Study On Stress Corrosion Behavior Of X70 Pipeline Steel Welded Joint In Marine Environment

China is rich in marine oil and gas resources.With the development strategy of marine engineering,the corrosion problem of submarine pipeline is increasingly prominent.In particular,there are many additional structural tensile stresses caused by topography,ocean current and geological factors,together with the joint action of the internal pressure of the pipeline,which will produce a strong coupling effect of composite load and complex environment,which will lead to stress corrosion cracking(SCC)and failure of the submarine pipeline in service,causing serious marine pollution,and even directly leading to major marine engineering accidents.Up to now,the most advanced steel used in submarine pipeline engineering at home and abroad is grade X70.Due to the pipelaying ship assembly line operation,the subsea pipeline requires high efficiency welding and high weldability.The weldability of high-strength steel grades above X70 is poor in the field construction,and it is easier to have stress corrosion cracking at the welded joints.Therefore,it is necessary to study the stress corrosion behavior of high-grade X70 pipeline steel welded joints in the marine environment and explore the mechanism of stress corrosion cracking in the complex marine environment.Based on the slow strain rate testing(SSRT)and scanning electron microscope(SEM),the stress corrosion behavior of X70 pipeline steel base metal and welded joint in the marine environment of South China Sea under different soaking time(10h,24h,72h,120h)was studied,and ABAQUS was used to simulate the distribution of residual stress of different regions(base metal,welded joint),the reason of the difference of stress corrosion sensitivity between base metal and welding joint of X70 pipeline steel was further explored.The results show that in the South China Sea simulation solution,the X70 pipeline steel base metal and the welded joint show obvious stress corrosion sensitivity compared with the air tension.Under the same conditions,the stress corrosion sensitivity of welded joint is higher than that of base metal.This is because the microstructure of the welded joint is mainly acicular ferrite and upper bainite,and its residual stress is significantly higher than that of the base metal.With the increase of soaking time,the SCC sensitivity of base metal and welded joint of X70 steel increases first,then decreases and increases again.The SCC sensitivity of steel is the highest when soaking for 24 hours.The law of SCC sensitivity coefficient I_SCCof X70steel base metal and welding joint is:I_SCC(10h)<I_SCC(72h)<I_SCC(120h)<I_SCC(24h).The reason of this phenomenon lies in the different structure of corrosion product film on the surface of metal electrode.Through potentiodynamic polarization technology,electrochemical impedance technology,SSRT,combined with metallographic microscopy to observe the corrosion morphology,XRD product film composition analysis,SEM characterization of the product film state,the influence of the corrosion product film on the stress corrosion behavior of X70 pipeline steel welded joints in the South China Sea environment was studied.The mechanism of stress corrosion cracking of X70 steel welded joint caused by membrane stress is analyzed by finite element method,and the mechanism of SCC of X70 steel welded joint in South China Sea environment is further explored.The results show that in the simulated solution of South China Sea,the corrosion product film on the welding joint surface of X70 steel has a double-layer structure,the outer layer is mainly flocculent Fe(OH)₃and Fe₂O₃,and the inner layer is mainly dense Fe₃O₄.With the increase of soaking time,from 10h to 120h,the density of corrosion product film was loose at 10h and24h,and dense at 72h.Cracks appeared and the density decreased at 120h.Under the same conditions,the free corrosion current density of metal also shows the characteristics of increasing first,then decreasing and then increasing,which is:I_corr(10h)<I_corr(72h)<I_corr(120h)<I_corr(24h).The film induced stress produced by the corrosion product film is mainly distributed in the front of the crack tip of X70 steel welded joint,which promotes the crack propagation to the front.