel fasteners are used withsuccess in many applications where thereis contact with metals such as zinc and inmild to moderately corrosive environ-ments, hot dip galvanized fasteners haveproved successful for connecting compo-nents manufactured from Corten steel.The use of an organic coating over one orboth metal coating interfaces of a jointprior to fastening, or the sealing of thatjoint after bolting, in an aggressiveatmosphere will substantially increasethe corrosion resistance of that joint.

Table 30 provides a guide to the compati-bility of various metals and alloys in contactin building applications. For example, it willbe observed from the table that a zinc coat-ed fastener (anode) connected to 300series stainless steel (cathode) is unaccept-able in a corrosive environment whereaszinc coated steel connected with 300series stainless steel is acceptable.

13.4 PROTECTIVE COATINGS

A coating applied to fasteners must, ofnecessity, be tightly adhering and resis-

45HDGASA © 2009

BBoolltteedd CCoonnnneeccttiioonnss

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Figure 98. An assortment of zinc coated boltsshowing the importance of coating thickness in

a particular environment.

Figures 99 (left) and 100 (right). Corrosion protection of holding down bolts should be equal to that provided for main structures.

CCHHAAPPTTEERR 1133

46 HDGASA © 2009

Figure 102. Demonstration of tensioning results obtained by the turn of the nutmethod.

Table 30. Metals and alloys between which direct contact is acceptable.

CONTACT MATERIAL (FASTENER/WASHER)

Aluminium and Copper and 300 series Zinc coated Aluminium/zinc Leadaluminium alloys copper alloys stainless steels steel and zinc coated steel

Sheeting Industrial & Industrial & Industrial & Industrial & Industrial & Industrial &

material marine Rural marine Rural marine Rural marine Rural marine Rural marine Rural

Aluminium andaluminium alloys A A C C B B B A A A C C

Copper andcopper alloys C C A A B B C C C C B B

300 seriesStainless Steels C B B B A A C C C B B B

Zinc coatedsteel and Zinc A A C C B B A A A A B A

Aluminium/Zinccoated steel A A C C B B B A A A C C

Lead C C A A A A B A C C A A

Legend: A = Acceptable. Increase in the corrosion rate of the sheeting or contact material will be zero or slight.B = Acceptable, but increase in the corrosion rate of the sheeting or contact material can occur.C = Do not use. Accelerated corrosion will occur, or the difference in the lives of the two materials is too great, or both.

200

150

100

50

150 300 450 600 750

Indu

ced

bolt

tens

ion,

kN

Torque, Nm

Minimum bolt tension

specified inSANS 10094

Galvanized andlubricant coated

Galvanized

Figure 103. Torque/induced tension-relation for M20 high strength structural bolts, only galvanized and galvanized and lubricant coated.

rolling or finishing operation that coulddamage the zinc coating. The zinc coat-ing of an external standard metricthread that has not been undercut shallbe such as to enable the threaded partto fit an oversized tapped nut (figure101) in accordance with the allowancesgiven in table 31 below.

On bolts greater than M24, undercut-ting of bolt threads is frequently pre-ferred to only oversizing of nut threads.

Refer to Note 1 in Chapter 10.

Influence of galvanized coatings onthread stripping strengthIn high strength bolting, correct tight-ening is essential, and the oversize tap-ping of galvanized nuts does not neces-sitate a reduction in the level of mini-mum tension which applies to uncoatedfasteners. To meet this requirement,galvanized high strength nuts have ahigher specified hardness than thatdemanded in the case of ungalvanizednuts.

Bolt relaxation The possible effect of bolt relaxation,caused by the relatively soft outer zinc

Table 31. Recommended oversize tappingallowance.

tant to damage during and after assembly.For this reason, metal coatings applied,prior to assembly, are preferred.Additional protection, after assembly bymeans of a paint coating, is beneficial inaggressive environments, particularlywhen these metal coatings have beenapplied.

Coating metals used include zinc andnoble metals such as nickel and tin. In thecase of the more reactive metals, such aszinc, coating thickness is of paramountimportance with corrosion life being moreor less proportional to the coating thick-ness. Where metals, such as nickel and tinare used, thinner coatings will usually pro-vide long term protection provided thatthese coatings are free from imperfectionsand not subjected to mechanical damagewhich, in corrosive conditions, will lead toaccelerated corrosion of exposed underly-ing steel. The cost of providing protectionby means of the more noble metals is highand this has restricted the general use ofthese coatings for the corrosion protectionof fasteners in the structural steel industry.

13.5 HOT DIP GALVANIZING OF FASTENERS

Hot dip galvanizing of fasteners is a spe-cialised process and the products should,therefore, be purchased via an SABSapproved bolt manufacturer who willensure that the correct manufacturing andgalvanizing procedures, including over-size tolerances, etc., are adhered to.

Hot dip galvanized fasteners in variousforms are available as ex stock items frombolt stockists countrywide. Contact theAssociation for further information.

Oversize tapping allowance for hotdip galvanized nutsThe zinc coating on external threadsshall be free from lumps and shall nothave been subjected to a cutting,

Figure 101.

1133

OVERSIZE TAPPING ALLOWANCE FOR HOT DIP GALVANIZED NUTS

Nominal Size of Allowance (mm)Thread

M8 to M12 0.33

M16 to M24 0.38

>M24 = M27 0.43

>M27 = M30 0.47

>M30 = M36 0.57

>M36 = M48 0.76

>M48 = M64 1.0

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Table 32. Nut Rotation from the snug-tight condition. Refer to SANS 10094.

Length of bolt, mm

Nominal Nut rotation 1/2 turn Nut rotation 3/4 turnbolt with 60° tolerance over with 60° tolerance overdiameter no tolerance under no tolerance under

M16 up to 120mm 120 up to 240mm





Figure 105. Where accurate tensioning is critical,permanent indication of the extent of part turntightening can be identified by match marking.

Before final tightening

After

13.7 WASHERS

High strength washers are required tobe through hardened prior to hot dipgalvanizing.

13.8 HIGH STRENGTH FASTENERS – CLASS 10.9 (Refer to SANS 10094)

Class 10.9 fasteners may be hot dip gal-vanized, provided that a certificate ofcompliance is issued by the galvanizer,stating that the hot dip galvanizing hasbeen carried out in accordance withSANS 10094:2005.

Procedure for hot dip galvanizing ofclass 10.9 fasteners

1. All pretreatment cleaning isachieved by lightly wheelabrating forless than 5 minutes instead of acidpickling. This is preferred in order toeliminate the liberation of hydrogen

layer of the galvanized coating on mat-ing surfaces has been investigated.Tests carried out by the Hot DipGalvanizers Association and the SABShave revealed no substantial relaxationand this confirms international studieswhich show that a maximum loss of boltload of 6,5% for galvanized plates andbolts can arise, as opposed to 2,5% foruncoated bolts and members. This lossoccurs within about five days and littlefurther loss is recorded. This loss can beallowed for indesign and is readilyaccommodated.

Slip factor effecting mating surfacesin friction type jointsIn the case of galvanized friction gripjoints the galvanized coating behavesinitially as a lubricant and the co-effi-cient of friction is normally less than0,2. After the first few cycles, underalternating stress, the galvanized sur-faces tend to lock up and further slip,under alternating stress, is negligible(figure 104). If initial slip is undesirable,the application of a zinc silicate paint, tomating surfaces prior to assembly, willprovide a slip factor in excess of 0,4and, this enables hot dip galvanizedassemblies to be designed for perfor-mance which is similar to that ofuncoated steel.

