Official Answer
Answer
Passivation is a chemical treatment designed to remove 'tramp' iron from the surface of stainless steel and enhance the protective chromium-oxide layer. For 316 Stainless Steel (containing $2-3\%$ Molybdenum for improved pitting resistance), passivation in a nitric or citric acid bath is essential after the coiling and heat-treating processes. If iron particles from the tooling remain on the surface, they can initiate localized galvanic cells, leading to pitting corrosion in saline (chloride-rich) environments. The process per ASTM A967 ensures that the $Cr/Fe$ ratio on the surface is maximized, providing the 'passivity' required for long-term immersion in seawater or medical applications where the spring is exposed to bodily fluids.
