Experimental Study Of Weld Corrosion In The Submarine Pipeline Expansion Bend Installed From WC13-1 Platform To FPSO

The marine oil and gas resources are rich,the pipeline construction develops rapidly,but the corrosion will seriously threaten the pipeline operation safety.The submarine pipeline expansion bend is installed from WC13-1 platform to FPSO,336 weld corrosion defects were found in the pipe by pipe internal detection,and corrosion defects depth reached 87%,internal corrosion is serious.Therefore,the research on the corrosion mechanism and the influence of different factors on the weld corrosion in the expansion curved pipe of submarine pipeline has great practical significance in engineering practice,which provides the theoretical basis for the corrosion and protection of submarine pipeline weld.In this paper,the types and causes of inner tube weld corrosion are simulated by EDX?XRD analysis of the on-site pipe corrosion products,combined with OLGA Software simulation pipeline flow pattern.The relationship between the mechanical properties and the corrosion resistance of the weld and the base material was analyzed by tensile test,impact test and hardness test.The microstructure difference between weld and base material and its relationship with corrosion resistance were analyzed by m etal lo graphic micro structure experiment.The galvanic corrosion behavior between different parts of the weld area of inner tube was studied by electrochemical experiments and galvanic corrosion experiments.The characteristics of polarization curves and impedance spectra of weld seam and base material under different temperature and Cl-concentration were studied by electrochemical experiment,and the corrosion law and mechanism were analyzed.The corrosion regularity and mechanism of internal pipe weld and base material under different CO2 pressure and medium velocity were studied by using the hanging-piece experiment and the microscopic corrosion topography.The corrosion mechanism of weld was obtained based on on-site sample testing and laboratory test results,and the inhibitor was screened from 4 kinds of inhibitors and optimum concentration of inhibitor.The following 5 conclusions are gotten through the research contents of this paper:(1)The main components of corrosion products in the field are FeCO3 and Fe3O4;The content of S element and P element in the weld specimen of the expansion bend inner tube is higher than that of the base material,which leads to the poor corrosion resistance of the weld;The flow rate near the end of the pipe is too large,causing pitting corrosion.(2)The tensile strength of the weld specimen is smaller than that of the base material,the tensile deformation ability of weld seam is worse than that of base material;The average impact work of weld impact specimen is significantly lower than that of base material impacting specimen,and the fracture resistance of weld is poor;The hardness value of weld is larger than that of base material,high carbon content and corrosion resistance;The inclusions in the metallographic microstructure of the sample were less and smaller,while the inclusions in the weld specimen were more and larger in size.(3)In the inner tube weld,the base material and the heat affected zone,the welding seam's potential is lowest,the corrosion current density is biggest,the local corrosion rate is greatest;in galvanic corrosion the weld and heat affected zone are accelerated as anode corrosion,and the base material is protected as cathode.(4)The corrosion rate of weld and base material increases with temperature increasing,however,the sensitivity of weld to temperature is stronger;The resistance of Cl-to the corrosion of weld and base material increases with the increase of Cl-concentration;The local corrosion of the weld is gradually increased with CO2 partial pressure increasing,and the corrosion product film of the base material has a certain protective effect on the substrate;With the increase of the medium velocity,the ionic depolarization gradually increased,the corrosion rate of weld and base material increased,the corrosion rate of weld was larger than that of the base material,and the pitting pit gradually increased,which showed a typical platform-like localized corrosion topography.(5)The 2-mercapto benzene and thiazole had the best corrosion inhibition effect to the weld.The corrosion inhibitor of 2-mercapto benzene and thiazole is the best corrosion inhibitor for 350mg/L,the corrosion rate of corroded hanging piece is reduced to 0.0617mm/a,and the inhibition rate is 93.9%.