Suitable Steels for Hot Dip Galvanizing

Steel Type & Embrittlement

Hot dip galvanized coatings are able to be achieved on most ferrous materials and general steel grades without difficulty. However, as hot dip galvanizing is a form of heat treatment and items are soaked in acid, some susceptible grades of steel maybe prone to embrittlement which is outside of our control.

• Strain Age Embrittlement Strain age embrittlement is caused in certain low quality steels when areas stressed by cold working are exposed to elevated temperatures (including hole punching and tight radius bending in thicker steel sections). Steels generally have many impurities which gather in high stress areas and in certain steels cracking may occur prior to galvanizing. It is recommended where possible that items are worked after galvanizing; any flaking or cracking will be limited to the zinc coating which can be repaired using zinc rich paint.

• Hydrogen Embrittlement Generally occurring in steels with a tensile strength equal to or higher than 1000 MPa and harder than 340 DPN, hydrogen embrittlement rarely affects structural steels. This form of embrittlement is likely to be observed when an item is in service and under load. Hydrogen is absorbed during the acid pre-treatment process and then discharged quickly during galvanizing. Specialised steels such as Bisalloy and other susceptible steels should be abrasive blasted immediately prior to galvanizing to eliminate the requirement for soaking in pre-treatment chemicals.

• Liquid Embrittlement Embrittlement in this form may occur on high carbon and stainless steel where zinc atoms are absorbed by the susceptible metal. In critical applications, stainless steel items should not be hot dip galvanized. When galvanizing non-critical stainless steel items, additional pre-treatment may be required to enable the zinc coating to form.

• Other Issues Other issues related to steel type are generally limited to old iron work items or castings which are often porous. Castings may have sand embedded which cannot be removed by pre-treatment processing. Items should be abrasive blasted prior to delivery. Of additional note, soft solder and aluminium rivets must not be used in any fabrication as they will not withstand galvanizing temperatures. Brazed items should be discussed with Hunter Galvanizing staff to confirm suitability.

Effects of Steel Chemistry

Hunter Galvanizing provides industrial zinc coatings formed by metallurgical reaction between steel and molten zinc. The smoothness, thickness and colour of hot dip galvanized coatings are not factors which can be controlled as the steel thickness combined with the steel chemistry will determine the aesthetics of the coating. Thicker steel will attract thicker zinc coatings which by nature will be darker in colour. Items also may display a bright sheen through to a dull or matt grey finish. It is impossible for galvanizers to conform to a specific shade of silver or grey or to control the lustre of a coating.

The metallurgical structure of the steel may encourage a variety of effects to appear in the coating. Localized areas can display a lacework or snakeskin pattern, dull grey patches or large bright spangles. These effects may appear in one area or across the entire surface of a piece of steel. Extreme levels of silicon and phosphorous have dramatic effects relating to colour, lustre and smoothness of a hot dip galvanized coating.

Stainless steel components may require additional processing to achieve successful coatings.

Sand trapped in castings will cause coating issues.

Variances in steel chemistry in different sections within one fabrication are clearly visible after galvanizing.

Some manufacturing processes of steel processing can also alter the formation of the free zinc layer creating a number of effects on hollow sections. Fish bone effect can occur on large diameter pipes where the difference in the surface chemistry will cause varying reaction rates between the steel and zinc.

Patches of dull grey can present in a striped or spiralled sequence along lengths of pipe, RHS and SHS, where zinc has reacted to stresses in the surface chemistry produced during manufacture of these sections.

As discussed in Welding, zinc is attracted to welding media high in silicon. Weld material used in the production of pipe and tube sections is highly reactive with zinc and welding seams will be highlighted by heavier coatings.

All of the above phenomenon have no effect on the corrosion protection properties of the hot dip galvanized coating or on the integrity of the steel section. Items displaying any of these effects are not cause for rejection.

Hunter Galvanizing accepts steel items for hot dip galvanizing based on their design and fabrication. Galvanizers can not be aware of the potential for high reactivity of the steel with molten zinc, unless specific material specifications have been supplied and discussed prior to hot dip galvanizing.

Extreme reactivity between steel chemistry and zinc will result in coatings which may not be smooth.

Reaction between zinc and surface stresses have formed dull grey areas on the above SHS sections.

