Compressors handling hydrogen (hydrogen at partial pressures greater than 100 psig, or concentrations greater than 90 molar-percent at any pressure) are susceptible to hydrogen embrittlement. This embrittlement occurs when a metal is stressed in a hydrogen-rich atmosphere.
Metals highly prone to embrittlement include high-strength steels and high-strength nickel base alloys. Those having only a slight tendency include titanium, copper, austenitic stainless steels and aluminum alloys, with most materials commonly used on centrifugals falling in between.
As in the previous cases, the most practical solution has been found in selection of material properties compatible with the process involved. API 617 limits impellers to 120,000 psi yield strength and a hardness less than Rockwell C34. Figure200-73 shows that this stress level is for overspeed RPM, and is therefore conservative at running speed.