Corrosion and corrosion resistance of stainless steel

Abst:1. Types and definitions of corrosion In many industrial applications, stainless steel can provide satisfactory corrosion resistance. According to exp...
1. Types and definitions of corrosion In many industrial applications, stainless steel can provide satisfactory corrosion resistance. According to experience in use, in addition to mechanical failure, the corrosion of stainless steel is mainly manifested in: a serious form of corrosion of stainless steel is localized corrosion (ie stress corrosion cracking, pitting corrosion, intergranular corrosion, corrosion fatigue and crevice corrosion) . The failure cases caused by these local corrosion accounted 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 streaks in a corrosive environment. Stress corrosion cracking has a 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 it is residual stress or applied stress, or both) and a specific corrosive medium. The formation and expansion of the pattern is roughly perpendicular to the direction of tensile stress. This stress value that causes stress corrosion cracking is much smaller than the stress value required for material fracture when there is no corrosive medium. Microscopically, the cracks passing through the grain are called transgranular cracks, and the cracks extending along the grain boundary are called intergranular cracks. When the stress corrosion cracking extends to a certain depth (here, the cross-section of the material under load When the stress reaches its breaking stress in the air), the material will be broken according to normal cracks (in ductile materials, usually through the aggregation of microscopic defects). Therefore, the section of the part that fails due to stress corrosion cracking will contain the characteristic area of ​​stress corrosion cracking and the "dimple" area associated with the aggregation of micro-defects.

Pitting corrosion: a form of localized corrosion that causes corrosion. Intergranular corrosion: The grain boundaries are the boundary cities where the crystal grains with different crystallographic orientations are disordered and mixed. Therefore, they are advantageous for the segregation of various solute elements in the steel or the precipitation of metal compounds (such as carbides and δ phases). 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. Most metals and alloys may exhibit intergranular corrosion in certain corrosive media. Crevice corrosion: It is a form of local corrosion, which may occur in the crevice 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, where they meet with rivets, bolts, gaskets, valve seats, loose surface deposits, and sea creatures. General corrosion: It is a term used to describe the corrosion phenomenon that occurs on the entire alloy surface in a relatively uniform manner. When overall corrosion occurs, the material will gradually become thinner due to corrosion, and even the material will fail to corrode. Stainless steel may exhibit general 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.

2. Corrosion resistance of various stainless steels 304: It is a universal stainless steel, which is widely used to make equipment and parts that require good comprehensive performance (corrosion resistance and formability). 301: Stainless steel exhibits obvious work hardening during deformation, and is used in various occasions requiring higher strength.

302: Stainless steel is essentially a variant of 304 stainless steel with higher carbon content, which can be made to obtain higher strength through cold rolling.

302B: It is a kind of stainless steel with high silicon content, which has high resistance to high temperature oxidation.

303 and 303Se: free-cutting stainless steel containing sulfur and selenium respectively, used in occasions that mainly require free cutting and high surface gloss. 303Se stainless steel is also used to make parts that require hot upsetting, because under such conditions, this stainless steel has good hot workability. 304L: It is a variant of 304 stainless steel with lower carbon content, used in occasions where welding is required. The lower carbon content minimizes the precipitation of carbides in the heat-affected zone near the weld, and the precipitation of carbides may cause stainless steel to produce intergranular corrosion (welding erosion) in certain environments.

04N: It is a nitrogen-containing stainless steel. Nitrogen is added to increase the strength of the steel.

305 and 384: Stainless steel contains high nickel, and its work hardening rate is low. It is suitable for various occasions that require high cold formability.

308: Stainless steel is used to make electrodes.

309, 310, 314 and 330: The nickel and chromium content of stainless steel is relatively high, in order to improve the oxidation resistance and creep strength of steel at high temperature. While 30S5 and 310S are variants of 309 and 310 stainless steel, the difference is that the carbon content is lower in order to minimize the carbides precipitated near the weld. 330 stainless steel has particularly high resistance to carburization and thermal shock resistance.

316 and 317: Type stainless steel contains aluminum, so its resistance to pitting corrosion in marine and chemical industrial environments is much better than that of 304 stainless steel. Among them, the 316 stainless steel variants include low-carbon stainless steel 316L, nitrogen-containing high-strength stainless steel 316N, and free-cutting stainless steel 316F with higher sulfur content. 321, 347 and 348 are stainless steel stabilized with titanium, niobium plus tantalum, and niobium, respectively, and are suitable for welding components used at high temperatures. 348 is a kind of stainless steel suitable for the nuclear power industry, and has certain restrictions on the combined amount of tantalum and drill.