Zinc Plating on Cast Iron

Why are cast iron parts coming out of chromating with dark areas?

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Q. We are a zinc plating shop with both cyanide and acid chloride plating lines. We are able to do quality work on all of the parts that we receive with the exception of components made of cast iron. When we plate cast iron we end up with dark areas on the surface after the chromating step. Prior to the trivalent chromating step, we do not see the dark areas. What are we doing wrong? –R.R.

A. You don't mention in your message whether you are plating the cast iron parts with cyanide zinc or the acid chloride zinc. Cyanide zinc is not the best choice for plating cast iron. Assuming you are using the acid zinc plating bath, your problem is most likely due to not having enough of a zinc layer on the cast iron parts. When the parts are placed in the chromate conversion bath, some of the zinc is removed from the part during the conversion step. If there isn’t enough zinc to start with, you’ll end up with dark spots in those areas. The solution is straightforward: apply a heavier zinc deposit. You should also discuss this problem with the vendor of your chromium conversion coating.

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