Important concepts – hydrogen embrittlement/caustic catalysis/stress corrosion cracking

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Important concepts – Hydrogen embrittlement/catalytic causticity/stress corrosion cracking 1. Hydrogen embrittlement: Hydrogen in steel can make the material’s mechanical properties more brittle; this phenomenon is known as hydrogen embrittlement. The sources of hydrogen in steel are mainly the following three: hydrogen dissolved in the molten steel during the smelting process, and hydrogen that remains in the steel as it fails to escape promptly during crystallization and cooling ； During welding, hydrogen generated by the decomposition of moisture or oil under the high temperature of the arc dissolves into the steel ； During the operation of the equipment, hydrogen in the working medium enters the steel. Hydrogen embrittlement fracture occurs when hydrogen is present in the steel and the stress exceeds a certain critical value. The embrittlement of steel by hydrogen is a process in which microcracks propagate under high stress. The brittle fracture stress can be as low as 20% of the yield limit. The higher the strength of the steel (the greater the stress it can withstand), the more sensitive it is to hydrogen embrittlement. The stress levels in the container, including service stress and residual stress, are important factors contributing to hydrogen embrittlement. Hydrogen embrittlement is a type of delayed fracture; the delay before fracture can be as short as a few minutes or as long as several days. Hydrogen embrittlement fracture occurs only within the temperature range of -100 to 150°C. Very low temperatures hinder the movement and accumulation of hydrogen, making hydrogen embrittlement less likely to occur, while higher temperatures allow hydrogen to escape from the steel, reducing its concentration and thus preventing embrittlement. Post-weld holding and heat treatment take advantage of the principle that hydrogen can diffuse out of steel at high temperatures, in order to reduce the hydrogen content in the welds; it is an effective measure for improving the mechanical properties of welded joints. |Technical exchanges in the thermal power industry, including aspects related to power plant boilers, turbines, electrical systems, and water treatment. Hydrogen poses a significant threat to steel materials; the degradation of materials caused by hydrogen is referred to as hydrogen damage. There are various forms of hydrogen damage – in addition to hydrogen embrittlement, there is also hydrogen blistering, which occurs as a result of hydrogen accumulating at the layers within the steel plates ； The clusters of fine cracks caused by the accumulation of hydrogen in the core area of steel are known as white spots ； As well as hydrogen corrosion phenomena in steel under high temperature and pressure: in the case of carbon steel, when the temperature is above 250°C and the hydrogen partial pressure is above 2 MPa, the microstructure undergoes decarburization, cementite decomposes, numerous microcracks appear along the grain boundaries, and the steel loses almost all of its strength and toughness. www.csgrd.comK+M S%n ~3],o 2. Alkali embrittlement is a form of embrittlement that occurs as a result of high concentrations of sodium hydroxide (NaOH) in the medium, which accelerates the corrosion of steel. Its mode of failure is the presence of numerous fine, branch-like cracks that are invisible to the naked eye, in addition to the main crack visible to the eye. When a component suffers from caustic embrittlement, the steel near the crack still retains good plastic and brittle properties. Caustic embrittlement generally occurs at the riveting and expansion joint areas of stressed components. d3. Stress corrosion brittle fracture: Low-stress brittle fracture caused by the combined action of tensile stress and a corrosive environment is known as stress corrosion. Stress corrosion can occur in both plastic and brittle materials. It differs from damage caused solely by stress or by corrosion; under certain conditions, it can be induced even at very low stress levels or in media with weak corrosivity. Damage caused by stress corrosion often shows no obvious signs of deformation in advance, and brittle fracture occurs suddenly, which makes it highly hazardous. The rate of stress corrosion is generally in the range of 10-3 mm/h, which is higher than the usual corrosion rate (10-4 mm/h), but lower than the fracture rate caused solely by stress. Stress corrosion occurs only under specific conditions: (a) the component is subjected to tensile stress. Tensile stress can be generated by external factors or during the processing process. Generally, even a very small tensile stress can cause stress corrosion. , (b) a specific corrosive medium environment that matches the type of material. Each material suffers from stress corrosion only in certain media, but not in others. For example, the media in which ordinary steel is susceptible to stress corrosion include: hydroxide solutions, aqueous solutions containing nitrates, carbonates, cyanates, or hydrogen sulfide, seawater, sulfuric acid–nitric acid mixtures, liquid ammonia, etc. The media in which austenitic stainless steel is susceptible to stress corrosion include acidic and neutral chloride solutions, seawater, hot fluoride solutions, and hydroxide solutions, among others. The most common stress corrosion environments for pressure vessels welded from carbon steel and low-alloy steel include: wet H2S environments, liquid ammonia environments, and NaOH solutions. The most common stress corrosion in austenitic stainless steel pressure vessels is caused by chloride ions. The sensitivity of materials to stress corrosion: For steel, the sensitivity to stress corrosion is related to factors such as the steel’s composition, microstructure, and heat treatment.