1. The role of each component in the bath
1) Chromic anhydride
The aqueous solution of chromic anhydride is chromic acid, which is the sole source of chrome plating. Practice has shown that the concentration of chromic anhydride can vary over a wide range. For example, when the temperature is 45~50°C, the cathode current density is l0A/dm2, the chromic anhydride concentration varies from 50 to 500g/L, and even up to 800g/L, the bright chrome plating layer can be obtained. However, this does not mean that the concentration of chromic anhydride can be changed at will. The concentration of chromic anhydride used in general production is between l50 and 400 g/L. The concentration of chromic anhydride plays a decisive role in the conductivity of the bath. Figure 4-19 shows the relationship between the concentration of chromic anhydride and the conductivity of the bath. It can be seen that at each temperature, there is a concentration of chromic anhydride corresponding to the highest conductivity; the temperature of the plating solution is increased, and the maximum conductivity is shifted to a slightly higher direction as the concentration of chromic anhydride increases. Therefore, in terms of conductivity, it is preferable to use a chrome plating solution having a higher concentration of chromic anhydride.
However, when a high-concentration chromic acid electrolyte is used, the loss due to the workpiece is severe, which causes unnecessary consumption of materials and causes certain pollution to the environment. The low concentration plating solution is sensitive to impurity metal ions and has poor coverage.
Too high or too low an anhydride concentration will narrow the range of temperature and current density at which the bright coating is obtained. The plating solution with a low concentration of chromic anhydride has high current efficiency and is mostly used for hard chrome plating. The more concentrated plating solution is mainly used for decorative plating. Although the performance of the plating solution is related to the chromic anhydride content, the most important depends on the ratio of chromic anhydride to sulfuric acid.
In addition to sulfate, fluorides, fluorosilicates, fluoroborates and mixtures of these anions are often used as catalysts for chrome plating. When the catalyst content is too low, the plating layer is not obtained or the plating layer obtained is small, mainly brown oxide. If the catalyst is excessive, it may cause poor coverage, current efficiency, and may result in partial or total no plating. The currently widely used catalyst is sulfuric acid.
The content of sulfuric acid depends on the ratio of chromic anhydride to sulfuric acid, generally controlled at Cr03: So4 = (80~100): 1, and the optimum value is 100:1. When the content of So42- is too high, the dissolution of the colloidal film is strong, the exposed area of the substrate is large, the real current density is small, the cathode polarization is small, the obtained coating layer is uneven, and sometimes the flower is formed, especially the concave portion may expose the substrate. When the above problems occur in production, according to the results of chemical analysis, an appropriate amount of strontium carbonate should be added to the plating solution, and then the precipitate of barium sulfate formed can be removed by filtration. When the So42- content is too low, the coating is grayish and rough, and the gloss is poor. Since the So42- content is too low, only a 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, the precipitation of chromium is hindered or the discharge grows in a local area, so the resulting coating is rough. At this time, an appropriate amount of sulfuric acid may be added to the plating solution.
When a fluorine-containing anion (F-, SiF62-, BF4-) is used as a catalyst, the concentration is 1.5% to 4% of the chromic anhydride content. The advantages of such a plating solution are: high cathode current efficiency of the plating solution, hardness of the plating layer Large, low current density, not only for hanging plating, but also for barrel plating.
Fluorine silicate ions are used in China, which has the function of activating the surface of the coating. When the current is interrupted or the secondary chrome is chromed, the bright coating can still be obtained, and the chrome plating can also be used. H2SiF4 or Na2SiF6 (or K2SiF6) is generally added as the main source of SiF62-. The plating solution containing siF2 ion has a wider working range than the So42-ion plating solution as the temperature increases. The disadvantage of the plating solution is that it is highly corrosive to the workpiece, the anode, and the plating tank, and has high maintenance requirements, so it is impossible to completely replace the plating solution containing So42-. At present, many manufacturers use So42- and siF62- in combination, and the effect is better.
3) Trivalent chromium
In the chrome plating solution, Cr6+ ions are reduced at the cathode to produce Cr3+, and at the same time, they are reoxidized on the anode, and the trivalent chromium concentration is quickly balanced. The equilibrium concentration depends on the ratio of the anode to the anode. Cr3+ ions are the main component of the cathode to form a colloidal film. Only when a certain amount of Cr3+ is contained in the plating solution can chromium deposition proceed normally. Therefore, the newly prepared plating solution must take appropriate measures to ensure that a certain amount of Cr3+ is contained.
A. Electrolytic treatment is carried out using a large-area cathode.
B. Adding a reducing agent to reduce Cr6+ to Cr3+, which can be used as a reducing agent, such
as alcohol, oxalic acid, rock sugar, etc., among which alcohol (98%) is commonly used, and the dosage is 0.5 mL/L. When adding alcohol, since the reaction is exothermic, it should be added with stirring, otherwise the chromic acid will be splashed. After adding alcohol, it can be put into use after a little electrolysis.
C. Add some old bath.
The content of Cr3+ in ordinary chrome plating solution is about 2~5g/L. It is also reported that it is 1%~2% of chromic acid content. The allowable content of trivalent chromium is related to the type of plating solution, the process and the content of impurities in the plating solution. . When the Cr3+ concentration is low, it is equivalent to the phenomenon that the content of So42- is high. The cathode film is discontinuous, the dispersing ability is poor, and chromium deposition occurs only at a higher current density; when the cr3+ concentration is high, the content of So42- is insufficient, and the cathode film is thickened, which not only significantly reduces the plating solution. Conductivity increases the cell voltage and reduces the range of current densities for bright chrome plating. In severe cases, only coarse, gray coatings can be produced.
When the content of Cr3+ is too high, the cathode current and the large-area anode are also used to maintain the anode current density of 1~1.5A/dm2 for electrolysis treatment. The treatment time depends on the content of Cr3+, from several hours to several days and nights. When the bath temperature is 50~60 °C, the effect is better.