Corrosion refers to the deterioration of metal. It is a natural process that takes place when the metal is converted into a more stable form such as hydroxide, oxide, or sulphide through a chemical reaction.
For corrosion to take place, the metal must be in an environment containing certain substances such as oxygen, hydrogen, or particular types of bacteria. Sometimes, it can even be caused by an electrical current or dissimilar metals.
Iron corrosion is the most common form of corrosion and occurs when the metal is exposed to both oxygen and water, creating a red iron oxide that we commonly refer to as rust. Iron alloys such as steel are also susceptible to rust. Less commonly, iron can rust when it reacts with chloride in an environment that lacks oxygen.
Different Forms of Corrosion
Corrosion can take several different forms, including:
This refers to corrosion that is evenly spread across a large area of material. It is the most common form of corrosion seen.
This is one of the most severe forms of corrosion as it can be difficult to spot. It is a localised form of damage and usually takes place when a local anodic or cathodic point generates a cell of corrosion on the metal’s surface. A hole or cavity will start to form, penetrating the material vertically from the surface. A common cause of pitting corrosion is non-uniformity of metal structures.
Galvanic corrosion takes place when differing metals are exposed to an electrolyte that produces an electrically conducting substance when dissolved in a solvent such as water. It can also take place when a metal is exposed to differing concentrations of an electrolyte.
Crevice corrosion occurs in environments with low levels of oxygen such as under bolt heads or washers. It is a localised form of damage and occurs when the lack of oxygen alters the pH balance of the materials.