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. 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. 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. 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. Variances in steel chemistry within the same fabrication or piece of steel is common. 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. Reaction between zinc and surface stresses have formed dull grey areas on the above SHS sections. Extreme reactivity between steel chemistry and zinc will result in coatings which may not be smooth.
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. 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. 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.
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. 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. 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. Delaminated coatings are caused by continued alloy growth in highly reactive steel and is outside the control of the galvanizer.
Suitable Surface Conditions Large Items or Thick Steels Adhesive residue contaminates steel surface. Very thick coatings may form on items which are large and consist of heavy steel sections. Longer immersion and handling times required to process these sections may result in the metallurgical properties of the steel having high reactivity with the zinc whilst in the galvanizing bath. Further information relating to zinc reactivity is available in Effects of Steel Chemistry. Rusted Items Light mill scale and surface rust on items can be removed within our pre-treatment process. Heavily rusted steel will require abrasive blasting prior to delivery to remove the rust layers. If the steel surface is pitted after blasting, this effect will be evident after the hot dip galvanized coating is applied. Surface rust is easily removed in our process. Cutting Oil Some cutting lubricants can become baked onto the steel surface during fabrication. Oil based fluids are not visible during pre-treatment processing and may contaminate the surface preventing the coating from forming. Cutting oils should be cleaned from the surface prior to delivery. Pitted surfaces will be evident after galvanizing. Steel Coatings The bond between the steel and zinc is unable to be achieved if any form of substance remains on the steel surface after chemical treatment. Resistant substances which can prevent the coating from forming include (however are not limited to) pre-existing zinc coatings, paint, lacquer and adhesive residue from identification labels and stickers. Adhesive Residue Manufacturer s steel identification stickers will deteriorate during pre-treatment processing; however non visible adhesive residue may remain and prevent successful formation of the zinc coating. Stickers should be removed and the immediate area prepared by suitable means to remove the adhesive substance prior to despatch. Marking Pens Paint pigmentation from marking pens may be resistant to chemical cleaning. During pre-treatment processing paint layers are removed, however, non-visible pigments may remain. The zinc coating will form around the residue pigmentation and remnants of workshop markings may remain evident after galvanizing. Oil based paint markings should be removed by suitable means from steel surfaces prior to delivery for galvanizing. Drilling lubricant can contaminate the steel surface preventing successful coating.
Oil based paint markings should be ground from the steel surface prior to hot dip galvanizing. Masking In some applications, small mating or threaded areas may be required to be uncoated. This can be achieved by applying a small amount of a suitable adhesive or sealing product on the area creating a barrier to pre-treatment acids and zinc. The following products will have varying success in preventing zinc coatings from forming and some clean up of the surrounding surfaces will be required by the fabricator after galvanizing. Tapes high temperature tape / duct tape Sealants silicone adhesive sealant / petroleum gel / household caulk Paints Maskote / Stop Galv Application of a high temperature tape or sealants will prevent zinc from forming in small areas. Penetrant Dye for Welds Dyes utilised for weld checks can create issues if unable to be removed by pre-treatment chemicals. Dyes should be removed from steel surfaces with a suitable paint removing solution or by light sanding prior to despatch for galvanizing. Painted Sections Most pipe and tube sections manufactured in Australia are painted with water based coatings which generally can be removed within our process. Some local manufacturers of pipe and tube products and most offshore producers coat their product range with clear varnish or black bituminous paint. These coatings are resistant to chemical removal within our galvanizing plant and are required to be abrasive blasted prior to delivery. To avoid additional costs and extended processing times, hollow sections should be stipulated that they be suitable for hot dip galvanizing when ordering from your steel supplier. All steel sections with powder coated, brush or spray paint coatings must be abrasive blasted prior to despatch to Hunter Galvanizing. Dyes utilised to check for weld penetration can contaminate the steel surface and should be removed prior to delivery. Primer paint coatings can be removed from most domestically produced pipe and tube sections during our process.
Pre Existing Zinc Coatings Pre existing zinc coatings must be stripped from all items including in-line produced lengths of hollow section, purlins, wire and mesh prior to hot dip galvanizing. Additional charges are necessary for this procedure and lead times may be increased. Some pipe and tube products have an external light coating of zinc and a painted internal surface. These items will require internal abrasive blasting prior to delivery to our facility in addition to acid stripping. Thermal Cut Edges Flame, laser and plasma cutting will change the structure of the steel composition in the immediate area of the heat source. These areas may present thinner coating thicknesses and a lack of adhesion, reducing the ability of the zinc alloy layers to bond with the base metal. The high temperatures utilised to cut material depletes the alloying elements in surface of the steel. As discussed in Hot Dip Galvanized Coatings the formation of the coating structure relies on the alloying of iron and zinc in the galvanizing bath. Should insufficient alloying elements be present, the minimum coating thickness may not be able to be achieved; and cohesion of the galvanized coating will also be limited. To eliminate such issues, grind the heat affected surface and bevel the cut edge. Pre existing zinc coatings must be removed prior to hot dip galvanizing. Flame cut edges should be ground prior to delivery for galvanizing.