Official Answer
Answer
Pitting corrosion occurs when the passive layer of the stainless steel is locally breached, usually by chloride ions. This creates a small pit that acts as a geometric stress concentrator ($K_t$). In a retaining ring subject to cyclic axial loads or vibration, the stress at the base of the pit can exceed the fatigue limit. A crack initiates at the pit and propagates through the wire thickness. Because retaining rings are under 'hoop stress' from being installed in a groove, the crack is driven by both the installation stress and the operational load, leading to a 'brittle-like' fracture despite the material's inherent ductility.
