Galvanized Coatings

8/13/2019 Galvanized Coatings

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. ANBUROSE.S

M.E (CIM)200721521

CEG, ANNA UNIVERSITY


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hot dip galvanizing


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h ot d ip g alvan izing

Galvanized coatings are formed by a metallurgicalreaction between the steel and the zinc.

This reaction occurs when the steel reaches thetemperature of the molten zinc 455oC.

This reaction forms a zinc-iron alloy of crystalline form,

that grows from the steels surface. The thickness of thisalloy layer is determined by the time the steel isimmersed in the molten zinc alloy.


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Zinc is among single most widely used coatingmaterials used to protect steel from corrosion.

It is applied to steel components by a number ofindustrial processes.

These include zinc electroplating, thecontinuous galvanizing of sheet, wire and hollowsections, and the hot dip galvanizing offabricated steel items.


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Zinc is the primary anti-corrosion component forprotecting steel in its various forms, and is used forelectroplating, continuous galvanizing of wire, sheet and

tube.

hot dip galvanizing of structural steel.


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The

h o t d ip g alvan izingprocess


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The hot-dip galvanizingprocess consists of

three basic steps:surfacepreparation,

galvanizing, andinspection.

Hot-dip Galvanizing Process


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Surface Preparation

It is essential for the steel surface to be clean anduncontaminated in order to obtain a uniform, adherentcoating. Surface preparation is usually performed in sequenceby caustic (alkaline) cleaning, water rinsing, pickling, asecond water rinsing, and fluxing.

The caustic cleaner removes organic contaminantsincluding dirt, water-based paint markings, grease, andoil. Next, scale and rust are removed by a pickling bathof hot sulfuric acid (150 F / 66 C) or room-temperaturehydrochloric acid. Water rinsing usually follows both

caustic cleaning and pickling.


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Hot dip galvanizing Molten zinc

Degreasing Hot caustic

Pickling Hydrochloric acid

Pre-fluxing Zinc am. chloride

Quenching Dichromate

Loading - jigging

The ho t d ip g alvanizing pro cess


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Loading - jigging

The steel items need to be supported during the hot dipgalvanizing process.

Solutions and molten zinc must be able to flow into allsections and drain off all sections.

Steel items must be designed to prevent air, solutions and

molten zinc becoming trapped

Check d es ign for p oss ib le p rob lems

The ho t d ip g alvanizing p roc ess


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Degreasing

A hot (90 oC) caustic bath is used to remove -Oil, grease, paint another organic compounds

If these contaminating materials are not removed, the nextstage of the process (pickling) is affected

Check i f any paint i s on i tem - so m e paints are not eas i lyremov ed and m ay require abras ive blas t ing before

galvanizing

Som e pipes and pipe f i t t ings have a b lack varnish c oat ingthat cannot be remov ed in the caust ic bath .

The ho t d ip ga lvan izing pro cess


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Acid pickling

The steel items are immersed in hydrochloric acid toremove - rust, mill scale and other metal oxides

The steel surface must be perfectly clean of these oxidesfor the molten zinc to react with the steel

Very heavy ru s t m ay not be eas i ly remo ved by p ickl ing -badly rus ted i tems sh ould b e abras ive blas ted or

m echanical ly c leaned f i rs t .



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Pre-fluxing

Steel items are water rinsed after pickling and immersed inhot (70-80 oC) zinc ammonium chloride (ZAC) solution .

The ZAC solution conditions the clean steel surface readyfor hot dip galvanizing.

Good pre-t reatm ent = goo d qual i ty galvanizing



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Surface PreparationZinc-iron metallurgical bond only occurs on clean steel

DegreasingRemoves dirt, oils, organicresidue

PicklingRemoves mill scale andoxides

FluxingMild cleaning, provides

protective layer

Degreasing/Caustic cleaning


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Quenching

After hot dip galvanized steel item is removed from thegalvanizing bath, it is immediately quenched in a sodiumdichromate solution

The dichromate quenching cools the item so that it can bequickly handled and passivates the surface of the galvanizedcoating to maintain its bright appearance.



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Hot dip galvanizing

The steel is immersed in molten zinc (temp 450 0C)

The clean steel surface reacts with the molten zinc to form a

zinc-iron alloy which is very strongly bonded to the surface.The hot dip galvanized coating forms in 3-5 minutes,depending on the steel thickness.

Galvanized coating

Steel

Free zinc layer

Micrograph - 100 X


Alloy layers


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QuenchingAfter hot dip galvanized steel item is removed from thegalvanizing bath, it is immediately quenched in a sodiumdichromate solution

The dichromate quenching cools the item so that it can bequickly handled and conditions the surface of the galvanizedcoating to maintain its bright appearance.

Chromate quenching wi l l r educe w hi te rus t ing prob lems .Whi te rus t fo rms when ra inwater reac t s wi th newlygalvanized s teel to form zinc hyd roxid e .

Poor s torage or HDG steel wi l l also p rom ote whi te rus t .



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Galvanizing

Steel articles areimmersed in a bath ofmolten zinc ( 830 F)

> 98% pure zinc, minorelements added forcoating properties (Al, Bi,Ni)

Zinc reacts with iron inthe steel to formgalvanized coating Zinc bath removal


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Inspection

Steel articles are inspectedafter galvanizing to verifyconformance to appropriatespecs.

Surface defects easilyidentified through visualinspection.