Zinc metal spraying or alternativelylight abrasive blasting of mating sur-faces will also provide acceptable slipfactors.

Lubrication of threadsFor high strength galvanized fastenersto be tensioned to the required level,thread lubrication, by means of molyb-denum disulphide based lubricant oralternatively a wax such as beeswax, isessential (figure 102).

13.6 BOLT AND NUT ASSEMBLIES

Hot dip galvanized bolts and nutsshould ideally be supplied in the nut-ted-up condition. This ensures thatbolts and nuts have been matched andsupplied by the same manufacturerwhile the possibility of bolts being sup-plied with clogged threads is avoided.

ions (H+1) and remove the potentialfor hydrogen embrittlement.

2. Thick hot dip galvanized coatings areavoided by limiting the immersiontimes to less than 2 minutes, agitatingin the molten zinc and ensuring thatall components are immersed for sim-ilar periods of time followed by effi-cient centrifuging (table 33).

3. No stripping and re-galvanizingof rejected sub-quality coating isallowed.

4. No uncoated areas are acceptable.

Note: Users of fasteners shall be awareof dangers during tightening proce-dures if they are not applied correctly.

13.9 BOLT TENSIONING PROCEDURES

Extensive tests have been carried out inorder to arrive at the most effectivemethod of tensioning hot dip galvanizedfasteners while ensuring that this can beperformed in a reliable fashion by semiskilled personnel. The torque required totension hot dip galvanized fasteners,even after lubrication, can vary substan-tially from one fastener to another and,while this fact also applies to uncoatedfasteners, the scatter is greater in the caseof galvanized fasteners. It is recommend-ed that reliable tensioning of highstrength hot dip galvanized fastenersshould not be based on torque/tensionvalues, particularly in the case of friction

Figure 104.

Stress versus slip for fatigue specimen subjected to alternating stress of 160MPa. Hot dip galvanized members and bolts. First, second and fifth stress cycles. W.H. Munse ‘Structural Behaviour of hot galvanized bolted connections’.

Proceedings 6th International Conference on Hot Dip Galvanizing 1968.

Threaded Articles Local Coating Mean Coating Maximum Coating Class 10.9 Fastener Thickness (min.) Thickness (min.) Thickness (min.)Diameter μm or gms/m2 μm or gms/m2 μm or gms/m2

ø > 6mm 40 (285) 50 (360) 65 (465)

Note: Excessively thick hot dip galvanized coatings (i.e. zinc immersion time of longer than 2 minutes), results in excessive growthof the hard Fe / Zn alloy layers and possible fatigue failure from crack propagation at stress raisers. Excessively thick coatings onthreads will interfere with thread tolerances.

Threads are to be clearly defined and free from excess solidified zinc, allowing for ease of nut fitting and tensioning.

Table 33.

grip connections. This recommendationis in line with results obtained in coun-tries elsewhere and, for this reason,torque control tensioning is not encour-aged either for coated or uncoated highstrength fasteners.

Recommended method of tensioning(turn of the nut method) If hot dip galvanized fasteners are to beused, it is recommended that the turn ofthe nut method of tensioning should beadopted. This method has proved to bereliable and slight variations in thedegree of final nut turning do not signifi-cantly influence the ultimate bolt tension(figure 102). The procedure is simple anddoes not entail the use of specialisedequipment. Nuts are tightened to a snugtight position and variations in tightnessat this stage do not significantly influencethe final result. Snug tight is defined inmany specifications as the full effort of aman on a standard podger spanner or thepoint at which there is a change in thenote or speed of rotation when a pneu-matic impact wrench begins impactingsolidly. Podger spanners are graded inlength, in relation to bolt size andstrength and, for example, a spanner ofsome 450mm in length is regarded asappropriate for an M20 high strengthstructural bolt. It must be repeated thatthe clamping force supplied by snug tightis highly variable but this is not significantwhen bolts are subsequently fully tight-ened. The bolt tension/bolt elongationcurve is relatively flat once the proof loadis exceeded and, hence, variations in thesnug tight condition result in only smallvariations in the final bolt tension.

For final tightening the standards intable 32 are recommended. The tableprovides for rotation up to 60° inexcess of the recommended nut rota-tion or a total of 240° in the case ofM16 and M20 fasteners up to a lengthof 120mm. This level of tension is wellwithin the capacity of high strength fas-teners as laid down in SANS 1700-7-7;SANS 1700-7-8; SANS 1700-14-8;SANS 1700-14-9; SANS 1700-14-10(SABS 1282 is to be replaced by specif-ic parts of SABS 1700) where, for testpurposes, fasteners of this length arerequired to be tensioned by nut rotationafter snug tightening to a minimum of300° without fracture or the stripping ofthreads.

Where accurate tensioning is criticalsuch as in the case of friction grip connections, permanent indication ofthe extent of part turn tightening can be identified by match marking the bolt end and nut, at the snug tighten-ing stage, before final tightening(figure 105).

Part torque - part turn methodThis procedure entails the use of a torquewrench to induce a snug tight conditionto all bolts prior to applying full tensionby turn of the nut procedures.

Alternative methods of tensioninghot dip galvanized fasteners The use of load indicator washers pro-vides ef fective tensioning but thisentails the use of specially manufac-tured washers with protrusions whichare flattened as tension increases and areduction of the gap by a specifiedamount, indicates that minimum bolttension has been reached.

Hydraulic tensioning equipment, whichstresses the bolt to the required extentprior to nut tightening, is also available.These alternative methods entail theuse of specialised equipment and forthis reason the use of the uncomplicat-ed and reliable turn of the nut methodis recommended.

13.10 THE EFFECT OF HOT DIP GALVANIZING ON STRENGTH PROPERTIES OF FASTENERS

The hot dip galvanizing process doesnot adversely effect the mechanicalproperties of high strength fastener

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INSTALLATION DO’S � Do Wash off any cement splashes with clean water as soon as possible.

� Do Remove wet storage stain (white rust) if necessary with the aid of a bristle brush and water (not a wire brush). Morestubborn stains can be removed in accordance with the methods given in Chapter 5.

� Do Avoid direct contact with dissimilar metals. If in doubt, insulate with some organic material or overcoat both at the interface.

� Do Use hot dip galvanized fasteners with properly oversized nuts, in preference to zinc electroplated fasteners on hot dipgalvanized structures.

� Do Seal the crevices formed between two bolted zinc coated mating surfaces in moist corrosive conditions.

� Do Use a hot flame, flexible grinding pads or abrasive paper when removing minor zinc protuberances left by the galvanizer.

� Do Use an approved lubricant on galvanized high strength steel bolts prior to bolt tensioning, e.g. molybdenum disulfied(Molyslip) or beeswax.

� Do Use a bristle brush and industrial abrasive (vim) with water to remove superficial rust stains (iron fillings, etc.) that haveadhered to the coating surface. Should this method not be effective, replace the bristle brush with a ‘scotchbrite’ pad (abrasivepad). Should the stain still remain, ascertain by means of an electro-magnetic thickness gauge the residual coating thickness.

INSTALLATION DON’TS � Don’t Use electro-plated fasteners for fixing hot dip galvanized components. If hot dip galvanized fasteners are not

available, overcoat suitably cleaned electro-plated fasteners with a reputable organic coating such as “Zincfix” (Chapter15 and 17).

