Official Answer
Answer
MP35N (a Cobalt-Nickel-Chrome-Moly alloy) is selected for the most demanding subsea environments because it offers an unparalleled combination of ultra-high strength (UTS up to $2000$ MPa) and exceptional resistance to Hydrogen Induced Stress Cracking (HISC) and Sulfide Stress Cracking (SSC). In the presence of cathodic protection systems and $H_2S$, standard stainless steels like 17-4PH or even 316 can fail. MP35N's face-centered cubic structure is highly stable. It also has a high modulus of elasticity ($E imes 233$ GPa), allowing for very high 'cling' forces in retaining rings that must withstand extreme pressures and corrosive 'sour' gas.
