In our daily life we would have seen some of the following things happening like silver spoons in our house get a black coating. Old copper bridges deteriorate and it turns green in colour. We would have also seen the gates of some old houses which are not painted for a long time gets a brown colour substance accumulated over it. Ever wondered the reason behind the change? Talking about it with respect of chemistry it happens due to the phenomenon which is called as corrosion. It is a natural process in which slow degradation of metals takes place. This process happens due to the chemical reactions that occur between the metal and the other substance in the surroundings. It occurs in metals, rubber, plastic, etc., but we generally consider metals because corrosion of metals happen at a high rate.
Silver or silver-plated objects have a bright shiny surface but they lose their shine after sometime because they react with sulphur and its compounds present in the surrounding to form a layer of silver sulphide i.e., black in colour.
Copper, in an open air, reacts with the carbon dioxide to form copper carbonate which is green in colour.
Let us understand the corrosion of iron with the help of an experiment:
Take three test tubes. Fill the first test tube half with the water and dip half of the iron nail in it. Now put cork above in the test tube so that half of the test tube is filled with water and half of it air. In the second test tube, completely immerse the iron nail in the water and pour some oil above the water. So, that there is no air contact with the iron nails. Clear the test tube and put calcium chloride(drying agent) in it so that there is no moisture content present in it. Now, observe the test tubes. You will find that only the iron nail present in the first tube has a brown colour coating i.e., rust. So the necessary condition for the rusting of iron is that it should be exposed to both air and water. Rusting process is generally seen everywhere.
To understand the concept of corrosion and know how it actually happens in the metal, please watch the video.