Dark, grey or light grey – it`s all galvanised. Protection is equal – and often better – with dark grey coatings than with light grey.
When rimmed or aluminium killed steels are hot dip galvanised, compact zinc-iron alloy layers are formed and covered by zinc, which has a light bluish grey metallic lustre. In some cases the zinc can form randomly orientated crystals to give a `spangle`finish but this is not a sign of either good or poor hot dip galvanising, nor does spangle affect the corrosion resistance of the zinc coating.
Silicon, which is sometimes added to steel as a deoxidant during the production, speeds up the reaction between the steel and molten zinc. When the steel part is removed from the galvanising bath but still remains hot, the reaction may continue and convert all or part of the surface zinc layers to zinc-iron alloys.
Zinc-iron alloys are dark grey compared with the light grey of zinc and are more abrasion resistant.
Usually zinc-alloy coatings form more thickly and hence give longer life than the coatings on rimmed or aluminium killed steels. The zinc-iron alloys in any case have at least as good corrosion resistance as zinc, thickness for thickness and can have better intrinsic corrosion resistance in acid industrial environments. These thick coatings can be more susceptible to mechanical damage particularly if handed roughly, and appropriate care must be taken.
The dark grey coating surface may develop iron oxide staining on atmospheric exposure, even in mild conditions when moist. This is only a surface effect and does not develop into nodular or flaking rust; the galvanised coating remains intact to protect the steel.
Learn more about Discolouration of Galvanised Steel
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Jim McErlean
