Comprehensive and Detailed Explanation From Exact Extract:

According to API RP 571 and API RP 941 (Nelson Curves), 1 Cr--0.5 Mo steel is no longer recommended for services susceptible to High-Temperature Hydrogen Attack (HTHA).

API RP 941 documents industry experience showing that 1 Cr--0.5 Mo steels have suffered HTHA damage below previously assumed safe operating limits, due to insufficient carbide stability. As a result, this material has been removed from the Nelson Curves as an acceptable choice for new construction in hydrogen service.

By contrast:

Mn--0.5 Mo, C--0.5 Mo, and 1.25 Cr--0.5 Mo steels retain higher resistance due to more stable carbide structures.

Referenced Documents (Study Basis):

API RP 571 -- Section on High-Temperature Hydrogen Attack

API RP 941 -- Updated Nelson Curves and Material Recommendations