Answer
Fretting corrosion occurs when there is high-frequency, low-amplitude relative motion between the wave spring crests and the mating surface. This motion, combined with the contact pressure, breaks down the protective oxide layer of the material (especially in stainless steel). The resulting metal-to-metal contact leads to cold welding and the tearing of microscopic particles, which then oxidize and act as an abrasive (often appearing as a reddish-brown powder for steel, known as 'cocoa'). During teardown, fretting is identified by pitted or worn areas specifically at the wave crests. It can lead to fatigue crack initiation. Solutions include applying a dry-film lubricant (like $\text{MoS}_2$) or increasing the spring's preload to prevent the relative motion.
