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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 ...
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. These coatings are commonly applied to protect the part against corrosion , to improve the adherence of other coatings, for lubrication , or for aesthetic ...
The process steps can typically involve chromate conversion coating the entire component, followed by a masking of the surface in areas where the chromate coating must remain intact. Beyond that, the chromate coating is then dissolved in unmasked areas. The component can then be anodized, with anodizing taking to the unmasked areas.
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.
This etches the work-piece surface and removes any scale. In some cases, the activation step is done in the chromium bath. In some cases, the activation step is done in the chromium bath. The chromium bath is a mixture of chromium trioxide and sulfuric acid , the ratio of which varies greatly between 75:1 to 250:1 by weight.
It is relevant to chromate conversion coating, which is used to passivate common metal alloys such as aluminium, zinc, cadmium, copper, silver, magnesium, and tin. [2] In conversion coating chromate reacts with these metals to prevent corrosion, retain electrical conductivity, and provide a finish for the appearance of the final alloy products. [3]
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]
Phosphate conversion coating is a chemical treatment applied to steel parts that creates a thin adhering layer of iron, zinc, or manganese phosphates to improve corrosion resistance or lubrication or as a foundation for subsequent coatings or painting. [1] [2] [3] It is one of the most common types of conversion coating.