Chromium oxide film on 304 stainless steel plate

 The surface of 304 stainless steel plate is widely used in modern industrial fields and daily life due to the formation of a dense chromium oxide film and its high corrosion resistance. However, while resisting uniform corrosion, local pitting corrosion (i.e. pitting corrosion) of stainless steel is difficult to avoid. The occurrence of pitting corrosion starts at the material surface and undergoes two stages of nucleation and growth, followed by rapid expansion in the depth below the material surface. Therefore, pitting damage has great concealment and suddenness. The location of corrosion is not random, they always occur around small pieces of manganese sulfide, in areas of several hundred nanometers; In the process of stainless steel manufacturing, high concentrations of sulfides are generated locally, leading to a decrease in chromium in the surrounding area. The chromium oxide produced by the reaction of chromium with oxygen can prevent corrosion. Low concentrations of chromium will first cause corrosion, and in water containing salt, this process will become more rapid. Force corrosion cracking refers to a general term used to describe the mutual failure of alloys under stress due to crack propagation in corrosive environments. It is a complex phenomenon involving many factors such as mechanics, electrochemistry, metallurgy, etc. The necessary condition for stress corrosion cracking is the existence of tensile stress (whether residual stress or external stress, or both) and specific corrosion medium. The formation and propagation of cracks are roughly perpendicular to the direction of tensile stress. During the processing and forming of the heat exchange plate, metal loss occurred at the very narrow front edge of the groove; In working condition, when the heat exchanger is subjected to alternating stress, this weak link will generate small cracks that gradually diffuse through the metal; In general, fatigue damage that occurs within the elastic range of the shallow surface of the metal will not affect the metal; However, the continuous deformation caused by pressure changes, especially at sharp fatigue damage areas, results in small plastic deformation, causing the passivation film on the metal surface in this area to continuously crack and re passivate at the grain boundary, resulting in slip step phenomenon; In this way, at the edge of the formed film and the sharp edge of the passivation film that constantly ruptures, a potential difference will be generated, and an electrochemical reaction will be added here, creating a spot that can increase local stress. 304 stainless steel plate is a highly ductile alloy, and such a groove with increased stress will produce a short brittle crack.