# Corrosion - Types, Methods of Prevention, Examples

Corrosion is the process of slow deterioration of solid material due to unwanted chemical reactions. It is commonly seen in most of the metals which have the capacity of going back to their native mineral state. Corrosion can occur in a small concentrated area to form a crack or a pit.

It can also occur throughout the entire surface of the metal and corrode it in a uniform way. Sometimes corrosion in metals is not very visible, hence making it hard to detect.

## What is Corrosion of Metals?

Corrosion of metals refers to the natural process wherein a refined metal is converted into its more chemically stable form. Examples for such stable forms of metals are metal oxides, metal hydroxides, and metal sulfides.

When metals undergo corrosion, they degrade and break down. A common example of this is the rusting of iron. Corrosion is generally an undesirable phenomenon and there are branches of study (such as corrosion engineering) which aim to control and stop this process.

For example, reactive metals such as potassium and sodium are kept in kerosene oil to avoid their exposure to the atmosphere.

### Factors affecting Corrosion of Metals

There are many factors governing the corrosion of metals.

• In the presence of oxygen, the corrosion of metals increases.
• The metals placed higher in the reactivity series gets corroded more easily than the ones place below them.
• The dissolved salts in water act as an electrolyte and hence the rate of corrosion is enhanced. For example, iron corrodes faster in seawater. The presence of pollutants, for example, NO2 and CO2 increases rusting.

## Examples of Corrosion

There are various examples of corrosion depending upon its type:

• The uniform corrosion is the most basic and rusting is a good example.
• Example of Pitting corrosion is pitting of stainless steel in the presence of water containing chloride.
• Crevice corrosion can be found in nuts and bolt. In the tiny gap between the nut and the threaded bolt and presence of a thin film of water in the humid condition.
• Galvanic corrosion’s example is a steel rivet in alloy 400 sheets which are made exposable to seawater.
• Corrosion of Iron is rusting. Rust is a hydrated ferric oxide (Fe2O3.xH2O). Silver gets tarnished due to the formation of silver oxide Ag2O.

## Types of Corrosion of Metals

There are various types of corrosion such as:

• Caustic Agent Corrosion
• Localized Corrosion
• General Corrosion
• Stress Corrosion Cracking
• Crevice Corrosion
• Pitting Corrosion
• Uniform Corrosion

Of these types, galvanic corrosion is the most common. Let us now understand all the different types of corrosion in detail.

### Pitting Corrosion

Pitting corrosion is very unpredictable and therefore is difficult to detect. It is considered one of the most dangerous types of corrosion. It occurs at a local point and proceeds with the formation of a corrosion cell surrounded by the normal metallic surface. Once this ‘Pit’ is formed, it continues to grow and can take various shapes. The pit slowly penetrates metal from the surface in a vertical direction, eventually leading to structural failure if left unchecked.

### Uniform Corrosion

This is considered the most common form of corrosion wherein an attack on the surface of the metal is executed by the atmosphere. The extent of the corrosion is easily discernible. This type of corrosion has a relatively low impact on the performance of the material.

### Crevice Corrosion

Whenever there is a difference in ionic concentration between any two local areas of a metal, a localized form of corrosion known as crevice corrosion can occur. Examples of areas where crevice corrosion can occur are gaskets, the undersurface of washers, and bolt heads.

### Stress Corrosion Cracking

Stress Corrosion Cracking can be abbreviated to ‘SCC’ and refers to the cracking of the metal as a result of the corrosive environment and the tensile stress placed on the metal. It often occurs at high temperatures.

### Intergranular Corrosion

Intergranular corrosion occurs due to the presence of impurities in the grain boundaries that separate the grains that are formed during the solidification of the metal alloy. It can also occur via the depletion or enrichment of the alloy at these grain boundaries.

### Galvanic Corrosion

When there exists an electric contact between two metals that are electrochemically dissimilar and are in an electrolytic environment, galvanic corrosion can arise. It refers to the degradation of one of these metals at a joint or at a junction. A good example of this type of corrosion would be the degradation that occurs when copper, in a salt-water environment, comes in contact with steel.

## Prevention of Corrosion

Prevention of Corrosion is a very important measure that must be taken to protect metals and to stop them from eroding away.

