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　　In the practical application of galvanized steel pipe, when the corrosion is along the metal grain boundary, the local selective corrosion is called intergranular corrosion. Intergranular corrosion is caused by the inhomogeneity of electrochemical potential between grain boundaries and grains. For stainless steel with intergranular corrosion, the surface of galvanized steel pipe looks bright, but it will be broken into small grains when it is knocked lightly.

　　Due to the difficulty of intergranular corrosion inspection of galvanized steel pipe, the damage is difficult to be predicted. According to the hazard data, this kind of corrosion accounts for about 10.2% of corrosion cracking.

　　The theory of impurity in grain boundary region and the theory of selective dissolution of another phase explain intergranular corrosion from different perspectives.

　　The reason of intergranular corrosion is that the electrochemical potential of grain boundary material is different from crystal itself, the energy ratio of grain boundary is higher, there are many edge dislocations and vacancies at grain boundary, solute atoms and impurity atoms are easy to form segregation at grain boundary, resulting in grain boundary adsorption phenomenon of galvanized steel pipe, resulting in disordered arrangement of grain boundary atoms and larger atomic spacing. New phases are easy to form at the grain boundary, resulting in the deposition of some alloying elements.

　　In addition, sometimes the newly formed phase itself has high energy and is easy to corrode, leading to increased corrosion tendency in the grain boundary area. 4. Stress analysis of bellows expansion joint tensile stress is one of the necessary conditions leading to stress corrosion of expansion joint. The actual tensile stress causing stress corrosion of galvanized steel pipe expansion joint can be roughly divided into four types: working stress, that is, the internal force generated under working load.

　　The displacement compensation process of bellows expansion joint is the continuous thermal expansion and cold expansion along the vertical direction. The stress produced in the service process is called working stress.

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