Due to the excellent performance of the chrome layer, it is widely used as the outer layer and functional coating of the protective-decorative coating system, and has always played an important role in the electroplating industry. With the development of science and technology and people's increasing attention to environmental protection, on the basis of traditional chrome plating, micro-crack and micro-porous chrome, black chrome, loose chrome, low-concentration chrome, high-efficiency hard chrome, trivalent chrome have been developed successively. New processes such as chrome plating and rare earth chrome plating further expand the application range of chrome plating.
According to the composition and performance of the chrome plating solution, the chrome plating solution can be divided into the following categories.
① Ordinary chrome plating solution is a chrome plating solution that uses sulfate as a catalyst. The plating solution only contains chromic anhydride and sulfuric acid, with simple composition and convenient use. It is the most widely used chrome plating solution at present. The ratio of chromic anhydride and sulfuric acid is generally controlled at 100:1, and the concentration of chromic anhydride varies from 150 to 450 g/L. According to the concentration of chromic anhydride, it can be divided into high concentration (350-500g/L), medium concentration (150-250g/L) and low concentration (50-150g/L) chrome plating solution. The low-concentration chrome plating solution has high current efficiency and high hardness of the chrome layer, but the covering ability is poor. It is mainly used for functional electroplating, such as hard chrome plating and wear-resistant chrome plating. Only a lower voltage is needed, and the covering ability is better than that of the dilute solution, but the current efficiency is lower. It is mainly used for decorative chrome plating and complex chrome plating.
② The composite chrome plating solution uses sulfuric acid and fluosilicic acid as catalysts. The addition of fluorosilicic acid improves the current efficiency, coverage and brightness range of the plating solution compared with ordinary chrome plating solutions. For example, the current efficiency of the cathode can reach more than 20%. However, fluorosilicic acid has a strong corrosive effect on the parts of the anode and cathode parts that cannot be plated with chromium and the lead lining of the plating tank, and certain protective measures must be taken. The lining and anode should be made of lead-tin alloy. This bath is mainly used for barrel chrome plating.
③ Automatically adjust the chrome plating solution using strontium sulfate and potassium fluorosilicate as catalysts. At a certain temperature and a certain concentration of chromic acid solution, strontium sulfate and potassium fluorosilicate have a precipitation-dissolution equilibrium, respectively, and have a solubility product constant Ksp, that is, when the concentration of [SO42-] or [SiF62-] in the solution increases When it is large, the corresponding ion concentration product will be greater than the solubility product constant, and excess SO42- or SiF62- will form SrSO4 or K2SiF6 precipitation and precipitate; on the contrary, when the concentration of [SO42-] or [SiF62-] in the solution is insufficient, the tank will The SrSO4 or K2SiF6 precipitates in the solution dissolve until the corresponding ion concentration product is equal to its solubility product. Therefore, when the temperature of the bath and the concentration of chromic anhydride are constant, the concentration of [SO42-] or [SiF62-] in the bath can be automatically adjusted by the equilibrium of dissolution and precipitation, and does not change with the continuation of the electroplating process.
This type of plating solution has the advantages of high current efficiency (270A), wide range of allowable current density (up to 80-100A/dm2), good dispersion ability and coverage ability of the plating solution, and fast deposition speed, so it is also called "high-speed automatic adjustment of chrome plating". ". But the plating solution is highly corrosive.
④ The rapid chrome plating solution is based on the common chrome plating solution, adding boric acid and magnesium oxide, allowing the use of a higher current density, thereby increasing the deposition rate, and the resulting coating has a small internal stress and good adhesion to the substrate.
⑤ The concentration of chromic anhydride in the tetrachromate chrome plating solution is relatively high. In addition to chromic anhydride and sulfuric acid, the plating solution also contains sodium hydroxide and sodium fluoride to improve the cathodic polarization. Sodium citrate was added to mask iron ions. The main advantages of this type of plating solution are high current efficiency (above 35%), fast deposition rate, and good dispersion ability of the plating solution, but the plating solution is only stable at room temperature, and the operating temperature should not exceed 24 °C. Cool the plating solution; the coating has poor brightness and low hardness, and needs to be polished after plating to meet the requirements of decorative chrome.
⑥ The room temperature chrome 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, so it is suitable for thin-layer electroplating. Its current efficiency and dispersion ability are high, and it can be used for rack and barrel plating.
⑦ The concentration of chromic anhydride in the low-concentration chromic acid chrome plating solution is 5 times lower than that of the standard chrome plating solution, which can reduce the pollution to the environment. The current efficiency and the hardness of the coating are between the standard chrome plating solution and the composite chrome plating solution, and its coverage and corrosion resistance are comparable to those of high-concentration chrome plating. However, the resistance of the plating solution is large, the tank voltage is high, and the requirements for rectifier equipment are strict. At the same time, the covering ability of the plating solution needs to be improved.
⑧ Cr3+ compound is the main salt in the trivalent chromium plating solution, and complexing agent, conductive salt and additives are added. The process eliminates or reduces environmental pollution, the dispersing ability and covering ability of the plating solution are higher than that of the chromic acid plating solution, and the cathode current efficiency is improved; the plating can be performed at room temperature; the tank pressure is low; However, the plating solution is sensitive to impurities, the gloss of the plating layer is dark, and the thickness of the plating layer is several microns, which cannot be arbitrarily increased; the hardness of the plating layer is low, and the composition of the plating solution is complex, which is not conducive to maintenance.
⑨ Rare earth chrome plating solution adds a certain amount of rare earth elements and fluoride ions on the basis of traditional chrome plating solution. The use of rare earth elements can reduce the concentration of chromic anhydride, broaden the plating temperature range (10-50°C) and the cathode current density range (5-30A/dm2), reduce the tank pressure, and improve the brightness and hardness of the coating. To achieve low temperature, low energy consumption, low pollution and high efficiency in chrome production, the so-called "three lows and one high" chrome plating process. However, there are still different views on the stability and reliability of the plating solution, especially the research on its mechanism needs to be in-depth.