Coating thickness verified

through magnetic thicknessgauge readings.


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Characteristics Typically 65 - 150 microns thick (1 mm = 1000 microns)

Zinc iron alloy layers make up 80% - 100% of the coating.

The zinc-iron alloy layers are harder than 250 grade steel

Steel

Zinc-iron alloys

Zinc

Micrograph of hot dip galvanized coating



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Coat ing com pari sonsCompared to zinc electroplating (ZEP) and continuousgalvanizing (CG) , hot dip galvanizing (HDG) is betterbecause..

- HDG is thicker and will last over 50 years or morewithout rusting

- HDG is harder and is 5 times more abrasion resistantthan ZEP and CG

- HDG coats all external and internal surfaces andedges. All CG coatings have cut, exposed edges afterprocessing.



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Coat ing th ickn ess o f coa tings o ngalvanized pro du cts

0

10

20

30

40

50

60

70

80

90

100

Z 3 5 0 G a

l . s h e e

t


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Metallurgical Bond


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Edge Protection

Same thickness at corner

Micrograph of galvanized edge


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Influencers of Coating Development

Steel SurfaceConditions

Steel Chemistry Silicon

Phosphorous


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The Sandelin Curve


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Coating Appearance

NewlyGalvanizedNo Spangle

NewlyGalvanizedHighlySpangle

NewlyGalvanizedDull Coating

NewlyInstalled

Shiny & DullCoating


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The Zinc Patina

Forms as zinc reacts with the environment

Consists of zinc oxide, zinc hydroxide, andzinc carbonate

Protects the galvanized coating by providing

an additional layer of corrosion resistance


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Passivation Cycle

Time0 48 hrs.

48 hrs. 6 mo

6 mo. 2 yrs.

1

2

3


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Environmental Performance

AtmosphericLiquid (Chemicals, Fresh H2O, Salt H2O)

SoilHigh TemperatureLow Temperature

Concrete


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Liquid: Effect of pH on HDG steel


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Performance in Soil

> 200 different soil typesComplex corrosion kinetics in soil

Variables include: Porosity Resistivity Organic material Moisture content pH Temperature


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Performance in VariousTemps

High Temperature< 392 F (200 C)

Low Temperature> -75 F (-60 C)


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Metallurgical BondHot-dip galvanizing is a applied coating that provides acombination of properties unmatched by other coatingsystems because of its unique metallurgical bond withthe steel.

Typical z inc- i ron a l loy layers


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Fatigue Strength

the fatigue resistance of galvanized steel reinforcementthat deformed reinforcing steel, exposed to anaggressive environment prior to testing under cyclictension loading, performs better galvanized.


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Barrier Protection

PaintPowder Coatings

Galvanizing


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Cathodic Protection

Impressed CurrentGalvanic Sacrificial Anode

Galvanic Zinc Application Zinc Metallizing Zinc-rich Paints

Hot-dip Galvanizing


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Cathodic Protection

Impressed Current

External source of direct current power is connected (orimpressed) between the structure to be protected andthe ground bed (anode).

Ideal impressed current systems use ground bed

material that can discharge large amounts of current andyet still have a long life expectancy.


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Cathodic Protection

Galvanic Sacrificial Anode Pieces of an active metal such as magnesiumor zinc are placed in contact with the corrosiveenvironment and are electrically connected tothe structure to be protected

Example: Docked Naval Ships


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Cathodic Protection

Galvanic Zinc Application

Zinc Metallizing (plating) Feeding zinc into a heated gun, where it is melted and sprayed on astructure or part using combustion gases and/or auxiliarycompressed air.

Zinc-rich PaintsZinc-rich paints contain various amounts of metallic zinc dust andare applied by brush or spray to properly prepared steel.

Hot-dip Galvanizing Complete immersion of steel into a kettle/vessel of molten zinc.


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Galvanic Zinc Applications

Zinc Metallizing Zinc-rich Paints


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Concrete: Rebar Corrosion

Staining

Spalling

Cracking

Complete Failure


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Concrete: Galvanized Rebar

Unprotected RebarGalvanized Rebar


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Features of HDG Coatings

Zinc-iron intermetallic layersHarder than the substrate steel

Zinc patinaBarrier protectionCathodic protectionMetallurgical bond to the substrate steelPaintableEdge and corner protectionZinc is a natural and healthy metal


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Benefits of HDG Coatings

Maintenance-free for 50 100 years in mostatmospheric environments

Long term performance in soils, water, andchemical environmentsNo touch-up requiredHigh & Low temperature performanceApplication independent of weather100% recyclable


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Dry Bridge Road Bridge

Date Galvanized1999

Sector

Bridge &Highway

EnvironmentRural

Location Alexander, NY


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Harrisburg Airport Transportation Facility

LocationHarrisburg, PA

EnvironmentUrban

SectorBuilding &Architecture

Date Galvanized

2004


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AES-PR Total Energy Power Plant

Date Galvanized2002SectorElectrical, Utility&CommunicationEnvironmentIndustrialLocation SanJuan, PuertoRico


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Leprino Foods

Date Galvanized 2002

Sector Food &Agriculture

Environment Rural

Location Waverly ,NY


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Aspinwall Water Treatment Plant

Date Galvanized 2001

Sector Water & Marine

Environment Industrial

Location Pittsburgh, PA

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Galvanized Coatings

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