Stress corrosion cracking of carbon steel in alkaline solutions
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Stress corrosion cracking of carbon steel in alkaline solutions: Alkaline stress corrosion cracking (also known as alkali embrittlement) refers to anodic dissolution-induced cracking that occurs under the combined action of tensile stress and NaOH in the medium. Alkali stress corrosion cracking mainly occurs in carbon steel and low-alloy steel equipment. Alkali stress corrosion cracks mainly occur intergranularly. The sensitivity of carbon steel and low-alloy steel to alkali corrosion cracking is primarily determined by the concentration of the alkaline solution, the metal temperature, and the level of tensile stress. Alkali stress corrosion cracking generally takes several years to occur, but it can also happen within a few days if the concentration of the alkaline solution or the temperature of the metal is increased to accelerate the cracking process. Pipelines for transporting alkaline solutions and storage tanks for such solutions are at risk of alkali embrittlement if the temperature exceeds 50°C. In particular, the weld bead and the heat-affected zone of the weld bead.
What material is suitable for pipes used with sodium hydroxide solution at an operating temperature of 80°C and a concentration of 40%–50%? And what material is appropriate for sodium hydrosulfide solution at a concentration of 40% and a temperature of 80°C?
Sodium hydroxide solutions with an operating temperature of 80°C and a concentration of 40%–50% represent quite demanding operating conditions. Because carbon steel pipes can be used at room temperature. We know that when the temperature of the alkaline solution exceeds 50 degrees, alkali embrittlement occurs in the weld bead and its heat-affected zone, leading to stress corrosion cracking of the weld bead. Therefore, under such conditions, when carbon steel pipes are used, heat treatment is applied to the welded seam and the heat-affected zone to reduce their hardness. The required hardness after treatment should be roughly the same as that of the base material, which helps to prevent alkali embrittlement in the pipes. There is another method: instead of welding for piping connections, a \"ferroalloy memory pipe joint\" can be used, which yields excellent results; check the relevant literature. For a sodium hydrosulfide solution at 80°C, we know that carbon steel pipes can be used when the concentration of the medium is less than 30%. It can no longer be used once this concentration is exceeded. 304 stainless steel and similar materials can be used.