Corrosion and its Prevention

Corrosion is a common phenomenon, which can be observed in our daily life easily. For example, you must have seen various iron objects covered with a reddish-brown layer. This layer indicates rusting of iron, which is a type of corrosion.

In corrosion, a pure metal is naturally converted to a more stable form such as its oxide, hydroxide or sulphide state involving the deterioration of the metal.

Corrosion is an electrochemical process in which redox reactions occur between the metal and water, oxygen and sulphur dioxide, etc. It is a spontaneous and irreversible process in which the metal changes into chemical compounds such as oxide, sulphides and hydroxides, etc. For example, due to corrosion or rusting, the iron changes into red iron oxide (rust) in the presence of moisture and oxygen present in the air.

Corrosion is an unwanted phenomenon as it affects the quality of metals. Although iron is known for its tensile strength and durability, due to rusting it starts deteriorating with time and becomes brittle and weak. So, it damages the properties that involve the use of iron.

In simple words, we can define corrosion as a natural process wherein the pure metals like iron is transformed into unwanted substances when the metals react with water and air.

Do all Metals Corrode?

Metals that are located higher in the reactivity series such as zinc and iron tend to corrode easily, whereas, less reactive or non-reactive metals like gold, platinum do not undergo corrosion. This is because in corrosion oxidation of metals occurs and the tendency to undergo oxidation reduces as we move down the reactivity series. So, all metals do not corrode.

Aluminium is an exception. It is a reactive metal but still, it does not corrode. This is because it already has a layer of aluminium oxide over its surface. This layer does not allow it to undergo further corrosion.

Causes of Corrosion

• The exposure and reaction of a metal to other substances like oxygen, hydrogen, carbon dioxide, sulphur dioxide, etc.
• The too much stress on metals like steel which causes it to crack also causes corrosion
• An increase in temperature also promotes corrosion

Factors Affecting Corrosion

• Contact of metals to air containing oxygen, carbon dioxide, sulphur dioxide, etc., causes corrosion.
• Exposure of metals to moisture or saltwater promotes corrosion.
• Impurities in metals like salt (NaCl) promotes corrosion
• High temperature also increases corrosion
• Some metal oxides have an insoluble protective layer that prevents further corrosion
• Contact with acid in the atmosphere also increases the rate of corrosion.

Examples of Corrosion

• The rusting of iron when it comes in contact with water and oxygen which leads to the formation of a brown coat over its surface is a type of corrosion. The chemical reaction involved in rusting is shown below;
  4Fe + 3O₂ → 2Fe₂O₃
  2Fe₂O₃ + xH₂O → Fe₂O₃.xH₂O (rust)
• The silver ore when comes in contact with oxygen in the air leads to the formation of a black layer or covering over it due to the formation of sulphur dioxide. It is called corrosion, however, it is also known as tarnishing of silver.
• Copper metal when exposed to carbon dioxide in the air, it causes the formation of a green coat over it due to the formation of copper carbonate. This corrosion is also called tarnishing of copper.

Types of Corrosion

i) Crevice Corrosion

This type of corrosion occurs when there is a difference in ionic concentration between any two local metal areas. This difference occurs when oxygen is restricted in some areas such as under the surface of washers and bolt heads. It is a localized type of corrosion as it affects certain parts, not the entire surface.

Besides this, crevice corrosion can occur at a low temperature and can be minimized by proper joint design.

ii) Stress Corrosion Cracking

It is caused due to the corrosive environment and the effect of tensile stress on the metal. It generally occurs at high temperatures. For example, cracking of stainless steel in chlorine solution due to stress corrosion. It generally occurs in alloys than in pure metals.

iii) Galvanic corrosion

It occurs when there is an electric contact between two metals with different electrochemical properties and are placed in an electrolytic environment. In this corrosion, one of the metals is degraded at a joint or at a junction. For example, when aluminium and steel are in contact and immersed in seawater, aluminium is corroded, while steel remains protected.

iv) Pitting Corrosion

This type of corrosion is hard to detect and is also unpredictable. So, it can be dangerous. It occurs at a local point and progresses with the formation of a corrosion pit surrounded by the normal metal surface. The pit keeps growing and takes various shapes. The pit gradually increases from the surface into the metal vertically and can cause structural failure if left unchecked.

For example, if a drop of water is resting on a steel surface, the pitting corrosion will start at the centre of the water drop then slowly progress into the metal (vertically downward).

v) Inter-granular Corrosion

This type of corrosion occurs on the granular area, however, this corrosion does not affect the grains. It occurs when grains and grains boundaries react differentially with impurities. The difference in reactivity is caused by defective welding, heat treatment, etc.

vi) Uniform Corrosion

As the name suggests, in this type of corrosion, a uniform layer of rust is formed over the entire surface of the metal. It occurs on metals that lack surface coating for protection. The common metals that are affected by this corrosion include aluminium, zinc and lead.

vii) Erosive Corrosion

It occurs when there is a relative movement between the metal surface and corrosive liquids. In this corrosion, the metal surface deteriorates gradually due to the abrasion of fast-moving liquid. It occurs in metals that carry moving fluid.

viii) Fretting Corrosion:

It occurs at the contact area of two metals that are joined together. The slips and variations between the contact areas cause it. It commonly occurs in bolted and riveted joints, clamped surfaces, etc.

How to prevent corrosion

There are lots of methods to prevent corrosion, some of them are described below;

i) Electroplating

In this method, an electric current is used to create a thin layer of metal over another metal. It is done to make cheaper metals more appealing as well as to protect them from corrosion.

This method requires two different metals, an electrolytic solution, and two electrodes in a tank and a battery or source of current that will pass the required current into the solution to carry out the electrolysis.

When current is passed one electrode gets a positive charge and another gets the negative charge. The ions of the positively charged metal shift to the surface of the negatively charged metal to create a thin layer. For example, when we take brass and copper for electroplating, the copper metal slowly gets deposited or covers the brass and thus a thin coating of copper covers the surface of the brass. Here, the electrolytic solution must contain copper sulphide.

ii) Galvanization

In this method, iron is coated with a layer of zinc. The iron is dipped in the molten zinc. The layer of zinc protects the iron from corrosion. This method has been in use for more than 200 years.

iii) Painting and Greasing

In this method, a layer is created over the metal surface by painting or greasing. This layer of paint or grease protects the metal from corrosion. Carbon fibre coating can be used for this purpose.

iv) Selection of Material

Select the materials that are not affected by corrosion. For example, stainless steel and aluminium are resistant to corrosion.

v) Using Corrosion Inhibitors

As the name suggests, these inhibitors when added to the corroded metal can prevent further corrosion.

vi) Dry and clean: Keep the metal surface dry and clean.