Types Of Corrosion

Corrosion is a phenomenon in which the surface of a metal is destroyed due to certain external factors which thin it down, make it lose its strength and luster, and make it fragile. Corrosion is a very common phenomenon amongst metals. Various factors work on the surface of different metals at various rates.

Corrosion strongly depends on the resistance of a metal towards it, like some metals such as gold and platinum are strong and resistant towards corrosion whereas iron is very vulnerable as it quickly corrodes on being exposed to moisture and excess oxygen.

When it comes to dividing corrosion into different types, there are various bases for its classification. If we want to classify corrosion based on their characteristics and mechanisms there are eight types of corrosion. Based on their Ionic or Electro-chemical theory there are three types of corrosion: Auto corrosion, Contact corrosion and Externally induced corrosion. Focusing on the former criteria, below given are the eight types of corrosion:

Uniform Attack

In the uniform attack the corrosive agent working on the surface of the metal reacts with the surface ions uniformly over the surface of the exposed metal making the metal thin and fragile.

After some time the metal completely loses its strength.

The uniform attack is the most common type of corrosion an example of which is the rusting of iron when exposed to moisture.

Metal erosion caused by winds near sea shore.
Metal erosion caused by winds near seashore.
Pitting Corrosion
Pitting Corrosion

Selective Leaching

Selective leaching takes place in metal alloys. In selective leaching one of the metals on the alloy undergoes corrosion.

Selective leaching may be done intentionally to remove one metal from the alloy.

The most common example of selective leaching is the removal of zinc from brass also known as dezincification.

Erosion

Erosion is yet another common type of corrosion which occurs naturally. In corrosion the surface of the metal slowly and gradually deteriorates by the action of the environment.

Eventually due to the constant attack of the corrosive agent on the metals, the surface of the metals wears away forming holes and gullies on the surface.

The pits formed are mostly directional and have a pattern.

Pitting

Pitting is perhaps the most dangerous and vicious of all the other types of corrosion. In pitting small or large and deep or shallow pits and holes are formed on the surface, completely destroying the metal.

If pitting occurs in a metal machine the machine completely stops working. It is caused due to some chemical corrosive reagents.

Another reason why this form of corrosion is considered the most dangerous is that this type is difficult to predict and although it may take place gradually and silently the function of the metal stops suddenly without a warning.

Microscopic view of a metal affected by Intergranular Corrosion
Microscopic view of a metal affected by Intergranular Corrosion
Stress Corrosion Cracking of a fuel pipe
Stress Corrosion Cracking of a fuel pipe

Intergranular Corrosion

The grain boundaries of some metals are very reactive.

Because of these active grain boundaries, when there is a corrosive attack on the metal the grains of the boundaries start to react with the agent, this causes Intergranular corrosion.

The Intergranular corrosion mostly occurs in alloys and may be at times due to some impurities present on the grain boundaries.

Stress Corrosion Cracking

The stress corrosion cracking is a unique type of corrosion in which cracks are formed on the surface of the metal due to a specific agent causing stress on the surface and eventually breaking the metal down.

The two common examples of stress corrosion cracking are: season cracking of brass, and the caustic embrittlement of steel.

During the process of corrosion, the rest of the surface remains intact while small and fines cracks proceed at one point.

These cracks then extend further breaking through the surface of the metal.

This type of corrosion is not very common and occurs only when the metal is exposed to a specific medium.

Galvanic Corrosion caused due to difference in anodic index of plates and bolts
Galvanic Corrosion caused due to difference in anodic index of plates and bolts
Silver Bridge collapse caused due to Crevice Corrosion in 1967
Silver Bridge collapse caused due to Crevice Corrosion in 1967

Galvanic Corrosion or Two-Metal Corrosion

In galvanic corrosion a less resistant metal and a high resistant metal are placed together in a corrosive medium.

Due to the potential difference between the two metals there is an electric flow between metals, this makes less resistant more vulnerable and the high resistant more strong.

This is called galvanic corrosion.

Crevice Corrosion

Crevice corrosion is an intense localized corrosion which occurs within crevices and other shielded areas on metal surfaces when it is exposed to corrosive agents.

This results in holes and puts on the surface of the metal.

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Comments 2 comments

agusfanani profile image

agusfanani 5 years ago from Indonesia

This hub has given me something I've never known before. Thank you.


jbosh1972 profile image

jbosh1972 4 years ago from Indianapolis, IN. USA

I appreciate the systematic break down of the different corrosion types. Perhaps you should right hubs going into detail of how the different kinds of corrosion can occur. Also, check out my hubs. My interest are welding, art, chemistry, and metallurgy.

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