TYPES OF CORROSION
By a process mechanism metal corrosion is divided into chemical and electrochemical.
Chemical Corrosion is a process, where the metal interacts with the corrosion environment, during which the metal is oxidized, while an environment oxidizer reduced. Interaction products are not spatially separated.
Electrochemical Corrosion is a process, where the metal interacts with the corrosion environment (an electrolytic solution), during which metal atom ionization and corrosion environment oxidizer reduction do not occur at the same time, and ionization and reduction rates depend on an electrode potential.
Depending on corrosion environment and behavior the corrosion occur in several forms.
Gas Corrosion is a metal deterioration in a gaseous medium at a minimum moisture content (as a rule, not more than 0,1%) or at high temperatures. Such corrosion is frequent in chemical and petrochemical industry. For example, when a sulfuric acid is recovered at a sulfur dioxide oxidation stage, in ammonia synthesis, nitric acid and hydrogen chloride recovering, organic alcohol synthesis processes, petroleum cracking and etc.
Atmospheric Corrosion is a metal deterioration in atmosphere or any wet gas.
Underground Corrosion is a metal deterioration in soils and grounds.
Biological Corrosion is a deterioration caused by the activity of microorganisms.
Contact Corrosion is a deterioration occurring, when metals having various stationary potentials in the electrolyte involved are in contact.
Radiation Corrosion is a deterioration caused by radioactive radiation.
External/Stray-Current Corrosion is a metal deterioration caused by an external/stray current.
Stress Corrosion is a deterioration occurring, when the corrosion environment and mechanical stresses act at the same time. If these stresses are tensile, a metal may crack. This is a very dangerous corrosion, especially for structures exposed to mechanical loads (axes, springs, pressure chambers, steam boilers, turbines and etc.). If metal products are exposed to cyclic tensile stresses, corrosion fatigue may occur and metal fatigue strength decreases. Exposed to such corrosion are car springs, ropes and mill rolls.
Corrosion Cavitation is a metal deterioration occurring, when a metal is simultaneously exposed to the corrosion and impact effects of environment.
Fretting Corrosion is a deterioration caused by vibration combined with a corrosion environment effect. This corrosion may be eliminated by selecting structural materials in a correct way, reducing a friction factor and applying coatings and etc.
Corrosion is called continuous, if it covers an entire metal surface. Continuous corrosion may be uniform, if a corrosion rate is equal across the entire metal surface, and non-uniform, if a corrosion rate varies in various surface sections. The uniform corrosion occurs, for example, when steel pipes corrode in air.
In case of selective corrosion one structural constituent or one component of an alloy is destructed. The examples of such corrosion are iron graying and brass dezincing.
Local Corrosion covers particular surface sections. The local corrosion may occur as isolated spots not deeply recessed in a metal; pits appearing as voids deeply recessed in the metal, or points deeply penetrating into the metal.
The first form is observed, for example, when the brass is exposed to sea water, while the pit and point corrosions occur in ground-driven steels and sub-sea austenitic chromium-nickel steel, respectively.
Undersurface Corrosion starts on a metal surface and then develops depth-ward. Corrosion products turn out to concentrate in metal cavities. This corrosion makes the metal products become swollen and laminated.
Intercrystalline Corrosion is described as metal deterioration across grain boundaries. This corrosion is especially dangerous, as the metal apparently does not change, but quickly loses its strength and elasticity, and easily deteriorates due to unconsolidated weak corrosion product formations between the grains. Especially exposed to such corrosion are chromium and chromium-nickel steels, and nickel and aluminum alloys.
Crevice Corrosion deteriorates metal surfaces under gaskets, in gaps, threaded joints and etc.
Chemical Corrosion is a process, where the metal interacts with the corrosion environment, during which the metal is oxidized, while an environment oxidizer reduced. Interaction products are not spatially separated.
Electrochemical Corrosion is a process, where the metal interacts with the corrosion environment (an electrolytic solution), during which metal atom ionization and corrosion environment oxidizer reduction do not occur at the same time, and ionization and reduction rates depend on an electrode potential.
Depending on corrosion environment and behavior the corrosion occur in several forms.
Gas Corrosion is a metal deterioration in a gaseous medium at a minimum moisture content (as a rule, not more than 0,1%) or at high temperatures. Such corrosion is frequent in chemical and petrochemical industry. For example, when a sulfuric acid is recovered at a sulfur dioxide oxidation stage, in ammonia synthesis, nitric acid and hydrogen chloride recovering, organic alcohol synthesis processes, petroleum cracking and etc.
Atmospheric Corrosion is a metal deterioration in atmosphere or any wet gas.
Underground Corrosion is a metal deterioration in soils and grounds.
Biological Corrosion is a deterioration caused by the activity of microorganisms.
Contact Corrosion is a deterioration occurring, when metals having various stationary potentials in the electrolyte involved are in contact.
Radiation Corrosion is a deterioration caused by radioactive radiation.
External/Stray-Current Corrosion is a metal deterioration caused by an external/stray current.
Stress Corrosion is a deterioration occurring, when the corrosion environment and mechanical stresses act at the same time. If these stresses are tensile, a metal may crack. This is a very dangerous corrosion, especially for structures exposed to mechanical loads (axes, springs, pressure chambers, steam boilers, turbines and etc.). If metal products are exposed to cyclic tensile stresses, corrosion fatigue may occur and metal fatigue strength decreases. Exposed to such corrosion are car springs, ropes and mill rolls.
Corrosion Cavitation is a metal deterioration occurring, when a metal is simultaneously exposed to the corrosion and impact effects of environment.
Fretting Corrosion is a deterioration caused by vibration combined with a corrosion environment effect. This corrosion may be eliminated by selecting structural materials in a correct way, reducing a friction factor and applying coatings and etc.
Corrosion is called continuous, if it covers an entire metal surface. Continuous corrosion may be uniform, if a corrosion rate is equal across the entire metal surface, and non-uniform, if a corrosion rate varies in various surface sections. The uniform corrosion occurs, for example, when steel pipes corrode in air.
In case of selective corrosion one structural constituent or one component of an alloy is destructed. The examples of such corrosion are iron graying and brass dezincing.
Local Corrosion covers particular surface sections. The local corrosion may occur as isolated spots not deeply recessed in a metal; pits appearing as voids deeply recessed in the metal, or points deeply penetrating into the metal.
The first form is observed, for example, when the brass is exposed to sea water, while the pit and point corrosions occur in ground-driven steels and sub-sea austenitic chromium-nickel steel, respectively.
Undersurface Corrosion starts on a metal surface and then develops depth-ward. Corrosion products turn out to concentrate in metal cavities. This corrosion makes the metal products become swollen and laminated.
Intercrystalline Corrosion is described as metal deterioration across grain boundaries. This corrosion is especially dangerous, as the metal apparently does not change, but quickly loses its strength and elasticity, and easily deteriorates due to unconsolidated weak corrosion product formations between the grains. Especially exposed to such corrosion are chromium and chromium-nickel steels, and nickel and aluminum alloys.
Crevice Corrosion deteriorates metal surfaces under gaskets, in gaps, threaded joints and etc.