Evaluate the use of Elgiloy or MP35N for spiral rings in ultra-high-pressure subsea valves.

Elgiloy (Co-Cr-Ni alloy) and MP35N are cobalt-based 'super-alloys' used in the most demanding subsea and medical applications. They offer tensile strengths up to 300 ksi and exceptional resistance to hydrogen sulfide ($H_2S$) and sea water. For a spiral ring in a subsea valve, these materials provide the highest possible 'clinging' force and thrust capacity while being immune to the galvanic corrosion that can occur between a stainless ring and a carbon steel housing. Their high cost is justified in 'zero-failure' environments where the cost of intervention (e.g., a ROV mission) exceeds the material cost by several orders of magnitude.