Grey Coatings on Hot Dip Galvanized Steel

8/13/2019 Grey Coatings on Hot Dip Galvanized Steel

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GREY COATINGS ON HOT DIP GALVANIZED STEELINTRODUCTION

A common phenomenon with hot dip galvanized structural steel is the grey appearance of part or all of the coating

after galvanizing, where the expectation of the customer is for the galvanized steel to be shiny. Grey coatings are

often a cause of contention between galvanizers and their customers as a result. This File has been produced to

explain the phenomenon of grey coatings, their cause and effect on the performance of hot dip galvanized steel.

WHY ARE SOME GALVANIZED COATINGS GREYHot dip galvanized coatings are the result of a metallurgical reaction between the zinc and the steel. This reaction

forms a series of zinc-iron alloys in the form of needle like crystals that grow from the steels surface. With

conventional galvanized coatings, the alloy layer makes up about 80% of the coating and the upper 20% of the

coating is zinc. This surface layer gives produced the shiny appearance. Where this surface coating of free zinc is

not present, the zinc-iron crystals are visible and it is the appearance of these that gives the coating matte silver or

grey appearance. When the steel emerges from the galvanizing bath, the coating is always shiny. The appearance of

the coating changes to grey as the residual heat from the galvanizing process allows the reaction between the steel

and the zinc to continue until all the fee zinc on the surface is consumed, leaving the coating with 100% alloy layers.

WHAT CAUSES SOME STEELS TO PRODUCE GREY COATINGS?

The reaction between zinc and steel in the galvanizing process is a function of a number of factors. The most

significant of these with respect to grey coatings are:

1. The chemical composition of the steel

2. The steel section thickness3. The galvanizing bath temperature

4. The cooling rate of the steel after galvanizing

Of these, the chemical composition of the steel is the most important. Two alloying elements in particular, silicon

and phosphorus, will increase the reaction rate of the zinc with the steel. If the silicon content exceeds 0.20% or the

combination of the percentage of silicon plus 2x the phosphorus level exceeds 0.25%, then the likelihood of grey

coatings forming is increased.

Most Australian-made steels are galvanizer friendly in this respect with silicon and phosphorus levels controlled

within acceptable limits. As about 35% of steel used in Australia is now imported, the variation in steel chemistry

makes control of grey coatings a more difficult issue. The steel section thickness is a factor with relatively thick

sections (over 20 mm) because the greater mass of steel retains heat longer.

The zinc-iron reaction will continue even when the zinc has solidified (at 420 degrees C) as a solid-state reaction

until the temperature falls below about 390 degrees C. For this reason, heavy plate fabrications will produce thicker,

grey coatings regardless of the steel chemistry. The galvanizing bath temperature will only have an effect where it is

possible to operate the galvanizing bath at above the normal 455 degrees C level. This can only be done in specialceramic lined galvanizing baths, as high operating temperatures will damage conventional steel galvanizing baths.

The cooling rate of the steel after galvanizing can affect the coating appearance. Galvanized items that are air-cooled

are more likely to develop grey or partly grey coating than items that are quenched immediately after withdrawal

from the galvanizing bath. This occurs because the quenching halts the solid-state zinc iron reaction before all the

free-zinc on the coatings surface is consumed.
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WHAT EFFECT DO GREY COATINGS HAVE ON COATING PERFORMANCEWithout exception, grey coatings are thicker than shiny galvanized coatings on equivalent steel sections. Australian

and international galvanizing standards require that on structural sections over 6 mm in thickness, the minimum

galvanized coating thickness is specified at 85 microns.

Grey galvanized coatings are more typically almost double this thickness, and on heavier sections will frequently

exceed 200 microns in thickness. As galvanized coating life is almost directly proportional to coating thickness, a

significant increase in service life can be expected from these heavier coatings.

The main problems associated with grey coatings are their aesthetic acceptability and the fact that the zinc-iron alloy

layers are hard and inflexible, and may be prone to mechanical damage if subjected to impacts during transport and

erection, where conventional shiny coatings have excellent resistance to quite severe impacts. One fringe benefit of

grey coatings on galvanized steel is that they provide a good substrate for painting, because of the matte surface.

