Have you noticed a reddish-brown or orangish layer on your gates, iron nails, park swings, or other metal items? This layer is formed due to a chemical process known as rusting, which is a form of Corrosion. In this process, refined metals are transformed into more stable compounds such as metal oxides, metal sulphides, or metal hydroxides. 

Similarly, when iron rusts, it forms iron oxides due to the action of air moisture and oxygen. We can also say, Corrosion is a process that converts refined metals into more stable compounds such as metal oxides, metal sulphides, or metal hydroxides. In this article, we have discussed more causes of corrosion, with some examples. 

What is Corrosion?

Corrosion is a natural process that converts a refined metal into a stable chemical form such as oxide, hydroxide, or sulphide. It is the gradual decomposition of materials (usual metals) due to chemical and/or electrochemical reactions with their surroundings. It deteriorates and corrodes the material by weakening its strength, appearance, and durability properties.

Definition

It is a natural process, which converts a refined metal to a more chemically stable form, such as its oxide, hydroxide, or sulphide. It is the gradual destruction of materials (usual metals) by chemical and/or electrochemical reactions with their environment.

Corrosion degrades the useful properties of materials and structures, including strength, appearance, and permeability to liquids and gases. In other words, it is a gradual process of eating of metals by their own oxides that results in the decaying or deterioration of metal or metal surface. It forms a reddish-brown or an orange layer on the metal.

Rusting

Corrosion of iron metal is called rusting. When iron or alloys of iron reacts with moist air present in the atmosphere, they form a reddish-brown layer of iron oxide, \(\rm{F{e_2}{O_3}}\) (rust). Rusting of iron causes severe damage to the iron substance.

Factors Affecting Corrosion

1. Moisture- Metals tend to corrode at a much faster rate in moist or wet environments.

2. Temperature- An increase in temperature increases the rate of corrosion.

3. Exposure of metals to air conditioning gases such as \(\rm{C{O_2}},\;\rm{S{O_2}},\;\rm{S{O_3}},\) etc. increases the rate of corrosion.

4. Presence of impurities such as salt \(\left( \rm{{NaCl}} \right)\) increases the rate of corrosion.

5. Nature of the first layer of oxide formed- Aluminium being a reactive metal does not corrode like other metals. It forms a protective layer of aluminium oxide on corrosion which prevents the metal from further reacting with the environment.

6. Type of metal- All metals do not corrode. The metals that are placed higher in the reactivity series such as iron, zinc etc., corrode very easily whereas metals placed lower in the reactivity series such as gold, platinum, and silver do not corrode.

Types of Corrosion

Some of the general types of corrosion are as follows-

1. Galvanic corrosion: This type of corrosion occurs when two metals with different electrochemical charges are linked via a conductive path. It occurs when metal ions move from the anodic metal to the cathodic metal. The anodic metal is the weaker, less resistant one, and loses ions to the stronger, positively charged cathodic metal.

2. Intergranular corrosion: This type of corrosion occurs when one impure metal is present. If a metal contains a combination of alloys that possess different charges, one of the metals can become corroded.

3. Stress-corrosion cracking (SCC): This type of corrosion can severely damage a component beyond repair. When a metal component is subjected to extreme tensile stress, stress caused by cold work, welding, and thermal treatment, cracks can form, which then becoming targets for further corrosion.

4. General corrosion: It is electrochemical corrosion. It occurs as a result of rust. When metal, specifical steel, is exposed to water, the surface is oxidized and a thin layer of rust appears.

5. Localized corrosion: occurs when a small part of a component comes in contact with specific corrosion-causing stress. Because the small area corrodes at a much faster rate than the rest of the component, and the corrosion works alongside other processes such as stress and fatigue, the result is much worse than stress or fatigue alone.

6. Caustic agent corrosion: This occurs when impure gas, liquids, or solids wear a material down. When exposed to moisture these gases dissolve to form harmful corrosive droplets. Hydrogen sulphide is an example of one such caustic agent.

7. Cavitation corrosion: This appears in areas where vapour bubbles are formed due to low pressure. When the bubbles implode on a surface, the protective oxide is destroyed and eroded away and after that built up again. The process is repeated and characteristic deep holes of cavitation corrosion are formed on the surface. It can be observed on the trailing edge of impellers and propellers.

