Study On Intergranular Corrosion Properties Of X65-inconel 625 Metallurgical Clad Pipe Welded Joints

For the exploitation of oil and gas fields with strong corrosive media,more and more corrosion resistant clad pipes are used.Under acidic corrosion conditions such as crude oil and natural gas,the inclusion of S^2-,H⁺and Cl^-may increase the intergranular corrosion sensitivity of the weld overlay.The welded joints of clad pipes include overlay layers and butt welds.Due to the effects of dilution and uneven microstructure,the welded joints of clad pipes may not meet the required corrosion rate.In the past,when evaluating the intergranular corrosion(IGC)performance of a welded structure,the influence of the thickness of the specimen was usually not considered.In this paper,by studying the IGC properties of specimens at different locations,it was found that the thickness of the specimens had a greater impact on the IGC behavior.Therefore,studying the intergranular corrosion performance of different locations of the weld overlay is of great significance for the long-term safe service of clad pipe welded joints.The main research contents of this thesis are as follows:In this study,the IGC resistance of the welded joint of X65-Inconel 625 clad pipe weld overlay was evaluated by chemical immersion method and electrochemical method.As the position of the specimen gradually approached the X65 side,the IGC resistance of the welded joint of the clad pipe was lowered.The increased susceptibility of the clad pipe welded joint to IGC was due to the combination of small grain size and high Fe content of the weld overlay microstructure.Corrosion defects in the BM and HAZ exhibited different forms of penetration into the interior of the material.The corrosion in the HAZ penetrated along the grain boundaries,while those in the BM formed corrosive penetration paths along inter-dendritic regions.As the test progressed,the pitting in the BM became deeper and evolved into inter-dendritic corrosion.The IGC resistance of the weld overlay near the X65 side was very low,which was of great significance when evaluating the IGC performance of the clad pipe welded joints.The IGC test specimens needed to represent as much of the tested material as possible.Therefore,it was necessary to include the bottom part of the weld overlay as much as possible.Through further research on nickel-based 625 superalloy,it was found that the IGC resistance of different characteristics was different,and the essential reason that the low angle grain boundaries(LAGBs)was more resistant to IGC than the high angle grain boundaries(HAGBs)was given.The strong IGC resistance exhibited by the LAGBs is not due to the state of the precipitates dominated by the grain boundary misorientation angle,but to the same crystal orientation of the grains adjacent to the grain boundary of the LAGBs.And the corrosion penetrates down the grain boundary mostly due to the existence of the grain boundary itself as a defect form,rather than due to segregation of elements.