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I. Factors causing pitting corrosion in stainless steel: The excellent corrosion resistance of stainless steel is due to the formation of an invisible oxide film on its surface, which puts it in a passive state. The formation of this passivation film is due to the reaction of steel with oxygen when it is exposed to the atmosphere, or as a result of contact with other oxygen-containing environments. If the passivation film is damaged, the stainless steel will continue to corrode. In many cases, the passivation film is only damaged on the metal surface and in localized areas; corrosion causes the formation of tiny pores or pits, resulting in irregularly distributed pitted corrosion on the surface of the material. Pitting is likely to occur due to the presence of chloride ions that react with depolarizers. Pitting in passive metals such as stainless steel is often caused by certain aggressive anions that locally damage the passivation film. Maintaining a passive state, which provides high corrosion resistance, usually requires an oxidizing environment – and this is precisely the condition that leads to pitting. The media that cause pitting are chloride solutions containing heavy metal ions such as Fe3+, Cu2+, and Hg2+ in C1-, Br-, I-, ClO4- solutions, or chloride solutions of Na+ and Ca2+ alkaline and alkaline earth metal ions that contain H2O2, O2, etc. The pitting rate increases with rising temperature. For example, in solutions with a sodium chloride concentration of 4%-10%, the maximum weight loss due to pitting occurs at 90°C ； For more dilute solutions, the maximum occurs at higher temperatures. Methods to prevent pitting: ① Avoid the accumulation of halide ions. ② Ensure the uniformity of the oxygen or oxidizing solution, stir the solution, and avoid areas where no liquid is flowing. ③ Either increase the concentration of oxygen, or remove oxygen. ④ Increase the pH value. Compared to neutral or acidic chlorides, significantly alkaline chloride solutions cause less pitting, or none at all (hydroxide ions act as a corrosion inhibitor). ⑤ Operate at the lowest possible temperature. ⑥ Add a passivator to the corrosive medium. Low concentrations of nitrates or chromates are effective in many media (by inhibiting the preferential adsorption of ions on metal surfaces, thereby preventing corrosion caused by the adsorption of chloride ions). ⑦ Cathodic corrosion protection is used. There is evidence that stainless steel protected by electrocoupling with low-carbon steel, aluminum, or zinc cathodes does not suffer from pitting in seawater. Austenitic stainless steels containing 2%-4% molybdenum exhibit good pitting resistance. The use of molybdenum-containing austenitic stainless steels can significantly reduce pitting or general corrosion in corrosive media such as sodium hydride solutions, seawater, sulfurous acid, sulfuric acid, phosphoric acid, and formic acid. II. Intergranular corrosion of stainless steel: Unstable austenitic stainless steels with a carbon content of over 0.03% (grades that do not contain titanium or niobium) are prone to intergranular corrosion in certain environments if not heat-treated properly. These steels suffer from intergranular corrosion when heated between 425–815°C, or when cooled slowly through this temperature range. Such heat treatment causes carbides to precipitate at the grain boundaries (sensitization), and it leads to chromium depletion in the adjacent areas, making these areas susceptible to corrosion. Sensitization can also occur during welding, causing subsequent local corrosion in the heat-affected zone. The most common method for testing the sensitivity of stainless steel is the 65% nitric acid corrosion test. During the test, the steel specimen was placed in boiling 65% nitric acid solution for a period of 48 hours per cycle, for a total of 5 cycles, with the weight loss measured at the end of each cycle. As a general rule, the average corrosion rate over 5 test cycles should not exceed 0.05 mm/month. Intergranular corrosion in welded austenitic stainless steel structures can be prevented by the following methods: ① Use low-carbon grades such as 00Cr19Ni10 or 00Cr17Ni14Mo2, or stable grades such as 0Cr18Ni11Ti or 0Cr18Ni11Nb. The use of these grades of stainless steel helps to prevent the precipitation of carbides in amounts that could have harmful effects during welding. ② If the structural components of the metal parts are small and can be heat-treated in a furnace, they can be heat-treated at 1040–1150°C to dissolve chromium carbide, and then rapidly cooled in the range of 425–815°C to prevent the precipitation of sigma phase. Welded ferritic stainless steels can also experience intergranular corrosion in certain media. This is caused by the precipitation of carbides or oxides and strain in the metal lattice when steel is cooled rapidly from above 925°C; stress-relief heat treatment after welding can eliminate this stress and restore corrosion resistance. Adding titanium in an amount more than 8 times the carbon content to 1Cr17 stainless steel can generally reduce intergranular corrosion of welded steel structures in certain media. However, adding titanium in concentrated nitric acid is not effective. III. Stress corrosion cracking in stainless steel: Stress corrosion cracking results from the combined effect of static stress and corrosion, which leads to crack formation and metal embrittlement. Only tensile stress causes this type of failure. In fact, all metals and alloys (with very few exceptions) are prone to stress corrosion cracking in certain environments. There are also differing opinions regarding whether the failure of some metals is due to \"stress corrosion\" or \"hydrogen embrittlement\" (such as the cracking of high-strength steels in hydrogen sulfide). For the purpose of discussion, all such damages caused by external environments are included under the category of stress corrosion cracks. Hardened (quenched and tempered) martensitic stainless steels are susceptible to stress corrosion cracking in solutions containing chlorides, thermal hydroxides or nitrates, or hydrogen sulfide. For austenitic stainless steels, hydroxide solutions of concentrated chlorides are the main agents causing stress corrosion cracking. It has been shown that several other environments can also cause stress corrosion cracking in austenitic and martensitic stainless steels. However, it should be noted that in many such environments, the presence of impurities may have already caused cracks. Sensitized austenitic stainless steels are susceptible to stress corrosion cracking in the intergranular mode. If sensitivity is high and/or stress is high, this type of crack may occur in what would otherwise be considered a weak environment. Sensitized and austenitic stainless steels must never be used in stressful conditions unless sufficient testing has been conducted to prove that the encountered environment will not cause intergranular stress corrosion cracking. The environments in which stress corrosion crack failure occurs are usually quite complex. For example. The stresses involved are usually not just service stresses, but rather a combination of residual stresses generated in the metal due to manufacturing, welding, or heat treatment. This situation can often be alleviated by using methods to relieve stress in the manufactured equipment. In the same category, as mentioned above, the corrosive agent that causes cracks is often merely an impurity present in the product being processed. In the overall solution, the amount of corrosive agents present may not be sufficient to cause cracks, but at the cracks or in the splashing areas above the liquid, the local concentration of these agents can lead to damage. Although there are several general methods to prevent stress corrosion cracking, the best approach is to use materials that are resistant to stress corrosion cracking in that environment. Therefore, in hot chloride environments, 0Cr18Ni13Si4 (American AISI LX M15) or ferritic stainless steel should be selected. In hydrogen sulfide environments, ferritic and austenitic stainless steels are generally suitable, while hardened martensitic stainless steels should not be used. Properties of various stainless steels.