Due to chemical or electrochemical attack of the environment on the surfaces of metal, compounds such as oxides, sulphides, carbonates etc are formed, which is known as corrosion. It is a slow process.
Types of Corrosion:
- Oxidation Corrosion:The metal reacts with atmospheric oxygen in the absence of moisture to form oxides.
2M + nO2 → 2Mn+ + 2No2- → M2 On
Besides oxygen, Gases like Cl2, CO2, and F2 also react with metals.
- Liquid metal Corrosion:It takes place when a liquid metal flows over a solid metal at high temperature. Eg. In nuclear reactor sodium metals corrodes the cadmium rods.
- Galvanic corrosion:When we joined two different metals directly or by a conductor and exposed it to a conducting solution. The more electro-positive metal will act as anode and the metal which is less electro-positive acts as cathode. Due to flow of current the metal which is acting as anode liberates its free electrons and its corrosion begins. The corrosion is more if the potential difference between the metals is more.
- Pitting Corrosion:When a metal is covered by some water or dirt, the part which is covered has low oxygen concentration acts as an anode. The part which is exposed to oxygen acts as cathode. The covered area gets corroded and small pits are formed on it.
Fe2+ + 2OH– → Fe (OH)2 → Fe (OH)3
- Crevice Corrosion:This type of corrosion is caused by Corrosion deposits and dirt deposits. This can be noticed near the rivets, bolts etc
- Waterline Corrosion:When water is stored in a metallic tank, it is observed that the metal below the waterline gets corroded. It is because the water below the waterline is poorly oxygenated and acts as an anode. The metal above the waterline is highly oxygenated and acts as a cathode. Hence corrosion of metal below the waterline takes place.
- Stress Corrosion:When the metal undergoes processes like bending, rolling etc we apply stress on it. The areas of the metal under stress acts as an anode and the remaining areas act as cathode. Due to the potential difference set up between the two areas the corrosion takes place in the strained portion.
- Inter granular Corrosion:This type of corrosion generally occurs in alloys. This type of corrosion occurs along the grain boundaries. It is due to the fact that the grain boundaries shows higher electrode potential than the grains at the centre. The material along the grain boundaries is more sensitive to corrosion.
- Erosion Corrosion:This type of corrosion occurs when the streams of gases, liquids strikes the surface of metal. The striking gases and liquids remove the protective films of metal surface. It occurs when the gases and liquids strike at high velocity.
- Wet or Immersed Corrosion: This type of corrosion occurs when a metal is immersed in a liquid for a long time. In this type the liquid slowly penetrates into the surface of metal or metal ions slowly dissolves into the liquid.