� Don’t Unnecessarily disturb the matt grey zinc carbonate surface film by wire brushing.

� Don’t Abuse hot dip galvanized articles by aggressive unloading, unnecessary hammering or banging when aligning twocomponents. In spite of the excellent abrasion resistant properties offered by the coating, thicker coatings can be brittle andeasily damaged.

� Don’t Allow hot dip galvanized articles to be installed in aggressive acidic environments or allow liquids with a pH of lessthan 6,0 or greater than 12,5 to be conveyed in hot dip galvanized pipes.

� Don’t Bend articles excessively after hot dip galvanizing for alignment or fitting.

� Don’t Use steel files or inflexible grinding pads with inexperienced personnel to remove protuberances left by the gal-vanizer.

Should any difficulty be experienced with the above, contact the Hot Dip Galvanizers Association Southern Africa.

Figure 106.

steel or even material such as springsteel. Hardened steels <1000MPa yieldstrength, are not considered to beprone to hydrogen embrittlement as aresult of pickling, prior to galvanizing,and any absorbed hydrogen would bedif fused during immersion in themolten zinc at 450°C.

In the case of high strength grade 10,9fasteners as well as products manufac-tured from spring steel, excessivelythick galvanized coatings (>65µm)should be avoided since excessivegrowth of the hard Fe/Zn alloy layerscan result in fatigue failure due to crackpropagation from these layers into thesubstrate where a potential stress raisermay be present. In any case, excessive-ly thick coatings on threads is undesir-able as this will interfere with threadtolerance and may also result in gaulingduring tensioning. Ideally a maximumcoating thickness of 65µm should applyto all male threaded components.

The use of hot dip galvanized Class 10.9bolts and nuts is permitted providedthat a certificate of compliance is issuedby the galvanizer that the fastenershave been processed in terms of SANS10094.

Coating by hot dip galvanizing is usuallycarried out after fabrication is complete.This provides a continuous corrosionresistant barrier to the component sothat a reasonable service life can beachieved.

Modular lengths of components smallerthan available bath sizes are recom-mended since single dipping of the com-ponent will generally achieve a betteroverall coating quality. Components areoccasionally double end dipped if joiningis appropriate and the available bath sizeis inadequate for a single dip.Components that have been designed inmodular lengths appropriate to the avail-able bath sizes occasionally have to bejoined on site. This can be achieved bybolting or welding. When welding ispreferred certain precautions arerequired to achieve quality joints. Themost common welding processes withtheir respective effects on the weldedjoint are provided below.

14.1 SHIELDED METAL ARC WELDING (SMAW)

Welding conditions are similar to thoseused on uncoated steel, except that theroot opening is increased in certain casesto give full penetration and allow fordrainage.

The welding arc should be advanced onto the zinc coating by a weaving actionahead of the molten weld pool to meltand vapourise the coating from the steel.

T-Joints using SMAWThe same basic welding technique usedfor welding butt joints should beemployed; that is, a slower travel speedthan normal and a slight whipping actionof the electrode. Undercut is the mostprevalent defect found in fillet weldsdeposited in the horizontal and verticalpositions with either rutile or basic cov-ered electrodes.

With general galvanizing, the zinc coat-ing is thicker than that deposited on con-tinuously galvanized sheet. This extrazinc may cause trouble in the verticalposition, because when it is molten ittends to run down into the weld pooland make the slag difficult to control.This can be minimised, and often pre-vented, by maintaining as short an arclength as possible.

14.2 GAS METAL ARC WELDING (GMAW)

Conditions for welding general hotdip galvanized steelThe short-circuiting transfer mode pro-duces less distortion and damage to thezinc coating than the spray transfermode.

GMAW spatter formationWith the carbon dioxide GMAW process,each short circuit during the transfer ofmetal causes a momentary rapid rise incurrent followed by extinguishing thearc. Re-ignition is accompanied by theejection of small particles of moltenmetal in the form of spatter. When eithercarbon dioxide or 80% argon, 20% car-bon dioxide shielding gas is used, spat-ter is increased when welding hot dipgalvanized steel compared with uncoat-ed steel.

if the spatter particles adhering to theworkpiece are unsightly, the problemcan be minimised by spraying an anti-spatter compound on to the workpiecebefore welding. Available anti-spatterproducts are based on silicone, petrole-um or graphite compounds. Applyingone of these products will allow thespatter particles to be brushed off easily.

Spatter formation increases with thethickness of the zinc coating and, istherefore greater on general galvanizedsteel than continuously galvanizedsheet. When general hot dip galvanizedsteel is welded in a T-joint, in a flat posi-tion, spatter particles tend to roll into thecorner of the joint causing difficult weld-ing. Spatter formation is also trouble-some when welding in the overheadposition, as spatter particles are apt tofall into the gas nozzle of the weldinggun. Spatter formation is reduced byreducing the diameter of the weldingwire.

14.3 GAS TUNGSTEN ARC WELDING(GTAW)

Gas tungsten arc welding of general gal-vanized steel is not recommendedunless the zinc coating is first removed.The zinc vapour may contaminate theelectrode which in turn will cause erraticarc operation and poor weld quality. Ifthe zinc coating is removed, the baresteel is welded using procedures suit-able for uncoated steel. Gas tungsten arcbraze welding with its attendant lowertemperatures, can be carried out withless joint preparation.

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14.4 FLUXED CORED ARC WELDING (FCAW)

Hot dip galvanized steels may be arcwelded with flux cored electrodes. Slagsystems have been developed for carbondioxide shielding as well as for gas-freeapplications. The self-shielded elec-trodes are favoured for fabricating sheetmetal because of the low penetrationand high travel speeds that are possible.The recommendations of the electrodemanufacturer should be followed and thewelding procedure should be qualifiedby appropriate tests.

14.5 SUBMERGED ARC WELDING (SAW)

Butt jointsButt joints can be welded using hot dipgalvanized steel using the same edgepreparation as for uncoated steel.

Low travel speed can reduce or elimi-nate porosity. By supporting the platesfree from the welding bench, so that zincvapour can escape from above as well asbelow the joint, thicker general hot dipgalvanized coatings can normally bewelded without porosity.

T-JointsSubmerged arc twin-fillet welds, inwhich both sides of a T-joint are weldedsimultaneously can be deposited on hotdip galvanized steel.

14.6 OXYFUEL GAS WELDING (OGW)

Hot dip galvanized steel may be oxyfuelgas welded using copper-coated mildsteel filler rods. Preparation for weldingis similar to that used for weldinguncoated steel; jigs and clamps are usedto prevent distortion caused by heatbuildup and any grease or dirt isremoved from the weld area. A neutralflame should be used, and the size of thetip should be the same as that used forwelding uncoated steel or similar thick-ness.

With oxyfuel welding, because of the lowtravel speed used, the zinc coating isvolatilised and completely removed for atleast 7mm on each side of the weld. For anadditional 7mm or so on each side, partialvolatilisation occurs. These changes resultin a reduction in corrosion resistance.Beyond this depleted region, for up to19mm, the appearance of the zinc coatingmay be degraded; however, this mattedregion has been observed to have nodeterioration in corrosion resistance.

CCHHAAPPTTEERR 1144

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14.7 BRAZING AND BRAZE WELDING

BrazingHigh-frequency induction brazing can beperformed on general galvanized sheetwith very good results using filler alloysof silicon bronze or 60% copper – 40%zinc. Careful control of heating rates canresult in sound joints with very littledamage to the zinc coating.

