Corrosion Behaviour Research Of X70 Pipeline Steel Welded Joints For Deep Sea

Pipeline transportation has become the first choice for oil and gas transportation because of its high efficiency and safety.Since 1954 the United States in the Gulf of Mexico established the first submarine pipeline,foreign deep-sea pipeline transportation developed rapidly.Domestic pipeline steel started late.Full length 261 km of the South China Sea Li wan gas field was China’s first deep-sea oil and gas projects.In this paper,the X70 pipeline steel with the outer diameter of 691 mm,wall thickness of 40.5mm,used in 3500 m deep sea,reaching the leading level at home and abroad in the thickness and depth of service.The corrosion can easily lead to the failure of materials,which can lead to huge economic losses and safety risks.Deep-sea pipeline in a long time in the complex environment of low temperature and high pressure,the corrosion resistance of pipeline steel is higher.In this paper,the corrosion behavior of X70 pipeline steel welded joints for 3500 m deep sea was studied by salt spray corrosion test,stress corrosion test,immersion test,electrochemical test.The neutral salt spray test(NSS test)was carried out on the welded joint of thick wall X70 pipeline steel in this paper,and the corrosion products of 1h,2h,4h and 8h were observed by metallographic microscope and scanning electron microscope.The corrosion products surface was analyzed by EDS,and the composition of corrosion product films was discussed by XRD.The results show that pitting corrosion occurs firstly in the salt spray corrosion process,and the pitting corrosion is gradually developed into an overall corrosion.Cl-causes pitting corrosion,pitting leads to the generation of cracks.The composition of the corrosion product is uniform,which is mainly composed of iron oxide,chloride and hydroxide.The corrosion rate increases slowly with the prolonging of the corrosion time.In the process of corrosion,Cl-reacts with the matrix easily through the outer loose corrosion product,and the dense corrosion product film on the innermost layer that blocks the further spread of the corrosive medium slows the reaction rate and improves the corrosion resistance.The tensile test was carried out at different temperatures and at different strain rates for the 3500 m deep sea X70 pipeline base metal and the welded joints.The stress corrosion behavior was analyzed.The fracture surface was observed by scanning electron microscopy(SEM).The fracture composition was analyzed by EDS.The fracture behavior was analyzed by slow stress curve.The stress corrosion mechanism is described.The stress corrosion has a reference significance to the use of deep-sea pipeline service.The results show that the stress corrosion tendency of the matrix and the welded joint at 3.3×10-6 s-1 and 1.0×10-6 s-1 is not obvious.The effect of temperature on the stress corrosion of the substrate and the welded joint is not obvious.The corrosion resistance of X70 pipeline welded joints for deep sea was studied by the immersion test,the weight loss test,the electrochemical test in this paper.The components of the passive film was analyzed by XRD and the microstructure was observed by scanning electron microscope.The results show that the corrosion resistance of the weld metal is the best,and the self corrosion potential is the highest.The corrosion resistance of the heat affected zone follows,the self corrosion potential is between the weld metal and the base metal.The corrosion resistance of the base metal is the worst,and the self corrosion potential is the most negative.And for the same area,the corrosion resistance of the inside welding is better than that of the outside welding.The formation of dense Fe3O4 passivation film can effectively slow down the progress of the reaction,and the corrosion products of Fe2O3,FeOOH and Fe(OH)3 which are loose in the outer layer,have no protective effect on the matrix.The microstructure of the weld metal of the best corrosion resistance is mostly intragranular nucleation of the ferrite and the M-A island is fine and uniform.The microstructure gradient of the heat affected zone is the largest,the M-A island is coarse and the corrosion resistance is inferior to the weld metal.The base metal consists of ferrite and bainite,the bainite is island-like distribution and the corrosion resistance is the worst.The influence of outside welding thermal cycle on microstructure of the inner part is more refined,the volume fraction of M-A island is more,the stress is released,and the corrosion resistance is better than the outside welding.