The main types of corrosion
Corrosion is destruction of metal by chemical or electrochemical interactions with the environment. Chemical called corrosion, which occurs in the environment, not conducting electric current. Such corrosion occurs when high temperature steel processing hot press or in the process of operation in a hostile environment. On the surface of the metal appears a film of oxide, sulfide and other chemical compounds. In some cases, the formation of a dense surface film can prevent the development of further corrosion. For example, tin, lead, aluminum, chromium and Nickel to form on its surface a continuous protective film. A film on the surface of steel and cast iron has a loose structure, cracking and corrosion penetrates deep into the metal.
It is associated with electric current flow. Rusting metal structures under the action of atmospheric moisture, ship hulls, steel structures in fresh and salt water are examples of such corrosion. A detailed examination of the mechanisms of corrosion chemical and electrochemical impossible to see between them clear distinctions. It happens that corrosion chemical blends in with corrosion, electrochemical, or Vice versa. And corrosion of metals in electrolytes generally has a dual nature.
Other types of corrosion
Depending on conditions of corrosion distinguish gas corrosion if the metal is in a gaseous environment at high temperature, corrosion in nonelectrolytes — if the metal is, for example, in gasoline. Atmospheric corrosion-prone metal structures in the air. In liquid media that conduct electric current, there is corrosion in electrolytes. Soil corrosion subjected to underground pipelines and underground metal structures. Stray currents can cause electrocorrosion. As a result of contact of different metals in the electrolyte (for example, if parts of copper in contact with aluminium) possible contact corrosion. Structural corrosion associated with the heterogeneity of the metal structure. For example, the destruction of iron in sulfuric acid solution, accelerated due to inclusions of graphite in the metal. Stress corrosion can change its value and direction, which often leads to corrosion fatigue — decrease the fatigue limit of the metal. Corrosion due to friction, may, for example, to break the neck of the shaft when rotating in water. Crevice corrosion is possible in the gaps between the parts. The corrosion products cause the secreted by microorganisms and sweat glands of a person.
There are local corrosion, solid and intergranular. Localized corrosion concentrates the individual sections on the surface of metals. This is a dangerous form of destruction. Its distribution is directed into the interior, leading to the loss of serviceability of the structure. Localized corrosion often occurs in places where there is mechanical damage to the surface. Continuous corrosion is characterized by the fact that the whole metal surface is covered with rust, gradually corrodes the product. This type of corrosion is relatively easily monitored and evaluated. If the process of destruction, begun on the surface extends deep into the grain boundaries, then we are dealing with intergranular erosion. It’s that it leads to brittle and metal and reduce its bearing capacity. This kind of destruction, which is very dangerous, often occurs during welding or heat treatment of metals. The degree of resistance of metals to intergranular corrosion depends on the percentage content of carbon. For example, alloyed stainless steel grade Х18Н9 reducing the carbon content to 0.015% leads to the complete elimination of this type of corrosion.
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