The reason for the tarnishing of Silver is corrosion. Tarnish is a thin layer of corrosion that occurs on copper, brass, iron, magnesium, neodymium and other related metals.
Corrosion is defined as the natural process that causes the transformation of pure metals to undesirable substances when they react with substances like water or air. Tarnish is a thin layer of corrosion that occurs on copper, brass, iron, magnesium, neodymium and other related metals. Silver, for instance, requires hydrogen sulphide to tarnish, but over time it can tarnish with oxygen.
Silver reacts with sulphur and sulphur compounds in the air give silver sulphide (Ag2S) which is black in colour. Exposed silver forms Ag2S as it reacts with the H2S(g) in the atmosphere which is present due to certain industrial process.
2Ag(s) + H2S(g) → Ag2S(s) + H+2+(g)