Cadmium Plating and Black Spots

Question:

Our shop does bright cadmium plating but we have a problem with black spots appearing after the parts have been processed and shipped to our customer. The base material is high-tensile strength steel so the parts must be baked. Our process consists of rack plating of the parts followed by a chromate conversion coating and then baking to relieve hydrogen embrittlement. What are we doing wrong? A. M.

Answer:

Your e-mail does not give me much to go on but I am pretty sure the black spots are caused by corrosion and it may be due to the baking step. You do not state at what temperature the bake is performed but if it is above 140°F you will dry out and crack the chromate film and the corrosion resistance of the finish will be reduced. Try baking your parts after the plating step and then applying the chromate.