Corrosion of Some Common Metals

1. Copper– When copper metal is exposed to air, the oxidation-reduction of copper metals in the environment occurs in several steps. Copper is oxidized to copper(I) oxide, which is red and then to copper (II) oxide, which is black.
\(\rm{2Cu + \frac{1}{2}{O_2}} \to \rm{C{u_2}O}\;\left(\rm{ {red}} \right)\)
\( {\text{C}} { {\text{u}}_ {\text{2}}} {\text{ + }}\frac{ {\text{1}}}{ {\text{2}}} { {\text{O}}_ {\text{2}}} \to {\text{2CuO (black)}}\)

 

2. Iron– When iron is exposed to oxygen and the presence of water, it creates a red iron oxide commonly called rust.
\(4 {\text{Fe}} + 3 { {\text{O}}_2} + 2x { {\text{H}}_2} {\text{O}} \to 2 {\text{F}} { {\text{e}}_2} { {\text{O}}_3}.x { {\text{H}}_2} {\text{O}}\)

 

3. Silver– When silver comes in contact with oxygen in the atmosphere it forms silver sulphide which has a black coating deposited over it. Silver reacts with hydrogen sulphide gas in the atmosphere to form silver sulphide and this is known as tarnishing of silver.
\(2 {\text{Ag}} + { {\text{H}}_2} {\text{S}} \to {\text{A}} { {\text{g}}_2} {\text{S}} + { {\text{H}}_2}\)

Prevention of Corrosion

Corrosion is a significant challenge because it causes disasters and massive financial loss. Prevention of corrosion is important because most materials and structures we use are made of metals. Some of the preventive measures are as follows:

1. Alloying- An alloy is a mixture of two or more metals or a metal and a nonmetal. Alloys often have properties different from the metals they contain and are generally stronger and harder than their main metals, less malleable and less ductile. This makes them more useful than pure metals. For example, Bronze is an alloy of copper and tin used in construction and shipbuilding.

2. Galvanization- It is the process of applying a protective zinc coating to steel or iron, to prevent corrosion.

3. Electroplating- It is the process of coating one metal or metal object with a very thin layer of another metal, typically by applying a direct electric current.

4. Painting and Greasing- Covering or painting the surface of the metals with paints or grease prevents the exposure of metals by creating a barrier between the metal surface and environment and thus prevents corrosion.

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Summary

This article studied how metals get degraded or deteriorated and various factors causing this degradation. We also studied some of the preventive measures to follow to increase the quality and life of metals.

FAQs on Corrosion

Q.1. Do non-metals undergo corrosion?
Ans: No, only active metals undergo the process.

Q.2. Do all metals corrode?
Ans. All metals do not corrode. The metals that are placed higher in the reactivity series such as iron, zinc etc., corrode very easily whereas metals placed lower in the reactivity series such as gold, platinum, and silver do not corrode.

Q.3. What are the methods to prevent corrosion?
Ans. The methods to prevent corrosion are as follows:
1. Alloying
2. Galvanization
3. Electroplating
4. Painting and greasing.

Q.4. What causes corrosion?
Ans: Some of the causes are as follows:
1. Metal corrodes when it reacts with another substance such as oxygen, hydrogen, an electrical current or even dirt and bacteria.
2. Corrosion can also happen when metals like steel are placed under too much stress causing the material to crack.

Q.5. What are the three types of corrosion?
Ans: The three types of corrosion are-
1. General corrosion– It occurs as a result of rust. When metal, specifical steel, is exposed to water, the surface is oxidized and a thin layer of rust appears.
2. Galvanic corrosion– This occurs when two metals with different electrochemical charges are linked via a conductive path. It occurs when metal ions move from the anodic metal to the cathodic metal.
3. Localised corrosion– This occurs when a small part of a component comes in contact with specific corrosion-causing stress. Because the small area corrodes at a much faster rate than the rest of the component, and it works alongside other processes such as stress and fatigue, the end result is much worse than the result of stress or fatigue alone.

Q.6. What is corrosion? Explain with an example.
Ans: The eating of metals by their own oxides by the action of air, water, and chemical reaction is called corrosion. The most common example of corrosion is rusting of iron. Iron nails, bicycle chains, and some machine parts form a reddish-brown layer of iron oxide on exposure to moist air.

Q.7. What are corrosion and its effects?
Ans: The effects of corrosion are that it degrades the quality of metals like appearance, strength, malleability, ductility, its tensile strength and decreases its life. It also affects the cost of maintaining and repairing the metal structures, objects, or articles.

Q.8. Do non-metals undergo corrosion?
Ans. No, only active metals undergo the process.

Q.9. What is the condition necessary for corrosion?
Ans. The condition necessary for corrosion to take place is moisture. This can be in the form of moist air, oxygen, and water.