Braze weldingBraze welds are made at a lower tem-perature than fusion welds. The basemetal is not melted and there is less lossof the zinc coating from the steel. Use ofsuitable brazing alloys produces strongcorrosion resistant welds.

14.8 SOLDERING

Hot dip galvanized steel can be solderedusing either an acid or an organic flux.Zinc chloride- and ammonium chloride-based fluxes are usually adequate whenusing tin-lead solders containing 20 to50% tin. The most popular solder com-position is 40% tin – 60% lead. The rec-ommended heat source is a solderingiron. A caustic treatment prior to solder-ing helps to improve wettability.

Sodium dichromate passivation, used toprevent wet storage staining, may inter-fere with solder flow. Sodium dichro-mate should be removed (see ChemicalCleaning - Chapter 17) prior to soldering.

Hot dip galvanized coatings that havebeen phosphated are difficult to solder.The phosphate films must be removedprior to soldering.

14.9 EMBRITTLEMENT OF STEEL BY LIQUID ZINC DURING ARC AND OXYFUEL GAS WELDING

Welding general galvanized steel jointsusing carbon-steel electrodes can beprone to cracking. This cracking is causedby intergranular penetration of zinc intothe weld metal. It occurs most oftenalong the throat of a fillet weld, in theweld root and is also observed in thebase metal in the heat-affected zone.

Properly designed welded joints usingthe procedures which follow can min-imise the occurrence of such embrittle-ment and the residual tensile stresseswhich exaggerate the problem. Selectingan electrode containing silicon below0.4% is also advisable.

The likelihood of cracking occurring in filletwelds depends upon several key factors:

� the thickness of the hot dip galva-nized coating;

� the method of hot dip galvanizing;

� the thickness of the hot dip galva-

nized steel;

� the width of the joint root opening;

� the method of joint restraint;

� the welding process; and

� the electrode classification.

Weld cracking is influenced by coatingthickness. For this reason, cracking occursmost often when thick coatings areapplied by general galvanizing. Crackingmay not develop at all with thin, electro-deposited coatings. Cracking tends to beless prevalent with low-penetratingshielded metal arc welding and moreprevalent with gas metal arc welding andmore prevalent with gas metal arc weld-ing, especially with carbon dioxideshielding gas. The higher heat input andslower welding speed with shieldedmetal arc welding allows more zinc tovolatilise ahead of the molten weld pool.

Methods for minimising fillet weld crack-ing on hot dip galvanized steel, due tozinc penetration fall into four categories;

� use correct root opening betweenthe plates, about 1,6mm is recom-mended;

� correct choice of consumable, withthe GMAW process low silicon E70S-3 electrodes are better than the highsilicon E70S-6 electrodes, also rutileE6012/13 are better than low hydro-gen E7015/16 types;

� selection of the galvanized base metalby suitable procedure tests; and

� preparation of the base plate toreduce the available zinc by burningthe zinc off by oxy-gas torch, grind-ing or abrasive blasting or preplan-ning by applying a mask prior to hotdip galvanizing.

14.10 RESISTANCE WELDING

Resistance welding or spot welding iscommonly used to join steel sections thin-ner than 5mm thick if the coating is lighterthan 300g/m2 (43µm thick). Coatings up to450g/m2 (65µm thick) have been success-fully welded, although the life of the cop-per electrode is much shorter than withlighter coatings. On heavy coatings, it isnecessary to frequently redress or replaceworn electrodes, due to the build up ofzinc on the electrode.

Coating damage by resistance welding isusually of minor significance, requiringvery little or no repair. If the galvanizedcoatings are thick, resistance welding isimpractical.

Resistance seam welding is not recom-mended due to zinc contamination of theelectrode wheel but projection welding ispossible without serious difficulty.

14.11 SAFE HEALTH PRACTICES

Fumes are always generated during thewelding of uncoated steel. They containvarying amounts of iron oxide, ozone,hydrogen, carbon monoxides, nitrousoxides and fluorides. Zinc oxide is gen-erated when welding or cutting zinccoated steel. Zinc oxide is a white com-pound that is clearly visible in the weld-ing fumes, unlike the gases mentionedabove.

Health effects of zinc oxideThe inhalation of newly formed zincoxide can cause a condition known aszinc fever, or zinc shivers. The symptomsare similar to those of influenza, i.e.fever, fits of shivering, increased salivarysecretion, head-aches and, in more seri-ous cases, nausea and vomiting.

Zinc is not however, retained in thebody in the same way as lead, cadmi-um, and other heavy metals, but isexcreted in urine and faeces. Thesymptoms of zinc fever usually disap-pear within a few hours and long termeffects are thought to be minimal.Temperature, however, does not seemto exceed 39°C and complete recoveryusually occurs within 24 - 28 hours. Asuggested Threshold Limit Value (TLV)of 5mg/m3, has been laid down forAmerican practice. A worker may beexposed to this level for a period ofeight hours without harmful effects.

Protection from welding fumesBy taking elementary precautions, partic-ularly in confined spaces, the effects ofzinc fume can be minimised as follows:

� provide positive ventilation such as asuction hose;

� use suction tube gun nozzles on gasmetal arc and flux cored arc weldingequipment;

� use face masks and respirators;

� weld out of doors if practicable;

� use copper chill bars if practicable, toabsorb the heat of welding;

� welders should position themselvesso as not to be overexposed tofume; and

� ensure that areas to be rewelded /repaired are cleaned of remainingzinc. Grinding is considered to bethe most effective way of removingsuch coatings.

Additional technical informationon welding of hot dip galvanizedsteel can be obtained from the

South African Institute of Weldingor the Hot Dip Galvanizers

Association Southern Africa.

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Figure 108. Site welded joint on hot dip galvanized highway sign gantry - repaired

by “Zincfix”.

(roughness 80 grit) or wire brushed thor-oughly. All dust and debris must be com-pletely removed. In the event of moisturebeing present, all surfaces are to be prop-erly dried.

A zinc rich paint or epoxy containing notless than 80% of zinc in the dry film (53%by volume), should be applied to a mini-mum coating thickness of 100µm, unlessthe purchaser advises the galvanizer oth-erwise, for example, when the galvanizedsurface is to be painted and the coatingthickness for renovated areas is to be thesame as for the hot dip galvanized coatingthickness. The paint coating should over-lap the surrounding zinc by at least 5mm(figure 108).

The preferred product is a two or threecomponent zinc rich epoxy.

15.2 SITE REPAIRS

The preferred method of repair is by zincmetal spraying. Due to the remoteness of

15.1 COATING REPAIR PROCEDURE BY THE GALVANIZER

In terms of SANS 121/ISO 1461:2009 agalvanizer may repair a coating by eitherzinc thermal spraying, zinc rich epoxy orpaint, suitable zinc flake or zinc pasteproducts. The use of a zinc alloy stick isalso acceptable. Zinc rich epoxy or paintmust conform to certain requirements inthe specification.

The total uncoated areas for renovation bythe galvanizer shall not exceed 0,5% ofthe total area of the component.

For articles equal to an area of 2m2; 0,5%represents a maximum area of 100cm2 or100mm x 100mm. For articles equal to anarea of 10 000mm2; 0,5% represents amaximum area of 50mm2 or 7mm x 7mm.No individual repair area shall exceed10cm2 or 10mm x 100mm.

If uncoated areas are greater than 0,5%,the article shall be regalvanized, unlessotherwise agreed between the purchaserand the galvanizer.

