How is the corrosion resistance of stainless steel and its type?

2019-03-25 15:59:49 4

Stainless steel in our lives involves many aspects, used in our production, today we will understand the corrosion resistance and classification of stainless steel.

       1. Types and definitions of corrosion

       In many industrial applications, stainless steel can provide satisfactory corrosion resistance. According to the experience of use, in addition to mechanical failure, the corrosion of stainless steel is mainly manifested in: a serious form of corrosion of stainless steel is local corrosion (ie stress corrosion cracking, pitting corrosion, intergranular corrosion, corrosion fatigue and crevice corrosion) . The failure cases caused by these localized corrosions account for almost half of the failure cases. In fact, many failures can be avoided through reasonable selection.

      (1) Stress Corrosion Cracking (SCC): A general term for stress-bearing alloys that fail in each other due to the expansion of the striated lines in corrosive environments. Stress corrosion cracking has a brittle fracture morphology, but it can 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 presence of specific corrosive media. The formation and expansion of the profile is approximately perpendicular to the direction of the tensile stress. This stress value that causes stress corrosion cracking is much smaller than the stress value required for material fracture without the presence of corrosive media. At the microscopic level, the crack passing through the grain is called a transgranular crack, and the crack along the grain boundary is called an intergranular crack. When the stress corrosion cracking spreads to a depth (here, the load is on the material section) The stress reaches its breaking stress in the air), and the material is broken by normal cracking (in the ductile material, usually by polymerization of microscopic defects). Thus, the section of the part that fails due to stress corrosion cracking will include a characteristic region of stress corrosion cracking and a "dimple" region associated with the polymerization of the microdefect.

      (2) Pitting corrosion: It is a form of local corrosion that causes corrosion.

      (3) Intergranular corrosion: grain boundaries are boundaries between crystal grains with different crystallographic orientations. Therefore, they are segregation of various solute elements in steel or precipitation of metal compounds (such as carbides and δ phases). A favorable city for precipitation. Therefore, in some corrosive media, it is not surprising that the grain boundaries may be corroded first. This type of corrosion is known as intergranular corrosion, and most metals and alloys may exhibit intergranular corrosion in certain corrosive media.

      (4) Crevice Corrosion: A form of localized corrosion that may occur in the gap where the solution stagnates or in the surface of the shield. Such gaps may be formed at the junction of metal and metal or metal and non-metal, for example, where rivets, bolts, gaskets, valve seats, loose surface deposits, and marine organisms are attached to the candle.

      (5) General Corrosion: A term used to describe the corrosion phenomenon occurring in a relatively uniform manner on the entire surface of the alloy. When total corrosion occurs, the village material becomes thinner due to corrosion, and even the material corrosion fails. Stainless steel may exhibit general corrosion in strong acids and bases. The failure problem caused by general corrosion is not very worrying because it can usually be predicted by simple immersion tests or by consulting the literature on corrosion.

2. Corrosion resistance of various stainless steel

      (1) 304 is a versatile stainless steel that is widely used to make equipment and parts that require good overall performance (corrosion resistance and formability).

      (2) 301 stainless steel exhibits obvious work hardening during deformation and is used in various applications requiring higher strength.

      (3) 302 Stainless steel is essentially a variant of 304 stainless steel with a higher carbon content, which can be obtained by cold rolling to obtain higher strength.

      (4) 302B is a stainless steel with a high silicon content, which has high resistance to high temperature oxidation.

      (5) 303 and 303Se are free-cutting stainless steels containing sulfur and selenium, respectively, for applications where the main requirements are easy cutting and high gloss. 303Se stainless steel is also used to make parts that require enthusiasm, because under these conditions, this stainless steel has good hot workability.

      (6) 304L is a variant of 304 stainless steel with a lower carbon content for applications requiring soldering. The lower carbon content minimizes the precipitation of carbides in the heat affected zone near the weld, which may result in intergranular corrosion (weld erosion) in certain environments.

      (7) 304N is a nitrogen-containing stainless steel added to increase the strength of the steel.

      (8) 305 and 384 stainless steels contain high nickel and have a low work hardening rate, which is suitable for various applications where high cold formability is required.

      (9)308 Stainless steel is used to make welding rods.

      (10) 309, 310, 314 and 330 stainless steel have higher nickel and chromium content in order to improve the oxidation resistance and creep strength of steel at high temperatures. The 30S5 and 310S are variants of the 309 and 310 stainless steels, except for the lower carbon content, in order to minimize the carbides precipitated near the weld. 330 stainless steel has a particularly high resistance to carburizing and thermal shock resistance.

      (11) Types 316 and 317 stainless steels contain aluminum and are therefore much more resistant to pitting corrosion in marine and chemical industrial environments than 304 stainless steel. Among them, 316 stainless steel variants include low carbon stainless steel 316L, nitrogen-containing high-strength stainless steel 316N and high-sulfur free-cutting stainless steel 316F.

      (12) 321, 347 and 348 are stainless steel stabilized by titanium, niobium and tantalum, respectively, and are suitable for use as welded members at high temperatures. 348 is a stainless steel suitable for the nuclear power industry, which has certain restrictions on the combination of boring and drilling.

       Because stainless steel has good corrosion resistance, it is widely used in ordinary production and work life, and it has many types, and the corresponding stainless steel types are selected according to people's own needs.