What guidance is available regarding material compatibility in relation to hydrogen piping? Is austenitic steel, such as 316SS, always the recommended choice?

Acceptability of materials is highly dependent on the specific application. Applied stress levels, exposure to contaminants, the operating temperature, the partial pressure, and number and magnitude of material stress cycles are some of the factors that affect material selection. Guidance is provided within documents such as ISO 11114, Gas cylinders - Compatibility of cylinder and valve materials with gas contents, ASME B31.12, Hydrogen Pipe and Pipelines, and NFPA 2, Hydrogen Technologies Code. The Sandia Technical Reference site (https://h2tools.org/technical-reference-for-hydrogen-compatibility-ofmaterials)
also provides information for users to make informed selections. For ambient temperature systems designed to rules described in accepted codes and standards, austenitic stainless steel is frequently a good choice.