Galvanising is the process on a substrate of metal, normally steel. There are two methods of deposit, namely hot dipping or electrolytic. Normally the former is a much heavier coat but may be to some degree irregular in thickness, whereas electrolytic is even and probably much thinner although the actual coating weight can be controlled.
Galvanised steel will oxidize if wetted, providing the material is in contact with other substances. These other substances may be wrapping paper or underlying similar products if in bundle or coil form. This damage shows up as zinc oxide and is normally called ‘white rust’. In severe cases this rusting may penetrate the entire zinc coat and expose the underlying metal which will normally show up as black patches.
When first coated the product has a bright, shiny surface but this will dull brown by exposure to air; this dulling action is not detrimental. During the manufacturing process base metal must be cleaned and fluxed. The cleaning is normally by acid and it is therefore essential that prior to coating the cleaning medium is removed in totality. If it is not, the residual acid may well attack the zinc coat from the underside.
Again, in all probability where there is residual acid, the acid of the zinc will be discoloured and could be confused with that generated by contact with sea water.
Zinc coated products cannot be chemically tested for salt with the standard silver nitrate test as there are products in the zinc which will react with the silver nitrate and produce a black precipitate. If necessary, or if there is any doubt, then the oxide should be submitted to a competent analyst to determine if the impregnating water is of a salt water origin or not.
It is possible to recover the base metal by immersing the product in hydrochloric acid and then re-coating, and this could well be a feasible option where one is dealing with component parts, e.g. galvanised steel conductor pylons, but it is extremely doubtful if this is an economical option where sheet or standard merchant bars are involved.