Corrosion

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What is Corrosion?

Elements are primarily divided into two groups based on physical and chemical properties. These two groups of elements are metals and nonmetals.

Chemically active metals get eroded in the presence of air and moisture in the atmosphere. Metals placed at higher reactivity series (such as Magnesium, Iron, Zinc, and Tin) are much susceptible to corrosion.

Corrosion is a natural phenomenon that transforms a refined material into a more chemically stable form, such as its oxide, hydroxide or sulphide. It is an immediate destruction of material (generally metals) by chemical and/or electrochemical reaction with their environment. Corrosion decays the useful properties of metal and structures including strength, appearances, and permeability to liquid and gases.

Corrosion is the decaying of metals on the grounds of the chemical reaction in the presence of the immediate environment.

In this article, we will discuss what is corrosion, corrosion definition, types of corrosion, prevention of corrosion, causes of corrosion, etc.


Corrosion Definition

Corrosion definition states that it is a natural process that causes the transformation of pure metals into undesirable substances when they react with substances such as water or air. This reaction causes damage or disintegration of metal as it interfaces with the environment and gradually spreads to the entire bulk of the metal.


Causes Of Corrosion

Metals begin to corrode when it reacts with other substances such as oxygen, hydrogen, an electric current, or even bacterias or dirt. Corrosion can also take place when metals such as steel are occupied under so much stress compelling materials to crack.

Corrosion Example

  • When an iron ore comes in contact with oxygen present in moist air, a brown coat is deposited over it because of the formation of carbon dioxide. This is known as the rusting of iron.

  • When a silver ore comes in contact with oxygen present in the air, a black coat is deposited over it because of the formation of sulphur dioxide. This is known as corrosion or tarnishing of silver.

  • When a copper metal comes in contact with the carbon dioxide present in the air, a green coat is deposited over it because of the formation of copper carbonate. This is known as corrosion or tarnishing of copper.


Corrosion and Its Types

The corrosion and its types are given below:

  1. Uniform Corrosion

In the Uniform Corrosion, a uniform layer of rust is formed on the surface of the metal and spread over the entire surface of the metal. This type of corrosion is widely seen on the metals which are not protected by surface coating. Aluminium, zinc, and lead are some metals commonly affected by uniform corrosion.

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  1. Erosive Corrosion

Erosive corrosion is caused by mechanical abrasion due to the relative movement between the metal surface and corrosive liquids. In this, the surface of metals gets deteriorated gradually by the abrasion of fast-moving liquid and cavities are also formed. This type of corrosion is commonly seen in metals carrying moving fluids in it.

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  1. Fretting Corrosion

Fretting corrosion occurs at the contact area of two materials that are joined together. This usually appears when the contact area is subjected to the slips and variations. This type of corrosion can be seen generally in bolted and riveted joints, clamped surface etc.

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  1. Corrosion Fatigue

Fatigue material is defined as the failure of material due to repeated use of the stress. If the fatigue of metal is developed in the corrosive environment, then it is known as corrosion fatigue. This can be improved by preventing the fatigue resistance of the material.

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  1. Inter- granular Corrosion

Intragranular corrosion is the corrosion that appears on the granular region and the grains are not affected in this case. It appears when there is a noticeable difference in reactivity against impurities between grains and grain boundaries. the difference in reactivity appears due to defective welding, heat treatment, stainless steel, copper, etc.


Prevention Of Corrosion 

The below prevention of corrosion points shows how corrosion can be prevented in different ways.

  1. Electroplating

Electroplating is a process that uses electric current to cover the thin layer of metal over the top of another less expensive metal. It is usually done to provide finishing to the cheaper metals and include certain properties such as anti-corrosion and anti-rust.

In order to electroplate a metal, two different metals, an electrolyte solution, two electrodes, a battery or other source will be required that will create the electric current.

Once the power is turned on, one metal gets positively charged and the other metal gets negatively charged. Over a certain period, the molecules of the positively charged metal shift to the surface of the negatively charged metal, which creates a very thin layer.

A common example of electroplating is brass and copper. In such a case, brass and copper would be placed in an electrolyte solution.

The electrolyte would be attached to each piece of metal and battery. Once the power is turned on, the copper metal will slowly attach themself to the brass, creating a thin copper coating on the surface of the brass. For such a case, you would probably require a solution that contains copper sulphide.

  1. Galvanization

This process includes a coating of iron with a thin layer of zinc. It is usually done by dipping iron in molten zinc. This zinc layer coating protects the inner part of the iron from corrosion.

  1. Painting And Greasing

Providing a layer of grease or paint on the metal can prevent the exposure of metal with the external environment thereby preventing corrosion.

  1. Selection of Optimum Material

Choosing an optimum material can also help in preventing corrosion. Aluminium and stainless steel are highly corrosion resistant.

  1. Using Corrosion Inhibitors

Corrosion inhibitors are the element which has the ability to diminish the rate of corrosion (when added to the corroded environment).


Factor Affecting Corrosion

The factors affecting corrosion are discussed below:

  • Presence of impurities such as salt (eg, NaCL).

  • An increase in temperature increases corrosion.

  • Exposure of metal to air conditioning gases such as CO₂, SO₂, and So₃ etc

  • Exposure of metal to moisture especially salt water ( which escalates the rate of corrosion).

  • An increase in temperature increases corrosion.

  • Some oxides such as AL₂0₃ form an insoluble protective layer which can further stop corrosion. Others like rust easily disintegrate and exhibit the remaining part of the metal.

FAQ (Frequently Asked Questions)

1. Are all Metals corroded easily?

Ans: Metals placed in higher reactivity series such as Potassium, Sodium etc are highly reactive whereas metals placed in lower reactivity series such as Gold, Silver, Mercury etc are less reactive. This explanation includes the factor that corrosion included the oxidation of metals. As we move down to the reactivity series, the tendency to get oxides diminishes (Potential of oxidation gets diminished).

Aluminium does not corrode like other metals even though it is highly reactive. It is because aluminium is covered by a layer of aluminium oxide. This layer of aluminium oxide which covers aluminium protects it from being corroded.

2. What are the advantages and disadvantages of Corrosion?

Ans: Here are some of the advantages and disadvantages of corrosion:

Advantages of Corrosion

  1. Protection- The oxide layer is formed on the surface corrosion which further protects the inner metal from being corroded.

  2. Galvanic Corrosion - This type of corrosion appears when two distinct types of metals are in electric contact in the presence of an electrolyte where the highly active metals are more liable to corrode.

  3. Sacrificial Anodes such as Zinc can be used as a preventive measure to prevent corrosion of other metals.

Disadvantages of Corrosion

  1. Loss of metal from the surface.

  2. The physical appearance of metal changes.

  3. The chemical properties of the metal vary.

  4. The life span of metallic objects decreases.

  5. Loose of efficiency of the metallic machines.