Zinc thermal sprayed coatingsThe preferred method of repair is by zincmetal spraying. Repair at the galvanizerwill only be necessary if bare spots arepresent, usually caused by inadequatecleaning, air entrapment or if mechanicaldamage has occurred (figure 107).

MethodThe damaged area is to be lightly blastedusing preferably a pencil blasting nozzle orthe surrounding coating should bemasked in order to limit damage.

A zinc thermal sprayed coating is thenapplied to a lightly blasted surface to aminimum coating thickness of 100µm,unless the purchaser advises the galvaniz-er otherwise, for example, when the gal-vanized surface is to be painted and thecoating thickness for renovated areas is tobe the same as for the hot dip galvanizedcoating thickness. The repaired area isthen wire brushed (preferably stainlesssteel) to remove loosely adhering oversprayed zinc. Wire brushing provides theadded benefit of sealing the pores thatmay be present in the sprayed coating.

Zinc rich epoxy or suitable zinc richpaintMethodThe defective area shall be blasted asabove or abraded with abrasive paper

most sites, however, and the unavailabilityof metal spraying equipment, repairs byzinc rich epoxy or zinc rich paint have todate generally been more popular.

Site repairs should be limited to smallcoating defects and areas that have beencut or welded on site.

Should excessive amounts of grease oroil be present at the affected area, it shallbe removed by means of an approvedsolvent. As far as possible, all residuesare to be thoroughly removed by wash-ing with clean water.

The affected area shall then be abradedwith abrasive paper (roughness 80 grit) oralternatively thoroughly cleaned using,preferably a stainless steel brush. All dustand debris shall be completely removed.

Repair can now be carried out using anapproved product.

Single pack zinc rich paints are goodmaterials and can easily be applied.They, however, require several coats toachieve the required dry film thickness interms of SANS 121/ISO 1461. Multiplecoats will necessitate longer dryingtimes between coats.

Site repairs by “Zincfix”Until recently, the approved products forrepair were only available in large con-tainers. Due to the large quantitiesinvolved and short pot life when mixed,the products proved to be expensiveand wasteful.

A product is now available in a two com-ponent, solvent free form, packed forconvenience in handy, easy to usesquish packs. The repair product is called“Zincfix” and is approved by and avail-able from the Hot Dip GalvanizersAssociation of Southern Africa and all ofits members.

The product has been tested against anumber of reputable products and hasperformed exceptionally well.

The packs are available in 100gm or400gm sizes. The quantity will coat anarea greater than 0,25m2 and 1,0m2

respectively, to a DFT (Dry film thickness)of 100 to 150µm in a single application.

The contents are easily mixed in accurateproportions.

CCHHAAPPTTEERR 1155

RReeccoonnddiittiioonniinngg DDaammaaggeedd CCooaattiinnggss OOrr SSiitteeMMooddiiffiieedd HHoott DDiipp GGaallvvaanniizzeedd CCoommppoonneennttss

Figure 107. Zinc metal spray applied to a small uncoated area on a pipe section,

adjacent to the flange.

The choice of rust prevention system isoften made only on the basis of pur-chase price. However, the purchaseprice says little about the overall econ-omy of the different rust preventionsystems.

The maintenance costs of one systemcan be considerably higher than thoseof another. This is especially true ifaccess to the structure is difficult; ifmaintenance causes operational dis-ruptions; if products and machineshave to be covered, or if scaffoldinghas to be erected.

Unfortunately, it is not practical to givea universally applicable answer to thecost of hot dip galvanizing or othersurface treatments. Structures andcomponents vary in size, which affectsthe ease with which they can be han-dled and therefore the cost of galva-nizing.

The price of hot dip galvanizing isbased on the mass of the goods,whereas the price of painting is usuallybased on the surface area (figure 111).The relationships between averagematerial thickness and surface inm2/tonne are given in figure 109.

The initial costs are often generallylower for hot dip galvanizing than forheavy duty painting (figure 110)because painting is more labour-inten-sive than hot dip galvanizing.

When the total costs of different rustprevention systems are to be com-pared, a number of complicationsbecome evident, since intervalsbetween individual maintenancerequirements can vary. The cost situa-tions at each such interval can alsovary. However, the long service lifegiven by zinc coatings, together withthe reduced risk of minor damage lead-ing to a significant reduction in protec-tion against corrosion, almost alwaysmakes hot dip galvanizing cheaperthan other methods of surface treat-ment in the long run.

Total lifetime costs of corrosion preven-tion systems vary because of differentservice intervals, different labour con-

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Figure 109. Diagram for re-calculation of material thickness in mm to material surface in m2 pertonne. (According to H-J Böttcher and J P Kleingarn)

Figure 110. Comparison between relative initial costs of hot dip galvanizingand a typical heavy duty paint system. The graph takes into account the thickness of hot dip galvanized coating for various profile thicknesses.

tents, the complexity of the mainte-nance task access costs and discountrates used in present value calcula-tions. The first cost of galvanizing willoften be higher than the first cost ofshort life paint systems but lower thanlong life paint systems. The cost of hotdip galvanizing complicated shapesand fabrications with high surface toweight ratio will usually be more com-petitive than the cost of painting. Hotdip galvanizing will also offer short turnround times with no danger of costlysite delays. These factors, plus long ser-vice life, will usually lead to corrosionprotection by hot dip galvanizing beingan extremely competitive engineeringsolution.

Figure 111. Converting from a mass based to an area based price.

CCHHAAPPTTEERR 1166

"Duplex Coating" is a term first introducedby JFH van Eijnsbergen, the eminent corro-sion expert, in the early fifties. It describesthe protection of steel by a layer of zincwhich is overcoated by a non-metallic coat-ing. The purpose is to provide additionalcorrosion resistance especially whenrequired or when a pleasing appearance isnecessary. The corrosion resisting life of aproperly applied duplex coating is normallygreater than the sum of the lives of the twoindividual coatings. Typically, in a severelyaggressive climate, the increase factor is 1,8to 2,0. In sea water it is 1,3 to 1,6 and innon-aggressive climates the factor is 2,0 to2,7.

Effective protection by a duplex system isonly possible if long term inter-coat adhe-sion is obtained by means of a paint coatingwhich will not react chemically with the zincsubstrate. Inadequate preparation andcleaning of the zinc surface, prior to theapplication of a compatible paint system orpowder coating, is the main cause of pre-mature failure.

Because paint films are pervious, to a lesseror greater degree, water can penetrate,over a period of time, to the zinc surfaceand may react with the zinc. The solid cor-rosion products of zinc are approximately20% greater in volume than the zinc fromwhich they arise, whereas steel corrosionproducts have about twice the volume ofthe steel from which they are formed. In thecase of duplex coatings, this can be benefi-cial since defects in the organic coating canbe partially sealed and undercreep retard-ed. However, excessive attack at the inter-face will result in peeling or blistering, butusually to a lesser extent than when themore voluminous corrosion products ofsteel are produced.

Hot dip galvanized coatings on silicon killedsteel are easier to paint than pure zinc coat-

ings on continuoully galvanized sheets dueto the presence of iron/zinc alloy layers.With thermally sprayed zinc coatings it isadvisable to apply an initial sealer coat toprevent the absorption of the paint mediainto the pores of the zinc coating thus leav-ing a pigment-rich layer which will be proneto disintegration. Powder coating with poly-esters, epoxy polyesters or epoxies is com-mon practice.