Variances in steel chemistry within the same fabrication or piece of steel is common.

Thicker coating along the seam is a result of high reactivity between the zinc and silicon in the weld material. Note the differing colours and coating thickness of other items.

Rough coating appearance reflects the metallurgical substrate of the pipe section. This differs from the attached piece of RHS.

Fish bone effect on large diameter pipe is a result of manufacturing stresses and not an acceptable cause for rejection.

For general fabricators it is not possible to determine steel chemistry accurately prior to processing. Steel analysis certificates can detail only batch testing levels. As silicon and phosphorus are not always distributed evenly throughout the steel making process, samples of the total heat may not be representative of each individual piece of steel. Where aesthetics is critical, trial samples of product can be galvanized; however, again they may not provide a true indication of chemistry across a product batch. Items displaying chemistry related issues are acceptable under the galvanizing standard and are not means for rejection.

Two chemical components in steel have the ability to affect coating thickness and appearance.

• Silicon Very high or very low levels of silicon will generate high reactivity with the zinc and in turn stimulate rapid growth of the zinc-iron layers. Silicon in the ranges between 0.04%–0.14% (low extreme) and silicon above 0.22% (high extreme) will have varying degree of effects. Zinc-iron layers will grow less in steels containing between 0.15%–0.22% silicon and in general will display lighter coloured coatings (subject to steel thickness).

• Phosphorous When phosphorous is present in steel in levels above 0.05%, reactivity is also increased and will result in thicker, matt coatings. Recommended level of phosphorous should be below 0.05%.

If steel has a combination of extreme levels of silicon (either very high or very low) and high levels of phosphorous, the coating produced will be excessively thick and the outer layers may be brittle and easily chipped during handling and transportation. The following formula can be used as a guide when determining the steel’s chemistry as to its suitability for hot dip galvanizing:

Suitable Steel = %Silicon + (2.5 x %Phosphorous) < 0.09%

Example of high reactivity steels: analysis displayed 0.19% silicon (acceptable range), 0.021% phosphorus (extreme). In combination 0.19+(2.5*0.021)=0.24% well above the recommended level for hot dip galvanizing.

• Delamination Extremely reactive steel (notably high levels of phosphorous) can form a void between the top two layers of the galvanized coating causing the outer layer to peel or crack. This is referred to as delamination. Sufficient zinc generally remains in the underlying (hard alloy) layers which continue to be metallurgically bonded to the base steel offering the protective qualities of a sound hot dip galvanized coating. This effect is often outside of our control and not a suitable means of rejection. Delamination occurring after galvanized items have been abrasive blasted in readiness for painting is not the responsibility of the galvanizer. Procedures for the correct blasting of hot dip galvanized coatings are detailed in Duplex Coatings.

As silicon and phosphorus are not always distributed evenly throughout the steel, some areas within the same piece of steel can have higher/lower silicon and/or phosphorus levels; creating coatings that grow differently than the surrounding areas.

• Differing Cooling Rates Some cut edges of sections within a fabricated item may cool much quicker than others and result in shiny effects surrounding dull grey areas. This different rate of cooling allows free zinc to form on top of the existing alloy layers causing zinc patterns, whilst other areas not affected by the free zinc produce a dull grey colouring. This effect is not deemed to be an issue, as the shiny areas will also change to the dull grey colour as the coating weathers and the coating patina forms.

Items with coating effects described are no less suitable in service than those with a bright finish or a spangle effect typically displayed on very thin steels. A dull grey coating is likely to indicate a thicker coating of zinc; and as service life is proportional to coating thickness, these coatings will perform longer, extending the life of the underlying steel. Further information relating to dull grey colouring and weathering is detailed in Hot Dip Galvanized Coatings. Control of colour, lustre and texture remains outside of the control of Hunter Galvanizing.

Delaminated coatings are caused by continued alloy growth in highly reactive steel and is outside the control of the galvanizer.

Differing cooling rates may produce dull grey areas, this is in stark contrast to the remaining bright shiny coating around edges of small plates and holes.

Example of high reactivity steels: analysis displayed 0.19% silicon (acceptable range), 0.021% phosphorus (extreme). In combination 0.19+(2.5*0.021)=0.24% well above the recommended level for hot dip galvanizing.