Chromate conversion coating

Chromate conversion coating is a type of conversion coating applied to passivate aluminum, zinc, cadmium, copper, silver, magnesium, tin and their alloys to slow corrosion. The process uses various toxic chromium compounds which may include hexavalent chromium. The industry is developing less toxic alternatives in order to comply with substance restriction legislation such as RoHS. One alternative is trivalent chromate conversion which is not as effective but less environmentally damaging.

Chromating is commonly used on zinc-plated parts to protect the zinc from white corrosion, which is primarily a cosmetic issue. It cannot be applied directly to steel or iron, and does not enhance zinc's anodic protection of the underlying steel from brown corrosion.^[1] It is also commonly used on aluminum alloy parts in the aircraft industry where it is often called chemical film. It has additional value as a primer for subsequent organic coatings, as untreated metal, especially aluminium, is difficult to paint or glue. Chromated parts retain their electrical conductivity to varying degrees, depending on coating thickness. The process may be used to add color for decorative or identification purposes.

Chromate coatings are soft and gelatinous when first applied but harden and become hydrophobic as they age. Curing can be accelerated by heating up to 70°C, but higher temperatures will gradually damage the coating over time.^[1] Some chromate conversion processes use brief degassing treatments at temperatures of up to 200°C.^[1] Coating thickness from a few nanometers to several hundred nanometers can be produced, but the Alodine and Modified Bauer-Vogel coatings on aluminium are typically a few micrometers thick.^[1]

The protective effect of chromate coatings on zinc is indicated by color, progressing from clear/blue to yellow, gold, olive drab and black. Darker coatings generally provide more corrosion resistance. Chromate conversion coatings are common on everyday items such as hardware and tools and usually have a distinctive yellow color. Steel parts must be plated with zinc or cadmium prior to chromating.

Phosphate coatings on iron and steel may also be treated with a chromic acid rinse to enhance the phosphate coating.^[1]

Additional recommended knowledge

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Standard Specifications

MIL-C-5541 specifies chromate conversion of aluminium alloy parts.

ASTM B633 Type II and III specify zinc plating plus chromate conversion on iron and steel parts.

Composition

The composition of chromate conversion solutions varies widely depending on the material to be coated and the desired effect. Most solution compositions are proprietary.

The widely used Cronak process for zinc and cadmium consists of 5-10 seconds of immersion at room temperature in a solution of 182 g/l sodium dichromate crystals (Na₂Cr₂O₇2H₂O) and 6 ml/l concentrated sulphuric acid.^[1]

Iridite 14-2, a chromate conversion coating for aluminium, contains chromium(IV) oxide barium nitrate and sodium silico fluoride.

See also

• Zinc chromate primer (paint)

References

1. ^ ^{a b c d e f} Edwards, Joseph (1997). Coating and Surface Treatment Systems for Metals. Finishing Publications Ltd. and ASM International, pp. 66-71. ISBN 0-904477-16-9. 

• Xia L., Akiyama E., Frankel G., McCreerya R., (2000). "Storage and Release of Soluble Hexavalent Chromium from Chromate Conversion Coatings - Equilibrium Aspects of CrVI Concentration". Journal of The Electrochemical Society 147: 2556-2562. doi:10.1149/1.1393568.
• Xia L., McCreerya R., (1998). "Chemistry of a Chromate Conversion Coating on Aluminum Alloy AA2024-T3 Probed by Vibrational Spectroscopy". Journal of The Electrochemical Society 145: 3083-3089. doi:10.1149/1.1838768.
• Kendig M.W., Buchheit R.G. (2003). "Corrosion Inhibition of Aluminum and Aluminum Alloys by Soluble Chromates, Chromate Coatings, and Chromate-Free Coatings". Corrosion 59: 379-400.