Effect of Stress on Groove Corrosion of ERW Pipe

The high-frequency resistance welding technology that directly rolls the steel material is a technique for rapidly producing pipes. At present, it is widely used in many fields such as underground casing, oil and gas gathering pipelines and the like. However, the welded area of??this pipe has a higher activity than the base metal and the HAZ, so the weld zone determines the service life of the pipe. Groove corrosion is a special crevice corrosion of a seam welded pipe, often forming grooves in the weld zone. Under natural conditions, the formation of the groove easily stagnates the corrosion medium in the seam, resulting in a decrease of dissolved oxygen concentration, and gradually forms an oxygen concentration cell inside and outside the seam to accelerate the corrosion of the metal in the seam. At the same time the process of etching the remaining metal ions in the seam, causing Cl-moved into the seam, it was observed immersed in neutral sodium chloride solution in the gap, which contains chloride as the main solution of 3 to 10 times, with the progress of corrosion, corrosion products accumulate around the weld, such as Fe2 + hydrolysis is easy to produce excess H + in the seam, and the role of stagnation makes the seam metal in acidic conditions, it has been reported in the literature its pH value can reach 2 to 3, so that the seam remain activated metal to promote corrosion. When anodizing it, the rate of corrosion of the base metal and the weld mainly depends on its electrochemical activity.

If the erw pipe is not subjected to the whole tube heat treatment, the residual tensile stress in the circumferential direction of the weld joint reaches about 350 MPa, which is close to the yield strength of the welded pipe. When the weld is processed into a working electrode, its residual stress is reduced to below 100MPa. Tensile tensile stress can reduce the corrosion potential of the weld, accelerate corrosion and increase the corrosion sensitivity coefficient of the trench. However, after the heat treatment, the residual tensile stress of welded pipe products in circumferential direction is small, which is 10 ~ 100MPa. After processed into working electrode, the circumferential tensile stress changes little. This conventional measurement method can not characterize the corrosion resistance of pipe products in practical applications, can not be a true distinction between heat treatment or not on the corrosion resistance of pipe.

A worker from a steel pipe company, which is located in China.