Study On Hydrogen Permeation Behavior Of X80 Steel And Its HAZ Under Interaction Of Cathodic Polarization And Stress

In recent years,the exploitation and utilization of marine oil and gas resources has developed rapidly,so the corrosion prevention of facilities used in the sea is more and more important.Researches and engineering practices show that cathodic protection technology can effectively protect the marine structures from corrosion.However,if the protection potential is too low,the hydrogen evolution reaction will occur easily,and then,the hydrogen gathering and diffusing into the steel will induce the hydrogen embrittlement.The high strength of steel structure and the significant changes of microstructure caused by the welding thermal process,besides the working load and complex stress will influence the hydrogen penetration into steel,this will affect the safety of marine steel structures under cathodic protection.Aimed at the unsubstantial welding heat affected zone(HAZ)and carrying out the hydrogen permeation study of high strength steel structure under cathodic protection is very important for extending the life of marine structure and has significant meaning for preventing the disastrous accident.Nowadays,the research on hydrogen permeation behavior of X80 steel & HAZ caused by cathodic protection and stress is relatively rare.In this paper,thermal simulation technology was used to prepare large specimens of HAZ,the hydrogen permeation behavior in different regions of HAZ was evaluated under the conditions of different cathodic protection potential and stress,and then the hydrogen permeation parameters under each condition were calculated.The experiment results showed that whatever the stress level was,the hydrogen penetration current density increased as the cathodic polarization potential decreases and the steel's sensitive hydrogen precipitation potential was lower than-1050 mV(vs.SCE).When the cathodic polarization potential of hydrogen permeation under different stress was-1100 mV(vs.SCE),the hydrogen permeation parameters of X80 steel & HAZ has the same change regularity,that is the hydrogen permeation current density increased with the increase of stress and then began to fall,then significantly increased when the tensile sample was completely going into the later stage of plastic deformation.The apparent hydrogen diffusion coefficient Da increased firstly and then decreased with the increase of stress.Under the same stress level,Da increased as the peak temperature of thermal cycle increases,on the other hand,the adsorption of hydrogen concentration C0 changed contrary to Da.The microstructure observation and Electron Backscattered Diffraction(EBSD)analysis results show that,X80 steel & HAZ has different microstructure and large-angle grain boundaries as the peak temperature of thermal cycle was imparity,the different amount of hydrogen trap density and binding energy is the reason for the difference of hydrogen diffusion coefficient Da.When under 60%Rt0.5 elastic stress,the lattice expanded and the space between metal atoms increased which make it easier for hydrogen to penetrate,and this led to the increase of hydrogen permeation current density and diffusion coefficient.The calculation of hydrogen trap energy confirmed that when appeared plastic deformation,the new generated dislocations acted as the reversible hydrogen trap to capture hydrogen,hydrogen trap density in sample was increased,so the hydrogen permeation current density and diffusion coefficient decreased with the increase of plastic deformation.