1. The corrosion types and definition
In many industrial applications, stainless steel, can provide satisfactory corrosion resistant performance today.According to the use of experience, in addition to the mechanical failure, the corrosion of stainless steel mainly displays in: the a serious corrosion of stainless steel in the form of local corrosion (i.e. stress corrosion cracking, pitting corrosion, intergranular corrosion, crevice corrosion and corrosion fatigue).The local corrosion failure cases caused by almost accounts for more than half the cases of failure.In fact, many by reasonable material selection and failure accidents can be avoided.
Stress corrosion cracking (SCC) : refers to bear the stress of the alloy in the corrosive environment due to the extension of fierce stripes alternate failure of a general term.Stress corrosion cracking is brittle fracture morphology, but it can also occur in the toughness of the material.Is a necessary condition for the stress corrosion cracking of tensile stress (both residual stress and applied stress, or both) and a specific corrosive medium.Type grain formation and extension of rough and vertical tensile stress direction.This leads to stress value of the stress corrosion cracking than when there are no corrosive medium stress value of the material fracture need much smaller.In the micro level, through the crack in grain called transgranular crack, and the cracks along the grain boundary expansion graph called the intergranular crack, when the stress corrosion cracking extended to one depth (here, the load stress on the section of materials to achieve its fracture stress) in the air, the material is according to the normal crack (in ductile material, usually through microscopic defect aggregation) and disconnect.Therefore, as a result of the stress corrosion cracking and failure parts section, will contain area has the characteristics of the stress corrosion cracking and polymerization with micro defects have associated "toughening nest" area.
Pitting corrosion, is a lead to corrosion forms of local corrosion.
Intergranular corrosion: grain boundary of different crystallographic orientation is intergranular disorder malocclusion of world city, therefore, they are all elements of the solute segregation in steel or metal compounds (e.g., carbide and delta facies) precipitate out the favorable area city.In some of the corrosive medium, therefore, grain boundary could be corrosion is not surprising that the first.This type of corrosion is known as intergranular corrosion, the majority of metals and alloys can be present in specific corrosive medium intergranular corrosion.
Crevice corrosion: is a form of local corrosion, it may send full in the gap of stagnation of solution or the surface of the shield.This gap can be in metal and the formation of joint of metal or metal and nonmetal, for example, with rivets, bolts, gaskets, valve seat, the surface of the loose sediment and formed into the sea creatures candle.
General corrosion: it is used to describe the spoon in the alloy on the surface to compare the way the corrosion phenomenon of term.When general corrosion occurs, the village of material thinning due to corrosion, and even material corrosion failure.Stainless steel general corrosion may be present in the strong acid and strong alkali.General corrosion caused by the failure of the problem is not how to worry, because, this kind of corrosion is often simple immersion test or consult the corrosion aspects of literature and predict it.
2. All kinds of stainless steel corrosion resistance
304 is a kind of common stainless steel, it is widely used to make for a good comprehensive performance, corrosion resistance and formability of equipment and parts.
301 stainless steel during deformation presents obvious work hardening phenomenon, a high intensity was used for various occasions.
302 stainless steel is essentially higher variations of 304 stainless steel, the carbon content by cold rolling can make its higher strength.
302 b is a kind of high silicon content in stainless steel, it has high resistance to high temperature oxidation performance.
Se is 303 and 303 respectively containing sulfur and selenium free cutting stainless steel, used for main requirements with high degrees of free cutting and table light hao occasion.303 se stainless steel is also used to make part of the need of hot upsetting, because in such conditions, the stainless steel has good hot workability.
304 l is a variation of carbon content lower 304 stainless steel, welding is applied to the occasion.Low carbon content is made in the heat affected zone near the weld in the precipitation of carbide to a minimum, and may result in precipitation of carbide stainless steel intergranular corrosion in some environments (welding erosion).
304 n is a kind of stainless steel containing nitrogen, and nitrogen is in order to improve the strength of the steel.
In 305 and 384 stainless steel containing high nickel, the work hardening rate is low, suitable for various occasions requiring cold formability.
308 stainless steel used to make electrode.
309, 310, 314 and 330 stainless steel and nickel, chromium content is higher, in order to improve the steel at high temperature oxidation resistance and creep strength.And 30 s5 and 310 s was 309 and 310 stainless steel varieties, the difference is only carbon content is low, in order to minimize the carbide precipitation near the weld.330 stainless steel has a particularly high carburizing resistance and thermal shock resistance.
316 and type 317 stainless steel containing aluminum, thus in the ocean of pitting corrosion resistance and chemical industrial environment is greatly better than that of 304 stainless steel.Among them, the type 316 stainless steel by varieties including low carbon stainless steel 316 l, nitrogen of high strength stainless steel 316 n and a high percentage of sulfur free cutting stainless steel 316 f.
Is 321, 347 and 348, respectively, in titanium, stainless steel, niobium and tantalum and niobium stabilization using suitable for high temperature of welded components.348 is a stainless steel is suitable for the nuclear power industry, for tantalum and has a certain limit the amount of drilling.