Experience summed up from a yellowing failure of trivalent chromium plating

Trivalent chromium sulfateChromium plating has the advantages of high corrosion resistance and close to the color of the hexavalent chromium plating. It is used instead of hexavalent chromium Chrome chrome plating has been widely used.

Because the domestic development and use of sulfate trivalent chromium plating is relatively short, the independent research and development process is not mature enough, and there are relatively many problems encountered in production. In order to improve the skills of trivalent chromium plating process maintenance, a chromium plating failure was analyzed and summarized.

01 Fault description and troubleshooting

The Trich-955l sulfate trivalent chromium plating process developed by the author was used in a Shenzhen electroplating plant to electroplate Samsung’s mobile phone accessories. After opening the tank, the plating solution and coating The performance meets the requirements of industry standards and has been well received by customers. The process formula and operating conditions are: Trich-9551M opener agent 10mL/L, Trich.9551B supplement 280mL/L, Trich-9551CS conductive salt 300g/L, where P(Cr3)=12～l8L, P(H3BO3) =65～75g/L, pH: 3.0～3.8, temperature 48～55℃, cathode current density 8～15A/dm2.

However, when the plating solution was used for the 3rd month, the quality of the plating appeared to be problematic. After the plated parts are dried, the appearance of the coating is checked to be qualified, and the package is sent to the customer. After the customer inspects the plated parts, the assembly process is entered. After the assembly, the chrome layer is slightly yellowed when the product is inspected. If qualified, return this batch of plated parts to the electroplating factory for re-plating.

The inspector of the electroplating plant inspected the plated parts again, and found that after touching the plated parts with hands or rubbing them with gloves, the plating layer would turn slightly yellow or darken in color. So the company sent someone to the electroplating plant to troubleshoot the cause of this failure. After chemical analysis of the plating solution, it was found that the content of trivalent chromium and boric acid in the plating solution was normal. Atomic absorption spectrometry did not detect copper and zinc impurities in the plating solution. Iron and nickel impurities were 13mg/L and 4mg/L, which are far below the allowable content. Hexavalent chromium was not detected by visible light spectrophotometry.

Adding Trich-9551A additive, Trich-9551CA complexing agent and Trich.9551CS conductive salt to the plating solution to adjust the plating solution can not eliminate the yellowing phenomenon of the plating layer. The yellowing phenomenon of the plating layer was reduced by treating the plating solution with a small current electrolysis method for 12 hours.

The chrome-plated layer was observed under a microscope with a magnification of 300 times, and it was found that there were fine granular yellow spots on the plating layer. After rubbing the coating with a soft cloth once and then observing it under a microscope, the coating spots turned into striped shadows. Wipe the coating back and forth several times with a soft cloth and observe again, the color of the coating becomes slightly darker. It can be seen that the chrome-plated layer adsorbs colloidal granular substances that are invisible to the naked eye. The plated parts look white after being dried, but when touched by hand, these granular colloidal substances are squeezed onto the entire coating. , The coating appears yellowing.

02 Analysis of the cause of failure

The trivalent chromium salt, complexing agent, boric acid and conductive salt of Trich-9551 trivalent chromium plating solution are all easily soluble substances in the plating solution and will not form colloidal particles suspended in the plating tank. The Trich-955lA additive is composed of several intermediates. One of the intermediates is difficult to dissolve in water. Alkaline substances are needed to dissolve it when formulating additives.

When this additive intermediate reaches a certain concentration in the plating solution, it will be supersaturated, forming colloidal particles suspended in the plating tank, and these colloidal particles will be adsorbed on the plating layer during electroplating, and the above-mentioned failure will occur. Electrolytic treatment of the plating solution can consume a part of the suspended solids, but it cannot effectively reduce the content of this intermediate in the plating solution.

03 Processing method

① Activated carbon adsorption

After investigating the cause of the fault, the plating solution is treated by the activated carbon adsorption method. Activated carbon can not only absorb the colloidal suspension in the plating solution, but also can effectively reduce the concentration of this additive intermediate in the plating solution. Add 1.4 kg of activated carbon powder to 700L plating solution, stir the plating solution, keep the plating tank at 50°C for 4 hours, filter the plating solution, and clean the filter element until the plating solution is clear.

② Trial plating

Use low-current electrolytic plating solution for 120min, test and adjust the pH of the plating solution to about 3.6, adjust the plating bath temperature to 52℃, test plating, check the quality of the plating, and find that the yellowing phenomenon of the plating disappears, and the fault of the plating solution is eliminated.

③ Improve additive formula

The oversaturation of an intermediate in the additive in the plating solution is the root cause of this failure. Excessive addition of Trich-955lA additive in electroplating or high content of this intermediate in the additive formulation will cause this failure.

The intermediate body in the Trich-9551A additive formula was reduced to half of the original concentration, and the Hull cell test was carried out continuously for 20 hours, and the bath performance remained good. On this basis, the Trich-955lA additive formula was improved. After the product was sent to the electroplating plant, no similar failure occurred.

It can be seen that this failure was caused by the high content of the insoluble intermediate body in the Trich-9551A additive.

04 Conclusion

The development process of a new process generally involves three stages: small test, pilot test and production. After the small-scale and pilot-scale process technology, it is generally not possible to find all the problems in the process, and some deep-seated problems can only be discovered through long-term production.

The enlightenment of this trivalent chromium plating failure is: After completing the development of new products and new technologies, R&D personnel must follow up the application of mass production, and discover, study and solve existing problems in production , Make product performance and technical level reach a new height.