Explain the role of 'Stress Corrosion Cracking' in the failure of 17-7PH retaining rings in sour gas environments.

Stress Corrosion Cracking (SCC) is the combined effect of tensile stress and a corrosive environment (like $H_2S$ in sour gas). In 17-7PH CH900 rings, the high internal residual stresses from coiling, combined with the operating load, make them susceptible. The $H_2S$ facilitates hydrogen entry into the metal, and the 'PH' phases can act as crack paths. Failure is often sudden and occurs without significant metal loss (unlike general corrosion). To prevent SCC, the material should be over-aged to a lower strength condition (like TH1050), which increases toughness, or more resistant materials like Inconel 718 or MP35N should be specified, which are NACE MR0175 compliant for sour service.