Research On Corrosion Behavior Of X80Pipeline Steel And Its Weld Joint In Simulated Ku’erle Soil Solution

X80pipeline steel is considered to be the future choice of natural gas transportation. Atpresent, X80pipeline steel is mainly used on the second West-to-East gas pipeline.However,there are much local corrosion and corrosion cracking since pipeline laying, especially thecorrosion phenomenon of weld joint is more obvious. It is a serious threat to safe operation ofthe buried pipeline.Electrochemical behaviors of X80pipeline steel and its weld joint in simulated Ku’erlesoil solution were investigated by mass loss measurement, potentiodynamic polarization andelectrochemical impedance spectroscopy (EIS). The corrosion products and morphologieswere observed by X-ray diffraction (XRD) and optical microscope (OM). The results showedthat the corrosion rate of base metal of X80steel decreased gradually, however the corrosionrate of weld joint first decreased and then increased with the increasing of immersion time.The metal was protected by the corrosion product films. Commonly the weld joint was moreserious to corrosion behavior than the base metal under the same immersion time.The effects of SO42-on corrosion behavior of X80pipeline steel and its weld joint insimulated Ku’erle soil solution were investigated by potentiodynamic polarization and EIS.The results showed that the corrosion rate of the metal first increased and then reduced withthe augment of SO42-concentration. When SO42-concentration is5%, the corrosion behaviorof X80steel was the most serious at this time. When the concentration was10%, there wereonly a few small corrosion pits on the surface of metal. The adsorption ability of SO42-wasstronger than Cl-, so that it was able to inhibit the corrosion behavior of X80steel arousedfrom Cl-. Electrochemical measurement technology was used to study the effect of temperature,pH value and dissolved oxygen concentration on electrochemical corrosion behavior of X80pipeline steel and its weld joint by orthogonal testing method. The results showed that thecorrosion current density of base metal first increased and then decreased with the increase oftemperature. With the increasing of pH value, the corrosion rate of metal presented decreasingtrend slightly. With the increasing of the dissolved oxygen concentration, the corrosion rate ofX80steel base metal and weld joint reduced obviously. The degree of association betweencorrosion rate and environment was dissolved oxygen, pH value, and temperature accordingto the correlation degree.Slow strain rate testing (SSRT) and electrochemical measurement technique wereemp1oyed to study the stress corrosion cracking (SCC) behavior of X80pipeline steel andweld joint. Fracture surfaces were observed by SEM under different strain rate and appliedcathode potential conditions. The results showed that when the strain rate was5×10-6s-1, thecorrosion behavior of base metal of X80steel was the most serious at this time. However, thecorrosion rate of weld joint increased persistently. The crack generation mechanism of X80steel base metal and weld joint was attributed to anodic dissolution. The base metal and weldjoint exhibited lower SCC sensitivity under cathodic protection potential and their crackinggeneration mechanism were anodic dissolution and hydrogen embrittlement. When theapplied potentials were lower than cathodic protection potential, the process of hydrogenevolution played the dominant role in SCC occurrence, meaning that the SCC mechanism washydrogen embrittlement. Commonly the weld joint was more sensitive to SCC than the basemetal under the same applied cathodic potential and strain rate.The influence of hydrogen on stress corrosion behavior of a welded X80steel insimulated Ku’erle soil solution was investigated by electrochemical hydrogen charging,dynamic potential scanning technology and SSRT. The results showed that the metalcorrosion reaction could be accelerated with the increase of hydrogen current density and time.The corrosion rate decreased with the increasing of hydrogen concentration. Hydrogen promoted corrosion development of X80steel and weld joint. Hydrogen induced cracking(HIC) played a decisive role on the SCC behavior of the sample after hydrogen charging.