A Study On Corrosion Characterristics And Monitoring Of The Welding Zone Of Inner Wall Of Marine Pipeline

In the process of offshore oil and gas exploration and transportation,CO2 gas acts as a coexisting medium and often merges with oil,gas and water,forming a complex corrosive environment in the gathering and transportation pipeline.Welding is an important connection method for marine long-distance gathering and transportation pipelines.Under different welding processes,the element composition and microstructure of the marine pipeline welding zone are significantly different.In addition,due to the existence of welding residual stress and welding defects,it faces severe risks of non-uniform corrosion,which directly threatens the safety of the pipeline.The non-uniform corrosion of marine pipeline weldment mainly includes two aspects.One is the corrosion damage caused by complex galvanic effect among the heterogeneous areas of the welding zone(base material,BM,heat-affected zone,HAZ,and welded mmaterial,WM),and the other is the corrosion damage caused by self-corrosion inside the homogenous area of the weldment.Due to the difference in materials and the nonuniformity of the structure of the marine pipeline welding zone,there is still a lack of effective non-uniform corrosion monitoring methods,and the non-uniform corrosion mechanism and influencing factors have not been fully revealed.Therefore,this work has established a variety of non-uniform corrosion monitoring methods for the weldment of marine pipelines.On the basis,the CO2 corrosion behavior of the weldment under different environmental factors such as solution temperature,pH and fluid state was further studied.The main work was summarized as follows:(1)A method for non-uniform corrosion monitoring of marine pipeline weldment based on traditional multi-electrode array sensor was established.The three heterogeneous areas of marine pipeline welding zone were separated into electrode arrays to form a new coupled multielectrode array.The galvanic corrosion behavior of weldment at different temperatures was studied.The test results indicate that when the temperature was 30℃,BM appeared as the cathode,and HAZ and WM appeared as anodes.When the temperature was 60℃,BM still appeared as the cathode,while WM transformed from anode to cathode due to the formation of a dense FeCO3 corrosion product film.The local area where no corrosion product film was formed in HAZ appeared as the anode and suffered severe corrosion.(2)In order to online separate and acquire the galvanic corrosion component and selfcorrosion component of marine pipeline weldment,a multiplexing method conjuncted with micro-resistance measurement and multi-electrode array was established.By monitoring the galvanic current among the heterogeneous areas and the total corrosion depth of a single homogeneous area,the non-uniform corrosion of the weldment under different medium temperatures was effectively evaluated.The test results show that when the temperature was low,the galvanic corrosion was not the main factor causing the corrosion damage of BM and WM.While as the temperature increased,the galvanic effect had a significant influence to the corrosion of HAZ.In addition,due to the high pearlite content and fine grain size of WM,it was conducive to the preferential formation of FeCO3 corrosion product film,resulting in the suppression of the corrosion.(3)By slicing the heterogeneous areas of the marine pipeline weldment separately and assembling them into the ring pair resistance probe,the multi-ring electrical resistance sensor array(MERSA)that can realize the simultaneous online monitoring of corrosion differences between heterogeneous areas and circumferential corrosion differences in homogeneous areas was established.And the non-uniform corrosion behavior of weldment under the influence of temperatures and fluid states in actual pipelines was studied.The test results show that the corrosion difference among the heterogeneous area was significantly affected by the difference in the microstructure.The Widmanite and acicular ferrite contained in WM led to poor corrosion resistance,and which was not conducive to the dense FeCO3 corrosion product film formation.Furthermore,the circumferential corrosion difference in the homogeneous area was related to the dissolved oxygen.The residual dissolved oxygen would cause the oxygen concentration difference cell between the top and bottom of the pipe.Combined with computational fluid dynamics(CFD)analysis,it is found that the increase of local flow velocity aggravated the degree of non-uniform corrosion.(4)The influence of media pH on the galvanic corrosion and self-corrosion of marine pipeline weldment was studied using coupled MERSA and three-dimensional morphology characterization.Through the analysis of the discrete interval and the coefficient of variation of the corrosion rate,the corrosion risk was evaluated.The test results indicate that when the solution pH was 4,the corrosion of HAZ was the most serious.The influence of the galvanic effect on the total corrosion of HAZ was gradually weakened.When the solution pH increased to 6.6,due to the compactness and integrity of the FeCO3 corrosion product film in WM was lower than BM,WM transformed from the cathode to the anode.HAZ was the most severely corroded area,and the proportion of galvanic corrosion to the total corrosion gradually increased.