Answer
Passivation is a chemical treatment intended to remove free iron from the surface of stainless steel, enhancing the protective chromium-oxide layer. For spiral retaining rings in medical or marine use, AMS 2700 (Method 1 using Nitric Acid or Method 2 using Citric Acid) is the standard. 316 Stainless Steel, containing $2$-$3\%$ Molybdenum, offers superior pitting resistance compared to 302/304. During the coiling of spiral rings, the material is work-hardened, and the edges may have microscopic iron contaminants from the tooling. Citric acid passivation is increasingly preferred as it is environmentally safer and more selective in removing only the free iron, ensuring the $360^{\circ}$ spiral surface remains inert in chloride-rich environments.
