Chromate conversion coating
Chromate conversion coating is a type of conversion coating used to passivate aluminum, zinc, cadmium, copper, silver, magnesium, and tin alloys. 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, the chemical most widely used in the immersion bath process whereby the coating is applied. However, hexavalent chromium is toxic, thus, highly regulated, so new, non-hexavalent chromium-based processes are becoming more readily available at a commercial level. 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.
Chromate conversion coatings on an aluminum substrate are known by the following terms: chemical film, yellow iridite, and the brand names Iridite and Alodine. 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.
The most commonly referred-to standard for applying chromate conversion coating to aluminum is MIL-DTL-5541.
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.
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. 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.
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.
Curing can be accelerated by heating up to 70 °C, but higher temperatures will gradually damage the coating. Some chromate conversion processes use brief de-gassing treatments at temperatures of up to 200 °C, to prevent hydrogen embrittlement of the electro-deposited substrate underlying the chromate layer. It is typically specified for electroplated steels > 180 ksi tensile strength, and is normally performed prior to conversion coating. It is the pre-conversion plating that requires embrittlement relief (reference QQ-P-416 or other plating specifications that use chromate conversion), not the chromate-conversion coating itself.
- 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.
- Occupational Exposure to Hexavalent Chromium, US Dept. of Labor, OSHA Federal Register # 71:10099-10385, 28 Feb 2006.
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- Henkel Alodine products home page, accessed 2009-03-27
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- MacDermid MSDS for Iridite 14-2, Product number 178659.
- Testing and evaluation of non-chromated 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.
- 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