Black Spots On SG Cast Iron

Q: We plate SG iron castings in a zinc acid chloride plating bath and then chromate with a classic hexavalent yellow chemical conversion bath. We recently we tried a trivalent yellow conversion coating, but after processing 30–40 jobs we observe black patches on the jobs immediately after processing. Plating thickness for the job is approximately 18 microns. We followed the temperature and pH specifications recommended by the supplier. Is it possible that the iron impurities from the casting contaminate the chemical conversion bath and if so, how we can control the process? N. K.

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A: We should start by describing what SG stands for. SG is spheroidal graphite cast iron. In this type of cast iron, the graphite is present in spheroidal form instead of flakes. This type of cast iron has higher mechanical strength, ductility and shock resistance compared to grey cast iron. So much for the metallurgy lesson!

Regarding your question about the black spots after processing 30–40 batches of material, it is very likely that your trivalent conversion bath is contaminated with iron. You do not state whether you change the process bath after a set period of time or continue to use it until you have this problem with black spots. It has been my experience that the trivalent conversion coatings are fussier than the hexavalent conversion coatings. Hence, the trivalent baths must be monitored and cared for to a greater degree than the older hexavalent baths.

I would suggest you make up a new trivalent bath and see if the spotting disappears. If it disappears, it is pretty certain that something is being dragged in concentrating in the conversion bath. You should contact the supplier of the bath and have him do some troubleshooting for you. Also, investigate the rinse steps between the plating process and entry into your conversion coating process.