Chromium plating classification

Chromium plating classification
Due to its excellent performance, chromium plating has been widely used as an outer layer and functional coating for protective and decorative coating systems, and has always played an important role in the electroplating industry. With the development of science and technology and the increasing emphasis on environmental protection, new processes such as microcrack and microporous chromium, black chromium, loose pore chromium, low concentration chromium plating, high-efficiency hard chromium plating, trivalent chromium plating, rare earth chromium plating have been developed on the basis of traditional chromium plating, further expanding the application range of chromium plating.
According to the composition and performance of the chromium plating solution, it can be divided into the following categories.
① A chromium plating solution that uses sulfate as a catalyst for ordinary chromium plating. The plating solution only contains chromic anhydride and sulfuric acid, with simple composition and convenient use, making it the most widely used chromium plating solution at present. The ratio of chromic anhydride to sulfuric acid is generally controlled at Cr03: H2SO4=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 baths. Traditionally, medium concentration chromium plating solutions of Cr03250g/L and H2SO42.5g/L are referred to as "standard chromium plating solutions", also known as "universal chromium plating solutions", used for decorative and functional chromium plating. Low concentration chromium plating solution has high current efficiency and high hardness of the chromium layer, but poor coverage ability. It is mainly used for functional electroplating, such as hard chromium plating and wear-resistant chromium plating; The high concentration plating solution is stable and has good conductivity. During electrolysis, only a lower voltage is required, and the coverage ability is better than that of a dilute solution. However, the current efficiency is low, and it is mainly used for decorative chromium plating and complex parts chromium plating.
② A composite chromium plating solution using sulfuric acid and fluorosilicic acid as catalysts. The addition of fluorosilicic acid improves the current efficiency, coverage capacity, and brightness range of the plating solution compared to ordinary chromium plating solutions. For example, the current efficiency of the cathode can reach over 20%. However, fluorosilicic acid has a strong corrosive effect on the parts of the anode and cathode that cannot be chrome plated, as well as the lead lining of the plating tank. It is necessary to take certain protective measures, and the lining and anode should preferably be made of lead tin alloy. This plating solution is mainly used for barrel chromium plating.
③ Automatically adjust the chromium plating solution using strontium sulfate and potassium fluorosilicate as catalysts. In chromic acid solution at a certain temperature and concentration, strontium sulfate and potassium fluorosilicate have precipitation dissolution equilibrium respectively, and have a solubility product constant Ksp, that is, when the concentration of [S042 -] or [SiF62 -] in the solution increases, the corresponding ion concentration product will be greater than the solubility product constant, and excessive S042 - or SiF62 - will generate SrS04 or K2SiF6 for precipitation; On the contrary, when the concentration of [S042 -] or [SiF62 -] in the solution is insufficient, SrS04 or K2SiF6 in the tank will precipitate and dissolve until the corresponding ion concentration product is equal to its solubility product. So, when the temperature of the plating solution and the concentration of chromic anhydride are constant, the concentration of [S042-] 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 advantages such as high current efficiency (270A), allowable current density range (up to 80-100A/dm2), good dispersion and coverage ability of the plating solution, and fast deposition speed (50/- m/h), hence it is also known as "high-speed automatic adjustment chromium plating". But the corrosion of the plating solution is strong.
④ Rapid chromium plating solution adds boric acid and magnesium oxide on the basis of ordinary chromium plating solution, allowing for the use of higher current density, thereby improving the deposition speed. The resulting coating has low internal stress and good adhesion to the substrate.
⑤ Tetrachromate chromium plating bath has a high concentration of chromic anhydride. In addition to chromic anhydride and sulfuric acid, the bath also contains sodium hydroxide and sodium fluoride to improve cathodic polarization. Add sugar or alcohol as a reducing agent to stabilize Crs+in the plating solution; Add sodium citrate to mask iron ions. The main advantages of this type of plating solution are high current efficiency (over 35%), fast deposition speed
The dispersion ability of the plating solution is good, but 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 brightness of the coating is poor, and the hardness is low. After plating, it needs to be polished to meet the requirements of decorative chromium.
⑥ The room temperature chromium plating solution consists of chromic anhydride and fluoride (NH4F or NaF), and a small amount of sulfuric acid can also be added. This plating solution has low working temperature (15-25 ℃) and current density (8-12A/dm2), slow deposition speed, and is suitable for thin layer electroplating. Its current efficiency and dispersion ability are high, and can be used for hanging and rolling plating.
⑦ The concentration of chromic anhydride in low concentration chromic acid chromium plating solution is 5 times lower than that in standard chromium plating solution, which can greatly 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 ability and corrosion resistance are comparable to those of high concentration chromium plating. However, the resistance of the plating solution is high, the voltage of the bath is high, and the requirements for rectification equipment are strict. At the same time, the coverage capacity 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 complexing agents, conductive salts, and additives. This process eliminates or reduces environmental pollution, and the dispersion and coverage ability of the plating solution are higher than those of chromic acid plating solution. The cathode current efficiency is improved; Can be coated at room temperature; Low tank pressure; Electroplating is not affected by current interruptions. But the plating solution is sensitive to impurities, and the luster of the coating is relatively dark. The thickness of the coating is a few micrometers, and it cannot be thickened arbitrarily; The hardness of the coating is low, and the composition of the plating solution is complex, which is not conducive to maintenance.
⑨ Rare earth chromium plating solution adds a certain amount of rare earth elements and fluorine ions to 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), reduce tank pressure, and improve the brightness and hardness of the coating. To achieve low temperature, low energy consumption, low pollution, and high efficiency in chromium plating production, known as the "three low and one high" chromium plating process. 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 in-depth.

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