Chromate conversion coating

Not to be confused with Chrome plating or Diffusion hardening.
Zinc chromate conversion coating on small steel parts.

Chromate conversion coating is a type of conversion coating used to passivate aluminum, zinc, cadmium, copper, silver, magnesium, and tin alloys.[1] It is primarily used as a corrosion inhibitor, primer, decorative finish, or to retain electrical conductivity. The process is named after the chromate found in chromic acid, also known as hexavalent chromium,[2] the chemical most widely used in the immersion bath process whereby the coating is applied. However, hexavalent chromium is toxic, thus, highly regulated,[3] so new, non-hexavalent chromium-based processes are becoming more readily available at a commercial level.[4] One alternative contains trivalent chromium. The RoHS (Restriction of Hazardous Substances) Directive is commonly referred to regarding elimination of hexavalent chromium.

Chromate conversion coatings are commonly applied to everyday items such as hardware and tools, and can usually be recognized by their distinctively iridescent, greenish-yellow color.

Substrates

Aluminium

Main article: MIL-DTL-5541

Chromate conversion coatings on an aluminum substrate are known by the following terms: chemical film,[5] yellow iridite,[5] and the brand names Iridite[5] and Alodine.[6] It is also commonly used on aluminum alloy parts in the aircraft industry.

Iridite NCP is a non-chromium type of conversion coating for aluminum substrates.[7]

The most commonly referred-to standard for applying chromate conversion coating to aluminum is MIL-DTL-5541.

Magnesium

Alodine may also refer to chromate-coating magnesium alloys.[8]

Phosphate coatings

Chromate conversion coatings can be applied over phosphate conversion coatings used on ferrous substrates. The process is used to enhance the phosphate coating.[9]

Zinc

Chromating is often performed on galvanized parts to make them more durable. The chromate coating acts as paint does, protecting the zinc from white corrosion, thus making the part considerably more durable, depending on the chromate layer's thickness. Steel and iron cannot be chromated directly. Chromated of zinc plated steel does not enhance zinc's cathodic protection of the underlying steel from rust.[9]

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.[10] However, the coating color can also be changed with dyes, so color is not a complete indicator of the process used.

ISO 4520 specifies chromate conversion coatings on electroplated zinc and cadmium coatings. 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 greatly, depending on the material to be coated and the desired effect. Most solution formulae are proprietary.

The widely used Cronak process for zinc and cadmium consists of 5–10 seconds of immersion in a room-temperature solution of 182 g/l sodium dichromate crystals (Na2Cr2O7 · 2H2O) and 6 ml/l concentrated sulfuric acid.[9]

Iridite 14-2, a chromate conversion coating for aluminum, contains chromium(IV) oxide, barium nitrate, sodium silicofluoride and ferricyanide.[11]

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.

References

  1. ↑ Buschow, K.H. Jürgen; Cahn, Robert W.; Flemings, Merton C.; Ilschner, Bernhard; Kramer, Edward J.; Mahajan, Subhash (Editors), Encyclopedia of Materials - Science and Technology (2001) p. 1265, Elsevier, Oxford, UK.
  2. ↑ http://books.google.com/books?id=O1DcJk1JpCMC&pg=PA430
  3. ↑ Occupational Exposure to Hexavalent Chromium, US Dept. of Labor, OSHA Federal Register # 71:10099-10385, 28 Feb 2006.
  4. ↑ http://www.epa.gov/nrmrl/std/mtb/pdf/web-powdercoatarticleversion1.pdf
  5. ↑ 5.0 5.1 5.2 http://www.engineersedge.com/iridite.htm
  6. ↑ New surface treatment for aluminum. Anthony, J. Iron Age (1946), 158(23), 64-7.
  7. ↑ http://www.macdermid.com/industrial/aluminum.html
  8. ↑ Henkel Alodine products home page, accessed 2009-03-27
  9. ↑ 9.0 9.1 9.2 Edwards, Joseph (1997). Coating and Surface Treatment Systems for Metals. Finishing Publications Ltd. and ASM International. pp. 66–71. ISBN 0-904477-16-9.
  10. ↑ Degarmo, E. Paul; Black, J T.; Kohser, Ronald A. (2003). Materials and Processes in Manufacturing (9th ed.). Wiley. p. 792. ISBN 0-471-65653-4.
  11. ↑ MacDermid MSDS for Iridite 14-2, Product number 178659.

External links