Corrosion Behavior Of X80 Steel Welded Joints

With the increasing demand of oil and gas resources, the construction of oil and gas pipeline is developing rapidly. At present, X80 pipeline steel is mainly used on the west-east gas project in China. However, local corrosion and corrosion cracking occurred repeatedly in the process of oil and gas pipeline working, especially the corrosion phenomenon on the weld joint is more obvious. It is a serious threat to safe operation for the buried pipeline. Therefore, it is of great significance to research on the corrosion behavior of X80 pipeline steel welded joints under different conditions to prolong the life of the pipeline.The X80 pipeline steel was welded by Shielded Metal Arc Welding (SMAW) and Gas Metal Arc Welding(GMAW), where the SMAW covered electrode selected E7096 and E9018. The GMAW solder wire selected ER70S-G and ER80S-G,root welding protection gases were 25%CO2+75%Ar, hot welding protection gas was 100%CO2, filling and capping welding protection gases were 15%CO2+85%Ar,and the gas flow rate was 25-30L/min. The results showed that in X80 pipeline steel welding joint, the microstructures of the base metal and SMAW weld was both made up of acicular ferrite+bainite and with a small amount of pearlite. The average value of tensile and hardness of SMAW weld was 670Mpa and 242HV10 respectively, and the average value of impact absorbing energy of weld and HAZ was 107J and 137J respectively.The microstructures of the GMAW weld were mainly composed of acicular ferrite、bainite and lath ferrite; The average value of tensile and hardness of GMAW weld was 700Mpa and 238HV10 respectively, and the average value of impact absorbing energy of weld and HAZ was 142J and 190J respectively.The corrosion behaviors of X80 pipeline steel and two kinds of welding joints under different conditions were investigated by electrochemical workstation, scanning electron microscope(SEM) and X-ray diffraction(XRD).The results showed that X80 steel and two kinds of welded joint corrosion rate decreased with the increase of immersion time at 30℃ from 3d to 7d in simulated soil solution, X80 steel and GMAW welding joints had better corrosion resistance than SMAW welding joints.The corrosion rate of X80 steel and two kinds of welded joints increased with the increase of soaking temperature from 30℃ to 70℃ for 3d.The corrosion products of X80 steel were mainly composed of FeO (OH)、Fe2O3 and Fe3O4, after soaking in simulated soil solution at 30℃ for 72h, where Fe3O4 was combined tightly and the holes was less. The corrosion rate of X80 pipeline steel and two kinds of welded joints decreased with the increase of pH value at 30℃ from pH6.5 to pH8.5 for 3d in simulated oilfield water solution. The corrosion rate of X80 steel and two kinds of welded joints increased with the increase of pressure at 120℃ from 3MPa to 7MPa for 3d, and the corrosion rate of the SMAW welding joint was the worst; The results showed that after soaking in 120℃、5MPa oilfield water for 72h, the surface of X80 steel base metal had a small amount of corrosion products, and the corrosion products were more compact but existed some cracks. The combination of weld corrosion product by SMAW was not tight enough and surface corrosion produced a large number of voids but weld surface corrosion products by GMAW were mainly shape of bamboo and sheets, which had a higher density than by the SMAW.