Stainless steel is widely used. Not only is it often us […]
Stainless steel is widely used. Not only is it often used in daily life, but because of its excellent corrosion resistance, stainless steel is also frequently seen in industrial production.
The corrosion of stainless steel is mainly manifested in: a serious form of corrosion of stainless steel is localized corrosion (that is, stress corrosion cracking, pitting corrosion, intergranular corrosion, corrosion fatigue, and crevice corrosion). The failure cases caused by these local corrosion account for almost half of the failure cases. In fact, many failure accidents can be avoided through reasonable selection of materials.
Stress corrosion cracking (SCC): A general term that refers to the mutual failure of stress-bearing alloys due to the expansion of strong lines in a corrosive environment. Stress corrosion cracking has a brittle fracture morphology, but it may also occur in materials with high toughness. The necessary conditions for the occurrence of stress corrosion cracking are the presence of tensile stress (whether residual stress or external stress, or both) and the presence of a specific corrosion medium. The formation and expansion of the pattern is approximately perpendicular to the direction of tensile stress. The stress value that leads to stress corrosion cracking is much smaller than the stress value required for material fracture in the absence of corrosive medium.
Microscopically, a crack that passes through the grain is called a transgranular crack, and a crack that expands along the grain boundary is called an intergranular crack. When the stress corrosion cracking expands to its depth (here, the load bearing material cross section The stress reaches its fracture stress in air), then the material is broken according to normal cracks (in ductile materials, usually through the polymerization of micro-defects). Therefore, the cross-section of a part that has failed due to stress corrosion cracking will contain the characteristic areas of stress corrosion cracking and "toughness" areas associated with the aggregation of micro-defects.
Intergranular corrosion: The grain boundaries are the disorderly inter-boundary cities with different crystallographic orientations. Therefore, they are favorable for the segregation of various solute elements in Stainless steel or the precipitation of metal compounds (such as carbide and δ phase) District City. Therefore, in some corrosive media, it is not surprising that the grain boundaries may be corroded first. This type of corrosion is called intergranular corrosion, and most metals and alloys may exhibit intergranular corrosion in certain corrosive media.
Total corrosion: is a term used to describe the corrosion phenomenon that occurs on the entire alloy surface in a relatively uniform manner. When full-scale corrosion occurs, the village materials gradually become thinner due to corrosion, and even the materials fail to corrode. Stainless steel may exhibit full corrosion in strong acids and alkalis. The failure problem caused by general corrosion is not very worrying, because this kind of corrosion can usually be predicted by a simple immersion test or by consulting the literature on corrosion.
Crevice corrosion: It is a form of localized corrosion, which may occur in the stagnation of the solution or in the shielding surface. Such gaps may be formed at the junction of metal to metal or metal to non-metal, for example, where they are in contact with rivets, bolts, gaskets, valve seats, loose surface deposits, and sea creatures.
Pitting corrosion: It is a form of local corrosion that causes corrosion.