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Equation for Rusting of Iron

You must have seen rusted water pipeline on roadsides. Do you observe green color copper pipes or red color iron pipes? The silvery color of iron turns to reddish-brown due to formation of hydrated oxide which is known as rust and the process is called as rusting. Rusting takes place in contact with water, oxygen and other atmospheric impurities. 

Rusting of iron involve a series of reaction. When metal surface comes in contact with water, the dissolved oxygen reacts with metal like iron causing the formation of iron oxides. The metal surface acts as a electrolytic cell in which water drop acts as cathode and metal surface acts as anode. Oxidation at anode produce metal ion , which dissolve in water to form metal oxide There are several processes which prevent the corrosion or minimise the rate of reaction. 

Anodization, plating, painting of anti-corrosive substances or coating of corrosion inhibitors used to prevent corrosion on metal surface. Let’s discuss the process in brief. If we left a piece of bare iron outside where it is exposed to moisture, it will rust very quickly. The rate of corrosion increases in salt water. It is an electro chemical process in which a water droplet severs as a voltaic cell when it comes in contact with the metal and oxidizing it. 

Just imagine a water droplet on the metal surface. In this process, metal supplies electrons at the edge of the droplet and reduces the atmospheric oxygen. The inner side of the water droplet serves as the anode in this process. The reaction of oxidation of metal atoms can be written as given below;
Fe(s) Fe2+(aq) + 2e-

Now these electrons move from metallic iron to the outside of the droplet and react with oxygen and water to form hydroxide ions;

O2(g) + 2H2O(l) + 4e- 4OH-(aq)

These hydroxide ions move inward the metal surface and again react with metal ions to form iron (II) hydroxide which is precipitated on the metal surface.
Fe2+(aq) + 2OH-(aq) Fe(OH)2(s)

Overall the formation of rusting is a redox reaction and involves the conversion of metal to metal oxide. The complete equation for rusting of iron is as given below;
4Fe(OH)2(s) + O2(g)   2Fe2O3 • H2O(s) + 2H2O(l)

Since it is an electrochemical process, therefore the unprotected iron can easily rust in the presence of air and water. However there are several protection methods to prevent this process such as use of magnesium rod with iron pipes protect the corrosion of iron because Mg is more reactive metal compare to Fe. Because of this Mg forms oxides and use O2 and H2O present in the surroundings and prevent Fe surface.

This method is called as cathodic protection or sacrificial protection. Corrosion of some metals prevents further corrosion such as formation of aluminium oxide over aluminium metal surface prevent further corrosion as the oxide layer serves as a barrier for metal surface. Similar oxide layer can be observed over silver surface in jewellery and utensil. The green colour of copper utensils is due to oxides of copper. This process cannot observe in less reactive metals such as gold or platinum.