Crevice Corrosion Behavior Of Steel Pipes In Oil And Gas Field Environment

During the extraction and transportation of oil and natural gas,steel pipes are generally connected by threads or welding.Once the corrosive media infiltrate into the threads or welds due to the structural design and process defects,severe crevice corrosion will occur.In addition,the connected threads of the downhole steel pipes are also subject to great stresses caused by the gravity of steel pipes.There is a potential risk of galvanic corrosion for the bimetal composite pipe composited with dissimilar metals.Therefore,the joints of steel pipes may fail due to the long-term effects of crevice,galvanic and stress,resulting in serious safety accidents.In consequence,conducting the research on the corrosion behavior of steel pipes in the oil and gas field environment under the coupling of stress and crevice and the influence of the galvanic effect of dissimilar metals on the crevice corrosion,and then further establishing reliable protective measures,not only has great engineering value for ensuring the safe exploitation of oil and gas but also has important theoretical significance and academic value for in-depth understanding the crevice corrosion mechanism of oil and gas steel pipes.In this thesis,X65 carbon steel-316 stainless steel bimetallic composite pipe and 13Cr stainless steel were selected as the research materials.The influence of the galvanic effect between X65 carbon steel and 316 stainless steel on the crevice corrosion behavior of X65carbon steel and the effect of applied stress on the crevice corrosion behavior of 13Cr stainless steel were studied by electrochemical tests and surface analysis.The main contents are as follows:(1)Through the self-designed setup for the electrochemical test under the coupling effect of galvanic and crevice,the crevice corrosion behavior of X65 carbon steel under the coupling effect of galvanic and crevice in CO₂-saturated NaCl solution was studied.The influence of the galvanic effect between X65 carbon steel and 316 stainless steel on the crevice corrosion behavior of X65 carbon steel was explored.The results show that X65carbon steel undergo crevice corrosion in CO₂-saturated 1%NaCl solution containing acetic acid.The X65 carbon steel inside crevice suffers from severe corrosion while the corrosion of the X65 carbon steel outside crevice is insignificant.When X65 carbon steel and 316stainless steel are coupled inside crevice,due to the presence of large potential difference,the X65 carbon steel inside crevice is subjected to greater anodic polarization,resulting in an increase in the corrosion rate and a deeper corrosion groove at the opening of the crevice,i.e.,the galvanic effect promotes the crevice corrosion of X65 carbon steel.(2)Through the self-designed setup for the electrochemical test under the coupling effect of stress and crevice,the crevice corrosion behavior of 13Cr stainless steel under the coupling effect of stress and crevice in high pressure CO₂/O₂ environment was studied,and then the effect of stress on crevice corrosion behavior of 13Cr stainless steel was explored.The results show that the 13Cr stainless steel behaves as a self-passivation state in high-pressure CO₂/O₂ environment and there is no activation/passivation transition region in the polarization curve.The applied tensile stress acclerates the anodic dissolution rate of 13Cr stainless steel inside crevice.In addition,there is a galvanic effect between the stressed steel inside crevice and the unstressed steel outside crevice.The applied stress shortens the induction stage of crevice corrosion and induces a larger galvanic current,promoting the development of crevice corrosion.Meanwhile,the existence of O₂ is helpful to form a more stable passive film but causes a greater galvanic effect.