Answer
Fretting corrosion occurs in wave springs subjected to low-amplitude, high-frequency oscillations. It is characterized by the removal of the protective oxide layer on the spring's surface (e.g., the chromium oxide layer on $17-7PH$), leading to rapid oxidation and the formation of 'pits'. Diagnostic signs include the presence of a reddish-brown or black powder (debris) at the contact points between the spring and the bore or between turns in a nested spring. These pits act as stress risers, significantly reducing the fatigue life. In subsea environments, this can lead to 'fretting-induced SCC'. To troubleshoot, engineers should check for excessive vibration in the system and consider applying a sacrificial coating or a high-viscosity lubricant to dampen the micro-motion.
