Trivalent chromium passivator generally contains the following components.
(1) trivalent chromium is the main component source of Cr (Ⅲ) passivation film. Trivalent chromium can be selected from chromium chloride, chromium sulfate, chromium nitrate, chromium phosphate, chromium acetate and chromium potassium sulfate, and reduction products of chromic acid and dichromate can also be used.
(2) The oxidant generates zinc ions, which promotes the formation of passive film. The oxidant can be hydrogen peroxide, sulfate, halide, persulfate, tetravalent cerium, etc. In the passivation process of passivator using oxidant, trivalent chromium will be oxidized into hexavalent chromium due to the automatic increase of pH value, which will be mixed into the coating, so that the coating contains hexavalent chromium. Therefore, the film formed by passivating agent containing oxidant is dark in color, and if the passivation film contains Cr, it will not meet the requirements of RoHS regulations.
③ The complexing agent controls the film forming speed and the stability of passivation solution. The complexity is too strong, the film forming speed is slow, and the film layer is thin, so it is even impossible to form a film. Complexing agent includes fluoride, organic carboxylic acid, amide of organic carboxylic acid, polyhydroxycarboxylic acid, etc. In the actual formula, 2 ~ 4 kinds of organic complexing agents are often used to complement each other, stabilize the solution and improve the service life of the passivation solution.
The organic carboxylic acids mentioned in the patent are nothing more than dicarboxylic acids (such as oxalic acid, malonic acid, succinic acid and adipic acid), oxygen-containing carboxylic acids (such as citric acid, tartaric acid, gluconic acid, malic acid and itaconic acid) or polycarboxylic acids.
④ Other metals are mainly used to adjust the appearance color and corrosion resistance, and mixtures of lanthanide rare earth metal salts such as Mn, Sb, M0, Ti, C0, Ni and ce can be used, such as LaReCl3(6H20). Their existence is of great benefit to the formation of passive film, the activation of treatment solution and the improvement of corrosion resistance of passive film. Cobalt salts and nickel salts are the most commonly used.
The change of pH value will not obviously affect the corrosion resistance of cobalt-containing film, but the corrosion resistance of cobalt-containing film is obviously better than that of cobalt-free film. And the corrosion resistance is directly proportional to the cobalt content. Adding cobalt can increase the film thickness and corrosion resistance more effectively than increasing the trivalent chromium content.
⑤ The film-forming accelerant regulates the color of the film. Organic and inorganic anions can be used.
⑥ Hole sealing agent In order to overcome the corrosion resistance of the second generation passivator process and meet the requirements of environmental protection and high corrosion resistance for electroplating of automobile parts. The results show that trivalent chromium passivation process has certain groove marks and no self-healing ability, and nano-water-soluble silicide can be added to improve the corrosion resistance and hardness of the passivation film. It is best to use organosilicate, which can increase the stability of passivation solution and prolong the life of bath solution.
⑦ Wetting agent can improve the uniformity of passivation film. Effective wetting agents generally choose anionic surfactants, such as sodium dodecyl sulfate and sodium dodecyl benzene sulfonate, and also include sulfonates of aliphatic fluorocarbons. ; The second type of wetting agent is a derivative of sulfosuccinate, such as sodium dodecyl sulfosuccinate; The third wetting agent is naphthalene sulfonate, which can increase the color, uniformity and hardness of passive film.