Chromate conversion coating
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 with regards to the elimination of hexavalent chromium in electrical and electronic equipment, and the REACH ("Registration, Evaluation, Authorisation and Restriction of Chemicals") Directive is typically referenced in the context of 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.
Chromate conversion coatings on an aluminium substrate are known by the following terms: chemical film, yellow iridite, and the brand names Iridite, Bonderite (formerly known as Alodine, in the UK, Alocrom). It is also commonly used on aluminium alloy parts in the aircraft industry.
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.
Steel and iron cannot be chromated directly. Steel plated with zinc may be chromated (see Zinc below).
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.
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.
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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- Yellow and green chromating chemistry on aluminium
- MIL-C-17711B, Chromate conversion coating specification for zinc and zinc alloy casting and hot dipped galvanized parts