Plating bright tin on hardware shrapnel can improve the cathodic polarization and dispersion ability, make the coating crystal fine and shiny, and can mask metal impurities. Inorganic brighteners include nickel sulfate or diamond salt. It can make the shrapnel have better weldability and corrosion resistance. But discoloration occurs after bright tin plating, how should we solve it?

The metal shrapnel changes color after bright tin plating. We can use 10% phosphoric acid solution to remove the film before the hardware shrapnel is tinned and dried, and then wash it several times with hot and cold water.

Tin-plated shrapnel cannot be washed with anti-current, which will make the tin-plated surface prone to water mist. We can first pour the shrapnel into the basin, immediately pour water and stir, pour out the water, and repeat three times. Finally, soak the shrapnel in sodium phosphate dilution solution at about 50°C and then wash it three times, and pre-plate it on the copper substrate.

Neutralization and passivation should be carried out after electroplating to neutralize residual acid and form a protective film on the tin surface. During electroplating in the electroplating factory, the plated metal or other insoluble materials are used as the anode, and the shrapnel to be plated is used as the cathode. The cations of the plated metal are reduced on the surface of the workpiece to be plated to form a coating. In order to eliminate the interference of other cations and make the coating uniform and firm, it is necessary to use a solution containing metal cations as the electroplating solution to keep the concentration of metal cations in the coating unchanged. The neutralizer generally uses weak alkaline phosphate. After neutralization, it is washed into the passivation process. The passivation solution is dichromate at 6080°C, and after passivation for 2 minutes, an oxide film is formed on the tin surface.

To prevent the tin-plated layer from turning yellow, pay attention to poor cleaning after electroplating, excessive current density, and excessive bath temperature. The passivation of the coating is effective for discoloration, but it must be cleaned and dipped in stearic acid solution to form a protective film on the surface of the metal shrapnel.