Official Answer
Answer
Passivation is a chemical treatment in a nitric or citric acid bath that removes 'free iron' from the surface of the stainless steel and enhances the formation of a dense, protective chromium-oxide ($Cr_2O_3$) layer. For $17-7PH$ CH900 rings, which contain both chromium and aluminum, passivation is vital after the precipitation hardening process. During heat treatment, the surface can become slightly depleted of chromium, or contaminants from the furnace can be embedded. Passivation ensures that the ring achieves its full corrosion resistance potential. In medical and aerospace standards (e.g., AMS 2700), passivation is a mandatory step to prevent premature 'staining' or 'rusting' in humid environments.
