A stainless steel can have good corrosion resistance in many media, but in some other media, it may be corroded due to low chemical stability. Therefore, a kind of stainless steel can not resist corrosion to all media. In many industrial applications, stainless steel can provide satisfactory corrosion resistance. In addition to the severe corrosion of stainless steel, intergranular Corrosion Resistance and fatigue corrosion are the main forms of corrosion (i.e. corrosion of stainless steel in addition to the use of intergranular corrosion and stress corrosion). These failure cases caused by local corrosion account for almost half of the failure cases. In fact, many failure accidents can be avoided through reasonable material selection.
According to the mechanism, metal Corrosion Resistance can be divided into three types: special Corrosion Resistance, chemical Corrosion Resistance and electrochemical corrosion. The vast majority of metal Corrosion Resistance in life and engineering practice belongs to electrochemical corrosion.
It is a general term that refers to the mutual failure of stressed alloys due to the expansion of strong lines in corrosive environment. Stress corrosion cracking has brittle fracture morphology, but it may also occur in materials with high toughness. The necessary conditions for stress corrosion cracking are tensile stress (whether residual stress or applied stress, or both) and the existence of specific corrosion medium. The formation and expansion of the pattern are roughly perpendicular to the direction of tensile stress. The stress value leading to stress corrosion cracking is much smaller than the stress value required for material fracture in the absence of corrosive medium. Microscopically, the crack passing through the grain is called transgranular crack, while the crack along the grain boundary expansion diagram is called intergranular crack. When the stress corrosion crack extends to its depth (here, the stress on the section of the loaded material reaches its fracture stress in the air), the material will be broken according to the normal crack (in ductile materials, it is usually through the polymerization of microscopic defects). Therefore, the cross-section of parts that fail 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 refers to the high degree of local corrosion that occurs on the surface of metal materials without corrosion or slightly scattered corrosion. The size of common corrosion spots is less than 1.00mm, and the depth is often greater than the surface aperture. Light ones have shallow corrosion pits, and serious ones even form perforation.
Intergranular corrosion intergranular boundaries are the boundaries of disordered dislocation between grains with different crystallographic orientations. Therefore, they are the segregation of various solute elements or metal compounds (such as carbides and metals) in steel δ Favorable area for precipitation. Therefore, it is not surprising that the grain boundary may be corroded first in some corrosive media. This type of corrosion is called intergranular corrosion. Most metals and alloys may exhibit intergranular corrosion in specific corrosion media. Intergranular corrosion is a kind of selective corrosion damage. The difference between it and general selective corrosion is that the locality of corrosion is micro scale, but not necessarily local in macro scale.
Crevice corrosion refers to the macroscopic pitting or ulceration at the crevices of metal components. It is a form of local corrosion. It may occur in the crevices where the solution stagnates or in the shielded surface. Such gaps can be formed at the junction of metal and metal or metal and non-metal, for example, at the junction with rivets, bolts, gaskets, valve seats, loose surface sediments and marine organisms.
It is a term used to describe the corrosion phenomenon on the whole alloy surface in a relatively uniform manner. When full-scale corrosion occurs, the village material gradually becomes thinner due to corrosion, and even the material corrosion fails. Stainless steel may show overall corrosion in strong acid and alkali. The failure problem caused by total corrosion is not very worrying, because this kind of corrosion can usually be predicted by simple immersion test or consulting the literature on corrosion.
Uniform corrosion refers to the phenomenon that all metal surfaces in contact with corrosive media are corroded
Different index requirements for corrosion resistance are put forward according to different service conditions, which can be generally divided into two categories:
1. Stainless steel refers to the corrosion-resistant steel in the atmosphere and weak corrosive medium. If the corrosion rate is less than 0.01mm/year, it is considered as “complete corrosion resistance”; If the corrosion rate is less than 0.1mm/year, it is considered as “corrosion resistant”.
2. Corrosion resistant steel refers to the steel that can resist corrosion in various strongly corrosive media.
301 stainless steel shows obvious work hardening phenomenon during deformation, which is used in various occasions requiring high strength.
302 stainless steel is essentially a variant of 304 stainless steel with higher carbon content. It can obtain higher strength by cold rolling.
302B is a stainless steel with high silicon content, which has high resistance to high temperature oxidation.
303 and 303se are free cutting stainless steels containing sulfur and selenium respectively. They are used in occasions where free cutting and surface gloss are mainly required. 303se stainless steel is also used to make parts requiring hot upsetting because it has good hot workability under such conditions.
304 is a universal stainless steel, which is widely used to make equipment and parts requiring good comprehensive performance (corrosion resistance and formability).
304L is a variant of 304 stainless steel with low carbon content, which is used for occasions requiring welding. The lower carbon content minimizes the precipitation of carbides in the heat affected zone close to the weld, which may lead to intergranular corrosion (welding corrosion) of stainless steel in some environments.
304n is a kind of stainless steel containing nitrogen. Nitrogen is added to improve the strength of steel.
305 and 384 stainless steels contain high nickel and have low work hardening rate. They are suitable for various occasions with high requirements for cold formability.
308 stainless steel is used to make welding rods.
The nickel and chromium contents of 309, 310, 314 and 330 stainless steels are relatively high in order to improve the oxidation resistance and creep strength of the steel at high temperature. 30s5 and 310S are variants of 309 and 310 stainless steel. The difference is that the carbon content is low in order to minimize the carbide precipitated near the weld. 330 stainless steel has particularly high carburizing resistance and thermal shock resistance
316 and 317 stainless steels contain aluminum, so their pitting corrosion resistance in marine and chemical industry environment is much better than 304 stainless steel. Among them, 316 stainless steel is made of variants, including low-carbon stainless steel 316L, high-strength stainless steel 316N containing nitrogen and free cutting stainless steel 316F with high sulfur content.
321, 347 and 348 are stainless steels stabilized with titanium, niobium, tantalum and niobium respectively, which are suitable for welding components at high temperature. 348 is a kind of stainless steel suitable for nuclear power industry, which has a certain limit on the amount of tantalum and drill.