There are many surface treatments / coatings available this is just a sample of the common types requested.
Zinc plating – Galvanizing
Zinc electro plating – Clear passivate, yellow passivate, hexavalent chrome free thick film, olive drab passivate, black Passivate
Powder coating – Polyester, Epoxy polyester
Painting – Epoxy,urethane,etch primer,primer, electro static
Blackening – Black oxide
Chrome plating – Hard chroming