Answer
MP35N (a nickel-cobalt base alloy) is the gold standard for subsea wave springs due to its immunity to Hydrogen Induced Stress Cracking (HISC) and Sulfide Stress Cracking (SSC) in 'sour' environments containing $H_2S$. MP35N achieves ultra-high strength (up to $300$ ksi) through work hardening and aging. Unlike 17-7PH, which may be susceptible to embrittlement in certain electrolytic subsea conditions, MP35N maintains its ductility and fatigue resistance. The alloy's high Modulus of Elasticity ($33.8 \times 10^6$ psi) allows for very high spring rates in compact envelopes. For a subsea valve actuator with a 25-year service life requirement, the material's resistance to chloride-induced SCC and its massive cathodic protection compatibility make it the only viable choice despite its extreme cost.
