Black and White Corrosion on Zinc Plating
I am a quality control manager for a company that builds mild steel wire shelving products. We then have these shelves zinc and chrome plated by an outside plating vendor. A chloride zinc process is used to electrodeposit zinc on a ferrous substrate to a minimum thickness of .0003". The zinc plating is applied in accordance to: ASTM B633 [link by ed. to spec at TechStreet] -78 Fe/Zn8 SC2.
In the past few months we have been having a lot of corrosion problems with items that have been packed in our warehouse for several months. The wire shelves are covered with white and black corrosion marks. Some persons here think it is from the shelves sitting inside the cardboard cartons they are packed in for too long.
My question is this...under normal conditions how long should zinc plating hold up? Also what might be causing our corrosion problem?
Please keep in mind when posting your replies...I am not a plating vendor. Thanks!Steve C.
- Baltimore, MD USA
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.
Rochester Hills, Michigan
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.
Bangalore, Karnataka, India
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.
materials testing laboratory
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.
Atotech - Rock Hill, South Carolina
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.
March 23, 2011
I would like to know if the variation of pH of rinse water can cause this black corrosion. Is it possible?Cezar Borges
- Ipatinga, MG, Brazil
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