This happens because of corrosion of copper , as the copper is exposed to air it reacts slowly with water from the moisture present in air, carbondioxide and oxygen. And the product formed is a mixture of copper hydroxide and copper carbonate as shown below.
Lets look from a different angle corrosion is basically oxidation and in oxidation the electrons are lost. Here we have zinc coated on iron being more reactive the zinc will tend to loose the electrons even if the iron is exposed, even then being good conductor the electrons are given to the iron by theContinue reading “The galvanised iron lost its zinc coating even after it doesn’t corrode. Why does this happen?”