Plastic plating-Single metal plating-Nickel plating
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Nickel is a silvery and slightly yellow metal that is ferromagnetic. The relative atomic mass of nickel is 58.7, the density is 8.9 g/cm3, and the standard electrode potential is-0.25V. An extremely thin passivation film can easily form on the surface of nickel in air, so it has extremely high chemical stability. At room temperature, nickel can well prevent erosion by the atmosphere, water, and alkali liquor; it is stable in alkali, salt and organic acids; it dissolves slowly in sulfuric acid and hydrochloric acid, and is easily soluble in dilute nitric acid.
The standard electrode potential of nickel is more positive than that of iron, and the electrode potential after surface passivation is corrected. Therefore, compared with the steel substrate, the nickel coating is a cathodic protective layer and can only mechanically protect the substrate metal. However, generally nickel coatings are porous, so in addition to the direct use of it in some medical devices and battery casings, nickel coatings often form a multi-layer combination system with other metal coatings to be used as the bottom layer or intermediate layer. These multi-layer combination coatings are widely used in daily hardware, light industry, home appliances, machinery and other industries. Currently, the solutions used for nickel plating can be roughly divided into two categories: acidic and alkaline. Acid solution? The value is only 2 to 6, mainly including sulfate-chloride system, chloride system, sulfamate system, fluoroborate system, etc.; alkaline solution mainly refers to pyrophosphate system.
In actual production, so-called Watt plating baths based on sulfate-chloride are most commonly used. It is characterized by stable plating solution, high current efficiency, and moderate coating stress. By adding appropriate additives, a mirror-bright coating can be obtained within a wide range of current density. The surface is then coated with a layer of corrosion-resistant surface layer such as imitation gold coating, gold coating, silver coating or chromium coating, etc., which can achieve the ideal decorative effect. Aminosulfonic acid system is mainly used for thick nickel plating and electroforming due to low coating stress.
When developing high-performance nickel plating brighteners, people found that the sulfur content in the nickel coating has a great impact on the potential of the coating. When the coating contains a certain amount of sulfur, its potential can differ by more than 100mV compared with the sulfur-free coating. Experiments also found that when the potential difference reaches more than 125mV, in primary batteries composed of sulfur-free nickel and sulfur-containing nickel, sulfur-containing nickel will serve as the anode and corrode first. Therefore, for products with high protective requirements such as automobiles, motorcycles, bicycles, etc., people use a multilayer nickel protective decorative plating system consisting of sulfur-free semi-bright nickel or dark nickel, high-sulfur nickel with a sulfur content of 0.3%-0.5% and bright nickel with a sulfur content of about 0.1%.
