Corrosion is the phenomenon due to which te metals start to get converted into unwanted compounds due to the action of the atmospheric substances like air,moisture, no2, so2 etc.
rusting of iron- Brown. iron oxide.
corrosion of silver- black coloured coating.
there one more metal called pittal in Punjabi and Peetal in Hindi. it's corrosion causes deposition of green coloured layer in it. Sorry but I don't know the name of this metal in english.
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1.Rusting/corrosion of iron occurs due to presence of oxygen and hydrogen/water moisture and forms a brown coloured  layer of ferrous oxide.
2. Silver reacts with sulphur from the air to form a coating of black silversulphide.
3.copper reacts with moist carbondiaoxide and forms a green layer of copper carbonate.