The American Petroleum Institute (API) issued guidelines in API 941 for steel selection for high-temperature and
high-pressure hydrogen service in petroleum refineries and petrochemical plants. The likelihood of HTHA occurring
is a function of temperature, hydrogen partial pressure, and whether there are unstable carbides in the material.
Figure 2 illustrates HTHA damage in C-0.5 Mo Steel (ASTM A 204-A) after 65,000 hours in a catalytic reformer at
a temperature of 790°F (421°C) and hydrogen partial pressure of 425 psig (2.9 MPa)[1].