For most pieces of equipment and piping, 304/304L or 316/316L would be the recommended materials of construction for gaseous hydrogen. There are several standards and publications that give detailed information about designing and safely operating hydrogen systems and include information on materials of construction. One such standard is ANSI/AIAA G-095 "Guide to Safety of Hydrogen and Hydrogen Systems". Given the hazardous nature of hydrogen, it might be worth obtaining one or more of these and reviewing the relevant sections for you specific application.