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What is Corrosion? Why Metal Corrodes?

Updated: Nov 18, 2021


Corrosion Definition

Corrosion describes the deterioration of metal due to chemical reactions between the metal and the environment. The reaction between the metal and its environment starts as soon as the water and the oxygen in the environment come into contact with the metal. The water acts as the electrolyte for the corrosion to start on the metal surface. The extent and type of deterioration due to corrosion depend on the metal type and the environmental conditions where the metal is exposed to.


The Nature of the Metals

Metals in their natural forms are found as ore, ore is extracted from the ground through mining and is processed to make it useful for various purposes. Metals, that are extracted from the ground are divided into two categories, i.e. less noble (i.e. ignoble) and more noble. Ignoble means a material can react with its surrounding environment when exposed to it.



Almost all metals are prone to corrosion attack, due to their tendency to react with the surrounding atmosphere, but the rate of corrosion differs based on the metal’s nobility. For instance, mild steel or carbon steel is less noble than stainless steel, thus the corrosion rate on mild steel is faster than stainless steel. Why stainless steel corrodes slowly is that it is a combination of metal and other alloys, this combination makes the stainless steel nobler than mild steel, and this is why it is often preferred to other available metals like mild steel.


Some other small groups of noble metals like silver, gold, platinum, etc. are less reactive to the surrounding atmosphere and are the only metal we use in their pure form, these metals rarely corrode; hence they are usually more valuable when compared with the other alternate metals and are very costly.



There are various causes of metal corrosion. Some of these causes can be prevented by combining pure metal with alloys. In other cases, a strategic combination of metals or the effective management of the metal’s environment can also prevent corrosion.


Here are the 2 common forms of corrosion


General Corrosion

This is a common type of corrosion that attacks the entire surface of a metal structure. This type of corrosion occurs on the metal due to electrochemical or chemical reactions. When this happens, the metal thickness eventually reduces to a point where the metals break down or fail to perform an intended work for which it was designed for. This form of corrosion is predictable, so it is easy to plan and manage the attack.


Localized Corrosion

As the name suggests, this type of corrosion attacks specific parts of a metal structure. It comes in three different forms and may cause catastrophic failure of a structure or equipment.

  • Pitting, which occurs on the localized area and creates small holes in the metal surface.

  • Crevice corrosion, which occurs on a metal surface at, or immediately adjacent to, the gap or crevice between two joining surfaces, including the hidden areas under gaskets.

  • Filiform corrosion, also known as under film corrosion, is a special form of corrosion that occurs under some thin coatings when water penetrates a coating.

Galvanic Corrosion

Commonly termed as bimetallic or dissimilar corrosion, takes place when there are two different metals connected in an electrolyte. When this happens, one of the metal which is less noble than other become the anode and start corroding, however, the other metal which becomes nobler being unaffected from corrosion.


Corrosion is responsible for about $2.5 trillion in revenue loss every year. Adopting well-researched and properly executed prevention techniques can help prevent up to 25% of this loss. Preventing corrosion will also help to preserve people’s health and safety. Corroded ships, buildings, bridges, and other metal structures all come with injury and death risks.


You may learn more on what is corrosion and the various methods used to control the corrosion; checkout the following courses;

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