Chromate conversion coating is a type of conversion coating used to passivate steel, aluminium, 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. In Europe the RoHS (Restriction of Hazardous Substances) Directive is commonly referred to regarding elimination of hexavalent chromium in electrical and electronic equipment, and the REACH ("Registration, Evaluation, Authorisation and Restriction of Chemicals") Directive to wider applications including chromate conversion coating processes, paint primers and other preparations.
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.
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Substrates
Aluminium
Chromate conversion coatings on an aluminium substrate are known by the following terms: chemical film, yellow iridite, and the brand names Iridite and Alodine (in the UK, Alocrom). It is also commonly used on aluminum alloy parts in the aircraft industry.
Iridite NCP is a non-chromium type of conversion coating for aluminium substrates.
The most commonly referred-to standard for applying chromate conversion coating to aluminium is MIL-DTL-5541 in the US, in the UK it is Def Stan 03/18.
Magnesium
Alodine may also refer to chromate-coating magnesium alloys.
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.
Steel
Steel and iron cannot be chromated directly. Steel plated with zinc may be chromated (see Zinc below).
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. Chromating 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.
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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 consisting of 182 g/l sodium dichromate crystals (Na2Cr2O7 · 2H2O) and 6 ml/l concentrated sulfuric acid.
Iridite 14-2, a chromate conversion coating for aluminum, contains chromium(IV) oxide, barium nitrate, sodium silicofluoride and ferricyanide.
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 (158 °F), but higher temperature will gradually damage the coating on steel.
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