Chromium oxide film of 304 stainless steel
plate The surface of 304 stainless steel plate
has high corrosion resistance due to the formation of a dense chromium oxide film, which can be widely used in modern industrial fields and daily life. However, while resisting uniform corrosion, local pitting corrosion (ie pitting) of stainless steel is difficult to avoid. The occurrence of pitting corrosion starts at the surface of the material, and goes through two stages of nucleation and growth, and then rapidly expands to the depth below the surface of the material. Therefore, pitting damage has great concealment and suddenness. Corrosion occurrences are not random, they are always generated around small pieces of manganese sulfide, in the area of u200bu200bseveral hundred nanometers; during the manufacturing process of stainless steel, sulfides are locally generated in high concentrations, resulting in a reduction of chromium in the surrounding area, The chromium oxide produced by the reaction between chromium and oxygen can prevent corrosion. Low-concentration chromium areas will corrode first, and in salt-containing water, this process will become more rapid. Force corrosion cracking refers to a general term for the mutual failure of alloys subjected to stress in a corrosive environment due to the propagation of cracks. It is a complex phenomenon involving: mechanics, electrochemistry, metallurgy and many other factors. The necessary condition for stress corrosion cracking to occur is to have tensile stress (whether residual stress or applied stress, or both) and specific Corrosive media are present. The formation and propagation of cracks are roughly perpendicular to the direction of tensile stress. When the heat exchange fin is processed and formed, metal loss occurs at the very narrow front of the groove; in the working state, when the heat exchange fin is subjected to alternating stress, this weak link will produce tiny cracks that gradually diffuse through the metal; Under normal circumstances, this kind of fatigue damage that occurs in the elastic range of the metal superficial layer will not affect the metal; but the pressure change produces continuous deformation, especially at the sharp fatigue damage, the tiny plastic deformation, which makes the The passivation film on the metal surface of the region is continuously broken and re-passivated at the grain boundary, resulting in a slip step phenomenon; in this way, at the edge of the formed film, and the sharp edge of the passivation film is continuously broken, a potential will be generated Poor, the electrochemical reaction is added here, producing a spot that can increase the local stress. 304 stainless steel is a highly ductile alloy, such a notch with increased stress will produce a short brittle crack.
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