999 silver refers to the silver content of the purest silver jewelry (we all know that the world we live in is realistic, there can be no absolute 100%, and the purest silicon in the world is 99.999%), while 999 silver is softer than other metals.
925 silver refers to silver products with a silver content of 92.5%. Other metals are added to improve the hardness and wear resistance, so that 925 silver can be used as some fine silver ornaments. 925 is the international standard silver for making silver ornaments.
Compared with 999, 925 silver is not easy to be oxidized, because 925 contains less silver than 999, and of course less silver participates in the oxidation reaction, so there are relatively few cases of "black out", so 925 is not easy to be oxidized in this respect.
The so-called 999 silver is not easy to be oxidized, that is, there will be a trace of water vapor on the metal surface in a humid environment. If your metal is impure (for example, 925) and there are a few other metals in it, under such conditions, a primary battery will be formed. (that is, the structure of the so-called Fu Nan battery). Silver loses electrons and oxidizes more easily than silver exposed to air. This is called galvanic oxidation. In contrast, 999 silver is pure and has no other metals, so it is certainly not easy to be oxidized.