There is no essential difference between gold passivation and rainbow passivation except for a slight difference in appearance.
Passivation solution is composed of chromic acid, sulfuric acid, nitric acid and other three basic components, but the proportion is slightly different, and there is no difference in the composition and corrosion resistance of passivation film, so it belongs to rainbow passivation.
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Function and principle of zinc plating
Galvanized coating is an anode coating, which is mainly used to prevent the corrosion of steel, and its protective performance is closely related to the coating thickness. After passivation treatment, dyeing or coating with photo-protective agent, the protective and decorative properties of zinc coating can be significantly improved.
In the plating bath containing zinc plating solution, the cleaned and specially pretreated workpiece is used as the cathode, the anode is made of electroplated metal, and the two poles are respectively connected with the positive electrode and the negative electrode of DC power supply. The zinc plating solution consists of an aqueous solution containing electroplating metal compounds, conductive salts, buffers, pH regulators and additives.
After electrifying, the metal ions in the zinc plating solution move to the cathode under the action of potential difference, forming a coating. The metal of the anode forms metal ions in the zinc plating solution to maintain the concentration of electroplating metal ions. In some cases, such as chromium plating, it is an insoluble anode made of lead and lead-antimony alloy, which only plays the role of transferring electrons and conducting current.
The concentration of chromium ions in the electrolyte is maintained by adding chromium compounds to the plating solution regularly. During zinc plating, the quality of anode material, composition of zinc plating solution, temperature, current density, electrifying time, stirring intensity, precipitated impurities, power waveform, etc. Will affect the quality of the coating and need to be controlled in time.
Causes of color difference of zinc plating
In the formula of color passivation, the passivation solution is always acidic. In acidic medium, the zinc layer will react with it. The main reaction here is the redox reaction between zinc coating and chromic acid in passivation solution.
Zinc is used as reducing agent to reduce chromic acid as oxidant to trivalent chromium. Passivation film is actually a compound of trivalent chromium and hexavalent chromium after redox reaction with zinc. Among them, the compounds of trivalent chromium and zinc are blue-green, and the compounds of hexavalent chromium and zinc are ochre red or brownish yellow. Due to the combination and mutual interference of different pigments, the zinc passivation film is rich in color and has rainbow-like beauty.
Trivalent chromium compounds are generally insoluble in water and have high strength, which plays a skeleton role in passivation films. Hexavalent chromium compounds are easily soluble in water, especially hot water, and the film layer is not firm before drying. It is attached to the skeleton of trivalent chromium compound and fills the space, so it can be regarded as "meat" vividly. With the support of meat and skeleton, the passive membrane can be full.
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