Effect of Stress on Groove Corrosion of Electric Resistance Welded Pipe

Electric resistance welding technology for direct rolling of steel is a rapid production pipeline technology, which is widely used in many fields such as downhole casing, oil and gas gathering and transportation, etc. However, the weld of this pipe has a higher activity than the base metal and the heat affected zone, so the weld zone determines the service life of the welded pipe.Trench corrosion is a special crevice corrosion phenomenon for the seam welded pipe, which usually forms a groove in the weld zone The Under natural conditions, the formation of the groove is easy to make the corrosive medium in the stagnant state, resulting in the reduction of dissolved oxygen concentration, and gradually formed in the seam and seam oxygen concentration of the battery to accelerate the corrosion of metal within the seam. At the same time, the residual metal ions in the process of corrosion, it was observed in the neutral sodium chloride solution in the gap, the chloride contained in the main solution of 3 to 10 times, with the corrosion,the corrosion products accumulate around the weld,, such as Fe2 +, tends to produce excess H + in the seam, and the effect of stagnation makes the metal in the seam pieces, the pH value can reach 2 to 3, so that the metal within the seam to maintain activation, and promote the conduct of corrosion. When subjected to anodic polarization, the corrosion rate of the base material and the weld mainly depends on its electrochemical activity.

Electric resistance welded pipe (ERW), if not through the tube heat treatment, the weld at the circumferential residual tensile stress of about 350MPa, close to the yield strength of the pipe. When the weld into the working electrode, the residual stress reduced to 100MPa or less. The circumferential tensile stress can reduce the corrosion potential of the weld, accelerate the corrosion, and increase the trench corrosion susceptibility coefficient. After the heat treatment, the residual tensile stress in the weld is small, 10 ~ 100MPa, and the tensile stress in the circumferential direction after processing into the working electrode is little. In this way, the conventional measurement method can not characterize the corrosion resistance of the welded pipe products in practical application, and can not really distinguish the effect of heat treatment on the corrosion resistance of the welded pipe.