The process of conversion of metals slowly into undesirable compounds (normally oxides) or the process of degradation of metals in the presence of harmful gases and moisture in the atmosphere is called corrosion.” Rusting of iron is the main example of corrosion. On its basis, we will try to understand the mechanism of corrosion. Chemically, rust is hydrated Ferric oxide (Fe2O3 xH2O). It occurs due to moisture, CO2 and O2 .

Rusting does not form in vacuum or dry air. Rust is a brown coloured substance that can be removed by scratching. For explaining the process of rusting, the electrochemical theory is the most suitable.

Electrochemical Theory of Rusting:

At a definite place of iron objects, the process of rusting takes place, and that place behaves as an anode.

At anode:

2Fe ———>2Fe2+ + 4e–

Efe/Fe² = 0.44 V.

Electrons released by the anode move to the other place (cathode) and reduce oxygen in the presence of H+ ions. This H+ ion is obtained by the ionisation of H2CO3 formed by the dissolution of CO₂ in water.

At Cathode :

O2(aq) + 4H + 4e– →2H₂O 

E= 1.23 V.

Complete cell reaction by combining anode and cathode reaction

2 Fe(s) + O2(g) + 4H(aq) →2Fe2+ +H₂O

E = 0.44 V + 1.23 V = 1.67 V. 

Thus, a potential of 1.67 V is established and the process takes place continuously.

Fe2+ ion formed at anode reacts with OH ions and forms Fe(OH)2(s). This iron (II) hydroxide by the oxygen of the atmosphere forms hydrated iron (III) oxide in the presence of moisture.

2Fe(OH)₂ + 1\2 O₂ + H₂O —> Fe₂O3.xH₂O.

This hydrated ferric oxide (rust) does not stick to the metal surface and comes off from the surface as a reddish-brown powder. Now, iron atoms present below the surface come in contact with the atmosphere and repeat the process by which iron goes on getting destroyed by rust.

In seawater, the process of corrosion speeds up because salts present in seawater increase the conductance of electrolyte solution present on the metal surface.

Factors affecting Corrosion

1. Nature of Metal: Higher the reactivity of metal, the higher is its tendency to undergo corrosion. In an electrochemical series, the higher (top) the position of metal, the higher is its reactivity, and thus, higher is its tendency to undergo corrosion.
2. Presence of impurities: Because of impurities, small corrosion cells are made, and corrosion occurs speedily. The rate of corrosion in impure ions is faster than in pure ions.
3. Nature of Atmosphere: If moisture, salinity or gases like CO₂, SO₂, SO, etc., are present as impurities in the atmosphere, then the rate of corrosion is increased because these impurities act as electrolytes and help to speed the cell reactions.
4. Distortion in Metals: Corrosion of metal of smooth and planar surface is less than rough metal because, in the folded positions of metals, molecules or atoms are in strain.
5. Presence of Electrolyte: Due to the presence of water containing acid, base or salt like electrolyte with metals, the rate of corrosion increases. Corrosion of metal also depends on pH. In an alkaline medium, the speed of corrosion is slow, and in an acidic medium, it is fast.

Preventions of Corrosion:-

Several methods are there for protecting metals from corrosion (or iron from rusting). Some important methods are discussed above:

1. Barrier protection: It is a method in which a film is introduced that acts as a barrier between iron particles and gases of atmosphere and moisture. It can be done by (i) Painting the surface of the metal, (ii) Coating the surface metal with a thin layer of oil or grease (iii) Electroplating iron.

2. Sacrificial protection: In this method of protection, iron is coated by a thin layer of metal that is more active towards the atmosphere than iron. This prevents the loss of electrons from the iron surface. The active metal which loses electrons in preference of iron which makes the metal go into the ionic state. The outer layer of the metal will get destroyed after a period of time, but the inner metal is protected at this time, and the protection is called sacrificial protection.

In the galvanisation process, zinc metal is commonly used to protect the metal. Galvanised iron keeps its lustre due to the coating of an invisible protective layer of basic zinc carbonate ZnO, Zn(OH)2, due to the reaction between Zn, O₂, CO, and moisture in the air.

However, if some destruction occurs on the protective zinc film, which is coated on iron, even in that situation, the iron will not be rusted. This is so because due to scratches, both Zinc and iron are exposed to oxidation, but Zinc oxidises first and only after it does iron get oxidised.

In the case of tin plating, the film protects from rust till it is intact. If scratches occur at the coating surface, both metals are exposed to oxygen. In this case, iron gets oxidised firstly and rusted because the reduction potential of tin is more than that of iron.

Zn, Al and Mg powders are mixed with paints and used as protective layers.

3. Electrical protection or Cathodic Protection: This method is useful for the protection of iron articles that are directly in contact with water, such as underground water pipes. The articles of iron are directly connected with more active metals than iron like Zn or Mg. The active metal which we choose has a lower reduction potential than iron. Hence, the active metal loses electrons (Oxidises) preferably than iron, and thus iron is protected from being rusted.

4. Using anti-rust solutions: The commonly used solutions for anti-rust are alkaline phosphatase and alkaline chromate solutions.

Iron articles are dipped in a strongly alkaline and boiling solution of sodium phosphate (Na3PO4), a protective layer of iron phosphate is formed on the iron.

This layer protects the iron article from rusting. Due to the alkaline nature of solutions, H ions are not available. So, in the absence of H+ ions, oxidations of Fe into Fe²+ cannot take place.

Conclusion:-

Corrosion is very common in our daily life, but now, you know that corrosion current also exists as a result of this chemical reaction (a very small current).Corrosion is a natural process that changes a refined metal into a stable chemical state like oxide, hydroxide, or sulphide. It is the slow degradation of materials (often metals) caused by chemical and electrochemical interactions with their environment. It corrodes and deteriorates the material by reducing its strength, appearance, and durability. Rusting is just an example of corrosion which is the process of deterioration of metals.Some of the preventive steps that may be taken to improve the quality and longevity of metals were also investigated. Natural corrosion happens to metal and non-metals, and it makes a dry, flaky layer on their surface that makes them break down. There are a lot of different metals and alloys that we use in our daily lives that have this kind of thing in them.