Due to its excellent performance, chrome plating has always played an important role in the electroplating industry as an outer layer and functional coating for protective and decorative coating systems. With the development of science and technology and people's increasing attention to environmental protection, new processes such as microcrack and microporous chromium, black chromium, loose hole chromium, low concentration chromium plating, high-efficiency hard chromium plating, trivalent chromium plating, rare earth chromium plating, etc. have been developed on the basis of traditional chromium plating, further expanding the application scope of chromium plating.
According to the composition and properties of chromium plating solution, it can be divided into the following categories.
① Ordinary chrome plating solution is a chrome plating solution catalyzed by sulfate ions. The plating solution contains only chromic anhydride and sulfuric acid, with simple composition and easy use, making it a widely used chromium plating solution at present. The ratio of chromic anhydride to sulfuric acid is generally controlled at 100:1, and the concentration of chromic anhydride varies between 150-450g/L. According to the different concentrations of chromic anhydride, it can be divided into high concentration (350-500g/L), medium concentration (150-250g/L), and low concentration (50-150g/L) chromium plating solutions. Low concentration chromium plating solution has high current efficiency and high hardness of the chromium layer, but its coverage ability is poor. It is mainly used for functional electroplating, such as hard chromium plating and wear-resistant chromium plating; High concentration plating solution is stable, has good conductivity, only requires lower voltage during electrolysis, has better coverage ability than dilute solutions, but has lower current efficiency. It is mainly used for decorative chrome plating and chrome plating of complex parts.
② Composite chromium plating solution is a chromium plating solution catalyzed by sulfuric acid and fluorosilicic acid. The addition of fluorosilicic acid improves the current efficiency, coverage ability, and brightness range of the plating solution compared to ordinary chrome plating solutions, such as achieving a cathode current efficiency of over 20%. However, fluorosilicic acid has a strong corrosive effect on the parts of the anode and cathode that cannot be plated with chromium, as well as the lead lining of the plating tank. Certain protective measures must be taken, and the lining and anode should be made of lead tin alloy. This plating solution is mainly used for rolling chrome plating.
③ Automatic adjustment of chrome plating solution using strontium sulfate and potassium fluorosilicate as catalysts. In a chromic acid solution at a certain temperature and concentration, strontium sulfate and potassium fluorosilicate each have a precipitation dissolution equilibrium and a solubility product constant Ksp. That is, when the concentration of [SO42-] or [SiF62-] in the solution increases, the corresponding ion concentration product will be greater than the solubility product constant. Excessive SO42- or SiF62- will generate SrSO4 or K2SiF6 precipitation and precipitate; On the contrary, when the concentration of [SO42-] or [SiF62-] in the solution is insufficient, the precipitation of SrSO4 or K2SiF6 in the tank dissolves until the corresponding ion concentration product equals its solubility product. So, when the temperature of the plating solution and the concentration of chromic anhydride are constant, the concentration of [SO42-] or [SiF62-] in the plating solution can be automatically adjusted through dissolution precipitation equilibrium, and does not change with the continuous electroplating process.
This type of plating solution has the advantages of high current efficiency (270A), allowing a wide range of current densities (up to 80-100A/dm2), good dispersion and coverage ability of the plating solution, and fast deposition speed. Therefore, it is also known as "high-speed automatic adjustment chrome plating". But the corrosiveness of the plating solution is strong.
④ The rapid chrome plating solution, based on ordinary chrome plating solution, adds boric acid and magnesium oxide, allowing for the use of higher current density, thereby increasing the deposition rate. The resulting coating has low internal stress and good adhesion with the substrate.
⑤ Tetrachromate chromium plating solution has a high concentration of chromic anhydride. In addition to chromic anhydride and sulfuric acid, the plating solution also contains sodium hydroxide and sodium fluoride to enhance cathodic polarization. Add sodium citrate to mask iron ions. The main advantages of this type of plating solution are high current efficiency (over 35%), fast deposition rate, and good dispersion ability of the plating solution. However, the plating solution is only stable at room temperature, and the operating temperature should not exceed 24 ℃. When using high current density, the plating solution needs to be cooled; The coating has poor brightness and low hardness, and needs to be polished after plating to meet the requirements of decorative chromium.
⑥ The room temperature chromium plating solution is composed of chromic anhydride and fluoride (NH4F or NaF), and a small amount of sulfuric acid can also be added. The working temperature (15-25 ℃) and current density (8-12A/dm2) of this plating solution are low, and the deposition rate is slow, making it suitable for thin-layer electroplating. Its current efficiency and dispersion ability are high, and it can be used for hanging plating and rolling plating.
⑦ The concentration of chromic anhydride in low concentration chromium plating solution is 5 times lower than that in standard chromium plating solution, which can reduce environmental pollution. The current efficiency and hardness of the coating are between standard chromium plating solution and composite chromium plating solution, and their coverage and corrosion resistance are comparable to high concentration chromium plating. However, the resistance of the plating solution is high, the slot voltage is high, and strict requirements are placed on the rectification equipment. At the same time, the coverage ability of the plating solution needs to be improved.
⑧ The trivalent chromium plating solution is mainly composed of Cr3+compounds as the main salt, with the addition of chelating agents, conductive salts, and additives. This process eliminates or reduces environmental pollution, and the dispersion and coverage of the plating solution are higher than those of chromic acid plating solution, resulting in improved cathode current efficiency; Can be plated at room temperature; Trough depression; Electroplating is not affected by current interruption. However, the plating solution is sensitive to impurities, and the gloss of the coating is relatively dark. The thickness of the coating is a few microns and cannot be arbitrarily thickened; The low hardness of the coating and the complex composition of the plating solution are not conducive to maintenance.
⑨ Rare earth chromium plating solution adds a certain amount of rare earth elements and fluoride ions on the basis of traditional chromium plating solution. The use of rare earth elements can reduce the concentration of chromic anhydride, broaden the coating temperature range (10-50 ℃) and cathode current density range (5-30A/dm2), lower the groove pressure, and improve the brightness and hardness of the coating. The so-called "three lows and one high" chrome plating process aims to achieve low temperature, low energy consumption, low pollution, and high efficiency in chrome plating production. However, there are still different opinions on the stability and reliability of the plating solution, especially the research on its mechanism still needs to be further explored.