17.1 WHEN TO PAINT HOT DIP GALVANIZED STEEL STRUCTURES

Existing structuresMaintenance painting on a cleaned, weath-ered galvanized surface is normally moreeffective than when such painting is carriedout on a rusted steel surface. This is becausezinc corrosion products are easier toremove thus providing a more stable sub-strate. As with ungalvanized steel, mainte-nance painting is rarely up to the standardof an original paint coat. Epoxies, whichhave been specifically formulated for themaintenance painting of steel, will usuallyalso be effective on weathered, but cleaned,galvanized surfaces. With the necessaryforesight, however, costly maintenancepainting could be avoided, or deferred, byapplying a duplex coating in the first place.Frequently, maintenance painting of galva-nized structural steel is carried out unneces-sarily, due to the mistaken belief that surface“rust stains” present on the coatingemanate from the steel substrate. It isimportant to appreciate that the iron/zincalloys make up a large proportion of theoverall coating. As gradual weatheringtakes place, rust staining, from these alloysis often observed, particularly in caseswhere extremely thick coatings, associatedwith the galvanizing of reactive silicon killedsteel are present. The only conclusive test isto determine the actual thickness of the

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Figure 112. Sweep blasted hot dip galvanized coating followed by an appropriate paint system.

CCHHAAPPTTEERR 1177

remaining coating by means of an electromagnetic thickness gauge. (Refer also toChapter 11).

New structuresBy far the most satisfactory duplexingresults are obtained by applying the paintsystem as soon as possible after galvaniz-ing. Weathered galvanizing, which is suit-ably cleaned, can provide a satisfactory sur-face on which to apply paint but this shouldonly be considered if material is situated ona site where corrosion is relatively mild anda stable weathered zinc surface has devel-oped. Under no circumstances should thepainting of freshly galvanized structures bedeferred in a marine environment wherethe need to remove unstable zinc corrosionproducts, prior to painting, will render thesystem less effective. In the case of boltedstructures, painting prior to erection pro-vides a distinct benefit in that mating sur-faces receive added protection while metalto metal contact is also prevented.

An alternative to applying the entire paintsystem immediately after galvanizing is toapply a suitable primer at this stage, fol-lowed by a compatible top coat on site. Thefinal coat should, however, be applied assoon as possible if primed material is deliv-ered to a corrosive site.

Ideally, material should be fully painted at,or near to, the galvanizer's works wherestrict quality control procedures can beenforced. Transit damage to painted mater-ial is mainly the result of poor stacking andrough handling. The use of plastic spacers,or similar material, as well as nylon slings,for loading and off-loading, usually results inminimal damage to a correctly applied paintcoating. The use of re-coatable paint prod-ucts is recommended so that any coatingdamage that may occur during transit isreadily repairable on site, either before orafter erection.

17.2 SURFACE PREPARATION FOR DUPLEX COATING

A zinc surface totally free from contami-nants is an essential pre-requisite for thesatisfactory painting of galvanized steel.Nearly all failures occur because of inade-quate preparation of the surface or re-cont-amination, after cleaning and prior to paint-ing, of the reactive zinc surface. Failure toinform the galvanizer that subsequentpainting is to take place will usually resultin the provision of a coating which hasbeen passivated and this may adverselyinfluence paint adhesion. When paintingis to be followed (see new structures)some time after the hot dip galvanizedcomponents are exposed to an aggres-sive marine environment, passivationafter hot dip galvanizing is to be encour-aged. Like-wise, zinc protuberances andlumps, which may be acceptable on gal-vanized surfaces, will not be removed ifthe galvanizer is unaware of the require-ment to paint.

Surface Roughness of Zinc CoatingAs in the case with a paint coating applieddirectly onto steel, weld spatter, slag andsteel surface defects will be apparent afterhot dip galvanizing if prior removal is notcarried out.

Irregularities in the surface of a galva-nized coating may consist of small drossparticles, zinc oxide, surface fluxdeposits and stains from unsealed inter-stitial spaces. Localised lumps may occurwhere drainage of excess zinc, duringwithdrawal from the galvanizing bath,was incomplete. Generally, these featuresdo not reduce corrosion resistance of thegalvanized coating but, if duplex systemsare to be applied, they will appear moreprominent after painting and may locallyreduce paint coating thickness.

A locally thinner paint film over a smalldross particle, or a zinc droplet, is less criti-cal than a thin paint film on a protuberanceon steel since the zinc corrosion products,formed after the thin paint area has corrod-ed away, will prevent accelerated corrosionprovided good adhesion of the overall paintfilm is maintained. This has been demon-strated, for example, in the case of powdercoatings where the presence of pinholeshad no influence on the corrosion resistanceof the duplex system upon atmosphericexposure. In severely corrosive immersedconditions, however, thinner localised paintfilms should not be tolerated.

Sweep blastingSweep blasting is a method often used forpreparing galvanized surfaces for painting.There is no doubt that, if this process is car-ried out correctly, excellent adhesion can beobtained. The use of an ultra-fine non-metallic grit and low nozzle pressure areessential but, if contaminated or powderyabrasive material is used, sweep blasting

can do more harm than good. High nozzleblasting pressure and the use of unsuitableabrasives can result in the delamination ofthe iron/zinc alloy layers particularly whenheavy zinc coatings, associated with thegalvanizing of silicon killed steel, are pre-sent. The process is less effective for prod-ucts such as grid flooring where inaccessiblesurfaces are under blasted while exposededges are inclined to be overblasted.Sweep blasting should only be used for thesurface preparation of galvanized steel if thecorrect equipment and materials are avail-able and operated by trained personnel (fig-ure 112).

Chemical cleaningZinc has a tendency to attract contaminants,such as oil and dust. All contaminantsshould be removed by cleaning with anapproved solvent detergent degreaser.Failure to do this is the main cause of duplexfailures. Galvanized cleaners, which containabrasives, have been shown to be effectiveprovided scrubbing is adequate. Abrasivecomponents tend to settle out in containerswhich have been standing and thoroughmixing, prior to use, is recommended. Afterdegreasing, bristle brushing and thoroughwashing and rinsing with running potablewater is essential in order to remove alltraces of the cleaning chemicals. The zincsurface should then be "water break free"and once this stage is reached, and the zincsurface is dry, painting should commence assoon as possible.

Chemical conversion coatingsChemical pre-treatment is designed to pro-vide a strong, durable and long lasting bondbetween metallic and non-metallic coatingsand also to prevent or retard undesirablechemical or physical action between thetwo coatings. The most widely used pre-treatment chemicals are phosphates andchromates and these are used extensivelyfor the painting of sheet and also for powdercoating. Certain products have been devel-oped which contain a small addition of cop-per salts which give the zinc coating a darkgrey colour. The advantage of these formu-lations is that it is possible to establish thatall surfaces have been treated merely byvisual inspection. These formulations havebeen shown to promote excellent adhesion,although it can be argued that copper saltsare theoretically harmful when in contactwith a zinc coating. Experience in theUnited Kingdom has shown that this onlyapplies if the subsequent organic coatingdeteriorates or has been applied to a surfacewhere the pre-treatment chemical is contin-uing to react.

17.3 SYSTEM SELECTION

In selecting a suitable system it is recom-mended that all products, wherever possi-ble, are purchased from the same paintmanufacturer. This will ensure compatibility.Certain paint formulae should not beapplied directly onto zinc surfaces.

