Corrosion Resistance of Various Stainless Steel

1. Corrosion resistance of various stainless steel
a. 304 is a versatile stainless steel, which is widely used in the production of equipment and parts that require good comprehensive performance (corrosion resistance and formability).
b. 301 stainless steel shows obvious work hardening phenomenon during deformation, and is used in various occasions requiring higher strength.
c. 302 stainless steel is essentially a variant of 304 stainless steel with higher carbon content, which can be made to have higher strength by cold rolling.
d. 302B is a stainless steel with high silicon content, which has high resistance to high temperature oxidation.
e. 303 and 303Se are free-cutting stainless steels containing sulfur and selenium, respectively, which are mainly used in the occasions where free-cutting and surface treatment are required, and high brightness. 303Se stainless steel is also used to make parts that require hot heading, because under such conditions, this stainless steel has good hot workability.
f. 304L is a variant of 304 stainless steel with low carbon content, which is used in occasions where welding is required. The low carbon content minimizes the precipitation of carbides in the heat-affected zone near the weld, and the precipitation of carbides may cause intergranular corrosion (welding corrosion) of stainless steel in certain environments.
g. 304N is a nitrogen-containing stainless steel. Nitrogen is added to increase the strength of the steel.
h. 305 and 384 stainless steels contain high nickel, and their work hardening rate is low, which is suitable for various occasions that require high cold formability.
i. 308 stainless steel is used to make welding rod.
j. 309, 310, 314 and 330 stainless steels have relatively high nickel and chromium content in order to improve the oxidation resistance and creep strength of the steel at high temperatures. The 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 precipitation of carbides near the weld. 330 stainless steel has a particularly high carburization resistance and thermal shock resistance.
k. 316 and 317 stainless steels contain aluminum, so their resistance to pitting corrosion in marine and chemical industrial environments is much better than 304 stainless steel. Among them, 316 stainless steel variants include low-carbon stainless steel 316L, nitrogen-containing high-strength stainless steel 316N, and free-cutting stainless steel 316F with a high sulfur content.
l. 321, 347, and 348 are stainless steel stabilized with titanium, niobium, tantalum, and niobium, respectively, and are suitable for welding components used at high temperatures. 348 is a stainless steel suitable for the nuclear power industry, which has certain restrictions on the amount of tantalum and drill.

2. Types and definitions of corrosion
In many industrial applications, stainless steel can provide satisfactory corrosion resistance. According to experience, 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 (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 material selection.
a. Stress corrosion cracking (SCC):
Refers to a general term for stress-bearing alloys that alternately fail 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 stress corrosion cracking to occur are tensile stress (whether it is 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. This 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, 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 a depth (here, the load section of the material) The stress reaches its fracture stress in the 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.
b. Pitting corrosion: It is a form of local corrosion that causes corrosion.
c. Intergranular corrosion:
The grain boundaries are the disorderly and intertwined boundaries between grains with different crystallographic orientations. Therefore, they are favorable zones for segregation of various solute elements in steel or precipitation of metal compounds (such as carbide and δ phase). 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.
d. Crevice corrosion:
It is a form of localized corrosion, which may occur in gaps where the solution is stagnant or in the shielded 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.
e. Comprehensive corrosion:
It 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 material corrosion fails. Stainless steel may exhibit full corrosion in strong acids and alkalis. The failure problem caused by full-scale 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.

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