Hot Dip Galvanizing for Steel Corrosion
Projection (fabricated steel items)
By Mike Ainsley
International Zinc Association (IZA)
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Steel is and will continue to be one of the most important materials for construction in the modern world
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Steel: a modern construction material
of un-paralleled functionality
Stable and well understood engineering properties Easily fabricated into a diverse range of shapes Plentiful supply Economical to use 100% recyclable
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BUT …. CORROSION CAN DESTROY STEEL’S USEFUL AND
UNIQUE ADVANTAGES
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Visibly unattractive rusting
Consequences of corrosion
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Visibly unattractive rusting
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Consequences of corrosion
Loss of structural integrity
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Example of corrosion failure
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Example of corrosion failure
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Costly repairs or replacement
Interruptions to normal service
Safety risk
Cost of Corrosion
Financial compensation
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Types of coatings Metallic coatings (zinc coatings are the most widely used) Non-metallic coatings (such as most paints)
The most commonly used method for corrosion control is to apply a protective
coating
“Galvanizing” refers to the application of a zinc coating to steel for corrosion protection
Hot Dip Galvanizing
Zinc electroplating (electro-galvanizing)
Zinc-rich painting (“cold galvanizing”)
Zinc thermal spraying (zinc metallizing)
Galvanizing methods:
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It is the process of applying a metallurgically bonded coating to steel by immersion into molten zinc. The steel items have already been fabricated into the final shape Sometimes this is called “after- fabrication” galvanizing
Molten zinc
Hot Dip Galvanizing (HDG)
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Zinc electroplating
Steel Zinc
(-) (+)
Zn ++ Zn ++
Zn ++ Zn ++
Zn ++
Zinc Coating
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‘Zinc-rich’ painting
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Paint contains a high content of zinc powder which forms a semi-continuous zinc layer as the paint dries
Electron microscope cross section of the paint coating showing the
dispersion of zinc particles
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Gas torch
nozzle High velocity molten zinc
spray
Zinc thermal spraying
Usage %
Hot dip galvanizing ( > 90% )
Other zinc coatings
< 10%
100%
0% 17
Global zinc coatings usage for fabricated structural steel items
Some introductory comments
HDG process
HDG coating attributes
HDG applications 18
Hot Dip Galvanizing (HDG) of fabricated structural steel
Some introductory facts about HDG
First used over 150 years ago (in Europe) Globally is now used to protect approximately 25 million tonnes of structural steel per year Used for a wide variety of engineering and architectural applications
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HDG Process Is the process of applying a zinc coating to steel by immersing (dipping) the steel into
molten zinc at 450oC
HDG PROCESS STEPS
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Example of galvanizing factory
The following picture sequence shows actual steel items being hot dip galvanized.
From commencing the entry into the molten zinc to the time of removal, the total
elapsed time is approx 6 minutes
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Steel articles to be galvanized
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Steel articles to be galvanized about to enter the galvanizing bath enclosure
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The galvanizing bath within an enclosure
Steel articles above the galvanizing bath
27 Steel articles above the galvanizing bath
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Steel articles fully immersed in the galvanizing bath
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Steel articles being withdrawn from the galvanizing bath
30 Fully withdrawn
31 Water cooling of the galvanized articles
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Steel
Molten Zinc
Zn migration into the steel surface
Steel Zinc – Iron Alloy Layer
Molten Zinc
Zinc “diffuses” into the steel surface and combines with the iron component. This results in the formation
of a Zinc – Iron alloy layer
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How does the coating form ?
As the steel emerges from the molten metal, a layer of pure zinc metal solidifies on top of the zinc - iron alloy layer
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STEEL
Zinc- iron alloy
Zinc metal HDG COATING
How does the coating form ?
Cross-section through an actual galvanized coating at high magnification
Steel base
Zn-Fe alloy layer
Zn layer
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Quality Control inspection
HDG coating quality conformance
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Examples of applicable HDG Standards are:
BS (EN) ISO 1461
ASTM A123
1. Minimum permitted coating thickness. 2. Maximum permitted size of any area on the steel surface where the coating is not visibly present. Suitable “repair” methods for small uncoated areas are also defined
HDG Standards define:
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Coating thickness specified by ISO 1461 for HDG structural steel
Steel thickness for structural shapes (mm)
Coating thickness (microns)
< 1.5 45
> or = 1.5 & upto < 3 55
> or = 3 & upto < 6 70
> or = 6 85
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Measuring coating thickness using portable meters is
simple, quick, and accurate 40
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HDG Standards define maximum permitted size of area where the coating is not visibly present
Sufficiently small uncoated areas can be on-site repaired instead of re-galvanizing the item
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Important comment on coating thicknesses as specified in
Galvanizing Standards ASTM123 or BS EN ISO 1461 )
Specifications for minimum average coating thickness defined in
Galvanizing Standards are not based on achieving a required service life
Instead they are based on the realistic capabilities of the galvanizing process.
