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.
Reasons for corrosion of stainless steel welded pipes:
First, from the corrosion site
1. Pitting corrosion: It is a form of local corrosion that causes corrosion.
2. Intergranular corrosion: The grain boundary is a boundary 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). Favorable district city. 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.
3. 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.
4. General Corrosion: A term used to describe the corrosion phenomena that occur in a uniform manner across 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.
Second, from the principle of corrosion
1 chemical corrosion
1.1 Surface contamination: Oil stains, dust and acids, alkalis, salts, etc. attached to the surface of the workpiece are converted into corrosive media under certain conditions, and chemically react with certain components in the stainless steel parts to cause chemical corrosion and rust.
1.2 Surface scratching: damage to the passivation film caused by various scratches, so that the protection ability of stainless steel is reduced, and it is easy to react with chemical medium, causing chemical corrosion and rust.
1.3 Cleaning: After washing and passivation, the cleaning is not clean and the residual liquid remains, directly corroding the stainless steel parts (chemical corrosion).
2 electrochemical corrosion
2.2.1 Carbon steel pollution: Scratches and corrosive media caused by contact with carbon steel parts form galvanic cells and cause electrochemical corrosion.
2.2.2 Cutting: The adhesion and corrosion of rust-prone substances such as cutting slag and splashing form a primary battery to produce electrochemical corrosion.
2.2.3 Roasting: The composition of the flame heating zone changes and the metallographic structure is uneven, and the primary battery is formed with the corrosive medium to cause electrochemical corrosion.
2.2.4 Welding: physical defects (biting, stomata, cracks, unfused, incomplete penetration, etc.) and chemical defects (grain coarse, grain boundary chrome, segregation, etc.) and corrosive medium forming the primary battery Electrochemical corrosion.
2.2.5 Material: Chemical defects in stainless steel (uneven components, S, P impurities, etc.) and physical defects on the surface (loose, blisters, cracks, etc.) are conducive to electrochemical corrosion caused by the formation of galvanic cells with corrosive media.
2.2.6 Passivation: The passivation effect of pickling is not good, resulting in uneven or thin passivation film on stainless steel surface, which is easy to form electrochemical corrosion.
2.2.7 Cleaning: The acid-washed passivation residue and the chemically corroded product of stainless steel form electrochemical corrosion with stainless steel parts.
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