Corrosion can be defined as the destruction or deterioration of a material because of reaction with its environment. The term can refer to a process or the damage caused by such a process.
Important to note
- In some cases of corrosion there is no visible deterioration. The effect of the reaction with the environment is to change the properties, often the strength, of the material. Such forms of corrosion are not common in day-to-day experience but are of great importance to the corrosion engineer.
- Loss due to purely mechanical forces is known as wear. However, there are cases when corrosion and loss by mechanical forces combine and these are described in these notes.
- The definition applies to materials other than metals. However, the term is usually applied to metals. (Metallic corrosion is mainly considered in these notes). The mechanisms of corrosion of non-metallic materials are governed by different physical and chemical principles.
- Rusting is a term reserved for corrosion of iron and steel.
Rusting of iron and steel is perhaps the most common example of metallic corrosion.
There are many well-known examples such as the rusting of exhaust systems and bodies of cars, water pipes and various types of structural steel-work.
There are other examples of corrosion in everyday life. Silver tarnishes by reacting with sulphur to form black silver sulphide.
Copper roofs develop a green patina by reacting with various components of the atmosphere.
Boat owners will be aware of a number of corrosion problems that can arise in a marine environment, even in metals such as stainless steels which are often erroneously believed to be corrosion resistant.
However, there are many other corrosion problems occurring in industry, especially in the petroleum and chemical industries.
The average person would be totally unaware of them.
While corrosion is largely a harmful process, there are some desirable and beneficial cases. The green patina on copper mentioned above is generally considered to be pleasing to the eye.
Chemical or electrochemical machining can be used to machine hard and difficult-to-machine parts economically. Anodising of aluminium is a beneficial corrosion process used for obtaining a better appearance and corrosion resistance. Corroding etchants are used in metallurgical investigation to enable examination of metallic structures.
Perhaps the most common example of beneficial corrosion is in batteries. Many of the chemical reactions which occur in batteries are corrosion reactions.