Investigate the failure of a spiral retaining ring due to 'fretting corrosion' in a high-vibration aerospace gearbox.

Fretting corrosion occurs at the interface between the retaining ring and the groove wall when subjected to small-magnitude oscillatory displacement (vibration). This micro-motion breaks down the protective oxide layer of the steel, leading to the formation of abrasive debris (typically $Fe_2O_3$). The debris acts as an abrasive, accelerating the wear and creating pits that serve as stress concentrators for fatigue cracks. In aerospace gearboxes, this is often mitigated by applying a dry-film lubricant (MoS2) per MIL-L-46010 or by using a silver-plated ring to provide a sacrificial lubricating layer. Inspection for 'red rust' or 'cocoa' powder at the groove interface is a definitive sign of fretting.