Chromate coatings are chemical conversion coatings. The substrate metal participates in the coating reaction and becomes a component of the coating. The collaboration has a profound effect on the properties of the coating. Among the metals commonly chromate are zinc, zinc die casting, aluminum and sometimes copper and silver. Chromate films are typically very thin, on the order or .0000001 in. and contribute no measurable thickness to the overall coating.
The chemistry involves a reaction between the metal surfaces and an aqueous solution containing chromates (chromium salts, either hexavalent or trivalent) and certain activators or film formers. Activators may include sulfates fluorides, phosphates, and sometimes complex cyanides. Normally, given chromate is designed to work on a particular metal, but in a few cases will work on two or more. The chromate solution is normally acidic. About 0.0005 in. of plating thickness is dissolved during the chromating process.
Clear chromate offers from 8 to 12 hours' resistance to white corrosion (zinc oxide or "white rust") and had a clear to slightly iridescent blue appearance. This is one of the earliest types of chromating solutions and is used today. Older style "clear" chromates often deposited as a golden yellow coating and were then bleached by immersion in a dilute alkali solution to obtain the clear appearance.
More up-to-date blue bright chromates are single-dip solutions, often using only trivalent chromium salts and are, therefore, more environmentally friendly. Some varieties of this type of baths may be dyed a variety of colors. These colors are often used for identification purposes.
Gold, or yellow, chromate coatings are deposited from baths that contain chromate, sulfate, or chloride activators and produce a distinct iridescent golden yellow color. Yellow chromates contain more hexavalent chromium than clear films, which accounts for their color. Yellow chromate offers in excess of 96 hours' neutral salt spray corrosion protection and is an excellent pain base.
Olive drab, or forest green, chromate is the ultimate in commonly available conversion coatings, with neutral salt spray resistance in excess of 150 hours. This chromate is most commonly specified for military applications. It is generally not possible to apply to barrel plate work. The color and the corrosion resistance are due to the inclusion of an organic acid modifier to the chromate formula. Many customers find the color is not especially pleasing, but specify it for nonmilitary applications for its function, rather than its decorative, value.
Black chromate is usually achieved by incorporating a soluble silver salt in a golden chromate formula, which produces a deposit of black silver chromate. This coating offers excellent corrosion resistance and a jet black semimatte to matte appearance. Black chromate has even found some applications in the space program, in which it is used on solar collectors.
Chromate conversion coatings are included in a great many specifications and are also covered in their own specification Mil-C-5541, although that specification applied exclusively to chromate conversion coatings on aluminum.
Different Chromate Conversions (Resistance to white corrosion)
Clear - (8 to 12 hours) It has a slightly iridescent blue appearance.
Black- (48 hours)
Yellow - (in excess of 96 hours) Excellent paint base.
Olive Drab- (150 hours) dark green finish
Seals are offered for after chromates to increase corrosion resistance.