Performance Damage Study Of Bimetallic Composite Pipe Welded Joints In Sour Environment Transport Media

X65/Incoloy825 bimetal composite pipe is widely used in petrochemical,nuclear power,seawater desalination,shipbuilding and other industries because of its excellent corrosion resistance and excellent mechanical properties.In the process of long-term use of composite pipe,the corrosion damage problem of pipe welded joint is particularly prominent under the comprehensive effects of geological movement,corrosive medium and uneven structure.In this paper,the corrosion damage behavior of X65/Incoloy825 bimetal composite pipe in H₂S corrosion medium was studied.First,,the microstructure of X65/Incoloy825 bimetallic composite pipe welded joint was analyzed by OM,SEM,EDS,XRD,and Vickers hardness tester.It is found that X65 base metal is mainly composed of quasi polygonal ferrite and acicular ferrite,with a small amount of pearlite in the middle;Incoloy825 cladding base metal is mainly composed of austenite,with flat grain boundary,massive and thin strip,and twins often appear at the grain boundary.The weld zone is mainly composed of cellular crystal,cellular dendrite and dendrite.Different heat input and heat dissipation mode of different weld bead will lead to different microstructure.The results show that the content of Cr,Ni,Mo and other corrosion-resistant elements in the weld zone is reduced,which affects the corrosion resistance of the weld metal.The overall distribution of hardness is as follows:hardness of upper weld zone>hardness of lower weld zone>hardness of heat affected zone>hardness of Incoloy825 cladding layer>hardness of X65 base layer>hardness of explosive welding interface.The corrosion resistance of samples in weld area was analyzed by immersion corrosion method.The morphology of etch pits at different corrosion time was observed by three-dimensional topography instrument,and the performance before and after corrosion was tested by Instrumented indentation.The corrosion behavior of weld metal in sour environment transport media was studied.It was found that the weld metal was slightly corroded in the corrosive medium and formed Cr O₃ and Mo O₃,which had passivation effect.The average corrosion rate of the weld is much lower than 0.01mm/a,and decreases with the corrosion.The average corrosion depth was 0.2058μm for 30 days to 0.7167μm for 180 days,and the maximum corrosion depth C was in accordance with the exponential relationship of=0.158^0.816.Finally,according to the corrosion test results,the cellular automata model of material corrosion is established.The results show that the greater the corrosion probability,the greater the depth and width of the corrosion pit,the more pitting the corrosion morphology tends to be;the passivation probability has little effect on the morphology;with the increase of passivation probability,the corrosion morphology curve is smoother;with the increase of de passivation probability,the greater the roughness of the corrosion morphology,the closer to uniform corrosion.When the cell A concentration is 0.5,the cell B concentration is 0.1,the cell X concentration is 0.1,the corrosion probability is 0.15,the passivation probability is 0.02,and the depassivation probability is 0.01,the simulated corrosion morphology is the closest to the corrosion morphology after the test.The errors of simulated corrosion depth and experimental corrosion depth are 6.61%,3.34%and 1.23%for 2D cellular automata,and 5.28%,14.31%and0.46%for 3D cellular automata.The predicted corrosion depths of 210d,240d,270d and 300d are 11.23μm,11.71μm,13.26μm and 12.77μm,respectively.The stress distribution of CA model with different corrosion time around the corrosion pit is simulated by finite element method.It is found that the maximum Mises stress appears at the bottom of the corrosion pit,and the stress level is low at other locations.