Answer
Elgiloy (a cobalt-chromium-nickel alloy, UNS R30003) offers the highest level of corrosion resistance for spiral retaining rings. It is virtually immune to hydrogen embrittlement and has exceptional resistance to stress corrosion cracking in sulfide and chloride environments. Mechanically, Elgiloy provides a very high modulus of elasticity ($28.5 \times 10^6$ psi) and can be heat-treated to a hardness of 45-55 HRC. It maintains these properties from cryogenic temperatures up to $850^{\circ}F$. This makes it ideal for downhole oil tools and chemical pumps where 17-7PH or 316SS would fail due to chemical attack. The primary disadvantage is the high material cost, which usually limits its use to critical safety-of-flight or mission-critical subsea components.
