Post-Galvanising Defects

There are a number of common types of defects arising from the hot-dip galvanising process. An explanation of the causes of defects and variations in appearance follows:


Coating misses on weld areas are caused by the presence of welding slag on the welds. All welding slag must be removed by the fabricator prior to despatch to the galvaniser.





Preparation chemicals entering unsealed overlaps or through poor quality welds boil out of the connection during galvanising and cause surface contamination and coating misses during galvanising. Also, anhydrous fluxing salts left in the connection will absorb atmospheric moisture and leach out onto the adjacent galvanised surface. Leaching of these salts will eventually reach equilibrium. Affected areas should be washed clean to remove slightly corrosive leachate.






Dross is formed in the galvanising process in the form of zinc-iron crystals (approx 95% zinc – 5% iron) with a higher melting point than the metal in the zinc bath. Dross trapped in the galvanised coating may give the coating a rough or gritty appearance. The presence of dross inclusions in the coatings is not detrimental to the coating's performance as the corrosion resistance of zinc dross is identical to that of the galvanising coating.






A bulky white or grey deposit, known as wet storage stain may form on the surface of closely stacked freshly galvanised articles which become damp under poorly ventilated conditions during storage or transit. As such, wet storage stain on new galvanised surfaces is readily prevented by attention to conditions of storage and transport. Heavier deposits can be removed by brushing with a 5% solution of sodium or potassium dichromate with 0.1% by volume of concentrate sulphuric acid.






Some hot-dip galvanised coatings exhibit a high level of 'spangling' caused by zinc crystal patterns on the surface. This phenomenon arises with galvanising alloys produced in particular smelting processes and these alloys are commonly used for hot-dip galvanising. There is no difference in coating performance.






Small localised flaws up to about 3mm wide in a galvanised coating are usually self-healing because of the cathodic protection provided by the surrounding coating. They have little effect on the life span of the coating. Bare spots may be caused by inadequate pretreatment, the presence of residual welding slags, rolling defects such as laps, field and laminations, and non-metallic impurities rolled onto the steel surface.






Grey coatings may appear as localised dull patches or lacework patterns on an otherwise normal galvanised coating or may extend over the entire surface. Dull grey coatings usually occur on steels with relatively high silicon content (>0.24%) which are highly reactive to molten zinc. Welds made with steel filler rods containing silicon may also produce localised grey dull patches. As such, it is rarely possible for the galvaniser to minimise or control the development of dull grey coatings which is dependent basically on the silicon content of the steel.






Zinc ash is formed in the galvanising process as the work is immersed in the zinc. The ash formed is skimmed off the surface of the molten zinc prior to withdrawing the work from the galvanising bath. Sometimes, ash is trapped inside inaccessible areas and sticks to the outside of the coating as the work exits the bath. Ash may leave a dull surface appearance or a light brown stain after removal. It does not affect the performance of the galvanising coating.






These defects are unavoidable in the hot-dip galvanising of general items and are acceptable as long as they do not interfere with the assembly of the function of the item or present a safety hazard in handling r service.






Rust staining on the surface of galvanised coatings is usually due to contact with or drainage from other corroded steel surfaces. Staining arises from corrosion of the iron content of zinc-iron alloy coating and is therefore outside the control of galvaniser. It has no effect on the corrosion resistance of the coating.






Heavily galvanised coatings (over 250 microns thick) may be brittle and delaminate from the surface under impact and requite more careful handling in transport and erection. Thin, cold rolled items with very smooth surface finish and manufactured from reactive steel may also give rise to coating delamination.