Corrosion of metal (why iron rusts)


From a thermodynamic point of view, most metals exist in nature in a complex (stable) state. To obtain pure metals, people need to spend a lot of energy extracting them from minerals (this is a metallurgical process), therefore, most metals (in an unstable state) have the ability to spontaneously interact with the environment and then go into an oxide state (compound), i.e. returning to its natural state (ore):

Therefore, the corrosion of metals is a spontaneous trend that can be observed everywhere, and is the reverse process of the metallurgical process. The so-called corrosion control is not about allowing people to change the laws of thermodynamics, but about using various anti-corrosion measures to reduce the rate of corrosion and control it as little as possible.

Corrosion of metals refers to the destruction of metals due to chemical changes, electrochemical changes or physical dissolution under the influence of environments (most often liquids and gases). For example, metal parts rust in the atmosphere, steel forms a large amount of oxide scale due to the action of high temperature and atmospheric oxygen during the rolling process. In the chemical industry, metal equipment, machinery, etc. come into contact with highly corrosive media (such as acid, alkali, salt, etc.), especially under the conditions of high temperature, high pressure and high-speed flow, all kinds of corrosion is extremely serious.

The definition of corrosion clearly indicates that the corrosion of metals must be influenced by the external environment, and this influence occurs at the interface between the metal and the environment. Thus, the corrosion of metals is a reactive system, including both the material and the environment.

With the rapid development of non-metallic materials, especially synthetic materials, their destruction has also attracted people's attention. The definition of corrosion applies to all materials, that is, it is defined as "corrosion is the damage to a substance due to its interaction with the environment." An extended definition of corrosion has been applied to damage to plastics, ceramics, wood, concrete, and other materials. However, metals and their alloys are still the most important structural materials.