BHP produces a galvanized sheet product called Zincanneal where the mill produced shiny galvanized coating is

converted to a 100% alloy layer coating by post heat treatment to improve the paintability of the product for

whitegoods manufacture.
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The typical characteristics of appearance:There are several characteristics to the appearance to include; dull gray color, rust stains, blisters, texture

(smooth, rough), lumps, pimples, bare spots, bulky white deposit, and dark spots. The following information

will tell more about these different appearance issues:

1. Dull gray coating: This type of coating is normally acceptable and is caused by growth of the zinc iron alloy

layers through to the surface of the galvanized coating .

2. Rust stains: The rust stains are acceptable when present as a surface stain, this is usually caused by contact with or

drainage with other corroded steel surfaces.

3. Blisters: Small intact blisters in the coating are acceptable and are usually due to absorption of hydrogen by the

steel during the pickling process being expelled as a result of the heat put off by the galvanizing process.

4 General roughness and thick coatings: General roughness of the outer material is normally acceptable as long asnot otherwise agreed on with the consumer. The cause of rough galvanized coatings is usually a result from uneven

growth of zinc iron alloys because of the composition or surface condition of the steel.

5. Lumpiness and runs: Lumps are normally acceptable unless otherwise specified and are caused by uneven

drainage. Lumps and runs have no impact on the coating life.

6. Pimples: Pimples are grounds for rejection depending on size and extent. Pimples are caused by inclusions of

dross in the coating. Dross, which is a zinc iron alloy particles has a similar corrosion rate to the galvanized coating

and its presence as finely dispersed pimples is not objectionable. Gross dross inclusions may be

grounds for rejection as they tend to embrittle the coating.

7. Bare spots: Bare spots are generally acceptable if small in area and suitably repaired, depending on the nature of

the product. Some reasons for bare spots are faulty processing, rolling defects, laminations and nonmetallic

impurities rolled into the surface.

8. Wet storage stain or bulky white deposit: A bulky white or gray deposit, known as wet storage stain may form on

the surface of closely stacked freshly galvanized articles which become damp under poorly ventilated conditions

during storage or were left damp as a result of the product not being dry before storing. reason for wet material

being stored is an insufficient wax concentration, either to much was or to little.

AppearanceA galvanized coating is normally smooth, continuous and free from gross surface imperfections and inclusions.

While the heavy zinc coating on general galvanized articles should be smooth and continuous it cannot be compared

for surface smoothness to continuously galvanized sheet steel or wire since these are produced by processes which

permit close control of coating thickness and appearance.

Differences in the lustre and colour of galvanized coatings do not significantly affect corrosion resistance and the

presence or absence of spangle has no effect on coating performance. As discussed under Dull grey coating below,
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uniform or patchy matt grey galvanized coatings give equal or better life than normal bright or spangled coatings. It

is recommended that inspection of galvanized work should be carried out by a designated party at the galvanizers

works in accordance with the following guidelines, and tested when

necessary as detailed under Non-destructive testing for coating thickness.

Variations in appearance and their relationship to coating quality Variations in appearance of galvanized coatings

listed below and their influence on coating quality are discussed on following pages.

Dull grey coatingGeneral comment: Acceptable. A dull grey appearance is caused by growth of the zinc-iron alloy layers through to

the surface of the galvanized coating. Grey coatings may appear as localized dull patches or

lacework patterns on an otherwise normal galvanized coating or may extend over the entire surface.

Dull grey coatings usually occur on steels with relatively high silicon content which are highly reactive to molten

zinc as discussed under Composition of steel.

Welds made with steel filler rods containing silicon may also produce localised

grey areas in an otherwise normal galvanized coating.

Dull grey coatings are often thicker than the normal bright or spangled coatings and therefore give longer life. It is

rarely possible for the galvanizer to minimise or control the development of dull grey coatings which is dependent

basically on steel composition.

BlistersGeneral comment: Small intact blisters acceptable.

Extremely rare. Small blisters in galvanized coatings are due to hydrogen absorbed by the steel during pickling

being expelled as a result of the heat of the galvanizing process. Their occurrence is due to the nature of the steel and

is outside the control of the galvanizer. Blisters do not reduce the corrosion resistance of the coating.

Rust staintGeneral comment: Acceptable when present as a surface stain.

