Trivalent chromium black passivation. It is generally a black passivation film obtained by adding copper, silver, iron, nickel and other metal ions as blackening agents in the passivation solution with pH between 2-4. However, due to the rapid film formation of black passivation film, the film layer arrangement structure is relatively loose, salt spray resistance is not high. In the unclosed black
Performance comparison of different colours of trivalent chromium passivation films
Author: Robby
1, Trivalent chromium black passivation. It is generally a black passivation film obtained by adding copper, silver, iron, nickel and other metal ions as blackening agents in the passivation solution with pH between 2-4. However, due to the rapid film formation of black passivation film, the film layer arrangement structure is relatively loose, salt spray resistance is not high. In the unclosed black passivation film layer, neutral salt spray test generally 36h white spot.
2、Trivalent chromium blue-white passivation. Trivalent chromium blue-white passivation solution consists of trivalent chromium salt, sulfuric acid, nitric acid and hydrofluoric acid and other components. In order to improve its corrosion resistance, some also add nickel salts, cobalt salts, rare earth metal salts and sol-gel type sealer. Due to the lack of hexavalent chromium components in the trivalent chromium blue-white passivation film, and hexavalent chromium has a self-repairing ability of the film layer, so the corrosion resistance is relatively poor. General neutral salt spray test can only reach 48-72 hours.
3, Trivalent chromium coloured passivation. Trivalent chromium coloured film layer and blue-white film layer, are through the dissolution of zinc to form zinc ions, so that the solution pH rises, trivalent chromium directly with zinc, hydroxide and other ions reaction, the formation of insoluble compounds condensed on the surface of the zinc layer to form a passivation film. However, their trivalent chromium forms are different and the resulting colours are not the same. Trivalent chromium coloured passivation film film thickness of 0.25μm-1μm, while trivalent chromium blue-white passivation film thickness is only 80nm or so, so trivalent chromium coloured passivation film corrosion resistance to greatly exceed the blue-white passivation film. Coloured passivate out of white rust time can reach more than 144h, so we do not have to worry about the corrosion resistance of trivalent chromium coloured passivation film.
Trivalent chromium passivation on galvanized layers can be obtained according to different process conditions with different colour appearance and thickness of the passivated film.