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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