Plastic plating-Single metal plating-Chromium plating
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Chromium is a silvery white metal with a slight bluish tint. The relative atomic mass of chromium is 52.00, the density of electrolytic chromium is 6.9 ~ 7.1 g/cm3, the melting point is 1890°C, and the hardness is 750-1050HV. The standard electrode potential when chromium is not passivated is-0.74V, which is more negative than iron. However, the surface of chromium is easily passivated in air. The potential after passivation is +1.36V tin alloy and bright nickel layer. Therefore, for steel substrates, chromium coatings are cathodic coatings and can only play a mechanical protection role.
Chrome plating can be divided into decorative chrome plating and functional chrome plating according to the purpose of the coating. Decorative chromium plating is to coat a chromium layer of 0.25 ~ 2μm on a bright bottom layer to improve the decoration of the part. The bottom layer is generally a polished or electrodeposited bright coating, such as copper-tin alloy, bright nickel layer, etc. The chromium coating is widely used on exposed parts such as instruments, instruments, daily hardware, household appliances, aircraft, automobiles, motorcycles, bicycles, etc.
Functional chromium plating includes hard chromium plating, loose pore chromium (porous chromium), black chromium, milky white chromium, etc. The hard chromium layer is mainly used on various measuring cards, gauges, cutting tools and various types of shafts. The surface hardness of the hard chromium layer is used to improve its service life. Loose pore chromium is mainly used on cylinder cavities, piston rings, etc. of internal combustion engines, and uses the lubricating oil sandwiched in its pores to improve the wear resistance of parts. Black chromium is used on parts that require extinction and wear resistance, such as aerospace instruments, optical instruments, photographic equipment, etc. Opal chromium is mainly used on various measuring tools.
At present, the main component of chromium plating solutions used in industrial production is not metallic chromium salts, but chromic acid, and contains a small amount of sulfuric acid and cathode reaction catalysts. Chromium plating layers with different properties can be obtained by changing the operating conditions of the chromium plating solution with the same composition. The current efficiency of these chromium plating solutions is low. Generally only 10% to 25%, and a large amount of electricity is consumed in the hydrogen evolution reaction. In addition, although the chromium plating solution with chromic acid as the main component has a simple composition and good stability, due to the serious harm of hexavalent chromium to the human body and the environment and the large energy consumption, people have been prompted to develop chromium plating solutions with trivalent chromium as the main component and various chromium-replacement coatings.
