What is Corrosion?- Definition, Factors, Types and FAQ’s

Corrosion

We should have bought metal things for various purposes in our houses. While doing so, we could have come across the term called corrosion. So, what is corrosion? Why should we consider it?

Corrosion is a major phenomenon that we have to encounter in our daily lives. On many occasions, we should have noticed that some iron or steel objects appear to be covered in a brown-coloured or reddish layer. This coverage of the metals will lead to a phenomenon called rusting. Rusting is one form of corrosion. 

This article will discuss everything related to corrosion while having a good look at other related concepts.

What is Corrosion?

Corrosion is the formation of a brown or reddish-coloured layer on the surface of the metals. It generally converts refined metals into more stable compounds, namely metal sulphides, metal oxides or metal hydroxides. Similarly, it involves the iron oxides form through oxygen and atmospheric moisture in the rusting of irons.

While going through the logic of science behind it, one can say that it is an irreversible and spontaneous reaction.

Corrosion Definition

Corrosion is nothing but a natural process that is capable of transforming pure metals into unpleasant objects or substances. It usually happens when it is exposed to air, water, or other external phenomena.

The causes of this chemical reaction include the damage and destruction of metal substances. It starts very small and reaches the whole bulk of the material over time. That is why it is essential to opt for highly durable and corrosion-resistant metal products so that they stay fresh longer.

Corrosion affects the desirable properties of the objects, so it is an undesirable phenomenon. It only has negative effects. For instance, iron contains good rigidity and better tensile strength. Its strength is even better, particularly when it is alloyed with certain elements. However, this metal loses its strength and becomes more brittle, structurally unsound and flaky when it is subjected to rusting.

In the meantime, we can say corrosion is a diffusion-controlled process. So, it mostly takes place on exposed objects. Hence, several manufacturers have been trying to cover the exposed surfaces using various techniques in order to reduce the effect of corrosion. Some steps are also taken to enhance the corrosion-resistant characteristics of the metals. 

For the improvement of corrosion resistance, processes like chromate conversion and passivation are widely used. However, even after all such tries, it is impossible to protect a few objects which are regarded as more vulnerable to exposure.

On the other hand, we can classify corrosion as one type of electrochemical process because it generally involves redox reactions. Mostly, these reactions are between the metal and several atmospheric agents, namely oxygen, water and sulphur dioxide.

Factors influencing Corrosion

Various factors affect corrosion. The following are a few of them:

• Presence of impurity contents such as salt, especially NaCl.
• One of the major factors includes the exposure of the materials to the environment, which contains gases such as SO₂, SO₃, CO₂, etc.
• Exposing the metals to moisture, particularly salt water. It can increase the corrosion rate.
• Another main factor is the temperature. The rise in temperature can increase the rate of corrosion.
• Acid content present in the atmosphere can increase the corrosion process.

Types of Corrosion

There are multiple types of corrosion available. Let us see what they are:

1. High-temperature corrosion

As per its name, it is one kind of corrosion of materials that occurs due to heating. A rise in temperature causes acceleration in the rate of corrosion. It mostly occurs in metals. Because of hot atmospheric gases, namely sulphur, oxygen and other compounds, there will be an occurrence of chemical deterioration. All these gas components can oxidise the metals easily.

For instance, materials utilised in car engines are obliged to withstand heat for a longer period. They go through combustion and other high-temperature processes in order to perform, so they possess high resistance to corrosion.

2. Microbial corrosion

Microbial corrosion, aka MIC (microbiologically influenced corrosion), is caused by microorganisms. One of the most popular types of MIC is the chemoautotrophs. This corrosion can affect both metallic and non-metallic materials even without oxygen.

3. Metal dusting

This type of corrosion takes place while exposing vulnerable materials to a certain type of surroundings. It consists of high carbon activities like synthetic gas.

Metal dusting involves the breaking up of bulk metals into powders. In metal dusting, corrosion occurs because the graphite layer is deposited on the metals’ surface from CO (carbon monoxide), which is present in its vapour state. In addition, metal atoms are capable of transferring directly into graphite layers.

4. Hydrogen grooving

This corrosion takes place in piping systems. Because of the interaction of hydrogen gas bubbles, corroded pipe constituents and corrosive agents with the pipes, a groove will be formed. The gas bubbles are highly capable of destroying the protective coating as soon as it interacts with the material.

5. Uniform corrosion

Uniform corrosion is regarded as the most popular type of corrosion. Here, the metal surface’s corrosion is caused by exposure to the atmosphere. Furthermore, the corrosion’s extent is easily perceptible. Uniform corrosion’s impact on the performance of the material is comparatively low. 

For example, immerse a piece of zinc and steel in sulphuric acid. This acid should be in a diluted form. Doing so will generally dissolve over the whole surface at a consistent rate and speed.

6. Pitting corrosion

It is almost impossible to detect pitting corrosion since it is unpredictable. Pitting corrosion is a dangerous form of corrosion. It takes place at a small point, corroding, forming a corrosion cell that tends to be surrounded by a normal metallic surface.

After the formation of this pit, it starts to grow and form multiple shapes corroding a bigger part of the surface and making the metal useless. Later, the pit will penetrate the metal surface, which ultimately leads to a failure in the metal structure. 

For example, imagine a water droplet on the surface of the steel. Pitting will be formed from the centre of the droplet.

7. Intergranular corrosion

The occurrence of intergranular corrosion is caused by impurity content present in the grain boundaries. It is capable of separating the formation of grain during a process called metal alloy solidification. It can also occur through the alloy’s enrichment or depletion at various grain boundaries.

For example, due to intergranular corrosion, aluminium base alloys are affected.

8. Stress corrosion cracking

The abbreviation of stress corrosion cracking is SCC. It indicates that the metals will crack due to the presence of tensile stress placed on the metal’s surface and the corrosive environment. It usually takes place at higher temperatures.

Example: SCC of stainless steel present in chloride solution.

Preventive measures for Corrosion

The main objectives of the manufacturing and other industrial units are to prevent the occurrence of corrosion for both safety and economical reasons. In today’s world, most of the items that we see are made of metals. So, it is important to ensure whether these metals have high resistivity towards corrosion.

Some of the metal products include:

• Automobiles
• Bridges
• Machinery
• Doors
• Cot
• Railway lines
• Window grills

In order to avoid the rusting of these metals, there are several preventive steps taken by the organisations. It is a concerning issue, and it must be addressed properly for the sake of the industries and the consumers who buy to use them. 

Certain treatments are done to prevent or slow down the corrosion process. It is specifically useful for objects frequently exposed to several hostile environments, including acids, saltwater, weather, etc. 

Below mentioned are a few corrosion preventive measures:

• Passivation
• Anodisation
• Electroplating
• Galvanisation
• Biofilm coatings
• Painting
• Greasing
• Anti-corrosion coatings
• Frequent cleaning of metal surface
• Drying agents
• Use of corrosion inhibitor

Conclusion

In this educational blog, we have understood the corrosion meaning, corrosion definition and various other concepts related to corrosion. It is clear that corrosion is a negative phenomenon and has the capability to destroy the metals. 

Furthermore, various types of corrosions were also discussed, along with the preventive actions.

Frequently Asked Questions