![]() ![]() It has long been thought that the iridescent colour of passivated films is due to the chemical composition. When the passivated film is knocked, scratched or bruised, hexavalent chromium can be dissolved in the water film in the humid air and the film is automatically repaired at the breakage, which is one of the important advantages of chromate films. The trivalent chromium is the skeleton of the passivation film and the hexavalent chromium is filled in the skeleton of the trivalent chromium by adsorption, inclusions and chemical bonding, so the content of the hexavalent chromium directly affects the corrosion resistance of the passivation film. The film was analysed to contain 28.2% trivalent chromium, 8.68% hexavalent chromium and 19.3% water. It is generally accepted that the zinc passivation film is a hydrate consisting of alkaline chromium chromate, alkaline zinc chromate and hydrated chromium trioxide. There is still debate about the mechanism of passivation film formation and the chemical composition of the film layer. When the pH at the interface between zinc and the solution rises above 3, a series of film-forming reactions occur and a gel-like passivate film is formed at the zinc interface. The action of zinc with the passivate solution leads to the dissolution of zinc, the reduction of hexavalent chromium to trivalent chromium and the consumption of hydrogen ions in the reaction. The chromate passivate solution consists of chromic acid, activator and inorganic acid. Currently, passivation is mainly available as hexavalent chromium passivation and trivalent chromium passivation.Ĭhromate passivation is not only used as a protective layer, but also as a protective-decorative use on some low-grade products by white passivation or white passivation by organic material colouring. This chromate film, which is just under 0.5 μm thick, increases the corrosion resistance of zinc by a factor of six to eight and gives it a beautiful decorative appearance and resistance to staining. After zinc plating, a chromate treatment is applied to cover the zinc with a chemical conversion film so that the active metal is in a dull state, this is called zinc chromate passivation. Zinc is chemically active and tends to oxidise and darken in the atmosphere, eventually producing "white rust" corrosion. Hexavalent Chromium Passivation Hexavalent chromium passivation
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