Formation mechanism of trivalent chromium passivation film on galvanized layer

- Sep 26, 2018 -

The formation mechanism of the trivalent chromium passivation film is similar to that of hexavalent chromium passivation, but does not include the step of reducing hexavalent chromium to trivalent chromium. The first is that in the acidic medium, zinc is oxidized by the oxidant and forms zinc chromium oxide with trivalent chromium, while the acid is consumed to raise the pH of the contact interface, and then the trivalent chromium compound precipitates on the surface when the pH is increased to form a The layer is a gelatinous film composed of zinc chromium oxide.

There are three processes for forming a trivalent chromium passivation film of a galvanized layer, namely, dissolution of zinc, formation of a passivation film, and dissolution of a passivation film. The passivating agent must contain an oxidizing agent to dissolve the zinc layer. The oxidizing agent used is a nitrate which reacts with zinc. The dissolution of zinc consumes H+ in the solution, so that the pH of the zinc surface solution rises, and the trivalent chromium directly reacts with ZN2+ and OH- ions to form an isolation layer composed of insoluble zinc-chromium oxide, that is, formed on the surface of zinc. Passivation film, this film does not contain hexavalent chromium, so it does not have self-healing ability. Once the film is damaged, corrosion can occur very quickly, especially when the plated parts are passivated, they are easily scratched and damage the film. This deficiency can be offset by the use of a sealer or a protective coating that uses a post-treatment coating.

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