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Metal caustic corrosion refers to the damage phenomenon that occurs due to the combined action of tensile stress and corrosive media on metals in high-temperature, high-concentration alkaline environments (such as sodium hydroxide and potassium hydroxide solutions). It is common in industrial scenarios such as boilers, pressure vessels, and chemical equipment. Its essence is a kind of stress corrosion cracking (SCC), also known as "alkali embrittlement" or "caustic embrittlement". It has the characteristics of strong concealment, high suddenness and great harm. 1. Conditions for occurrence: All three are indispensable. The occurrence of caustic alkali corrosion must meet the following three conditions at the same time.: 1. High-concentration alkaline environments usually require the NaOH mass fraction to exceed 5%, and the risk increases significantly when the local concentration can reach more than 10%. Even if the initial concentration is very low (such as 10~20mg/L), repeated concentration under evaporation, sediments or in gaps may form local high-alkali areas. 2. The existence of tensile stress includes stress generated by working load, welding residual stress, assembly stress or thermal stress. When the stress is close to the yield strength of the material, cracks are more likely to occur. 3. Sensitive metal materials such as carbon steel and low alloy steel are the most common, especially in boiler riveting and expansion joints. Austenitic stainless steel may also undergo transgranular cracking in high-temperature concentrated alkali. 4. Typical scenario: boiler water contains trace amounts of NaOH → locally concentrated on the high-temperature evaporation surface → metal is subject to compressive stress → intergranular crack initiation → brittle fracture. 2. Corrosion mechanism and characteristics 1. During the corrosion process, under the action of high concentration of OH⁻, the protective oxide film (such as Fe₃O₄) on the metal surface is dissolved.: Fe₃O₄ +4NaOH → 2NaFeO₂ + Na₂FeO₂ +2H₂O The exposed base metal continues to react to form soluble ferrite, causing corrosion to continue. At the same time, the hydrogen atoms produced by the cathode reaction penetrate into the metal, causing hydrogen embrittlement and accelerating crack expansion. 2. The crack characteristics are mainly along the grain propagation.: The cracks develop along the grain boundaries and branch out like dendrites. No obvious plastic deformation: There are no signs before the rupture, and the damage is sudden. Surface is dark black: Cracked areas are often accompanied by oxide deposition. 3. Susceptible parts and industry cases Electric boiler tube plates, smoke tubes, and superheaters cause alkali embrittlement explosions due to improper water quality control Chemical autoclave, evaporator, waste heat recovery system Electrolytic aluminum plant evaporator alkali leakage accident Papermaking alkali recovery furnace NaOH concentration causes carbon steel cracking Nuclear power primary circuit cooling system local alkali concentration induces stress corrosion 4. Prevention and control measures Control water quality and alkali concentration. Strictly monitor the pH value of boiler water to avoid excessive free base. Prevent local concentration: Clean the deposits regularly and optimize the structural design to reduce gaps. Eliminate or reduce stress. Perform post-weld heat treatment (stress relief annealing) on the welded parts. Optimize structural design and reduce stress concentration. Material selection optimization. In high-temperature and concentrated alkali environments, corrosion-resistant materials such as austenitic stainless steel (such as 304, 316L) or nickel-based alloys can be used. Avoid using ordinary carbon steel for long-term operation in an alkali-containing environment >46°C. Add corrosion inhibitor. Corrosion inhibitors such as polyphosphate and fluorosilicate are used in alkaline environments to inhibit corrosion reactions. Regular inspection and maintenance. Early cracks are monitored using electrochemical testing, ultrasonic flaw detection, metallographic analysis and other means. Focus on high-risk areas such as riveting, expansion joints, and welds.