Stainless steel corrosion is due to the surface of the stainless steel generated a layer of very thin, good adhesion, translucent chromium oxide film. Once the film is destroyed, the chromium in the steel chemically reacts with the oxygen in the atmosphere to quickly restore the film, while mechanical damage can quickly regenerate a protective film. However, if it is chemically attacked by ions, such as chloride, it may be difficult to resist erosion, which may exacerbate corrosion due to the unobstructed entry of oxygen. Corrosion is a special term that refers specifically to the surface is very uniform tarnished, it may be the surface of the formation of a layer of interference membrane. Usually there is a slight color change, and a certain degree of brightness loss, especially small dirt into the surface of the membrane. Some improvement can be achieved by cleaning the surface. In all cases, all efforts in appearance have had little effect, especially when viewed from a distance. Pitting corrosion is the usual form of stainless steel corrosion. The general beginning of needle-like corrosion, due to the corrosion of the corrosive parts turn black or dark brown. In most severe corrosive environments, the amount and depth of pitting increases, giving the surface a corroded appearance. In weakly corrosive conditions, pitting itself can not be significantly reduced from the surface, but corrosion may occur on the surface resulting in a thin film that may tarnish the surrounding area when leaking from the rust. Crevice corrosion is produced in the absence of oxygen. For example, it can be produced by metal cleaning agents or non-metal cleaning agents. Aqueous electrolytes formed by rain or condensate can also cause crevice corrosion. Low-alloy steels are more susceptible to this corrosion, especially where crevices are very small and crevices are hard to penetrate. Design to minimize crevice corrosion to give special attention. In particular, easy to encounter water vapor, we must strive to avoid the gap. If the gap can not be avoided, you should consider the use of more corrosion-resistant, higher alloy steel content. Electrochemical Corrosion: This corrosion can occur when two metals with very different electrochemical potential differences contact each other. If water vapor connects the two metals to create a current loop, the combined current will dramatically increase the corrosion rate of the metal that is prone to chemical reactions. The difference in potential between any two stainless steels is not enough to cause such corrosion, only a few effects, without exponentially increasing corrosion. However, carbon steel and large areas of stainless steel together, carbon steel will be rapidly corroded, so different metals to be connected together to avoid water vapor in these areas to gather. If it is impossible to avoid water vapor, the two metals must be electrically isolated from each other. Stress Corrosion Cracking (SCC): There are two situations where stress corrosion cracking may occur. Chloride stress corrosion cracking may occur when stainless steel is in an aqueous chloride environment. For example, in a sea-fog environment where steel is under very high tensile stress and temperatures exceed normal ambient temperatures (typically above 60 ° C), it is unlikely that there will be no impact on the building unless the steel used passes the following The sensitization treatment. At lower temperatures, in unusual harsh environments, including organic chemicals, stress corrosion cracking can also occur, and these conditions are, in most cases, inevitable. Sensitization: Carbon in steel (usually 0.08%) combines with chromium and precipitates at grain boundaries during heat treatment or during welding. The formation of carbides makes the grain boundary appear poorly chrome, and the formation of anti-corrosion film in the grain boundary at the same time the local grain boundary corrosion, reducing the material stress corrosion resistance. In the manufacturing process to avoid the sensitization of the environment, the need for rapid cooling steel in the final heat treatment to prevent the precipitation of chromium carbide particles. During the welding process, the thin section stainless steel usually cools very fast enough to get the same effect as the precipitation of chromium carbide particles. Sensitization can also be avoided by using low carbon stainless steel such as 304L or 316L in thick section stainless steel welding. In other words, stabilized stainless steel such as 321 or 347 can be included in the specification. Although it is almost unnecessary to do so, the stabilized stainless steels contain either titanium or niobium. These stabilizing elements combine with carbon during heating to prevent the combination of carbon and chromium.
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