Wear and Corrosion Alternatives - Chromates

Chromates – what are they used for and why?

Chromates (salts containing hexavalent chrome in the chemistry Metal-CrO₃) have the unusual property of affording corrosion protection even when scratched or damaged.  They do this by going into solution in the corrosive environment, migrating to the exposed bare metal surface, and forming complexes to inhibit further corrosion.  In addition, chromates often form good bonding layers, improving the bond between a paint system and a surface, which also helps to improve corrosion resistance.  Consequently chromates are widely used as inhibitors and sealers wherever corrosion is a serious concern, especially where damage to the surface coating is quite likely.  Examples include

• Chromate conversion coatings for metal surfaces

  • Aluminum alloys are typically chromate converted to resist chloride attack in marine environments

  • Sacrificial corrosion resistant coatings on steels (such as Cd, Zn, Al) are frequently chromate converted to supplement the corrosion resistance of the sacrificial coating.  This type of chromate treatment is often termed a “sealer”.

  • Phosphate passivation coatings on steels are typically chromate sealed

• Primers and paints

  • Metal surfaces are commonly chromated to enhance paint adhesion

  • Paint primers often contain chromates so that if scratched they will continue to protect the surface.

  • Metal-ceramic paint systems used on aircraft engines are usually finished with chromate-containing sealing layers for corrosion resistance.

Typical applications of chromates

• Galvanized steel sheet

• Galvanized fasteners (bolts)

• Painted steels used in corrosive service

• Aircraft skins and structural members made of aluminum alloys

• Magnesium alloy components such as gearbox housings used in aircraft

• Cadmium plated aircraft landing gear

• Electrical connectors and cabinets

• Castings and extrusions

The movie below shows why chromates work so well to inhibit corrosion

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