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Factors affecting the corrosion of stainless steel. Stress corrosion cracking: Not all media can cause stress corrosion in metals; a particular metal will only experience stress corrosion cracking in specific media. As shown in the table below: Certain media that can cause stress corrosion cracking in alloys. Steel that is resistant to corrosion in air is called “stainless steel,” while steel that is resistant to corrosion in various aggressive media is called “acid-resistant steel.” Generally, stainless steel and acid-resistant steel are collectively referred to as stainless acid-resistant steel, or simply stainless steel. They can be classified based on their composition, structure, and application: Based on composition, they include chromium steel, chromium-nickel steel, chromium-manganese steel, etc ； Classified by structure, they include austenitic steel, ferritic steel, martensitic steel, austenite-ferrite duplex steel, etc. Based on their application, they can be classified into stainless steels resistant to seawater corrosion, those resistant to pitting corrosion, those resistant to stress corrosion, and stainless steels resistant to nitric acid, among others. In seawater, when the corrosion potential reaches the pitting potential Eb, the passivation film is locally damaged, leading to pitting of the stainless steel. The corrosion resistance of stainless steel is due to the presence of chromium in the alloy; when the chromium concentration exceeds 11%, it can resist atmospheric corrosion, and as the chromium content increases, so does the corrosion resistance. (But there is also a limit.) When stainless steel is exposed to high-temperature, high-pressure deoxygenated water, the corrosion rate is high during the first few hundred hours, but it gradually decreases thereafter to reach a constant value, resulting in uniform corrosion. The reason for this decrease in corrosion rate is the formation on the surface of the stainless steel of a spinel-type oxide film whose general chemical formula is M3O4 (where M refers to iron, chromium, and nickel; chromium being a strong passivating metal that makes hydrogen evolution corrosion difficult to occur). This film adheres closely to the surface of the base metal, is firm and stable, resistant to corrosion and wear, and provides excellent protection. In hot water, the presence of dissolved oxygen often makes the oxide (a-Fe2O3) formed on the surface of stainless steel relatively loose, making it susceptible to erosion by hydraulic forces and other factors, which leads to the loss of its protective function. Therefore, when the concentration of dissolved oxygen in water exceeds a certain level, corrosion is exacerbated. The corrosion of austenitic stainless steels of various grades in pure water is influenced by the temperature and flow rate of the water, as well as the surface condition of the metal itself. The initial uniform corrosion rate is generally 60–240 mg/dm2·a. Improving the surface finish can enhance the corrosion resistance of the stainless steel, while increasing the pH value of the water can boost its stability. Irradiation has little effect on the corrosion of stainless steel, but prolonged neutron irradiation will cause changes in its mechanical properties. Stress corrosion poses the greatest threat to stainless steel equipment. Damage caused by stress corrosion in stainless steel is always of a brittle fracture type; even highly plastic austenitic stainless steels do not exhibit significant plastic deformation when they suffer from stress corrosion cracking. The microcracks of stress corrosion are transgranular or intergranular, or a combination of both. Stainless steel is most susceptible to stress corrosion in water containing oxygen and chloride ions; this type of corrosion has caused severe damage to key equipment such as steam generators in nuclear power plants. The probability of failure in austenitic stainless steel increases as the chloride ion concentration rises, and this is particularly evident in water with a high oxygen content. Oxygen acts as a catalyst for chloride-induced stress corrosion cracking in austenitic stainless steel; therefore, in pressurized water reactors, to prevent chloride-induced stress corrosion in stainless steel, it is necessary to strictly control the levels of dissolved oxygen and chloride ions in the coolant. Austenitic stainless steels are prone to stress corrosion in neutral chloride solutions, but this does not occur if the oxygen content in the medium is below 1 mg/L; therefore, stress corrosion can be controlled by removing dissolved oxygen and oxidizing agents from the medium. It is generally believed that F- can cause stress corrosion cracking in stainless steel in both high-temperature and low-temperature water. No other halide ions were found to affect the stress corrosion of stainless steel, except for F- and chloride ions. Increasing the pH value of the solution can delay the corrosion fracture process, as changes in pH affect the kinetics of metal dissolution and the electrode processes. Adjusting the pH value with phosphates is beneficial for suppressing stress corrosion, but its concentration must be appropriate to avoid caustic stress corrosion (concentration at gaps leads to the formation of favorable hydroxide ions, according to the author). The caustic stress corrosion (intergranular corrosion) of stainless steel is different from chloride stress corrosion; the former does not require oxygen, and it does not occur as easily as the latter. Tests have shown that a sodium hydroxide solution concentration of over 50,000 mg/L is required to cause caustic stress corrosion cracking in 347 stainless steel. Caustic alkali has no adverse effect on the surface of stainless steel, but local concentration of caustic alkali in heated gaps can cause the stainless steel to break. (The pipes for transporting alkaline solutions can be made of stainless steel, but it is necessary to control the temperature and gaps. Alkali embrittlement due to local concentration in the steam generator is the main cause of stress corrosion cracking in stainless steels and nickel-based alloys