Black and White Corrosion on Zinc Plating

A. The first thing you will want to look at is the environment the parts are being stored in. Does it get extremely hot? Is it very humid? Is it sitting near any acid? If these parts are stored near the plating operation, chemical fumes from the plating process can actually rust them.

The other thing you will want to check is the quality of the plating. Plating shops sometimes run into problems that need to be addressed. Check plating thickness on your parts to see if it is the right amount (this can be done easily with equipment your plater has). Make sure your plater is maintaining the chemical baths properly. Make sure the parts are being cleaned properly. Introduction of a new contaminant in the manufacturing steps can stick to the part through the plating process. If the part does not get properly cleaned, then the plating will be bad.

Something is definitely wrong. Zinc plating should last a long time, and cardboard boxes should not affect it at all. You will need to do some troubleshooting, so good luck.

A. I have seen cardboard do this to zinc plating. Something to do with the glues used in manufacturing cardboard. PVC though undesirable, may still be required for longer storage periods. Still, checkout the plating and porosity of the wire before blaming the packing.

A. Are the parts stacked close together or nested? Zinc will form white corrosion products, then black, if allowed to remain wet and tightly packed. There is plenty of documentation on the phenomenon under the term "white rust". White rust is very common when galvanized sheets are stacked together, then become wet. I suppose this could also happen with the parts stacked tight with coated cardboard.

A. The white corrosion products are usually Zn(OH)2 and/or ZnCO3 from the moisture in the air. The black corrosion product is usually ZnS2 from Sulfur compounds in the cardboard. There are special cardboards and paper that are Sulfur free. You should contact your supplier of these materials and if they do not give you a good answer about the composition of their product find another vendor.

A. Plating Zn from chloride solution or from a phosphate bath may result in the formation of white spots on the plated component if the pH of the rinsing and plating solution was not controlled. I agree that either the precipitated of (ZnOH)2 or ZnCO3 corrosion material would causes the white corrosion coloration in the Zn deposits. The black discoloration co-deposit is likely to be a ferri-ferrous oxide. The white or black stains found in the films lattice crystal has nothing to do with cardboard phenomenon that others writers have already stated. The solution to this unwanted stains on the Zn films or its alloy deposits is to make sure that the temperature of the bath, the precleaning solution's pH and the final rinses was adequate to assure that any insoluble residual salts and unreacted Zn impurities such as the (Fe2)O3, oil or ZnCo3 was removed.

A. Hi Kevin. This part is deeply scratched. If it was scratched before plating, it probably should not have been plated without mechanical finishing first.

If it is a part that is in service, which I suspect, I doubt that it's a plating problem. It looks like it was exposed to tiny droplets of corrosive solution from a spray, which also ran down the part. Almost like somebody tried to spray it with oil or WD-40 ⇦this on eBay or Amazon [affil links] but sprayed it with rust converter ⇦this on eBay or Amazon [affil links] or some other acid instead. It's probably unrealistically optimistic to expect no corrosion after severe scratching and exposure to corrosive substances. If you can find out what service this part saw, it would be helpful.

I think maybe you are going out of your way to not tell us what this part is in an attempt to be fair & impartial. But I think what it is may be the key to the problem. Were there bolts in those holes and slots?  :-)

Regards,