Chemical corrosion

Corrosion is the destruction of metal due to chemical or electrochemical interactions with the environment. Chemical corrosion is corrosion that occurs in an environment that is not electrically conductive. This corrosion occurs when steel is subjected to high temperature hot-pressure treatment or when it is used in an aggressive environment. A film of oxide, sulfide, and other chemical compounds appears on the surface of the metal. In some cases, the formation of a dense surface film can prevent further corrosion. For example, tin, lead, aluminum, chromium, and nickel form a continuous protective film on their surface. The film on steel and cast iron, on the other hand, has a loose structure, cracks and corrosion penetrates deep into the metal.

Electrochemical corrosion

It is caused by the flow of electrical current. Structures corroded by atmospheric humidity, ship hulls, and hydraulic steel reinforcements in salt or fresh water can serve as examples of this type of corrosion. In a detailed consideration of the mechanisms of chemical and electrochemical corrosion it is impossible to see any clear distinction between them. It happens that chemical corrosion flows smoothly into electrochemical corrosion or vice versa. In general, corrosion of metals in electrolytes is of a dual nature.

Other types of corrosion

Depending on the conditions under which corrosion occurs, a distinction is made between gas corrosion, when a metal is in a gas medium at a high temperature, and non-electrolyte corrosion, when a metal is in gasoline, for example. Atmospheric corrosion affects metal structures in the air. In liquids which conduct electricity, corrosion occurs in electrolytes. Soil corrosion affects underground pipelines and underground metal structures. Stray currents can lead to electrical corrosion. Contact corrosion may occur when different metals come into contact with each other in the electrolyte (e.g., when copper parts come into contact with aluminum). Structural corrosion is related to the heterogeneity of the metal structure. For example, the degradation of cast iron in a sulfuric acid solution is accelerated by graphite inclusions in the metal. Stress corrosion can change its value and direction, often resulting in corrosion fatigue - a decrease in the endurance limit of the metal. Frictional corrosion, for example, can destroy a shaft journal as it rotates in water. Crevice corrosion can occur in the gaps between parts. Biocorrosion is caused by products released by microorganisms and human sweat glands.

According to localization

A distinction is made between local corrosion, solid corrosion and intergranular corrosion. Localized corrosion is concentrated in isolated areas on the surface of metals. This is a dangerous form of damage. Its spreading is directed inward resulting in loss of serviceability. Localized corrosion often occurs where there is mechanical damage to the surface. Solid corrosion is characterized by the entire surface of the metal being covered in rust which gradually corrodes the product. This type of corrosion is relatively easy to monitor and evaluate. If the process of destruction starts on the surface and extends deep into the grain boundaries, then we are dealing with intergranular erosion. It is terrible because it leads to brittleness of the metal and to a decrease in its load-bearing capacity. This type of destruction, which is very dangerous, is often found in welding or heat treating metals. The degree to which metals are resistant to intergranular corrosion depends on the percentage of carbon content. For example, in alloy chromium-nickel steel grade X18H9 reduction of carbon content to 0.015% leads to the complete elimination of this type of corrosion.

Buy, price

Evek GmbH always has high quality rolled steel of alloyed steel grades, as well as carbon steel with anti-corrosion coating in stock at the best prices. It is formed taking into account technological features of production and does not include additional costs. All products are certified. Lead times are minimal. Wholesalers get preferential discounts.