Official Answer
Answer
In subsea 'sour' gas environments containing $\text{H}_2\text{S}$, standard stainless steels are prone to Sulfide Stress Cracking (SSC). Elgiloy (Co-Cr-Ni-Mo alloy) and MP35N meet NACE MR0175 standards for these conditions. These alloys possess extraordinary corrosion resistance and can be work-hardened and aged to tensile strengths exceeding $250$ ksi. For spiral retaining rings, this allows for very high thrust capacities in extremely compact grooves. Their resistance to hydrogen-induced stress cracking (HISC) and high fatigue limit makes them the gold standard for permanent downhole tools where failure would result in multi-million dollar 'fishing' operations to retrieve equipment.
