The role of various components in the chromium plating solution

- Mar 21, 2018 -

1) Chromic anhydride

The aqueous solution of chromic anhydride is chromic acid, the only source of chromium plating. Practice has shown that the concentration of chromic anhydride can be varied within a wide range. For example, when the temperature is in the range of 45-50° C. and the cathode current density is 10 A/dm 2 , the chromic anhydride concentration varies in the range of 50-500 g/L, and even when it is as high as 800 g/L, a bright chromium plating layer can be obtained. However, this does not mean that the concentration of chromic anhydride can be arbitrarily changed. In general production, the concentration of chromic anhydride used is between 150 and 400 g/L. The concentration of chromic anhydride plays a decisive role in the conductivity of the bath. At each temperature, there is a concentration of chromic anhydride corresponding to the highest conductivity; as the temperature of the bath increases, the maximum conductivity increases to a slightly higher value with increasing chromic anhydride concentration. The direction of the move. Therefore, in terms of conductivity alone, it is preferable to use a chromium plating solution with a high concentration of chromic anhydride. However, when high-concentration chromic acid electrolytes are used, the loss caused by the workpieces is severe. On the one hand, it leads to unnecessary consumption of materials, and it also causes certain pollution to the environment. The low concentration bath is more sensitive to impurity metal ions and has poor coverage. An excessively high or low chromic anhydride concentration will narrow the range of the temperature and current density at which bright plating is obtained. The bath containing a low concentration of chromic anhydride has a high current efficiency and is mostly used for hard chrome plating. The thicker plating bath is mainly used for decorative plating. Although the performance of the bath is related to the chromic anhydride content, the most important factor depends on the ratio of chromic anhydride and sulfuric acid.

2) Catalyst

In addition to sulfate, fluorides, fluorosilicates, fluoroborates, and mixtures of these anions are often used as chrome-plated catalysts. When the catalyst content is too low, no coating is obtained or the resulting coating is very little, mainly brown oxides. If the catalyst is excessive, it will result in poor coverage, reduced current efficiency, and may result in partial or no plating. The most widely used catalyst at present is sulfuric acid. The content of sulfuric acid depends on the ratio of chromic anhydride to sulfuric acid, generally controlled at 80 to 100:1, and the optimal value is 100:1. When the sulfate content is too high, the colloidal membrane has a strong dissolution effect, the exposed area of the matrix is large, the true current density is small, the cathodic polarization is small, the resulting coating is not uniform, and sometimes the flower is produced, and in particular the recesses may also expose the substrate. metal. When the above problems occur in production, an appropriate amount of cesium carbonate should be added to the plating solution according to the results of the chemical analysis, and then the resulting barium sulfate precipitate can be removed by filtration. When the sulfate content is too low, the coating is grayish and has poor gloss. Because the content of sulfate ion is too low, only a very small portion of the film on the surface of the cathode is dissolved, that is, the rate of film formation is greater than the rate of dissolution, and the deposition of chromium is blocked or the discharge is grown in a local area, so the resulting coating is rough. At this point, the appropriate amount of sulfuric acid can be added to the plating solution. When the fluorinated anions (F-, SiF62-, BF4-) are used as the catalyst, the concentration thereof is 1.5% to 4% of the chromic anhydride content. The advantage of this type of plating solution is that the cathodic current efficiency of the plating solution is high and the hardness of the plating layer is high. Large, the use of low current density, not only for rack plating, also applies to barrel plating. In China, fluorosilicate ions are used more often than not. It has the function of activating the surface of the coating. When the current is interrupted or secondary chrome plating, a bright coating can still be obtained. It can also be used for chrome plating. H2SiF4 or Na2SiF6 (or K2SiF6) is generally added as the main source of SiF62-. The bath containing SiF2- ions, with increasing temperature, has a wider working range than that of sulfate ions. The disadvantage of this plating solution is its high corrosion to workpieces, anodes and plating baths and high maintenance requirements, so it is not possible to completely replace plating solutions containing sulfate ions. At present, many manufacturers use sulfate ions and SiF62-mixed with better results.

3) Trivalent chromium

The Cr6+ ions in the chromium plating solution are reduced at the cathode to produce Cr3+, which is oxidized at the same time on the anode. The trivalent chromium concentration quickly reaches a balance and the equilibrium concentration depends on the area ratio of cathode and anode. Cr3+ ion is the main component of the cathode-forming colloidal film. Only when the plating solution contains a certain amount of Cr3+, the deposition of chromium can be performed normally. Therefore, the newly formulated bath must take appropriate measures to ensure that it contains a certain amount of Cr3+.

①Use a large area cathode for electrolytic treatment.

②Add reducing agent to reduce Cr6+ to Cr3+, which can be used as reducing agent in alcohol, oxalic acid, rock sugar, etc. Among them, alcohol (98%) is more commonly used, and dosage is 0.5mL/L. When adding alcohol, because the reaction is exothermic, it should be added while stirring, otherwise it will cause splashing of chromic acid. After alcohol is added, electrolysis can be carried out for use.

③Add some old bath solution.

The content of Cr3+ in the ordinary chromium plating solution is about 2~5g/L, and the data is reported as 1%~2% of the chromic acid content. The allowable content of trivalent chromium is related to the type of the bath, the process, and the content of impurities in the bath. . When the Cr3+ concentration is low, it is equivalent to the phenomenon that the sulfate ion content is high. The cathode film is discontinuous and has poor dispersion ability, and the deposition of chromium occurs only at higher current densities. When the Cr3+ concentration is high, it corresponds to insufficient content of sulfate ions, and the cathode film becomes thick, which not only reduces the bath significantly. The conductivity increases the cell voltage and reduces the current density range for bright chrome plating. In severe cases, it can only produce rough, gray coatings. When the content of Cr3+ is high, small-area cathodes and large-area anodes are also used, and the anode current density is maintained at 1 to 1.5 A/dm2 for electrolytic treatment. The processing time depends on the content of Cr3+, from several hours to several days and nights. When the bath temperature is 50 to 60°C, the effect is good.

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