Corrosion & its Prevention: Definition, Types & Factors Involved

Corrosion and its Prevention is an important chapter in CBSE Class 10 Chemistry. The surface of a reactive metal slowly reacts with the moisture and air present in the atmosphere; as a result, a brownish compound is formed on the surface. The compound formed on the surface makes the metal appear dull. Corrosion is referred to as a phenomenon that involves the reaction of metal with moisture and other chemicals.

Rust is the reddish-brown flaky coating that we see on iron gates, unused cars, metal pipes and so on. Corrosion of iron objects on exposure to damp air is known as rusting. Corrosion is an undesirable process as it leads to the damage and wastage of metals. So, in this article, let’s understand what corrosion is and its preventing process such as painting, galvanising, electroplating, oiling. Read on to find more.

What is Corrosion?

Corrosion is defined as the demolition of metals or alloys by the surrounding moist and humid environment through chemical and electrochemical changes. Corrosion takes place in the presence of \({{\text{O}}_2}\) gas, hence it is an oxidation reaction. Almost all the metals corrode but, the rate of corrosion of different metals differ.

Corrosion is a spontaneous and irreversible process by which metals form stable compounds in the form of new compounds such as oxides, hydroxides, and sulphides.

Factors Impacting Corrosion

1. Position of metals in Electrochemical series – Electrochemical series plays a vital role in corrosion. More reactive metals lose electrons quickly and corrode faster. For example, iron gets oxidised easily, but gold does not because gold is less reactive than iron.
2. Impurities in metals – The presence of contaminants in metals generally increases the corrosion rate as these impurities act as a tiny electrochemical cell that leads to corrosion.
3. Presence of electrolytes – The dissolved salt in water behaves as an electrolyte as they conduct ions. The presence of electrolytes in water increases the rate of corrosion.
4. The concentration of Oxygen – The Corrosion accelerates with an increased concentration of oxygen. The regions with less oxygen concentration act as an anode, while regions with more oxygen concentration act as a cathode. Thus, corrosion occurs.
5. Humidity in weather – When the weather is humid, the presence of moisture and high temperature increases the corrosion rate as ions gain energy and start moving faster in increased temperature, colliding more frequently.

Various Examples of Corrosion

Metals placed higher at the reactivity series like iron, copper, zinc, etc., corrodes faster. Some of the typical examples of corrosion that we observe in our daily life are as follows:

Rusting of Iron

Corrosion of iron metal in the presence of air and water to form a reddish-brown surface is known as rusting. It is the oxidation of iron in the presence of air and water. A new compound that is the hydrated form of Iron \(\left({{\text{III}}} \right)\) oxide having the chemical formula \({\text{F}}{{\text{e}}_2}{{\text{O}}_3}.{\text{x}}{{\text{H}}_2}{\text{O}}\) is rust (where \({\text{x}}\) is variable).

Though rusting of iron is a natural phenomenon, but this process resembles a typical electrochemical cell reaction.

Iron loses electrons to form \({\text{F}}{{\text{e}}^{2 + }},\) this can be considered as the anode.

At Anode

\({\rm{Fe}}\left( {\rm{s}} \right) \to {\rm{F}}{{\rm{e}}^{2 + }}\left( {{\rm{aq}}} \right) + 2{{\rm{e}}^ – }\)

\({{\text{H}}_2}{\text{O}} \to {{\text{H}}^ + } + {\text{O}}{{\text{H}}^ – }\)

\({\text{C}}{{\text{O}}_2} + {{\text{H}}_2}{\text{O}} \to {{\text{H}}^ + } + {\text{HC}}{{\text{O}}^ – }_3\)

At the cathodic position, \({{\text{H}}^ + }\) ions obtained from \({{\text{H}}_2}{\text{O}}\) or acid in the environment take up the electrons released by the anode. Thus \({{\text{H}}_2}{\text{O}}\) is formed.

At Cathode

\({{\text{H}}^ + } + {{\text{e}}^ – } \to {\text{H}}\)

\({\text{4H}} + {{\text{O}}_2} \to 2{{\text{H}}_2}{\text{O}}\)

\({\text{4}}{{\text{H}}^ + } + {{\text{O}}_2} + 4{{\text{e}}^ – } \to 2{{\text{H}}_2}{\text{O}}\)

The ferrous oxides are further oxidised to ferric ions to form a compound with \({{\text{H}}_2}{\text{O}}{\text{.}}\)

Oxidation Half-reaction

\({\rm{Fe}}\left( {\rm{s}} \right) \to {\rm{F}}{{\rm{e}}^{2 + }}\left( {{\rm{aq}}} \right) + 2{{\rm{e}}^ – } \times 2\left( {{\rm{E}}^\circ = – 0.44{\rm{V}}} \right)\)

Reduction Half-reaction

\(4{{\rm{H}}^ + } + {{\rm{O}}_2} + 4{{\rm{e}}^ – } \to 2{{\rm{H}}_2}{\rm{O}}\left( {{\rm{E}}^\circ = 1.23{\rm{V}}} \right)\)

