Molybdates are classified as anodic or passivating inhibitors. When a coating film that contains molybdate pigments is exposed to water, a small amount of molybdate ions is released into the coating film. When these ions contact the metal substrate, they promote the formation of a protective, passive oxide layer on the metal that prevents corrosion of the underlying substrate. The presence of the molybdate ion raises the electrode potential and moves iron into a region of passivation. The interaction of molybdates with other metal substrates such as aluminum, although studied less extensively, is expected to be similar to that with iron.
Chromates inhibit corrosion in a similar fashion.
Molybdates have been used commercially for more than 20 years. Zinc molybdates are primarily used in solvent-based coatings, particularly alkyd and epoxy systems. Calcium molybdates are considered more compatible and stable in water-based coatings because their solubility is lower than that of zinc molybdates. Phosphomolybdates, the newest of the three molybdate pigments, incorporate a synergistic combination of phosphate and molybdate compounds. They have exhibited cost and performance advantages over their calcium and zinc molybdate counterparts.