Stainless steel will rust, corrosion will be?

In industrial production, the use of corrosive medium, many users in corrosive liquid conveying, material selection of equipment is key, especially the pump material selection, usually in people's concept of commonly used stainless steel material is common, but for the strong corrosion liquid, stainless steel can be corrosion, the user will choose fluoride plastic water pump, fluorine plastic anti-corrosion is very suitable for, acid and alkali salt can be used in any concentration. In some environments, stainless steel will also rust, so why are stainless steel rust? When the stainless steel tube surface appear brown rusty spot ( Point) When people big surprised: think & other; Stainless steel is not rusty, rust is not stainless steel, the problems may be steel & throughout; 。 Actually, this is a one-sided error for a lack of understanding of stainless steel. Stainless steel under certain conditions will be rusty. Why was a, stainless steel corrosion of stainless steel application environment is extremely complex, the oxidation of pure chromium passivation membrane can't adapt to the requirement of high corrosion resistance. Therefore according to the different using conditions also need to add molybdenum in steel ( Mo） , copper, Cu） , nitrogen ( N) Elements such as to improve the composition of passive film, further improve the corrosion resistance of stainless steel. Add Mo, MoO2 - due to the corrosion products Near the substrate and strongly promote the collective passivation, prevent the corrosion of substrate; Adding Cu make stainless steel surface passivation membrane containing the CuCl, due to it not interact with corrosive medium can improve the corrosion resistance; Adding N, due to the passivation membrane enrichment Cr2N, Cr in the passivation membrane concentration increased, thus improves the corrosion resistance of stainless steel. The corrosion resistance of stainless steel is conditional, a brand of stainless steel in a particular medium is corrosion resistant, but in another medium may be damaged. At the same time, the corrosion resistance of stainless steel is relative, so far, is not a kind of stainless steel in all environments are definitely not corrosion. Stainless steel has the ability of resistance to atmospheric oxidation & ndash; — Namely, stainless, also has in the medium containing acid, alkali, salt is the ability of corrosion & ndash; — The corrosion resistance. But the size of its corrosion resistance is with the chemical composition of steel itself, intensive state, using conditions and environmental medium type and change. As 304 steel tube, in dry, clean atmosphere, have absolute good corrosion resistance, but it will be moved to the coastal areas, in contains a great deal of sea salt fog, will soon be rusty; And 316 steel tube is good. So, not any kind of stainless steel, in any environment can not rust corrosion. Second, why stainless steel rust stainless steel will also generate an oxide in surface. Currently on the market all the stainless mechanism of stainless steel because of Cr element. The root cause of the stainless steel corrosion resistant ( The mechanism of) Of the theory of the passive film. The so-called passive film is on the surface of stainless steel with a layer of film is given priority to with Cr2O3. As a result of the existence of the film make the stainless steel substrate in a variety of medium corrosion, this phenomenon is called passivation. The two cases, the formation of the passivation film of one is stainless steel itself has the ability to the passivation, this since the passivation ability with the increase of the content of chromium is accelerated, therefore it is rust resistance; Another more widely forming condition is stainless steel in various aqueous solution ( Electrolytes) , in the process of corrosion form passivation film and make the corrosion. Immediately after the passivation film is damaged, can form new passivation film again. Stainless steel passivation membrane has the ability of resistance to corrosion, has three characteristics: one is the thickness of the passivation film very thin, the chromium content & gt; 10. Under the condition of 5%, average only a few microns; The second is the proportion of the passivation film is greater than the proportion of matrix; These two characteristics shows that the passive film is thin and dense, therefore, the passivation film is very difficult to be corrosive medium breakdown to rapid corrosion substrate; The third feature is the passivation film of chromium concentration more than three times more likely than the matrix; So the passivation membrane has high corrosion resistance.
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