Overall Cell reaction

\({\rm{E}}_{{\rm{cell}}}^{\rm{^\circ }}\,{\rm{ = }}\,{\rm{E}}_{{\rm{red}}}^{\rm{^\circ }}\,{\rm{ – }}\,{\rm{E}}_{{\rm{oxd}}}^{\rm{^\circ }}\)

\({\rm{E}}_{{\rm{cell}}}^{\rm{^\circ }} = 1.23 – \left( { – 0.44} \right){\rm{V}}\)

\({\rm{E}}_{{\rm{cell}}}^{\rm{^\circ }} = 1.67{\rm{V}}\)

\(2{\rm{Fe}}\left( {\rm{s}} \right) + 4{{\rm{H}}^ + } + {{\rm{O}}_2} \to 2{\rm{F}}{{\rm{e}}^{2 + }}\left( {{\rm{aq}}} \right) + 2{{\rm{H}}_2}{\rm{O}}{\mkern 1mu} {\mkern 1mu} \left( {{\rm{E}}^\circ  = 1.67{\rm{V}}} \right)\)

\(2{\text{F}}{{\text{e}}^{3 + }} + 3{{\text{O}}_2} \to 2{\text{F}}{{\text{e}}_2}{{\text{O}}_3}\)

\({\text{F}}{{\text{e}}_2}{{\text{O}}_3} + {\text{x}}{{\text{H}}_2}{\text{O}} \to {\text{F}}{{\text{e}}_2}{{\text{O}}_3}.{\text{x}}{{\text{H}}_2}{\text{O}}\left({{\text{rust}}} \right)\)

Corrosion of Copper

Copper materials or copper alloys get corroded when exposed to the atmosphere. The bright copper surface gets tarnished when copper oxidises, and it gradually changes to dark brown or black, and finally to green. This leaching causes the corrosion of copper.

A typical example of corrosion of copper metal is the blue-green colour of the “Statue of Liberty“. When it was constructed, it was not of the same colour as it looks today. Instead, it was brown coloured. But with time, copper got oxidised and reacted with sulphur trioxide, carbon dioxide, and water in the environment to form a new compound that gave it the characteristic blue-green patina look today!

The chemical equation involved in the corrosion of copper metal is given below:

Tarnishing of Silver

The silver metal gets tarnished due to a chemical reaction between the silver and sulphur-containing substances in the air. The silver is combined with sulphur and forms silver sulphide. This coating of silver sulphide gives a black appearance to the silver metal that is known as tarnishing.

The chemical equation involved in the tarnishing of silver metal is given below:

\(\underset{{{\text{Silver}}}}{\mathop {2{\text{Ag}}}} + \underset{{{\text{Hydrogen}}\,\,{\text{Sulphide}}}}{\mathop {{{\text{H}}_2}{\text{S}}}} \to \underset{{\left({{\text{Black}}\,{\text{coating}}} \right)}}{\mathop {{\text{A}}{{\text{g}}_2}{\text{S}} + {{\text{H}}_2}}} \)

Types of Corrosion

1. Uniform Corrosion

It is a very common type of corrosion. There is the uniform loss of metal over an entire surface in this type of corrosion. Example: Rusting, tarnishing of silver, etc.

2. Pitting Corrosion

Pitting corrosion is a localised Corrosion that attacks the metal surface and produces spots or pits.

For example, pitting corrosion may be observed in stainless steels in neutral or acid solutions containing halides, such as seawater containing \({\text{NaCl}}{\text{.}}\)

3. Crevice Corrosion

Crevice corrosion attacks the metal surfaces by a stagnant solution in crevices, like nuts and rivet heads around the edges. It can take place between two metals or between a metal and a non-metal.

Example: Corrosion on gaskets, couplings, and joints of metals or non-metals.

4. Galvanic Corrosion

Galvanic corrosion is the damage induced when two dissimilar materials are coupled and brought into electrical contact in a corrosive electrolyte.

Example: A sheet of iron or steel is covered with a zinc coating in a galvanised iron.

5. Stress Corrosion Cracking

It is a type of corrosion where cracks occur on the surface of the metal due to the combined influence of tensile strength and corrosive environment.

Example: Stress Corrosion Cracking (SCS) occurring in aircraft components.

6. Intergranular Corrosion

It is a type of corrosion that occurs at the grain boundaries containing impurities of a metal or alloy than inside the metal or alloy.

Example: Intergranular Corrosion is exhibited by Die-cast zinc alloys containing aluminium exposed to steam in a marine atmosphere.

7. Dealloying

It is an unusual type of corrosion, occurring mainly in certain alloys such as copper alloys and grey cast iron. In the dealloying process, the alloy metal loses its reactive element and forms the more stable corrosion-resistant element in a porous state.

Example: Some examples of dealloying include de-nickelification, de-zincification, and graphitic corrosion.

8. Erosion Corrosion

Erosion corrosion is a degradation of the metal surface due to mechanical action. It is caused due to the relative movement between fluid particles and solid surfaces in a corrosive atmosphere.

