Research On The Selective Corrosion Mechanism In The Welded Zone Of An Electric Resistance Welding Pipe For Oil Production

In this thesis, an optical microscope was used to analyze microstructures of electrical resistance welding（ERW） pipe under welded and post-weld heat treatment for investigating the effect of ERW process and post-weld heat treatment, microhardness measurement to reveal distribution and change of microhardness. electrochemical measurement to study the beheavior of electrochemical corrosion and immersion test to obtain the groove corrosion sensitivity of ERW pipe welding area. These investigations can provide a reference for the quality control of ERW pipe.Narrow welding heat affected zone formed on both sides of weld line after the ERW welding. Because the high cooling rate existed at ERW welded area after welding the microstructures were, in turn, the coarse martensite martensite+ferrite mixture form fusion line to base material. Online whole heat treatment at temperature of 980 C can eliminate the hardening microstructure of welding area in ERW pipe such that the microstructure and properties is quietly uniform. Fusion line mainly consisted of ferrite, and the tube body consists of ferrite and pearlite. Some similar treads can be seen between microhardness and microstructure measurements. The weld hardness of welding line near line section is obviously higher than that of the parent metal and welding line hardness of pipe with heat treatment after welding is slightly lower than that of the parent metal.In 3.5 wt % NaCl solution, open circuit potential of parent metal specimen shows a slowly decreasing trend and finally reaches a stable value. Parent metal samples present active anodic dissolution characteristics. With the increase of corrosion environment temperature, the polarization curve moves up the top right. In 3.5 wt % NaCl solution saturated CO₂, there is one time constant in the electrochemical impedance spectrum（EIS） of parent metal specimen, i.e. the high frequency capacitive reactance arc. The radius of high frequency capacitive reactance arc decrease as corrosion environment temperature increases, the radius of high frequency capacitive reactance arc is the largest at 30 C while that is the minimum at 90 °C.In the 3.5 wt % Na Cl solution under flowing, serious selective corrosion exists in the ABSTRACT welding zone of ERW welded pipe. The width of the corrosion is wider than that of the welding zone width, and the groove corrosion sensitivity coefficient is high. After post weld heat treatment at temperature 980 ℃ of With 980 ℃, selective corrosion mainly concentrated at the fusion lines, and the groove corrosion sensitivity coefficient decreases. Therefore, post-weld heat treatment can reduce groove corrosion sensitivity and lighten the degree of selective corrosion.