There are many different types of corrosion. Here we will cover the basics of the reaction and the three main types.
What is Corrosion?
Corrosion is a natural reaction that can occur in a variety of scenarios. The reaction results in a metal being converted to a more chemically stable form such as oxides, sulphides and hydroxide, which is a one-way reaction and cannot be reversed. It is a reaction that can occur on all metals however some are less reactive than others. For example, noble metals like gold are less reactive and rarely corrode.
The Most Common Types Of Corrosion
The most well-known form is rusting. Rusting is the formation of iron oxides which produces the distinctive orange colour. The products of the reaction reflect the metals natural state. The process can lead to degradation of the properties of the material e.g. strength and appearance which can result in catastrophe. Due to this, metals need anti-corrosion measures in place.
Why Does It Occur?
To understand corrosion, we must first understand how metal is produced. See the article, How is steel made. To produce metal, it is extracted from its natural stable form, ore, resulting in reduced stability. Therefore, some metals, such as iron, favour their natural state and spontaneously revert to it.
During the casting process, chemicals and coatings are added to the metals to make them stable in the environment and inhibit corrosion.
For corrosion to occur there are three main components:
- A metal (steel)
- Oxygen (generally from the atmosphere)
- An electrolyte (water)
Many structural alloys corrode at a gradual rate due to the natural moisture in the air. However, certain substances have a strong effect on the reaction. The process occurs in locations where the metal is exposed.
3 Different Types Of Corrosion
Uniform attack
This is the most common type and, as the name suggests, occurs uniformly across an exposed area of metal. The process is continual and leads to thinning and a reduction in mechanical properties. Steel and cast iron are types of metals that experience uniform corrosion when exposed to the elements. It is easily identified as it often has a large presence, therefore, meaning this type of corrosion doesn’t lead to catastrophic failures.
Galvanic Corrosion
Galvanic corrosion is the process when one metal corrodes preferentially compared to another when it is in electrical contact, via an electrolyte. The process occurs when two dissimilar metals are submerged in a conductive solution. This type of reaction is commonly exploited in the classroom to show students the use of primary cells. One metal (the cathode) is shielded, as the other is corroded (the anode). The rate of the reaction for the same metal uncoupled is slower compared to being coupled in a conductive solution. This reaction is encouraged in some situations.
For example, inexpensive household batteries tend to contain carbon-zinc cells. The zinc cell inside of the circuit will corrode preferentially which is essential for the battery to produce electricity. The most famous instance of galvanic corrosion was the Statue of Liberty. This reaction took place between the outer layer of copper skin and the wrought iron support structures. The insulation material, shellac, between the two metals failed over time resulting in the reaction.
Pitting Corrosion
Pitting corrosion is localised, which gives the appearance of holes in the material due to taking places cavities. The driving force behind this is the removal of the protective barrier, leaving small areas of the metal exposed to where the reaction will occur.
As the layer is removed, the metal becomes anodic while an unknown and potentially large area becomes cathodic, which results in galvanic corrosion in a localised area. It is challenging to detect, prevent and design against leading it to be considered one of the most dangerous forms.
Related Posts: How To Stop Corrosion, How is Steel Made?