Chrome plated classification

- Mar 19, 2018 -

Due to its excellent performance, the chrome plated layer is widely used as a protective outer coating and functional coating for a decorative plating system, and has always occupied an important position 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 plated, micro-cracks and micro-pores of chromium, black chrome, loose-hole chromium, low-concentration chrome plating, high-efficiency hard chrome plated, and tri-valent prices have been successively developed. Chromium chromium plated, rare earth chromium plated and other new technologies have further expanded the application of the chrome plated layer.

According to the composition and performance of the chrome plated solution, the chrome plated solution can be divided into the following categories.

① Ordinary chromium plating solution Chrome plated solution with sulfate as catalyst.. The plating solution contains only chromic anhydride and sulfuric acid, and has a simple composition and is easy to use. It is currently the most widely used chromium plating solution. 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 difference of chromic anhydride concentration, it can be divided into high concentration (350 ~ 500g/L), medium concentration (150 ~ 250g/L) and low concentration (50 ~ 150g/L) chromium plating solution. The low concentration of chromium plating solution has high current efficiency and high chromium layer hardness, but its coverage is poor. It is mainly used for functional electroplating, such as hard chromium plating, wear-resistant chromium, etc.; high concentration of plating solution is stable, good conductivity, electrolysis When only a lower voltage is needed, the coverage ability is better than that of a dilute solution, but the current efficiency is low, and it is mainly used for decorative chrome plated and complex parts chrome plated.

② Composite Chrome Plated Solution Chrome plated solution with sulfuric acid and fluorosilicic acid as catalyst. The addition of fluorosilicic acid improves the current efficiency, coverage and brightening range of the bath compared to ordinary chrome plated baths. For example, the cathode current efficiency can reach more than 20%. However, fluorosilicic acid has a strong corrosion effect on the parts where the anode and cathode parts cannot be plated with chromium and the lead lining of the plating tank. Therefore, certain protective measures must be taken. The lining and the anode are preferably made of lead-tin alloy. This bath is mainly used for chrome plated.

③Automatic adjustment of the chromium plating solution The chromium plating solution using barium sulfate and potassium fluorosilicate as the catalyst. At a certain temperature and a certain concentration of chromic acid solution, barium sulfate and potassium fluorosilicate each have a precipitation and dissolution equilibrium, and each has a solubility product constant Ksp, that is, when the concentration of [SO42-] or [SiF62-] is increased in the solution. When large, the product of the corresponding ion concentration will be greater than the solubility product constant. Excess SO42- or SiF62- will precipitate SrSO4 or K2SiF6 and precipitate; on the contrary, when the concentration of [SO42-] or [SiF62-] is insufficient in the solution, the bath The SrSO4 or K2SiF6 precipitates are dissolved until the product of the corresponding ion concentration equals 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 dissolution and precipitation balance, and does not change as the plating process continues.

These baths have high current efficiency (270A), allow for a wide range of current density (up to 80-100 A/dm2), good dispersion and coverage of the bath, and high deposition speed, and are also called "high-speed automatic adjustment of chrome plating. ". However, the plating solution is highly corrosive.

④Quick Chrome Plating Solution Adding boric acid and magnesium oxide on the basis of the common chromium plating solution allows the use of higher current density, thereby increasing the deposition rate, resulting in a low internal stress of the resulting coating and good adhesion to the substrate.

 Four chromate chromium plating bath of chromic anhydride such higher concentrations in addition to the bath containing sulfuric acid and chromic anhydride, also containing sodium hydroxide and sodium fluoride, in order to improve cathode polarization. Sodium citrate was added to mask the iron ions. The main advantages of this type of bath are high current efficiency (above 35%), fast deposition.

The dispersion of the bath is good, but the bath is only stable at room temperature, the operating temperature should not exceed 24 °C, when the use of high current density cooling bath; plating brightness is poor, low hardness, after plating to be polished to meet the decorative Chrome requirements.

⑥ At room temperature, the chrome plating bath consists of chromic anhydride and fluoride (NH4F or NaF). A small amount of sulfuric acid can also be added. The plating bath has a low operating temperature (15 to 25°C) and current density (8 to 12 A/dm2) and a slow deposition rate and is suitable for thin layer plating. Its high current efficiency and dispersion capability can be used for rack plating and barrel plating.

⑦ The concentration of chromic anhydride in low-concentration chromate plating bath is 5 times lower than that of standard chromium plating bath, which can greatly reduce the environmental pollution. The current efficiency and the hardness of the coating are between the standard chrome plating solution and the composite chrome plating solution, and its covering ability and corrosion resistance are equivalent to high-concentration chrome plating. However, the resistance of the bath is large, the voltage of the bath is high, and the rectification equipment is required to be strict. At the same time, the coverage of the bath needs to be improved.

⑧ Cr3+ compound is the main salt in the trivalent chromium plating bath, adding complexing agent, conductive salt and additives. The process eliminates or reduces environmental pollution, the plating solution has higher dispersibility and covering ability than the chromic acid plating solution, and the cathode current efficiency is improved; it can be plated at room temperature; the groove pressure is low; plating is not affected by current interruption. However, the plating solution is sensitive to impurities, the gloss of the plating layer is dark, the thickness of the plating layer is several micrometers, and it cannot be arbitrarily thickened; the hardness of the plating layer is low, and the composition of the plating solution is complicated and unfavorable for maintenance.

⑨Rare earth chromium plating solution in the traditional chromium plating solution based on a certain amount of rare earth elements and fluoride ions. The use of rare earth elements can reduce the concentration of chromic anhydride, broaden the plating temperature range (10 to 50°C) and cathodic current density range (5 to 30A/dm2), reduce the cell pressure, and improve the brightness and hardness of the coating. The chrome plating production achieves low temperature, low energy consumption, low pollution and high efficiency, which is the so-called "three low one high" chrome plating process. However, there are still different views on the stability and reliability of the bath, especially the study of its mechanism remains to be in-depth.

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