For example, the ASTM 123 specified coating thicknesses were
established following tests on many thousands of steel items of different thicknesses that were galvanized using the traditional galvanizing process. The results of these tests formed the basis for defining the average minimum coating thickness for different steel
thicknesses
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Requests for greater coating thicknesses than specified in Galvanizing Standards ASTM123 or BS EN ISO 1461 )
As stated previously, the specified coating thickness as defined in Galvanizing Standards is the “natural” thickness due to the technical nature of the galvanizing process.
Greater thicknesses (such as 120-150 microns) which may be requested to give extra corrosion protection may therefore be impossible to achieve in the normal conventional galvanizing process.
It may be possible to achieve extra thickness coatings by including a sandblast of the steel surface before galvanizing. This is usually at an added cost and there is a risk that extra thick coatings will not be as adherent to the steel as normal thickness coatings
HDG coating attributes The HDG coating provides a UNIQUE COMBINATION of advantages which is not available from other coatings
Resistant to damage
Excellent corrosion protection
Proven long service life
Reliable and predictable service performance
Good corrosion protection economics
Environmental compatibility
Versatility (able to galvanize most steel shapes and sizes)
Assured coating integrity
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Resistant to damage due to strong adhesion, high hardness & toughness.
Zinc Harder than steel Zn - Fe Alloy
STEEL
HDG coating attributes
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Bond strength
10x more than paint
Excellent corrosion protection due to both ‘barrier protection’ + ‘cathodic protection’
ZINC
STEEL
Zinc has a naturally low corrosion rate (1/20th to
1/80th that of steel)
Barrier protection
STEEL
Cathodic protection
ZINC ZINC
protection Zn ++ Zn ++
e _ e
_
HDG coating attributes
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Comparison with conventional paints No cathodic protection means the corrosion will
spread beneath the paint coating.
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Side by side performance comparison of HDG and a conventional paint coating
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Proven long service life
Typical service lives are: Rural location: up to 100 years Urban & mild coastal location: 30 - 40 years Severe coastal & industrial location: 20 - 30 years
HDG coating attributes
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Reliable protection: The fact that HDG coatings have been used for over 150 years testifies to its reputation for reliability
HDG coating attributes
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30 20 10 0
Coa
ting
thic
knes
s (m
icro
ns)
Service Life (years) 40
Initial coating thickness
Measured coating
thickness after 10 years
Projected service life
Predictable service life Rate of coating loss is approximately linear. Service life expectancy can therefore be extrapolated
HDG coating attributes
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Assured coating integrity
HDG coating attributes
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It is often stated that the HDG process is an “honest process” since there can be no
short-cuts to achieving a successfully applied coating
Economic corrosion protection
HDG’s long service life means little or zero maintenance is required. Therefore HDG often provides lower cost corrosion protection than a paint coating on a “life-cycle” cost basis
Coa
ting
cost
HDG 1st paint coat
1st maintenance paint
3rd maintenance paint 2nd maintenance paint
Cost Savings
HDG coating attributes
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Environmentally compatible 1. Zinc coatings are not toxic
2. Galvanizing conforms to the needs for “green”
building and construction practices. This has been demonstrated in “Life Cycle Assessment” studies which compare galvanizing and paint coatings
HDG coating attributes
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LCA case study performed on a multi-storey steel car-parking structure
Comparison of Hot Dip Galvanizing vs
3-coat epoxy paint system Study undertaken by the Institute of
Environmental Protection Technology (University of Berlin, 2006)
Assumes 60 year service life, maintenance
painting required at 20 and 40 years
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Results of the study
100%
HD
G
Pain
t Resource and Energy Consumption 50%
0%
Pollution effects - greenhouse gas - reduction of ozone layer - atmospheric acidification
HD
G
Pain
t
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Versatility HDG coating attributes
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Diverse range of steel shapes
and sizes can be galvanized
Versatility
HDG coating attributes
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Diverse range of steel shapes and sizes can be
galvanized
Summarizing the benefits of Hot-Dip Galvanizing
Provides reliable, long-term corrosion protection Provides economical corrosion protection Is environmentally compatible Can be applied to a diverse range of steel shapes and sizes
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