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 chromating step we do not see the dark areas. R. R.
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 will end up with dark spots in those areas. The solution is straightforward: Apply a heavier zinc deposit. I assume you are using the acid chloride zinc bath to plate the parts. The cyanide zinc plating bath is not a good choice for plating cast iron.