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Chromate conversion coating
Chromate conversion coating or alodine coating is a type of conversion coating used to passivate steel, aluminium, zinc, cadmium, copper, silver, titanium, magnesium, and tin alloys. The coating serves as a corrosion inhibitor, as a primer to improve the adherence of paints and adhesives, as a decorative finish, or to preserve electrical conductivity. It also provides some resistance to abrasion and light chemical attack (such as dirty fingers) on soft metals.
Chromate conversion coatings are commonly applied to items such as screws, hardware and tools. They usually impart a distinctively iridescent, greenish-yellow color to otherwise white or gray metals. The coating has a complex composition including chromium salts, and a complex structure.
The process is sometimes called alodine coating, a term used specifically in reference to the trademarked Alodine process of Henkel Surface Technologies.
Chromate conversion coatings are usually applied by immersing the part in a chemical bath until a film of the desired thickness has formed, removing the part, rinsing it and letting it dry. The process is usually carried out at room temperature, with a few minutes of immersion. Alternatively, the solution can be sprayed, or the part can be briefly dipped in the bath, in which case the coating reactions take place while the part is still wet.
The coating is soft and gelatinous when first applied, but hardens and becomes hydrophobic as it dries, typically in 24 hours or less. Curing can be accelerated by heating to 70 °C (158 °F), but higher temperature will gradually damage the coating on steel.
The composition of the bath varies greatly, depending on the material to be coated and the desired effect. Most bath formulae are proprietary.
The formulations typically contain hexavalent chromium compounds, such as chromates and dichromates.
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 (Na2Cr2O7 · 2H2O) and 6 mL/L concentrated sulfuric acid.
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Chromate conversion coating
Chromate conversion coating or alodine coating is a type of conversion coating used to passivate steel, aluminium, zinc, cadmium, copper, silver, titanium, magnesium, and tin alloys. The coating serves as a corrosion inhibitor, as a primer to improve the adherence of paints and adhesives, as a decorative finish, or to preserve electrical conductivity. It also provides some resistance to abrasion and light chemical attack (such as dirty fingers) on soft metals.
Chromate conversion coatings are commonly applied to items such as screws, hardware and tools. They usually impart a distinctively iridescent, greenish-yellow color to otherwise white or gray metals. The coating has a complex composition including chromium salts, and a complex structure.
The process is sometimes called alodine coating, a term used specifically in reference to the trademarked Alodine process of Henkel Surface Technologies.
Chromate conversion coatings are usually applied by immersing the part in a chemical bath until a film of the desired thickness has formed, removing the part, rinsing it and letting it dry. The process is usually carried out at room temperature, with a few minutes of immersion. Alternatively, the solution can be sprayed, or the part can be briefly dipped in the bath, in which case the coating reactions take place while the part is still wet.
The coating is soft and gelatinous when first applied, but hardens and becomes hydrophobic as it dries, typically in 24 hours or less. Curing can be accelerated by heating to 70 °C (158 °F), but higher temperature will gradually damage the coating on steel.
The composition of the bath varies greatly, depending on the material to be coated and the desired effect. Most bath formulae are proprietary.
The formulations typically contain hexavalent chromium compounds, such as chromates and dichromates.
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 (Na2Cr2O7 · 2H2O) and 6 mL/L concentrated sulfuric acid.