What is the cause of yellow spot on high pressure vacuum switch tube

The reason for the yellow spot of the high pressure vacuum switch tube is that the silver coating is not enough for the nickel coating, and the oxidation occurs. The specific reasons are as follows: when there is only one layer of nickel on the surface of high-pressure vacuum switch tube, the nickel layer will not be oxidized after baking exhaust, so there will be no yellow spot phenomenon. When the metal surface is covered with double layer of nickel and silver layer, when the thickness of silver layer is thin, the nickel surface is exposed, in this case, the nickel layer is easy to be oxidized, and yellow spot phenomenon appears, which is common with silver and nickel When nickel loses electrons easily, the reason why silver is protected is related.