After silver plating, it is found that the coating is easy to change color, which is due to the corrosion and discoloration of the silver coating surface when encountering harmful gases such as H2S, SO2, C02, Cl2, etc. in the natural atmospheric environment;The discoloration is more obvious in the environment of high temperature, humidity and the presence of sulfur.
Discoloration treatment method:
① Chemical passivation
Dichromate solution is generally used to form silver oxide and silver chromate films on the surface.Chromate passivation has the advantages of low cost, simple operation and convenient maintenance, but the anti change ability of this film is poor.
② Electrochemical passivation
Electrochemical passivation is to immerse the silver plated parts in dichromate solution and conduct electrochemical passivation at the cathode to form a passive film on the surface of the coating.Its anti discoloration ability is better than chemical passivation, and almost does not change the appearance color and weldability.
③ Pre nickel plating
Pre nickel plating before silver plating can prevent the direct contact between the base metal and silver, separate some sulfur and oxygen in the base metal from the silver layer, and prevent the possibility of oxidation of the silver coating from the inside.
④ Pulse plating is adopted.
In silver plating, if pulse current plating is used, it is also a method to prevent silver discoloration.Because intermittent power supply is adopted in pulse electroplating, the cathode potential changes periodically with time, which can improve the cathode current efficiency, reduce pinholes, increase the density of silver layer, and prevent silver discoloration.