Amongst others, alkyds may result insaponification with the formation of formicacid which will attack the zinc substrate andresult in long-term adhesion failure. Somepaints applied onto suitably cleaned zincsurfaces will provide good adhesionbetween the zinc coating and the paintwithout the application of a primer. This isparticularly true if zinc surfaces are correctlysweep blasted prior to painting. Excellentadhesion of certain specially formulatedhigh build epoxy and polyurethane coatingscan now be achieved without the use of aprimer or tie coat.

PrimersPrimers which are applied onto suitablycleaned galvanized surfaces and which haveproved extremely successful in providingthe required long term adhesion betweenthe organic coating and the zinc substrateinclude:

� Twin component, solvent carried,epoxy amine primer containing zincoxides and silicates.

� Single pack water borne modifiedacrylic copolymer primers. Suchprimers should not be used where per-manent saturation, in service, is antici-pated.

Finishing coatsThe material to be used will be determinedby the conditions to be encountered in ser-vice. Polyamide cured high-build epoxycoatings have been used successfully in thecorrosive mining industry but problemsassociated with abrasion and age embrittle-ment have been encountered with certainproducts. High-build, high volume solids,twin pack, aluminium filled epoxies are suc-cessfully applied to primed galvanized sur-faces. These products can be supplied withmicacious iron oxide (m.i.o.) pigments, orstraight colour pigments, if chalking is not adisadvantage. They are also used success-fully for maintenance painting. Polyure-thanes are becoming popular in situationswhere bright colour and gloss retention is arequirement. Over-coating with chlorinatedrubber has been successful but this productis, in many cases, being superseded byhigh-build vinyl coatings. Considerable suc-cess has been achieved with specially for-mulated epoxy tars which, when applied tosuitably prepared zinc surfaces, can providelong-term maintenance free protectioneven in situations of permanent saturation.For more detailed information contact theAssociation and refer to the publications,Code of Practice For Surface Preparation andApplication of Organic Coatings HDGASA01-1990, Specification for the PerformanceRequirements of Coating Systems HDGASA02-1990 and Hot Dip Galvanizing andDuplex Corrosion Protection, including –Quality Surveillance, Handling, Loading,Unloading, Stacking and Site Repair.HDGASA 03-2006. For continuously coatedgalvanized sheet, refer to Chapter 5.

54 HDGASA © 20091177

55HDGASA © 20091188

CCHHAAPPTTEERR 1188

Photos 12 & 13: Residual coating thickness readings of 90 and 85μm, taken on the 132/88kV terminal tower and two of the isolator supports.

Corrosion engineering is the specialistdiscipline of applying scientific knowl-edge, natural laws and physical resourcesin order to design and implement mate-rials, structures, devices, systems andprocedures to manage the natural phe-nomenon known as RUST.

While some efforts to reduce corrosionmerely redirect the damage into less vis-ible and predictable forms, controlledcorrosion treatments such as hot dip gal-vanizing and duplex coatings, willincrease a material’s corrosion resistancebut will also allow service life predictionsto be established.

This predictable performance is signifi-cantly highlighted by the following threerecorded case histories.

Eskom has for many years relied on hotdip galvanizing to protect their assetsamongst other things such as the steel-work required for power stations, pylonstructures and sub-station steelwork thatare exposed to the many environmentalconditions throughout South Africa.

Hot dip galvanizing is normally specifiedprimarily for corrosion protection. For thisreason, the two most important inspec-tion criteria of the coating taken at anytime during the life of the coating arecoating thickness and coating continuity.

As life of a zinc coating, no matter howapplied is more or less proportional to itsthickness in a given environment, a thick-er coating will provide a substantiallylonger life than a thinner coating.

18.1 PENTRICH SUB-STATION – MKONDENI , PIETERMARITZBURG

The sub-station was built around 1967and exposed to the atmosphere ofMkondeni. According to ISO 9223 –Corrosion of Metals and Alloys –Corrosivity of Atmospheres –Classification (see Chapter 12), suggeststhat this part of Pietermaritzburg is a C2or at worse a C3 environment.

Our findingsThe hot dip galvanized coating thicknesson several components within the sub-station was scrapped clean of atmospher-ic contaminants, measured using a cali-brated electromagnetic coating thicknessgauge and the results tabulated below.

Although SABS 763 was the hot dip gal-vanizing specification at the time ofinstallation, coating thickness require-

ments are similar to SANS 121 (ISO1461), the current specification. SeeChapter 10.

In spite of the atmospheric conditions thehot dip galvanized coating has corrodedvery little in the 40 years of exposure.

The holding down nuts and bolts of allthe structures from the coating thicknessreadings taken, seemed to have beenoriginally zinc electroplated and werebreaking down. The holding down boltshave subsequently been over coatedusing some zinc rich paint, which wasscrapped off to measure the residualmetallic zinc coating thickness. Althoughbecause of their size and the local atmos-pheric corrosion conditions, corrosion ofthese bolts in the medium term wouldnever really compromise the life of thestructure, they however should berepaired using an appropriate material.

Sub-conclusionThe residual hot dip galvanized coatingon the structural steel after 40 years ofexposure to the atmosphere ofMkondeni, Pietermaritzburg, is in asound condition and will not require anyrefurbishment for another 40 years.

COATING THICKNESS (μm)

Georgedale / Pentrich 3 – 132/88kV Terminal Tower

Mean Max Min No. of readings

90 x 90 x 8mm L 95 114 83 9

30 x 30 x 3mm L 134 161 114 11

M12 Hex Nut 78 141 55 10

M12 Hex Bolt 97 132 65 12

132/88kV Isolator Support

50 x 50 x 6mm L 88 109 61 22

150 x 75 Channel 123 179 89 18

70 x 70 x 12 L 155 162 145 14

M16 Hex Nut 73 88 59 4

Hot dip galvanized coating thickness readings taken on various exposed components at Pentrich Sub-Station.

18.2 BLOUWATER SUB-STATION – SALDANHA BAY

Blouwater Sub-Station is situated approxi-mately 130km north west of Cape Town.The area is routinely subjected to earlymorning mists that last well into mid-morn-ing. The location of the site selected is wellwithin 20km of the coast with the prevailingwinds being either South Easterly or NorthWesterly. Steel structures exposed to theseconditions are therefore subjected to highlevels of moisture as well as coastal salineatmospheres. It was built in 1970.

Our findingsIn general, the coating is in remarkablygood condition despite misleading surface

Photo 11: General photo of Pentrich Sub-Station.

PPrroovveenn CCooaattiinngg PPeerrffoorrmmaannccee –– CCAASSEE HHIISSTTOORRIIEESS

contamination. Interestingly, some of thebolts and nuts showed signs of distress.This appeared to be limited to fastenerassemblies underneath the lower end of theinverted diagonal angle bracings. The rea-son for this is due to an extended period ofaccumulated wetness and being shieldedfrom the sunlight.

The use of zinc-electroplated fasteners(electro-galvanizing) is unacceptable due tothe extremely thin zinc coating.

Sub-conclusionAfter approximately 35 years of service, thehot dip galvanized coatings on steel com-ponents installed at the Blouwater sub-sta-tion will continue to provide adequate andeffective corrosion protection for at leastanother 35 years.

There is no doubt that hot dip galvanizingcan and does provide a cost effective solu-tion to the vexed question of steel corrosionprotection, not only within 20km of thecoast, but also in the more aggressive areasexperienced within Southern Africa.

