Stress corrosion
The corrosion form of metal materials under the combined action of corrosive medium and mechanical stress is called stress corrosion. Corrosion cracks or even fractures are formed during the corrosion process, which is a kind of extremely dangerous localized corrosion. According to the different stress of metal in the process of stress corrosion, stress corrosion can be divided into stress corrosion cracking and corrosion fatigue. Stress corrosion is widespread in the water vapor system of thermal equipment, such as water wall tubes, high pressure deaerators, superheaters, reheaters, main feed water pipelines, steam pipelines, impeller and steam turbine blades, and condenser tubes, etc. and stress corrosion cracking or corrosion fatigue may occur in different situations.
Stress corrosion cracking is a kind of stress corrosion caused by metal under the combined action of specific corrosive medium and tensile stress. Tensile stress originates from residual stress generated during manufacturing and installation of metal components, internal stress caused by corrosion products, working stress generated during equipment operation, and thermal stress generated by temperature changes. The specific media that cause stress corrosion cracking of carbon steel are NaOH solution and boiling HNO3 solution. The stress corrosion cracking of carbon steel in NaOH solution is also called alkali embrittlement or caustic embrittlement. This type of corrosion used to occur in small boilers with softened water as make-up water or in boiler systems with free alkali in the boiler water. The specific medium that causes stress corrosion cracking of austenitic stainless steel is usually chloride solution. As long as there is a trace amount of chloride ions in the solution, austenitic stainless steel is in danger of corrosion cracking under the action of internal stress. The specific medium that causes stress corrosion cracking of copper alloys is ammonia vapor or amines.
The occurrence of corrosion fatigue does not require a specific corrosive medium. Under the action of alternating stress, corrosion fatigue may occur in most metals. Under the combined action of alternating stress and corrosive medium, the protective film on the metal surface is damaged by the stress of different directions and sizes, forming a corrosion galvanic cell and causing local corrosion. The generated corrosion cracks are mainly transgranular cracks, and the fracture has three regions: fatigue source, fatigue crack propagation zone and final fracture zone. The fatigue crack propagation zone is in the shape of shell or beach. The parts of thermal equipment that are prone to corrosion fatigue are: the junction between the steam drum and the water supply pipeline, the junction between the steam drum and the pipeline adding phosphate solution, and the junction between the regular sewage pipe and the lower header. Boilers are frequently started, and metal equipment is partially exposed to alternating heat and cold, alternating dry and wet, and the mixture of soda and water in the pipeline is fast and slow, and corrosion fatigue will also occur. In addition, if corrosion occurs when the equipment is out of operation, local pitting pits will become a source of fatigue under the action of alternating stress, making corrosion fatigue more likely to occur. Corrosion fatigue is also prone to occur when the state of the evaporative heating surface of the once-through boiler fluctuates, or the steam-water layer occurs in the horizontal boiling tube.
Intergranular corrosion
Corrosion is concentrated on the grain boundary, and other areas are not corroded or corroded very slightly. This corrosion is called intergranular corrosion. The occurrence of intergranular corrosion is mainly due to the difference in the chemical composition of grains and grain boundaries, and the potential of grain boundaries is lower than that of grains. For example, the intergranular corrosion of stainless steel is mainly due to the fact that the stainless steel is kept in the sensitization temperature range (450~850℃) for a long time, and the grain boundary is deficient in chromium due to the precipitation of (Fe, Cr) 23C6, and the corrosion resistance of the grain boundary is greatly reduced. Intergranular corrosion often occurs on alloy materials such as stainless steel, nickel-based alloys, aluminum alloys, and magnesium alloys.
