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.
Alodine may also refer to chromate-coating magnesium alloys.
Chromate conversion coatings can be applied over phosphate conversion coatings used on ferrous substrates. The process is used to enhance the phosphate coating.
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.
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, but higher temperature will gradually damage the coating on steel.