Stress corrosion cracking
It is a general term that refers to the mutual failure of stressed alloys due to the propagation of severe cracks in corrosive environments. SCC has brittle fracture morphology, but it may also occur in materials with high toughness. The necessary condition for stress corrosion cracking is the existence of tensile stress (whether residual stress or external stress, or both) and specific corrosion medium. The formation and expansion of the dimple is roughly perpendicular to the direction of tensile stress. The stress value that leads to stress corrosion cracking is much smaller than that required for material fracture in the absence of corrosion medium. Microscopically, cracks passing through grains are called transgranular cracks, while cracks extending along grain boundaries are called intergranular cracks. When stress corrosion cracking extends to a certain depth (here, the stress on the section of the material under load reaches its fracture stress in air), the material will be broken as normal cracks (in ductile materials, usually through the polymerization of microscopic defects). Therefore, the section of a part that fails due to stress corrosion cracking will contain the characteristic area of stress corrosion cracking and the "dimple" area associated with the polymerization of micro defects.
Pitting corrosion
It is a form of localized corrosion that causes corrosion.
Intergranular corrosion: intergranular boundaries are the boundaries of disorder and mismatch between grains with different crystallographic orientations, so they are the segregation of various solute elements or metal compounds (such as carbides and δ Favorable area for precipitation. Therefore, in some corrosive media, it is not surprising that the grain boundary may be corroded first. This type of corrosion is called intergranular corrosion. Most metals and alloys may exhibit intergranular corrosion in a specific corrosion medium.
Crevice corrosion: It is a form of local corrosion, which may occur in the crevice where the solution is stagnant or in the shielded surface. Such a gap can be formed at the joint of metal and metal or metal and nonmetal, for example, at the joint of rivet, bolt, gasket, valve seat, loose surface sediment and marine organism.
Overall corrosion
It is a term used to describe the corrosion phenomenon that occurs on the whole alloy surface in a relatively uniform manner. In case of overall corrosion, the material will gradually become thinner due to corrosion, and even the material will become invalid due to corrosion. Stainless steel may show overall corrosion in strong acid and alkali. The failure problem caused by overall corrosion is not very worrying, because this kind of corrosion can usually be predicted by simple immersion test or consulting corrosion literature.
