Official Answer
Answer
Passivation (per ASTM A967) involves treating the ring in a nitric or citric acid bath to remove free iron from the surface and enhance the protective chromium-oxide layer. For 17-7PH, this is typically performed after the CH900 heat treatment. The heat treatment itself creates a light discoloration (oxide scale), which must be removed before passivation to ensure a clean surface. In aerospace, this process is critical because even microscopic iron contaminants can initiate localized galvanic corrosion, leading to pitting and eventual fatigue failure in flight-critical hydraulic actuators.
