Varieties And Definition Of Corrosion
Among the numerous industrial purposes, stainless steel can always act with satisfying anticorrosive performance. Apart from machinery breakdown, according to practical experience, the corrosion of stainless steel has mainly several following aspects as: local corrosion (it also means stress corrosion cracking, pitting corrosion, intercrystalline corrosion, wear out corrosion and crevice corrosion) is a serious variety of the corrosion of stainless steel. Among all instances of invalidation, the ones brought by local corrosion take almost more than half. As a matter of fact, many of the instances of invalidation can be avoided by sound material selecting.
Stress corrosion cracking (SCC): it is a general term which means the crack of alloy in caustical surroundings will be expanded when under stress which leads to alternating invalidation. SCC always appears with brittleness fracture, but it may also appear to high toughness materials. The necessary condition for SCC is tensile stress (no matter it is residual stress or adscititious stress, or even the two) and certain corrosive medium. The formation and expansion of cracking is approximately perpendicular with the direction of tensile stress. The value of this stress which lead to SCC is very much smaller than the stress value of the fracture without the existence of corrosive medium. In microcosmic way, the crackings traverse the crystallines are called as transcrystalline crackings. And the crackings along the intercrystalline boundary is called as along-crystalline crackings. When SCC expands to a certain depth (right here, the stress of the fracture of load will reach up to the stress causes cracking in the air), the fracture will go along the normal cracking (in toughness material, it is usually done through aggregation of micro flaw). Consequently, the invalidated fractures of parts caused by SCC should contain areas with characteristics of SCC and “dimple”areas related to aggregation of micro flaws.
Pitting corrosion: is a kind of local corrosion which brings corrosion.
Intercrystalline corrosion: intercrystalline boundary is turbulent boundary layers in beteen separate crystallines in crystallographic tropism. Therefore, they are available areas segregated by various solute elements or seperated out by metal compound (as carbide and δ face) sedimentation inside the steel. Consequently, among some corrosive mediums, it is not a wonder if intercrystalline boundary might be antecedently become corrosion. This type of corrosin is called intercrystalline corrosion. Most of the metal and alloy will take on intercrystalline corrosion apperance shen inside certain corrosive medium.
Crevice corrosion: is a kind of local corrosion. It may happen inside the crevice where liquor stays still or the inner side of any shield. There is this sort of crecive in the joints of metal and metal or metal and metalloid, e.g. joints with rivet, bolt, washer, valve seat, loosened exterior sediment and sea creatures.
General corrosion: it is a term to describe corrosion spreaded uniformly on the surface of the entire alloy. When it happens, the material will be attenuating because of the corrosionSome times, it will cause invalidation even. In strong acid or alkali, stainless steel will take on general corrosion apperance. The invalidation matter brought by general corrosion is not something to be solicitous about, since the corrosion can always be pre-detected by simple marinate test or turning to literature files.
The corrosion resistance of various stainless steel
Grade 304 - The most commonly stainless steel. 304 is available in virtually all product forms and finishes with good anticorrosion and molding.
Grade 301 present the strain hardening when forming. So 301 can be supplied with a tensile strength situation.
Grade 302 is a slightly higher carbon version of type 304. After cold rolled 302 can obtain higher tensile strength.
Grade 302B is kind of stainless steel with high silicon rate. It resists oxidation in high temperature.
Grade 303 with the Sulphur addition, represents the optimum in machinability. It is primarily used when production involves extensive machining in automatic screw machines. Grade 303Se has a selenium rather than sulphur addition, improving the hot and cold forming characteristics over those of 303 and providing a smoother machined surface finish.
Grade 304L is the low carbon version of 304. It is used in heavy gauge components for improved weldability. The low carbon component reduce the separated carbide minimum beside the welding line. Because the separated carbide will cause the stainless steel intergranular corrosion in some condition.
Grade 304N is a kind of stainless steel within Nitrogen, 304N have a higher tensile strength as the Nitrogen addition.
Grade 305 and 384 have the lowest work and strain hardening rate by the high component rate of Nickle. They were designed for maximum formability condition.
Grade 308 is used for welding rod.
Grade 309 310 314 and 330 have a high component rate of Nickle and Chromium. So they combining excellent high temperature properties with good ductility and weldability, were designed for high temperature service. They also resist oxidation in continuous service at high temperatures. Grade 309S and 310S were the low carbon edition of 309 and 310 with the need of reducing the separated carbide beside the welding line. Grade 330 have the high resistance in cementite and thermal.
Grade 316 and 317 stainless steel containing Molybdenum. The inclusion of Molybdenum gives 316 and 317 the excellent corrosion resistance when exposed to a range of corrosive media such as marine and chemical environments. 316 has other edition as low carbon style 316L, high tensile strength with Nitrogen style 316N and optimum in machinability with Sulphur style 316F.
Grades 321 and 347 are the basic austenitic stainless steel (Grade 304) stabilized by Titanium or Niobium additions respectively. These grades are used for welding in high temperature because they are not sensitive to intergranular corrosion after heating. Grade 348 is a kind of stainless steel used for nuclear-power industry, has the stick request of the component for Tantalum and Cobalt.