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Corrosion

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Corrosion - Lesson Summary

Corrosion is the term used to describe the process of the surface of metal objects getting covered by oxides (or) other salts of the metal.

In corrosion, the metal is oxidised by losing electrons to oxygen, forming oxides. Corrosion is essentially an electrochemical process.

Ex: Corrosion of iron, which is commonly known as rusting. Iron corrodes in the presence of water and air. The spot where the corrosion begins can be considered the anode. During rusting process, iron loses two electrons to form the iron (II) ion.

2Fe(s) + O2(g) + 4H+ ------------→ 2Fe2+ + 2H2O(l)
   Iron   Oxygen  hydrogen ion     Iron(II) Ion     Water

The electrons thus released then move to another spot on the metal, which behaves as the cathode. At this spot, oxygen is reduced in the presence of a hydrogen ion to form water. The hydrogen ions are believed to be available either from carbonic acid that is formed when carbon dioxide dissolves in water (or) due to the dissolution of other acidic oxides from the atmosphere.

The ferrous ions formed are then further oxidised by atmospheric oxygen to ferric ions in the form of hydrated iron (III) oxide, commonly known as rust.

4Fe2+ + O2 + 4H20 → 2Fe2O3 + 8H+
            Fe2O3 + xH2O → Fe2O3.xH2O
                                     Hydrated Iron(III) Oxide (Rust)
                                                             
It is very important to prevent corrosion because it damages electrical towers, bridges, buildings, which can collapse as a result, and ships, which can develop a breach. Corrosion can also force major industrial plants, such as power plants or chemical processing plants, to be shut down.

Methods to avoid Corrosion:

Cover the surface with paint (or) some chemicals like bisphenol. This creates a physical barrier that protects the metal.

Cover the surface with other metals, such as tin (or) zinc.

Ex: Paint, prevents oxygen and water from reaching the metal surface.

Use of electrochemistry to prevent corrosion:

In cathodic protection, the metal is coated with another metal which is more easily oxidised. In cathodic protection of iron, iron becomes the cathode and the metal coating becomes the sacrificial anode. Cathodic protection can be used to protect steel pipes from corrosion.

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