What are the signs of 'Fretting Corrosion' in wave spring assemblies, and how can it be mitigated?

Fretting corrosion appears as reddish-brown (for steel) or black (for stainless) debris at the contact points between the spring peaks and the mating surface. It is caused by micro-oscillations ($< 100 \mu m$) under load. The debris acts as an abrasive, accelerating the wear. Mitigation strategies include: 1) Increasing the preload to stop the movement; 2) Applying a dry-film lubricant (e.g., $MoS_2$) to reduce the coefficient of friction; 3) Hardening the mating surface; or 4) Changing the spring material to one with better galling resistance, such as a phosphor bronze or a specialized nickel alloy.