Example: This type of corrosion mainly occurs due to faulty workmanship where burrs left at the cut tube ends in pipelines can upset smooth water flow, leading to localised turbulence and high flow velocities erosion-corrosion.

Methods to Prevent Corrosion

Corrosion is an undesirable phenomenon that we need to prevent because it causes much damage to metallic structures like bridges, statues, monuments, metal utensils, and many more.

All corrosion prevention methods are based on a fundamental concept, i.e., to cut off the water and air supply to the metals. Some of the preventive measures of corrosions are discussed below:

1. Painting

Painting iron articles such as iron gates, tracks, etc., stop metal exposure to air and water. Thus, corrosion is prevented by painting metallic objects.

2. Oiling and Greasing

Like painting, oiling and greasing also form a protective layer on the metal surface and protects it from corrosion.

3. Galvanisation

Galvanisation is a process by which a thin protective layer of zinc is made on iron and steel objects. Even after this zinc layer is broken, the galvanised articles are protected from corrosion or rusting.

4. Alloying

 Alloys are the homogeneous mixture of one or more metals and non-metals.

Through alloying, we can get the desired properties of the metals. For example, iron is a very useful metal, but rusting occurs easily. Thus, when this iron is mixed with Nickel and Chromium, we get stainless steel that does not rust. Nowadays, this stainless steel is very commonly used to make utensils.

Summary

Corrosion is a natural degenerative phenomenon affecting metal and non-metals by forming a dry, flaky layer on the surface that destroys them. It is common in some metals like iron, zinc, copper, aluminium, and some alloys like brass, bronze, etc., used in our daily lives.

Corrosion especially causes great loss to oil and gas pipeline sectors and other engineering materials as it reduces the life of these materials. Due to this, they must be replaced from time to time, accompanied by huge expenditure.

Thus, we should focus on the various methods to reduce or prevent corrosion, such as painting, oiling, galvanising, alloying, etc. We should also know the factors that accelerate the corrosion rate and take significant measures to avoid such conditions as we all know that “Prevention is better than cure”.

FAQs on Corrosion and Prevention

Frequently asked questions related to corrosion and prevention is listed as follows:

Q.1. What is corrosion, its causes and prevention?
Ans: Corrosion is a natural phenomenon of eating up metal by moisture, air, and chemicals in the atmosphere. Various processes can prevent corrosion: painting, galvanising, electroplating, oiling, alloying, etc.

Q.2. What is Corrosion? Explain.
Ans: Corrosion is defined as the degradation of metals or alloys by the surrounding moist and humid environment through chemical and electrochemical changes. Corrosion takes place in the presence of \({{\text{O}}_2}\) gas, hence it is an oxidation reaction.

Q.3. What are three ways to prevent corrosion?
Ans: The three ways to prevent corrosion are:
(i) Painting, Oiling, and Greasing – Painting, oiling, and greasing iron articles stops metal exposure to air and water. Thus, corrosion is prevented.
(ii) Galvanisation – It is the process by which a thin protective layer of zinc is made on iron and steel objects. Even after this zinc layer is broken, the galvanised articles are protected from corrosion or rusting.
(iii) Through alloying, we can get the desired properties of the metals and can prevent corrosion.

Q.4. What are corrosion examples and reactions?
Ans: Some examples of corrosion include: (i) Rusting – Iron gets corroded in the presence of air and water to form iron oxide forming rust. Chemical reaction: \({\text{F}}{{\text{e}}_2}{{\text{O}}_3} +{\text{x}}{{\text{H}}_2}{\text{O}} \to {\text{F}}{{\text{e}}_2}{{\text{O}}_3}.{\text{x}}{{\text{H}}_2}{\text{O}}\left({{\text{rust}}}\right),\) (ii) Corrosion of copper – Copper gets corroded when exposed to the atmosphere. The bright copper surface gets tarnished when copper oxidises, and it gradually changes to dark brown or black, and finally to green. Chemical reaction: \({\text{Cu}} + {{\text{H}}_2}{\text{O}} + {\text{C}}{{\text{O}}_2} + {{\text{O}}_2} \to {\text{Cu}}\left({{\text{O}}{{\text{H}}_2}}\right) + {\text{CuC}}{{\text{O}}_3}.\) (iii) Silver tarnishing – The silver metal gets tarnished due to a chemical reaction between the silver and sulphur-containing substances in the air. Chemical reaction: \(2{\text{Ag}} + {{\text{H}}_2}{\text{S}} \to {\text{A}}{{\text{g}}_2}{\text{S}} + {{\text{H}}_2}.\)

Q.5. What are Factors Affecting Corrosion?
Ans: Factors Affecting corrosion are the position of metals in the Electrochemical series, Impurities in metals, Presence of electrolytes, Concentration of Oxygen, Humidity in weather, and other environmental factors such as high temperature, presence of various gases in the atmosphere that cause corrosion, etc.

Q.6. What are Types of Corrosion?
Ans: The various types of corrosion include Uniform Corrosion, Pitting Corrosion, Crevice Corrosion, Galvanic corrosion, Stress Corrosion Cracking, Intergranular Corrosion, Dealloying, Erosion Corrosion.

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