I have a customer who wishes to passivate parts made from 416 stainless steel. I have been told that this is not possible. Is this correct or am I being snowed? Also, will this process reduce the size of the part? P.P.
Passivation of 416 stainless steel is possible. The usual formulation contains nitric acid and, in some cases, small amounts of sodium dichromate. A part with a dull or non-reflective surface can be passivated in a 20-50% nitric acid solution at 110-130F for 20-30 min. A part with a polished surface can be passivated in 20-25% nitric acid solution with 2-6% weight sodium dichromate added. A temperature of 120-130F is required with a time of 15-30 min. As you might guess, there are a number of variations on these formulations.
The one problem with the second formulation is that a hexavalent chromium compound is used. Many finishers are not set up to deal with the proper handling of hexavalent chromium compounds.
If the passivation process is done properly, the dimensions of your parts will not change significantly.