Cleaning Before Passivating

Question:

I need some advice. We have a new customer with 17-2 stainless parts requiring passivation. The parts have a heavy brown oxide on them as received. We need to remove this before the passivation treatment. What do you suggest to do this with?

Answer:

The 17-2 designation indicates that the stainless steel has 17% chromium and 2% nickel. While the designation you provided is not followed by a “PH,” I will assume it is a precipitation hardening stainless steel since that is how I have typically seen them named (i.e., 17-4 PH, 17-7 PH, etc.). Regardless of the type of stainless steel, I do not believe the product you are using will be very effective since it is caustic-based and intended primarily for mild steel. Aggressive cleaning methods employing mineral acids or abrasive blasting are typically required to remove heavy oxides from stainless steels.

If the 17-2 is in the annealed condition, an acidic treatment can be used to remove oxides. One example would be a mixture of approximately 10% nitric acid and about 3% hydrofluoric acid at room temperature for a few minutes. If the 17-2 is in the hardened condition, it should probably be abrasive blasted to remove the oxide. Following oxide removal, it can be passivated with a standard solution of approximately 40% nitric acid between room temperature and 140F for 30-60 min. This should be followed by a thorough rinsing in clean water.