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 the chromic acid used in the bath, more commonly known as hexavalent chromium.^[2] This type of bath is still the most widely used, however hexavalent chromium is toxic and, thus, highly regulated,^[3] so new non-hexavalent chromium based processes are becoming commercially available.^[4]

Chromate conversion coatings are common on everyday items such as hardware and tools and usually have a distinctive yellow color.

Substrates

Aluminum

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 referenced standard for chromate conversion coating aluminum is MIL-DTL-5541.

Magnesium

Alodine may also refer to chromating magnesium alloys.^[8]

Phosphate coatings

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

Zinc

Chromating is commonly performed on zinc-plated parts to make them more durable. The chromate coating acts like a paint, protecting the zinc from white corrosion, this can make the part several times more durable depending on chromate layer thickness.^{[citation needed]} It cannot be applied directly to steel or iron, and does not enhance zinc's cathodic protection of the underlying steel from brown corrosion.^[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]

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 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 sulfuric acid.^[9]

Iridite 14-2, a chromate conversion coating for aluminum, contains chromium(IV) oxide, barium nitrate and sodium silico fluoride.^{[citation needed]}

Chromate coatings are soft and gelatinous when first applied but harden and become hydrophobic as they age.^[11] Curing can be accelerated by heating up to 70 °C, but higher temperatures will gradually damage the coating over time. Some chromate conversion processes use brief degassing treatments at temperatures of up to 200 °C, to prevent hydrogen embrittlement of the substrate. Coating thickness vary from a few nanometers to a few micrometers thick.^[9]

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. ^ ^{a b c} 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. ^ ^{a b c d} 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. ^ Testing and evaluation of nonchromated coating systems for aerospace applications. Osborne, J. H.; Blohowiak, K. Y.; Taylor, S. R.; Hunter, C.; Bierwagon, G.; Carlson, B.; Bernard, D.; Donley, M. S. The Boeing Company, Seattle, WA, USA. Progress in Organic Coatings (2001), 41(4), 217-225.

Chromate coatings are soft and gelatinous when first applied but harden and become hydrophobic as they age.[11] Curing can be accelerated by heating up to 70 °C, but higher temperatures will gradually damage the coating over time. Some chromate conversion processes use brief degassing treatments at temperatures of up to 200 °C, to prevent hydrogen embrittlement of the substrate. Coating thickness vary from a few nanometers to a few micrometers thick.[9

The embrittlement relief operation isn't performed to eliminate embrittlement from the chromate conversion but the plating substrate that is being chromated. The embrittlement relief is typically specified for electroplated steels > 180 ksi tensile strength and it is normally required prior to conversion coating. It is not the chromate conversion coating but the electrodeposition process that requires embrittlement relief (reference QQ-P-416 or other plating specifications that use chromate conversion)!!

Kelly Ar

External links

• Overview of different chromating processes
• yellow and green chromating chemistry on aluminum
• MIL-C-17711B, Chromate conversion coating specification for zinc and zinc alloy casting and hot dipped galvanized parts