Rust staining on the surface of galvanized coatings is usually due to contact

with or drainage from other corroded steel surfaces. Steel filings or saw-chips produced during erection and

fabrication operations should be removed from galvanized surfaces to prevent possible localised rust staining. Rust

staining may also be caused by the weeping of pickling acid from seams and joints causing damage to the

galvanized coating, and in such cases requires a modification in design as discussed under Overlapping surfaces.A thin brown surface staining sometimes occurs in service when the galvanized coating comprises entirely zinc-iron

alloys as discussed under Dull grey coating. Staining arises from corrosion of the iron content of the zinciron alloy

coating and is therefore outside the control of the galvanizer. It has no effect on the corrosion resistance of th

coating. Long term exposure testing has shown that the corrosion resistance of zinc-iron alloys is similar to that of

normal galvanized coatings.

General roughness and thick coatings on welds.General comment: Acceptable, unless otherwise agreed.
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Rough galvanized coatings usually result from uneven growth of zinc-iron

alloys because of the composition or surface condition of the steel. Where welding electrodes containing silicon

have been used, the galvanized coating on the weld area may be thicker than normal and may also be brittle. Rough

coatings of this type are usually thicker than normal and therefore provide longer protective life. General roughness

may also be caused by over-pickling, prolonged immersion in the galvanizing bath, or excessive bath temperature,

factors which are frequently dictated by the nature of the work and may be beyond the control of the

galvanizer. In architectural applications where a rough finish is aesthetically or functionally unacceptable, the steel

composition and surface preparation should be closely specified and the galvanizer consulted at an early stage. It is

rarely possible for the galvanizer to effect any later improvement.

Lumpiness and runsGeneral comment: Acceptable unless otherwise specified.

Australian/New Zealand Standard 4680 Hot dip galvanized (zinc) coatings on fabricated ferrous articles demandsthat a galvanized coating shall be smooth but points out that smoothness is a relative term and that coatings on

fabricated articles should not be judged by the same standards as those applied to continuously galvanized products

such as sheet steel and wire, since these are produced by processes which permit a high degree of control over

coating thickness and appearance. Lumps and runs arising from uneven

drainage are not detrimental to coating life. When zinc drainage spikes are

present on galvanized articles

and their size and position is such that there is a danger they may be knocked off in service removing the coating

down to the alloy layers, they should be filed off by the galvanizer and, where necessary, the coating should be

repaired as described. For architectural applications the galvanizer can sometimes

achieve a smoother finish than the normal commercial coating, depending on the shape and nature of the product.

The steel should be carefully specified and the galvanizer consulted at the design stage and advised when the order

is placed. Extra cost may be involved.

Pimples

General comment: May be grounds for rejection depending on size and extent.

Pimples are caused by inclusions of dross in the coating. Dross, which comprises zinc-iron alloy particles, has asimilar corrosion rate to the galvanized coating and its presence as finely dispersed pimples is not objectionable.

Gross dross inclusions may be grounds for rejection as they tend to embrittle the coating.
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Bare spotsGeneral comment: Acceptable if small in area and suitably repaired, depending on the nature of the product.

Small localised flaws up to about 3 mm wide in a galvanized coating are usually self-healing because of the cathodic

protection provided by the surrounding coating as discussed under Cathodic Protection .

They have little effect on the life of the coating. Australian/New Zealand

Standard 4680 Appendix E

Renovation of damaged or uncoated areas specifies that the sum total of the damaged or uncoated areas shall

not exceed 0.5% of the total surface area or 250cm2, whichever is the lesser, and no individual damaged or uncoated

areas greater than 40cm2. However, as an exception, uncoated areas greater than 40cm2, which have been caused by

unavoidable air locks during the galvanizing operation, shall be repaired. Repa ir methods which accord with

AS/NZS 4680 Appendix E are detailed.

Bare spots may be caused by under-preparation by the galvanizer and by a number of factors outside his control, andfor which he cannot be responsible, including the presence of residual welding slags, rolling defects such as laps,

folds and laminations in the steel, and non-metallic impurities rolled into the steel surface.

