Two conditions decide it: the part must conduct electricity so the powder can be attracted and held, and it must withstand curing temperature without deforming.
Carbon and structural steel are the most common materials — profiles, sheet metal fabrications, enclosures and machine parts all coat very well after proper cleaning.
Aluminium holds powder coating extremely well and is standard for facade profiles, joinery and window systems; the coating also protects against weathering.
Galvanised and zinc-coated sheet can be coated, but the surface needs careful pre-treatment: gas released from the zinc layer during curing is a known cause of pinholing.
Stainless steel, brass and copper are also suitable. Not suitable: plastic, wood, and heat-sensitive assemblies containing gaskets, bearings or electronics that can't go into the oven.