Fe-based shape memory alloy pipe fittings can prevent stress corrosion cracking in carbon steel alkali pipelines

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Fe-based shape memory alloy pipe joints can prevent stress corrosion cracking in carbon steel alkali pipelines. 1. Overview The alkali pipelines in the refinery of a petrochemical company are used to supply alkali solutions with a concentration of 30%–40% to the production units; these pipelines are made of carbon steel, connected by welding, operate at a pressure of 0.6–0.7 Mpa, and function in an intermittent manner. During winter operations, steam heating at a pressure of 0.3 Mpa is required; due to the high temperature of the alkaline solution, the welds in the pipes crack, resulting in frequent leaks of the alkaline solution and creating difficulties in production. At the same time, the leaking alkali corrodes other pipes, resulting in high annual maintenance costs; this problem has not been resolved since 1994. 2. Analysis of corrosion causes: Ordinary carbon steel forms a surface film composed mainly of Fe3O4 or Fe2O3 in alkaline solutions. Meanwhile, the precipitation of carbides and nitrides at the grain boundaries renders the surface film there unstable, making it prone to dissolution. Under the action of external stress, grain boundary cracks are formed, causing selective dissolution of FeO2- in the newly exposed iron, thereby resulting in stress corrosion. 　　Carbon steel is susceptible to alkali embrittlement in all concentration ranges above 5% NaOH solution, with concentrations around 30% being the most hazardous. The lowest temperature at which alkali embrittlement occurs is 50°C, and it tends to happen most frequently in high-temperature areas near the boiling point. See Figure 1. 　　During the use of the pipeline, in summer or when the pipeline is not heated, an alkaline solution with a concentration of 30–40% does not cause alkali embrittlement ； In winter, when the pipes are heated and the temperature exceeds 50°C while the alkali concentration remains at 30–40%, alkali embrittlement occurs, as actual alkali-containing pipes usually reach temperatures above 50°C when heated. 　　Furthermore, an alkaline solution will dissolve iron only under highly concentrated conditions, through the following reactions:
3Fe + 7NaOH → Na3FeO3·2Na2FeO2 + 7H
Na3FeO3·2Na2FeO2 + 4H2O → 7NaOH + Fe3O4 + H
7H + H → 4H2
3Fe + 4H2O → Fe3O4 + 4H2
Alkali is supplied to this pipeline approximately every 10 days, which makes it easy for the alkaline solution to accumulate at the welds. Pipes are generally welded on one side, and the inner wall often has incomplete welds as well as gaps. As time passes, the alkali solution becomes more concentrated, leading to its accumulation at the weld area; this causes corrosion to occur first at that location. Moreover, there is no metal passivation layer in the weld seam, leaving the fresh metal surface exposed. According to the principles of electrochemical corrosion, the metal surface in that area is usually at the anode and is in a state of corrosion. 　　The joints of the original pipe were welded, and the microstructure near the weld seams had larger grains compared to that of the base material. Additionally, the welding structure was uneven and there were numerous defects after welding; as a result, there were significant differences in the mechanical properties and chemical composition of the surface of the weld seam and the base metal. 　　When this pipeline is used and shut down alternately at temperatures above 50°C, stress corrosion cracking occurs rapidly due to the combined effect of alkali accumulation and high temperatures, resulting in frequent leaks during winter. 3. Basis for material selection 　Through a comparison of several anti-corrosion methods, the \"Fe-based shape memory alloy pipe joint\" proved to be the most effective. This is because the effect of shape memory alloys lies in the fact that after being deformed (usually below or around the Ms temperature), the material can return to its original shape when heated to a temperature above a certain threshold. Such materials with shape memory effects are called memory materials. The principle of using iron-based shape memory alloys to connect pipes is shown in Figure 2. At room temperature, the pipe fitting is expanded through deformation; as a result, the inner diameter of the fitting becomes larger than the outer diameter of the pipes to be connected, which makes it easier to insert those pipes into the fitting. Then, the fitting is heated to a certain temperature (above Af), and it contracts back to its original smaller diameter, thereby gripping the pipes tightly. To achieve the purpose of connecting pipes. Using iron-based shape memory alloy pipe fittings to connect pipes can prevent the changes in the microstructure that occur as a result of welding, as well as stress corrosion cracking at the pipe welds. 4. Application results: After more than 10 years of use, no leakage was observed in this pipeline, indicating that this method can fully address the issue of stress corrosion in pipelines, and it is also relatively cost-effective.