API
API 941
Steels for Hydrogen Service at Elevated Temperatures and Pressures
The home of the Nelson curves, which map hydrogen partial pressure against temperature to show which steels resist high temperature hydrogen attack. Operating above the curve for a given alloy means irreversible internal damage.
This is an explanation, not the standard. PipingInspector is not affiliated with API and reproduces no part of the document. Always work to the current edition of the standard itself and to your project specification.