Wet storage stain or bulky white depositGeneral comment: Not the galvanizers responsibility unless present before first shipment. Acceptable if non-

adherent deposit is removed and the coating meets coating mass requirements. A bulky white or grey deposit, known

as wet storage stain may form on the surface of closely stacked freshly galvanized

articles which become damp under poorly ventilated conditions during storage or transit. In extreme cases, the

protective value of the zinc coating may be seriously impaired but the attack is often very light despite the bulky

appearance of the deposit. Initiation and development of wet storage staining on new

galvanized surfaces is readily prevented as detailed by attention to conditions of storage and transport and by

application of a chromate passivation treatment. Where the surface staining is light and smooth without

growth of the zinc oxide layer as judged by lightly rubbing fingertips across the surface, the staining will gradually

disappear in service and blend in with the surrounding zinc surface as a result of normal weathering.

When the affected area will not be fully exposed in service, particularly on the underside of steelwork and in

condensation areas, or when it will be subject to a humid environment, wet storage staining must be removed as

detailed below, even if it is superficial. Removal is necessary to allow formation of the basic zinc carbonate film

which normally contributes to the corrosion resistance of galvanized

coatings. Medium to heavy buildup of white corrosion product must be removed to allow formation of a basic zinc

carbonate film in service. Light deposits can be removed by brushing with a stiff bristle brush. Heavier deposits can

be removed by brushing with a 5 percent solution of sodium or potassium dichromate with the addition of 0.1

percent by volume of concentrated sulphuric acid. This is applied with a stiff brush and left for about 30 seconds

before thorough rinsing and drying. A check should be made to ensure that the coating

thickness in affected areas is not less than the minimum specified in relevant standards for the various classes of

galvanized coatings as detailed. In extreme cases, where heavy white deposit or red rust has been allowed to form as
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a result of prolonged storage under poor conditions, corrosion products must be removed by thorough wire brushing

and the damaged area repaired as detailed.

Dark spots/Flux stainingGeneral comments: Acceptable if flux residues have been removed.

Smuts of dirt may be picked up on the surface of the galvanized coating from floors and trucks or from contact with

other articles. These smuts are readily washed off to reveal a sound coating and are not harmful.

Where a flux blanket is used in the galvanizing process, stale flux may adhere to the surface of the work during

immersion and appear as a black inclusion in the coating. Such inclusions tend to pick up moisture forming a

corrosive solution and coatings containing them should be rejected. Black stains or deposits of flux picked up on the

surface. the object is withdrawn from galvanizing the bath do not warrant rejection provided the underlying coating

is sound and the deposit is removed.Service-Life Chart for Hot-Dip Galvanized Coatings
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Continous and Batch Galvanize

There are many types of coatings that are specified as hot dip galvanized. The process involves immersing steel in

molten zinc. The zinc reacts with the steel to form the galvanized coatings. The time the steel is immersed in the

zinc along with post-galvanizing treatment controls the coating thickness, appearance and other characteristics

Hot dip galvanized coatings are applied to steel to improve the anti-corrosion performance of the steel to ensure that

it lasts as long as possible with a minimum of maintenance. Standards currently being developed for the housingindustry have set a benchmark of at least 50 years as the acceptable life of structural building products. Only hot dip

galvanized steel products with the heaviest galvanized coatings are capable of meeting this requirement.

The Australian Standard AS 46801999 , Hot Dipped Galvanized Coatings on Ferrous Articles, includes

galvanized coating standards on sheet, wire, tube and general articles. A great deal of confusion exists through the

inclusion of galvanized coatings with significantly different coating characteristics within the same Australian

Standard.

COATING THICKNESS COUNTS
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All sheet, wire and many tube products are CONTINUOUSLY galvanized. This means that the coating is

applied at high speed and the coating thickness is controlled by the process. Immersion time in the zinc is

measured in seconds. Alternatively, in the BATCH hot dip galvanizing process steel items are immersed

for periods ranging from 3-10 minutes, depending on the mass of the items being galvanized.

These completely different methods of applying galvanized coatings produce different types of coatings.

There are 4 main differences that impact on anti-corrosion performance of BATCH galvanized steelcompared to CONTINUOUSLY galvanized steel. These are:

1. Coating thicknessBATCH galvanized items of the same section thickness are typically at least 3

TIMES thicker than similar CONTINUOUSLY galvanized coatings on sheet and tube.

2. Coating hardnessBATCH galvanized items have much thicker zinc/iron alloy layers in the coatings

which gives BATCH galvanized items 5 TIMES the abrasion resistance of CONTINUOUSLY galvanized

coatings.

