Answer
For medical implants, wave springs made of 316 Stainless Steel must be free of all manufacturing oils and surface contaminants to ensure biocompatibility and corrosion resistance. Vapor degreasing removes residual lubricants from the coiling process. Passivation, typically per ASTM A967 (Nitric or Citric acid), is then used to dissolve 'tramp iron' embedded in the surface from the tooling and to enrich the chromium oxide ($Cr_2 O_3$) protective layer. Without passivation, these iron particles act as initiation sites for pitting corrosion in the saline environment of the human body. For 316SS, which is non-magnetic and highly ductile, this process ensures that the spring's fatigue life is not prematurely ended by localized corrosion-fatigue mechanisms.
