Corrosion (corrosion, English: corrosion) refers to an object or its condition whose appearance and function have been impaired by chemical or biological actions.
Corrosion of metal means that it chemically reacts with the surrounding environment (adjacent metal, gas, etc.) to melt or produce corrosion products (so-called "rust"). This includes not only the generally-known “rust” on the surface, but also the decrease in thickness and the opening of holes due to corrosion.
Corrosion other than metal
Generally, corrosion is considered only for metals, but ceramics and plastics also cause corrosion and deterioration.
In addition, the expression "chemical corrosion" is sometimes used as the cause of burns (especially chemical burns) (see "burns" for details), and erosion or decay of living organisms or substances derived from living organisms is also called corrosion. There is. Hereinafter, metal corrosion will be mainly described.
Principle of metal corrosion
Metal corrosion proceeds when the metal on the surface loses electrons due to the redox reaction, is ionized, and falls off the metal surface. The generated ions become oxides, hydroxides or carbonates (in the case of patina) due to oxygen and are often deposited on the surface. When the crystal structure and physical properties change significantly in the process of replacing metal ions with oxides, when the metal corrodes, the shape and strength are impaired and it is regarded as rust.
Metal surface instability
Normally, the metal surface is covered with a thin (tens of Å) oxide. This is because the newly formed metal surface has a surface (that is, a surface) in which no adjacent metal exists, and the contribution of energy stabilization due to the delocalization of free electrons is smaller, so that the metal surface becomes energetically unstable. There is. Since this unstable metal rapidly reacts with oxygen molecules in the atmosphere, it forms a barrier layer of oxide, and the exposed metal surface does not exist in a natural state.
Since the oxide barrier layer existing on the surface differs depending on the type of metal, environment, processing, adhesion of foreign matter, etc., the corrosion resistance of the surface also changes. In other words, if the oxide barrier layer is easily damaged by physical or chemical action, the barrier layer is repeatedly peeled off and the surface is oxidized, and the corroded surface is invaded inside the metal.
In particular, the presence of water and trace amounts of acid accelerates the metal corrosion process. If the ionization tendency of the metal is greater than H +, the metal surface is easily ionized, and if the metal oxide is water-soluble, the barrier layer is also peeled off.
Such a state in which corrosion invades from the punctate defects of the barrier layer or the plated surface is called pitting corrosion (pitting corrosion, pitting corrosion). The presence of halide ions (mainly chloride ions, Cl-) is known to trigger pitting corrosion. The pores created by corrosion cause electrical delocalization on the metal surface and promote corrosion.
When stainless steel is manufactured at 600-800 ° C, precipitation of chromium carbide occurs, and grain boundaries with low chromium are generated by the amount of precipitation. The place where this chromium is low is vulnerable to corrosion and corrosion is preferentially performed. This phenomenon is called intergranular corrosion.
In addition, since there is a redox reaction at the center of metal corrosion, the part in contact with dissimilar metals becomes a factor that accelerates corrosion in order to form a galvanic cell. Corrosion due to this cause (anode reaction of the battery) is contact corrosion of dissimilar metals (galvanic corrosion).