The surface of malleable iron pipe fittings can be different. Normally, it can divide to black, baked galvanize and hot dipped galvanize.
Black: as per the meaning of the word, factory don't do any further procedures for surface. in case of rusting, factory will fill with anti-rusting oil for protection.
Baked galvanize: Similar as cold galvanize or electro galvanize. It is electrochemistry processes.
Hot dipped galvanize: The process is to put the fittings into 500% liquid zinc. The zinc thickness on fittings is thicker than cold galvanize, so price is higher as well.