Black Spots on Cadmium Plating

Q. We have been getting black spots on parts that are cadmium plated. The parts look great when shipped, but they are black when they get returned to us. We have had several labs do test on samples and they all say that it seems to be improper cleaning prior to plating, however this really seems unlikely to me. What are your thoughts are on the issue at hand? The material we are plating is C1018. —S.M.

A. I agree with you and do not think the blackening of the parts has to do with cleaning of the base material prior to plating. There are two possible causes for blackening of your cadmium plate. One may be how the parts are stored or shipped. The blackening of cadmium deposit can be caused by presence of sulfur-containing materials such as wrapping papers and corrugated board. Try shipping your parts in non-corrugated containers and use wrapping paper that does not contain sulfur.

Another possibility is that your plating bath contains copper contamination. You should analyze your bath and look for copper and other heavy metals. If you do have copper contamination in your cadmium plating bath, it can be removed by adding small amounts of sodium sulfide and precipitating the copper as copper sulfide. The amount of sodium sulfide and must be determined by determining the amount of copper present. If your analysis does show the presence of copper, you must also address where this is coming from. The most likely source is your buss bars.