- Sep 28, 2018 -
The trivalent chromium passivation solution has various compositional compositions, and generally contains a film-forming salt (ie, trivalent chromium ion), an oxidizing agent, a complexing agent, and an additive.
1. Film-forming salt. Trivalent chromium is a main component of the passivation film to form zinc chromium oxide. It is insoluble in water in the passivation film and has high strength, which constitutes the skeleton of the passivation film. The trivalent chromium ion is mainly obtained by two methods, one method is directly obtained by dissolving a trivalent chromium salt, the inorganic trivalent chromium salt is chromium sulfate, chromium nitrate, chromium chloride, etc., and the organic trivalent chromium salt has oxalic acid. Chromium, chromium maleate, etc.; another method is prepared by reducing hexavalent chromium, and the organic reducing agent for reducing hexavalent chromium may be a substance containing a reactive hydroxyl group, an aldehyde group or a carboxyl group such as formaldehyde, methanol, etc., inorganic The reducing agent is an alkali metal iodide, sulfur dioxide, and an alkali metal sulfite.
2. complexing agent. The role of the complexing agent is to participate in the formation of a mixture of trivalent chromium complexes to modulate the dynamic stability of the hydrated trivalent chromium ions. Since trivalent chromium exists in the form of a stable hexahydrate in water at room temperature, the rate constant of the substitution reaction of the internal coordination water molecule is small, so it is necessary to try to replace some water molecules of the hydrated ion with other ligands. In order to form a mixed ligand complex with less dynamic kinetics.
3. Oxidizer. The oxidant is a basic component of the passivation solution. Its function is to dissolve the metal zinc, accelerate the film formation speed, increase the thickness of the film layer, and form an oxide isolation layer with zinc and chromium during the passivation reaction. Commonly used such substances are peroxides, nitrates, halides and the like. Current trivalent chromium passivation tends to use nitrate as an oxidant
4. Metal ions. Other metal ions may also be contained in the passivation solution, which may have different effects in different passivation solutions. Some participate in the color formation of the passivation film; some can activate the passivation solution and promote the formation of the passivation film, and act as a catalyst to accelerate the passivation reaction; some can increase the overall hardness of the passivation film and increase the bluntness The corrosion resistance of the film. Such metal ions are iron, cobalt, nickel, molybdenum, manganese and rare earth metals.
5. Other substances. Different passivation fluids may also be added with other substances. For the purpose of making the passivation film more uniform and fine, some surfactants may be added; in order to increase the hardness of the passivation film, some halogen ions may be added; in order to increase the corrosion resistance, some corrosion resistant salts may be added; in order to make the passivation film not Streaks may be added, and a thickener may be added; in order to stabilize the passivation solution, a stabilizer may be added.