Here is  a general recommendation for compressed air piping.  You can also use galvanized steel but it will require more fittings, is heavier and harder to handle.

Pipe Schedule: 5 - 10  # Allows piping to be ben as to minimize the number of fittings

Welding:  Butt weld using GTAW

Material: UNS S30400 (304 SS)  316SS can also be used