I have had similar problems most notably on 303 and 416 free machining stainless. The pits may be caused by the acid or caustic removing the micro sulfur inclusions that are used to make the material-free machining. Parts made from these materials are better off not having any passivation applied than doing one of these processes. The article in the Feb 2005 Products Finishing magazine did not specify the material. I feel that may be the key to the problem. S.F.
The following was received following my column in February that discussed pitting of stainless steel following a pickling and passivating procedure. SF brings up a very good point. The original writer did not mention what type of stainless steel they were having problems with. If it were a free-machining stainless (similar to the 303 or 416 mentioned below), I agree that the sulfur inclusion would be etched right out of the surrounding stainless steel matrix. This would be most apparent with the first step the writer was describing which was a combination of hydrofluoric and nitric acids. Thanks for the feedback, it is always good to know people are reading.
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