There are various methods that are implemented to prevent or slow the corrosion of metals which are exposed to electrolytic environments such as salt water or acids. Some of these methods are described below.

### Cathode Protection:

The metal object that is to be protected from rusting is connected to a piece of more electropositive metal like zinc. The anode is made up of a more reactive element which loses electrons and gets oxidized. The anode goes on disappearing and thus saves the cathode form rusting.

### Barrier Protection:

In this process, the metal surface is not allowed to come in contact with atmospheric agents like air and water. Hence it protects the metal from corrosion. The barrier protection can be done by a coating of one metal with another metal with the help of electricity (electroplating).

### Sacrificial Protection:

In this method, the metal to be protected is covered with more electropositive metal such as zinc or magnesium, which gets oxidized and save the metal from corrosion. By using artificial anode more anodic metals like Zn, Mg, Al is connected to the iron article by wire

### Galvanizing:

Covering the iron article with Zinc is Galvanization. Aluminium also serve the process.

⇒ Also Read: Electrolysis and Electroplating

### Other Methods of Prevention of Corrosion

• Choosing the right type of metal can help to prevent corrosion and reduce the requirement for additional preventional measures. For example, metals like aluminium and stainless steel are highly resistant to corrosion.
• The control over the constituents of the environment to which the metal is exposed can also help in the prevention of corrosion. An example of this would be treating hard water with softeners before putting it in a boiler to prevent corrosion inside the boiler.
• One of the cheapest ways for prevention of corrosion is by applying a layer of paint on the surface of the metal. This coating of paint acts like a barrier and protects the metal from corrosion by preventing the flow of electrochemical charge.
• Corrosion can be controlled by coating the metal with a layer of a more reactive metal, zinc for example. The use of this type of sacrificial coating is called cathodic protection. Corrosion can also be checked by coating a metal with a layer of another, less reactive metal such as tin. This method is also known as anodic protection.
• The usage of chemicals that can react with the metal or the environment to prevent corrosive reactions can help curb the corrosion process. These chemicals are called corrosion inhibitors.

The design of the metallic structure can be modified in a way that there is no trapping of water on the metal. These design modifications can also help in the prevention of corrosion.

## Why Prevention of Corrosion is Necessary?

Corrosion can lead to the loss in the purity of the metal. Some important properties of the metal might also be lost due to corrosion. For example, the sturdiness of iron is lost when it undergoes rusting and the rusted iron crumbles easily.

When a metal structure undergoes corrosion, it loses its strength and the tendency to undergo structural collapse increases. The appearance of the metal is also compromised as it loses its lustre.

If measures for the prevention of corrosion are not taken, the corrosive attacks on the metal can greatly increase the cost of maintenance of the structure.

Corrosion can also prove dangerous to humans. A good example would be the cuts that pierce the skin from rusted iron leading to tetanus.

### Rusting of Iron – Electro Chemical Phenomenon

#### Conditions for the Rusting of Iron

The most favourable conditions for the rusting of Iron are,

• Contact with air
• Contact with moisture

Corrosion of Metals – Rusting of Iron

Rusting involves a galvanic cell with cell potentials.

At a point Iron acts as anode and gets oxidized, liberates electrons

$Fe\to F{{e}^{2+}}+2{{e}^{-}}$ ⇒ $E{}^\circ F{{e}^{+2}}/Fe=-0.44V$

Ions which are believed to be obtained from the dissociation of carbonic acid in the atmosphere, gets reducing oxygen to form water

$2{{H}^{+}}+\frac{1}{2}{{O}_{2}}+2{{e}^{-}}\to {{H}_{2}}O$

⇒ $E{}^\circ =1.23V$

Net reaction is:

$Fe+2{{H}^{+}}+\frac{1}{2}{{O}_{2}}\to F{{e}^{2+}}+{{H}_{2}}O$

i.e. $E{}^\circ =1.23-\left( -0.44 \right)$

Or, $E{}^\circ =1.67V$

⇒ $2F{{e}^{+2}}+\frac{1}{2}{{O}_{2}}+2{{H}_{2}}O\to F{{e}_{2}}{{O}_{3}}+4{{H}^{\oplus }}$

⇒ $F{{e}_{2}}{{O}_{3}}+x{{H}_{2}}O\to F{{e}_{2}}{{O}_{3}}.x{{H}_{2}}O$