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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. [ 1 ] : p.1265 [ 2 ] The coating serves as a corrosion inhibitor , as a primer to improve the adherence of paints and adhesives , [ 2 ] as a decorative finish, or ...
Zinc chromate, Zn Cr O 4, is a chemical compound, a salt containing the chromate anion, appearing as odorless yellow powder or yellow-green crystals, but, when used for coatings, pigments are often added. [2] [3] [4] It is used industrially in chromate conversion coatings, having been developed by the Ford Motor Company in the 1920s. [5]
Special kinds of primer (rust-converters) can be used to chemically convert rust to the solid metal salts. [7] This process is not recommended for structural steel. Painting and gluing aluminum are especially important in the aircraft industry, which uses toxic zinc chromate primers and chromating to add the necessary adhesion properties. In ...
Zinc phosphate is an inorganic compound with the formula Zn 3 (PO 4) 2. This white powder is widely used as a corrosion resistant coating on metal surfaces either as part of an electroplating process or applied as a primer pigment (see also red lead ).
A phosphate coating is usually obtained by applying to the steel part a dilute solution of phosphoric acid, possibly with soluble iron, zinc, and/or manganese salts. The solution may be applied by sponging, spraying, or immersion. [6] Phosphate conversion coatings can also be used on aluminium, zinc, cadmium, silver and tin. [7] [8]
A conversion coating is a chemical or electro-chemical treatment applied to manufactured parts that superficially converts the material into a thin adhering coating of an insoluble compound.
Zinc is rarely anodized, but a process was developed by the International Lead Zinc Research Organization and covered by MIL-A-81801. [17] A solution of ammonium phosphate, chromate and fluoride with voltages of up to 200 V can produce olive green coatings up to 80 μm thick. [17] The coatings are hard and corrosion resistant.
Aluminium chromate conversion coatings are amorphous in structure with a gel-like composition hydrated with water. [18] Chromate conversion is a common way of passivating not only aluminium, but also zinc, cadmium, copper, silver, magnesium, and tin alloys. Anodizing is an electrolytic process that forms a thicker oxide layer.