Crystal corrosion

Intergranular corrosion is a type of corrosion that occurs in a narrow area between grains of a metallic material and is a type of localized corrosion. Early nickel-base and iron-nickel-based corrosion-resistant alloys were almost completely corroded. In the middle of the 20th century, the corrosion problem of corrosion resistance of crystal grains was the focus of research. After extensive and in-depth research, the cause, mechanism and countermeasures of the corrosion of crystal corrosion of corrosion-resistant alloys have reached consensus. It is agreed that the intergranular corrosion of corrosion-resistant alloys is due to the precipitation of carbides and intermetallic phases along the grain boundaries due to the heat of precipitation of the harmful carbides and intermetallic phases. These harmful precipitates are mostly rich in chromium and rich. Molybdenum, the precipitation of which is accompanied by the depletion of these effective elements of corrosion resistance around them, forms a network-like depleted zone narrow along grain boundaries. In a corrosive environment, this depleted zone is not sufficiently corrosion resistant and therefore preferentially corrodes, eventually constituting intergranular corrosion, which is currently recognized as a depletion theory for intergranular corrosion of sensitized alloys.

Precipitation of carbides and metals is the root cause of intergranular corrosion in sensitized corrosion resistant alloys. Preventing such harmful precipitation or reducing the degree of depletion of effective elements due to harmful precipitation becomes an elimination or reduction of intergranular corrosion. Technical measures. The difference in the type of alloy and the degree of alloying determines the type and degree of precipitation of the sensitized alloy. Some alloys only precipitate carbides, so when the alloy composition is designed, satisfactory results can be obtained by means of reducing carbon or adding stabilizing elements, and for those alloys that have both carbide precipitation and metal phase precipitation sensitivity, In addition to reducing the carbon content of the alloy and adding stabilizing elements, it is necessary to adopt carbon reduction or addition of stabilizing elements in the design of the alloy composition to obtain satisfactory results, and to be sensitive to both existing carbide precipitation and metal phase precipitation. Alloys, in addition to reducing the alloy’s carbon and adding stabilizing elements, must limit alloying elements that favor the precipitation of intermetallic phases or add elements that inhibit the precipitation of metals to phase precipitation in order to reduce harmful metal precipitation during alloy composition design. Or prolonging its incubation period, the facts have proved that these technical measures can ensure the resistance to intergranular corrosion of alloy welded parts.

The precipitation behavior of corrosion resistant alloys in the sensitized state is closely related to the alloy series and the degree of alloying. Some alloys only precipitate carbides; some alloys containing high chromium and molybdenum not only precipitate carbides but also precipitate metallic compounds. Therefore, in order to prevent the intergranular corrosion problem of its welding, different technical measures should be taken.