Trivalent chromium passivation on the zinc coating can produce different color appearance and thickness of the passivation film according to different process conditions.
Author: Marisa
After the electroplated zinc and zinc alloy coatings are treated with trivalent chromium passivation, the salt spray resistance of the coating can be effectively improved. Different color appearances can be obtained according to different passivation solutions. Now there are more black, colorful and blue-white color passivations on the market.
1. Trivalent chromium black passivation. Generally, a black passivation film is obtained by adding metal ions such as copper, silver, iron, and nickel as blackening agents to a passivation solution with a pH between 2-4. However, due to the fast film formation speed of the black passivation film, the arrangement structure of the film layer is relatively loose, and the salt spray resistance is not high. In the unsealed black passivation film layer, white spots generally appear in 36 hours in the neutral salt spray test.
2. Trivalent chromium blue-white passivation. The trivalent chromium blue-white passivation solution is composed of trivalent chromium salts, sulfuric acid, nitric acid, and hydrofluoric acid. In order to improve its corrosion resistance, some also add nickel salts, cobalt salts, rare earth metal salts, and sol-type sealing agents. Since the trivalent chromium blue-white passivation film lacks hexavalent chromium components, and hexavalent chromium has the ability to self-repair the film layer, its corrosion resistance is relatively poor. The general neutral salt spray test can only reach 48-72 hours.
3. Trivalent chromium color passivation. The trivalent chromium color film layer is the same as the blue-white film layer. It forms zinc ions by dissolving zinc, which increases the pH value of the solution. The trivalent chromium directly reacts with zinc, hydroxide ions, etc. to form insoluble compounds that condense on the surface of the zinc layer to form a passivation film. However, their trivalent chromium forms are different, and the colors produced are different. The film thickness of the trivalent chromium color passivation film is 0.25μm-1μm, while the thickness of the trivalent chromium blue-white passivation film is only about 80nm, so the corrosion resistance of the trivalent chromium color passivation film is much greater than that of the blue-white passivation film. The time for the color passivator to produce white rust can reach more than 144h, so we don’t have to worry about the corrosion resistance of the trivalent chromium color passivation film.
The trivalent chromium passivation on the galvanized layer can obtain passivation films with different color appearances and thicknesses according to different process conditions. REST can provide corresponding technical support for the different needs of customers.