How do you evaluate 'fretting corrosion' on a spiral ring in a high-frequency oscillating assembly?

Fretting corrosion appears as a reddish-brown powder (in steel) or black pits (in stainless) at the contact points between the ring and the groove. It is caused by microscopic relative motion (slippage) under load. In a spiral ring, this usually occurs if the 'clinging' force is not high enough to overcome the inertial forces of the ring. To diagnose, look for 'polishing' or 'galling' on the ring turns. To mitigate, apply a dry-film lubricant (like $MoS_2$), increase the ring's radial tension, or use a material with a higher surface hardness. If left unchecked, fretting will lead to fatigue cracks and sudden failure of the ring.