Corrosion: Types, Effects and Prevention

Corrosion is a natural process observed in our daily lives. It can be seen everywhere around us - on gates, railings, etc. Such a process is more particular to objects made of iron. After corrosion, the object is covered with a reddish-brown coloured layer. This layer is a result of a chemical process where the metal is reacting with oxides in the air, leading to rusting, which is a form of corrosion. When such a chemical process occurs, refined metals are converted into more stable compounds such as metal oxides or metal hydroxides. However, this process is undesirable as it weakens the properties of metal. This can be understood by the fact that while iron is known to have good tensile strength and rigidity when subjected to rusting, iron objects become brittle, flaky, and structurally unsound. Another important point to note is that Corrosion involves redox reaction as it involves reactions between the metal and certain atmospheric agents such as water, oxygen, sulphur dioxide, etc. This undesirable process is irreversible wherein the metals turn into a much stable chemical compound. In this article, we will learn about corrosion, its types, how to prevent it, and more.

Keyterms: Corrosion, Iron, Metal, Metal oxides, Air, Metal hydroxide, Redox reaction, Water, Oxygen, Sulphur dioxide, Chemical Compound, Atmosphere, Chemical reactions

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Corrosion

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Corrosion transforms pure metals into undesirable substances, after reaction with water or air, causing damage and breakdown of the metal. This means that portions of the metal exposed to the environment as well as the entire bulk of the metal are damaged through this process.

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Do All Metals Corrode?

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Since we have mechanical machines everywhere around us, it is obvious that not all metals corrode equally. This means Metals placed higher in the reactivity series such as iron, zinc, etc. get corroded very easily. However, the metals placed lower in the reactivity series like gold, platinum, and palladium do not corrode. The explanation lies in the fact that corrosion involves oxidation of the metals and as we go down the reactivity series, the tendency to get oxidised is very low. Thus, the upper layer gets the most rust.

One interesting point is that aluminium doesn’t corrode like other metals even though it is reactive, because aluminium is covered by a layer of aluminium oxide which protects it from further corrosion.

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Factors affecting Corrosion

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Various factors affecting corrosion include 

• The exposure of the metals to air-containing gases like CO₂, SO₂, SO₃, etc.
• Exposure of metals to moisture, especially saltwater, increases the rate of corrosion.
• Change in temperature like an increase in temperature increases corrosion.
• Nature of the first layer of oxide formed like Al₂O₃ forms an insoluble protecting layer preventing further corrosion. 
• Another factor that affects corrosion is the presence of acid in the atmosphere that can easily accelerate the process of corrosion.

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Types of Corrosion

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(i) Crevice Corrosion

• Crevice is a type of corrosion that occurs when there is a difference in ionic concentration in the metal itself. 
• Thus, crevice corrosion is a localized form of corrosion that can occur in gaskets, the undersurface of washers, and bolt heads. 
• Crevice corrosion happens to all grades of aluminium alloys and stainless steel.

(ii) Stress Corrosion Cracking

• Stress Corrosion Cracking refers to the cracking of the metal as a result of the corrosive environment and the tensile stress placed on the metal often occurring at high temperatures. 
• It can be seen on the austenitic stainless steel set up in chloride Ans.

(iii) Intergranular Corrosion

• This type of corrosion occurs due to the presence of impurities in the grain boundaries that separate the grain formed during the solidification of the metal alloy. 
• It can be seen in the aluminium alloys.

(iv) Galvanic Corrosion

• Galvanic Corrosion refers to the degradation of one of these metals at a joint or at a junction, particularly when there exists an electric contact between two metals that are electrochemically dissimilar and are in an electrolytic environment. 
• For example, the degradation of copper in a salt-water environment after coming in contact with steel.
• In seawater, aluminium corrodes faster and steel is protected when aluminium and carbon steel are connected together. 

(v) Pitting Corrosion

• Pitting Corrosion is difficult to detect and is considered one of the most dangerous types of corrosion as it occurs at a local point. 
• It proceeds with the formation of a corrosion cell surrounded by the normal metallic surface, slowly penetrating metal from the surface in a vertical direction, eventually leading to structural failure it left unchecked. 
• One of the important examples of this type of corrosion is a droplet of water on a steel surface, pitting will initiate at the centre of the water droplet (anodic site).

(vi) Uniform Corrosion

• It is the most common form of corrosion.
• Here, the surface of a metal is rusted due to the atmosphere and this type of corrosion has a relatively low impact on the performance of the material.

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