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Corrosion resistance of stainless steel

Types and definitions of corrosion:A kind of stainless steel can have good corrosion resistance in many media, but it may be corrosive for chemical stability due to the low chemical stability in other medium. So, a stainless steel can not be all media corrosion. In many industrial applications, stainless steel can provide satisfactory corrosion resistance. According to the experience of use, except mechanical failure, the corrosion of stainless steel mainly in: stainless steel a serious corrosion form is localized corrosion (i.e. should stress corrosion cracking, pitting corrosion, intergranular corrosion, fatigue corrosion and crevice corrosion). The failure cases caused by these local corrosion cases accounted for almost half of the failure cases.. In fact, many failure accidents can be avoided through reasonable selection of material..According to the mechanism of corrosion of metals, can be divided into physical, chemical corrosion and electrochemical corrosion of three. The corrosion of metal in real life and actual engineering is of electrochemical corrosion..Stress corrosion cracking (SCC): refers to the stress of alloy in the corrosive environment due to the strong sheath expansion and alternate failure of a generic term. Stress corrosion cracking has a brittle fracture morphology, but it may also occur in the toughness of the material. The essential condition for the occurrence of stress corrosion cracking is to have tensile stress (whether the residual stress or the applied stress, or both) and the specific corrosive medium. The formation and expansion of the pattern is roughly perpendicular to the tensile stress direction.. The stress value of the stress corrosion cracking is much smaller than that of the material fracture when there is no corrosive medium.. On the microcosmic, across the grain crack transgranular crack and crack along the grain boundary expansion plans called intergranular crack. When the stress corrosion cracking expansion to its depth (here, bear the load of the cross section of the material on the stress reached it in the air in the fracture stress), materials according to the normal crack (in ductile materials, typically by micro defects of polymerization) and disconnect. Therefore, the stress corrosion cracking and the failure of the part of the section, will contain the stress corrosion cracking characteristic area and the polymer with the micro defects of the "tough nest" area.Corrosion: corrosion is refers to in the surface of metal material most corrosion or slight corrosion and scattered local corrosion occurred in height and common corrosion spot size of less than 1.00, depth is often greater than the aperture surface, the light has a shallow pit, serious and even the formation of perforation.

Intergranular corrosion: grain boundary is between the grains with different crystallographic orientation disorder malocclusion of the city boundary, therefore, they are steel all solute segregation or metal compound (such as carbide and delta phase precipitation favorable areas of the city. Therefore, in some corrosive medium, the grain boundary may be corroded first and not surprising.. This type of corrosion is known as intergranular corrosion, most metals and alloys in a specific corrosive medium can be intergranular corrosion. Intergranular corrosion is a kind of selective corrosion damage. It is different from common selective corrosion. The local character of corrosion is micro - scale, but it is not necessarily local..Crevice corrosion: is a mottled or ulceration of macro etch pits in the metal components cracks, is a form of localized corrosion, it may all stuck on the solution in the gap or shield inner surface. This gap can in metal and metal or metal and nonmetal joint form, for example, in formation and the rivets, bolts, gaskets, valve seat, loose surface sediment and marine organisms connected candle.Total corrosion: the term used to describe the corrosion phenomena occurring in the entire alloy surface in a comparison. When the full corrosion occurs, the village material gradually becomes thinner due to corrosion, and even the material corrosion failure.. May present a comprehensive stainless steel in acid and alkali corrosion. The failure problem caused by total corrosion is not very worrying, because this corrosion can be predicted by simple immersion test or the literature of corrosion..Uniform corrosion: refers to the metal surface of the corrosive medium all the phenomenon of corrosion. According to the different usage of the corrosion resistance of different indicators, generally can be divided into two categories:1 stainless steel refers to the corrosion resistant steel in the atmosphere and in the weak corrosive medium. The corrosion rate is less than 0.01mm / year, and it is considered as "complete corrosion"; the corrosion rate is less than 0.1mm / year and it is considered as "corrosion resistant"..2 corrosion resistant steel means the steel that can be corrosion-resistant in a variety of corrosive media.Corrosion resistance of various stainless steels:304 stainless steel is a kind of universal, it widely used for requirements of making good comprehensive properties, resistance to corrosion and formability of equipment and parts.301 stainless steel in the deformation of the phenomenon of apparent hardening, is used for a variety of occasions of higher strength.302 stainless steel is essentially the carbon content of 304 stainless steel variants, through the cold rolling can make it get higher strength.302B is a kind of stainless steel with high silicon content, which has high oxidation resistance of high temperature.303 and 303 se is respectively containing sulfur and selenium free cutting stainless steel, used mainly for easy cutting and table and high degree of Guanghao occasions. 303 se stainless steel is also used for making machine parts of hot upsetting, because under this kind of condition, the stainless steel has good hot workability.304L is a carbon content of 304 stainless steel variants for the occasion of the need for welding. Low carbon content makes in * near the weld heat affected zone of the precipitated carbides reduced to a minimum, and carbide precipitation may result in stainless steel in some environments, intergranular corrosion (welding erosion).304N is a nitrogen containing stainless steel, nitrogen is to improve the strength of the steel.305 and 384 stainless steel containing higher nickel, its processing hardening rate is low, it is suitable for all kinds of occasions of high cold forming requirements.308 stainless steel for welding electrode.310, 314, 309 and 330 stainless steel nickel, chromium content are relatively high, in order to improve the oxidation resistance and creep strength of steel at high temperature. The 30S5 and 310S are the variants of 309 and 310 stainless steel, but only the carbon content is lower, so that the carbide can be reduced to the minimum of the weld.. 330 stainless steel has a special high anti - carburizing ability and thermal shock resistance.316 and 317 stainless steel containing aluminum, and thus the corrosion resistance in the marine and chemical industry environment greatly better than 304 stainless steel. Among them, the 316 type stainless steel by variants including low carbon stainless steel 316L, nitrogen containing high strength stainless steel 316N and sulfur content higher easy cutting stainless steel 316F.321, 347 and 348 are titanium, niobium and tantalum, niobium stabilized stainless steel, suitable for high temperature welding component. 348 is a stainless steel for nuclear power industry, the amount of tantalum and cobalt has a certain limit