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(1) Introduction. The main forms of failure in pressure vessels include: strength failure, stiffness failure, sealing failure, low-temperature brittle fracture, high-temperature creep, stress corrosion cracking, and so on. Among these, the most critical and potentially harmful are low-temperature brittle fracture and stress corrosion cracking. Today’s article focuses on the mechanisms underlying stress corrosion and the main countermeasures. Under the combined action of stress (mainly tensile stress) and corrosive agents, chemical processing equipment components are prone to brittle cracking that occurs below the material’s strength limit, leading to the failure of the equipment and components. This phenomenon is known as stress corrosion cracking (abbreviated as SCC). Media with a tendency for stress corrosion mainly include liquid ammonia, wet hydrogen sulfide, chlorides, hydroxides, nitrates, and oxygenated water. A key characteristic of stress corrosion cracking is that it tends to occur at relatively low stress levels; it is completely different from cracking caused solely by mechanical stress, and generally shows no precursors ； It also differs from cracking caused solely by corrosion, as it does not result in widespread, uniform corrosion-induced thinning. Since stress corrosion often involves ionized corrosion, the corrosion rate is very high; sudden fracture or bulging and perforation occur without any signs of deformation, making it one of the most destructive forms of corrosion in industrial production. Cracks caused by stress corrosion often penetrate deep into the metal, making it easy for through-cracks to form. Once such cracks occur, they are difficult to repair, and in most cases the entire device has to be discarded. Stainless steel is highly susceptible to stress corrosion in an environment with chloride ions ； Carbon steel and low-alloy steel are prone to sulfide stress corrosion in a wet hydrogen sulfide environment. (II) Basic conditions for formation: (1) Metal materials are alloys, including those containing trace elements; pure metals generally do not develop stress corrosion cracks ； (2) The compatibility between the material and the corrosive medium: Not every metal material will develop stress corrosion cracks in combination with every medium; there is a certain degree of compatibility required ； (3) Tensile stress must be present; this tensile stress can be service stress or welding residual stress. Welding residual stresses are usually tensile in the weld and the area adjacent to it, sometimes reaching up to the material’s yield strength. Therefore, even if the welded structure is not under load, stress corrosion cracks can still occur as long as the material is compatible with the corrosive environment. (III) Main characteristics of stress corrosion (1) The rate of stress corrosion cracking is much higher than that of other types of local corrosion ； (2) Stress corrosion fracture often occurs suddenly, with no obvious signs beforehand, which makes it extremely hazardous ； (3) The morphologies of cracks mainly include intergranular, transgranular, and mixed types ； (4) Fracture morphology: macroscopically, it is a brittle fracture; microscopically, traces of plastic flow can still be observed on the fracture surface. (IV) Stress corrosion testing methods Depending on the method of applying stress, stress corrosion testing methods are mainly divided into the constant deformation method, the constant load method, the slow strain rate method, and the fracture mechanics method. (1) Constant deformation method: A certain degree of deformation is applied to the specimen in order to assess its susceptibility to cracking under test conditions ； (2) Constant load method: A uniaxially tensile specimen is subjected to stress along its axis and tested in a corrosive medium; the length of time until fracture is compared, or the relationship curve between stress and fracture time is utilized to determine the critical stress for stress corrosion cracking, σscc ； (3) Slow strain rate method: This involves using a specially designed slow strain rate stress testing machine to stretch the specimen in a corrosive medium at a certain strain rate until it breaks. Analyze the fracture condition and fracture surface characteristics of the specimen to assess its sensitivity to stress corrosion cracking ； (4) Fracture mechanics method: Studies are conducted using wedge-shaped loading specimens; specimens pre-existing with cracks are subjected to various K values to determine the critical value KISCC at which crack propagation stops. (5) Main testing standards: GB/T 4157-2006 \"Method for tensile testing of metals under constant load to evaluate resistance to sulfide stress corrosion cracking\", GB/T 17898-1999 \"Method for testing stress corrosion of stainless steels in boiling magnesium chloride solution\", GB/T 10126-2002 \"Method for testing stress corrosion of iron-chromium-nickel alloys in hot water\". (V) The main situations in which stress corrosion failure can occur in engineering applications are as follows: (1) For carbon steel and low-alloy steel, the media involved include alkaline solutions, nitrate solutions, anhydrous ammonia, wet hydrogen sulfide, acetic acid, etc ； For medium to high strength low-alloy steels, special attention is also required when using liquid ammonia as a medium ； (2) For austenitic stainless steel, the media include chloride ions, chlorides + steam, hydrogen sulfide, alkaline solutions, etc ； (3) For molybdenum-containing austenitic stainless steels, the media include alkaline solutions, chloride aqueous solutions, sulfuric acid+copper sulfate aqueous solutions, etc ； (4) For brass, the media include ammonia gas and solutions, ferric chloride, wet sulfur dioxide, etc ； (5) For titanium, the media include methanol or ethanol containing hydrochloric acid, molten sodium chloride, etc ； Pay special attention to situations where hydrochloric acid is formed due to medium leakage; for example, titanium gaskets are prohibited for liquid chlorine media ； (VI) Approaches to controlling stress corrosion cracking: (1) Reduce stress and eliminate residual stress ； For example, carrying out effective post-weld stress-relief heat treatment ； (2) Use stress-corrosion resistant materials ； For example, for liquid ammonia, carbon steel and low-alloy steel with lower strength should be preferred ； (3) Use cathodic protection ； (4) Add a corrosion inhibitor or remove harmful components from the medium. For example, an appropriate amount of water should be added to liquid ammonia, and in cases where the medium contains hydrogen sulfide, the water content in the medium must be strictly controlled ； (5) In environments with low concentrations of hydrogen sulfide, stress corrosion can be suppressed by using materials with improved purity, reducing the S and P content in the materials, and optimizing the material structure.