3. Coating integrityBATCH galvanized coatings apply a uniform heavy coating to all internal and

external surfaces, edges and cavities. CONTINUOUSLY galvanized coating will always have exposed

bare steel at cut edges. CONTINUOUSLY galvanized hollow sections are fully galvanized on the external

surfaces only.

4. Coating massThe cathodic protection of exposed steel by zinc depends of the mass of the zinc in

relation to the area of exposed steel. Because of the drainage characteristics of BATCH galvanized

coatings, the coating mass on BATCH galvanized products is significantly higher (typically 3-5 times) in

proportion to thickness than CONTINUOUSLY galvanized coatings. Hot rolled medium structural sections

commonly achieve coating mass levels exceeding 1000 g/m2.

MORE COATING THICKNESS = LONGER COATING LIFE

150 years of field testing has determined that all things being equal, galvanized coating life is equivalent

to galvanized coating thickness. When comparing BATCH galvanized coatings to CONTINUOUSLY

galvanized coating, all things are not equal.

THE CUT EDGE FACTOR

All CONTINUOUSLY galvanized sections have exposed steel at cut edges and rely on the adjacent zinc

in the coating to provide cathodic protection to the bare steel. This requirement accelerates the rate of

corrosion of the galvanized coating at cut edges. The thicker the CONTINUOUSLY galvanized section,

the faster the rate of coating corrosion at cut edges because of the greater area of bare steel exposed.

Even if it was possible to apply a CONTINUOUSLY galvanized coating to a steel item to the same

thickness as a BATCH galvanized item, the cut edge factor gives the BATCH galvanized coating a life

typically 1.5 TIMES greater.

COMPARISON OF GALVANIZED COATINGS

CONTINUOUSLY galvanized coatings comply very closely to their specified coating mass. BATCH

galvanized coatings on hot rolled steel sections almost always exceed their minimum specified coating

mass.


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VENTING AND DRAININGHOW TO GET THE HOLE SIZE RIGHTOne of the most common issues in designing fabrications for hot dip galvanizing is ensuring that

fabrications are vented and drained correctly. All steel to be galvanized needs to be immersed in molten

zinc and the zinc needs to be able to flow freely into and out of all hollow sections and corners. The flow

of molten zinc into, off, and out of the fabrication is one of the most important factors in determining the

final quality of the coating. Inadequate venting and draining can cause the following galvanized coating

defects:

misses in the coating caused by air locks preventing molten zinc contactin g the steel surface.

puddling of zinc in corners, wasting zinc and interfering with subsequent assembly

ash trapped on zinc surface causing surface defects

irregularities in surface appearance caused by erratic immersion and withdrawal because of item

floating or trapping zinc internally

thick zinc runs on surface caused by zinc freezing during draining

steel is only about 15% heavier than zinc. A relatively small amount of air trapped inside a hollow

section will prevent the section from sinking in the molten zinc
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any water trapped inside a hollow section will expand 1750 times its original volume as steam and

generate pressures as high as 50 MPa (7250 psi).

BASIC VENTING RULES

no vent hole should be smaller than 8 mm

the preferred minimum size is 12 mm

about 200 grams of zinc ash will be produced for each square metre of steel surface galvanized.

This ash is a solid powder and will not pass through small openings. Venting large internal areas required

larger vent holes to allow ash to escape

hollow vessels require 1250 mm2 of vent hole for each cubic metre of enclosed volume. This

means that a 40 mm2 diameter hole is required for each cubic metre of volume

hollow sections such as tube, RHS and SHS require minimum vent hole area equival ent to 25% of the

section diagonal cross section

vent holes should be at the edges of hollow sections

BASIC DRAINING RULES no drain hole should be less than 10 mm

preferred minimum drain hole size is 25 mm

large hollow sections ( tan ks, pressure vessels)

require a 100 mm diameter drain hole for each

cubic metre of enclosed volume

drain holes should be at the edges of hollow sections.
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hollow sections such as tube, RHS and SHS require minimum drain hole area equivalent to 25% of the

section diagonal cross section. The preferred design option is to leave the ends of tubes, RHS and SHS

open.

Documents

Grey Coatings on Hot Dip Galvanized Steel