Chromate conversion coating is a type of conversion coating applied to passivate aluminium, zinc, cadmium, copper, silver, magnesium, tin and their alloys to slow corrosion.[1] The process uses various toxic chromium compounds which may include hexavalent chromium.[2] The industry is developing less toxic alternatives in order to comply with substance restriction legislation such as RoHS.[3] One alternative is trivalent chromate conversion which is not as effective but less environmentally damaging.
Chromating is commonly used 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.[4] It is also commonly used on aluminium alloy parts in the aircraft industry where it is often called chemical film, or the well known brand name Alodine.[5] 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.[6] Curing can be accelerated by heating up to 70°C, but higher temperatures will gradually damage the coating over time.[4] Some chromate conversion processes use brief degassing treatments at temperatures of up to 200°C, to prevent hygdrogen embrittlement of the substrate.[4] Coating thickness vary from a few nanometers to a few micrometers thick.[4]
The protective effect of chromate coatings on zinc is indicated by color, progressing from clear/blue to yellow, gold, olive drab and black.[7] 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.
Phosphate coatings on iron and steel may also be treated with a chromic acid rinse to enhance the phosphate coating.[4]
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Standard specifications
ISO 4520 spesifies chromate conversion coatings on electroplated zinc and cadmium coatings.
MIL-DTL-5541F 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 sulfuric acid.[4]
Iridite 14-2, a chromate conversion coating for aluminium, contains chromium(IV) oxide barium nitrate and sodium silico fluoride.
See also
References
- ^ 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.
- ^ Buchheit, R. G.; Drewien, C. A.; Martinez, M. A.; Stoner, G. E. Chromate -free corrosion resistant conversion coatings for aluminum alloys. Sandia National Laboratories [Technical Report]SAND(1995),(94-3010, Minutes of the Fourth Annual Workshop on Chromate Replacements in Light Metal Finishing, 1994),125-7.
- ^ 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.
- ^ New surface treatment for aluminum. Anthony, J. Iron Age (1946), 158(23), 64-7.
- ^ 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.
- ^ *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.
External links
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