It is interesting to note that even where thehot dip galvanized steel appears to exhibit“red rust”, once the contaminated surfacehas been cleaned a substantial amount ofzinc and zinc iron alloys remain. It is anestablished fact that the zinc iron alloy lay-ers provide approximately 30% greater cor-rosion protection than that of pure zinc onits own. However, as the zinc iron alloyscorrode, speckles of red rust appear due tothe iron content within the alloys. This issometimes seen as representing a potentialfailure of the structure, but in reality thesteel remains unaffected and capable of per-forming the functions for which it was orig-inally designed.

18.3 ELECTRICAL TRANSMISSION TOWERS

South Africa is known for its many severecorrosive atmospheric conditions. Theseenvironmental conditions are not onlyrestricted to the coastal regions, but includemany inland industrial areas as well.

Selected sites and our findings The three sites, range across the full spec-trum of climatic conditions.

Site No. 1

� Relatively Benign Conditions.� 53kV DC line from Cabora Basa to

Eskom’s Apollo Sub-Station South ofPretoria installed 1973.

� After removing the “apparent” rustdiscolouration, the underlying zinc hotdip galvanized coating thickness mea-sured 65.4µm.

Site No. 2

� Inland Industrial site selected for its rel-atively severe corrosive conditions.

56 HDGASA © 20091188

Photos 17 & 18: Despite the apparent “rust” contamination of the galvanized surface, once removedthe galvanized coating measured 126μm.

Photos 14 & 15: Site No. 1 – After removing “apparent” rust discoloration, the underlying hot dip galvanized coating measured 65.4μm.

This site is located within GermistonIndustrial area.

� The specific tower is believed to havebeen in service for the past 40 years.

� After the removal of the discoloura-tion, i.e. corrosion products, a mea-surement of the remaining hot dip gal-vanized coating revealed 119µm.

Site No. 3

� Severe Marine Coastal Conditions� Buffalo-Port Rex Transmission Line

situated in East London. At the timeof the inspection, the towers hadbeen in service for 25 years.

� General condition of the tower after25 years of service, was found to besuch that an over coating of a paintsystem was recommended in orderto extend the service life of the struc-ture.

� The site selected for the severemarine conditions consists of trans-mission towers on the Buffalo-PortRex Transmission Lines situated inEast London.

� The tower that was inspected is situ-ated approximately 3km from theocean next to the Buffalo River salt-water estuary, as well as alongside acity dump. At the time of the inspec-tion the tower had been in service for25 years. The initial corrosion protec-tion comprised hot dip galvanizingand had never been over coated withan organic coating system. Severecorrosion with subsequent metal losswas observed on some of the struc-tural members. In one isolatedinstance the degree of metal loss wasso severe that it had resulted in theperforation of the member. The nuts

and bolts of these corroded memberswere also severely rusted. The mostsevere corrosion was mainly locatedon the inner surfaces of the members.The members that showed severesigns of corrosion were either perfo-rated or the degree of metal loss wasin the 1 to 2mm range. The outer sur-faces of these members were onlysuperficially corroded and the hot dipgalvanized thickness readings rangedfrom 87 and 104µm.

ConclusionHot dip galvanized structures in numerousapplications have been shown to exhibitoutstanding performance over the fullspectrum of environmental conditions.Where severe environmental conditionsare encountered, Duplex coatings (hot dipgalvanizing plus a top paint coating)should be considered.

We acknowledge and thank Eskom fortheir input in the above case histories.

Photo 16: Severe marine coastal conditions.Buffalo-Port Rex Transmission Line situated in EastLondon. At the time of the coating inspection, thetowers had been in service for about 25 years.

HDGASA © 2009

RREELLIIAABBIILLIITTYYThe hot dip galvanized coating is formed by a metallurgicalreaction between suitably cleaned steel and molten zinc. Thisresults in the formation of a series of iron/zinc alloys which areovercoated with relatively pure zinc. The process entails totalimmersion of components in both pretreatment chemicals andmolten zinc. This ensures uniform protection and coating reliabilityeven on surfaces which would be inaccessible for coating by othermethods.

DDEEPPEENNDDAABBIILLIITTYYEase of inspection and dependability in service are beneficialfeatures of a hot dip galvanized coating. The cathodic protectionof steel by zinc ensures that corrosion of the underlying steelcannot occur as long as zinc is present. Even at discontinuities onthe coating, corrosion creep under the surrounding zinc is notpossible.

PPRREEDDIICCTTAABBIILLIITTYYThe durability of a hot dip galvanized coating is determined by thedegree of corrosion of zinc in a specific environment and thethickness of the coating. Corrosion of zinc is normally uniform,thus durability of a hot dip galvanized coating is predictable inmost applications.

HHOOTT DD IIPP GGAALLVVAANNIIZZ IINNGG HHAASS BBEEEENN UUSSEEDD TTOO PPRROOTTEECCTT SSTTEEEELL FFRROOMM CCOORRRROOSS IIOONNFFOORR MMOORREE TTHHAANN 117700 YYEEAARRSS .. AAPPPPLL IICCAATT IIOONNSS FFOORR WWHHIICCHH HHOOTT DD IIPPGGAALLVVAANNIIZZ IINNGG IISS SSUU IITTAABBLLEE AARREE NNUUMMEERROOUUSS AANNDD VVAARRIIEEDD AANNDD TTHHEE DDEEMMAANNDDCCOONNTT IINNUUEESS TTOO IINNCCRREEAASSEE ..

PPUUBBLLIICCAATTIIOONNSSAAVVAAIILLAABBLLEE FFRROOMM TTHHEE AASSSSOOCCIIAATTIIOONN� Association Journal – Hot

Dip Galvanizing Today.

� Steel Protection by Hot DipGalvanizing and DuplexSystems.

� Practical Guidelines for theInspection and Repair of HotDip Galvanized Coatings.

� Specification for thePerformance Requirementsof Duplex Coating Systems.

� Code of Practice for SurfacePreparation and Application ofOrganic Coatings Applied to New Unweathered Hot DipGalvanized Steel.

� Guidelines for the Use of HotDip Galvanized Products inthe Mining Industry.

� Wall Chart - Design for HotDip Galvanizing.

ISSUE DATE: SEPTEMBER 2009

MEMBER’S LABEL

The material included in thispublication has been developed toprovide accurate and authorative

information regarding iron and steelproducts hot dip galvanized after

fabrication and is based onrecognized engineering principles andinspection practices. This material is for

general information only and is notintended as a substitute for competent

professional examination andverification as to accuracy, suitabilityand/or applicability. The publicationof the material contained herein is not

intended as a representation orwarranty on the part of Hot Dip

Galvanizers Association SouthernAfrica. Anyone making use of this

information assumes all liability arisingfrom such use.

SSHHEEEETT HHOOTT DDIIPP GGAALLVVAANNIIZZIINNGGSSHHEEEETT HHOOTT DDIIPP GGAALLVVAANNIIZZIINNGG

TTUUBBEE HHOOTT DDIIPP GGAALLVVAANNIIZZIINNGGTTUUBBEE HHOOTT DDIIPP GGAALLVVAANNIIZZIINNGG

CCEENNTTRRIIFFUUGGEE HHOOTT DDIIPP GGAALLVVAANNIIZZIINNGGCCEENNTTRRIIFFUUGGEE HHOOTT DDIIPP GGAALLVVAANNIIZZIINNGG

WWIIRREE HHOOTT DDIIPP GGAALLVVAANNIIZZIINNGGWWIIRREE HHOOTT DDIIPP GGAALLVVAANNIIZZIINNGG

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