Influence Of Coal Gas Environmental Factors On Hydrogen Permeation Behavior And Hydrogen Embrittlement Sensibility Of X80 Pipeline Steel

Coal gas has been developed in order to relieve the pressure of natural gas shortage and achieve the effective utilization of clean coal.However,unlike the traditional natural gas,coal gas usually contains a certain amount of hydrogen gas,so hydrogen-induced embrittlement becomes a potential safety issue on long-distance pipeline.While coal gas is transported with pipeline,the existence of other gas,the variation of hydrogen gas pressure,and the corrosion film formed inside the pipeline can affect the sensitivity of pipeline steel to hydrogen embrittlement?HE?by changing hydrogen adsorption and permeation behavior.Nowadays,the relevant research aiming at influence of coal gas environment on hydrogen adsorption,permeation and HE is rare.Therefore,it is necessary to study the hydrogen permeation and hydrogen embrittlement sensibility of X80 steel in coal gas environment.In this paper,Vienna Ab-initio Simulation was performed to study the effects of four main constitutes in coal gas?H₂?CH₄?CO₂?N₂?on hydrogen adsorption on Fe,and meanwhile,the hydrogen permeation test was conducted under high pressure to testify the simulation results.The results shown that,H₂ molecule was dissociated into H atoms on Fe?111?,and these hydrogen atoms were adsorbed on the surface by chemisorption.Conversely,the other three molecules still remained original structures by physical adsorption.The hydrogen permeation test under 12 MPa hydrogen gas shown that,hydrogen permeation current was only associated with hydrogen pressure and independent with the total pressure.Hydrogen gas pressure determined the steady current during hydrogen permeation,while other gas components could not cause a significant change of current.Therefore,pure hydrogen gas was used to substitute coal gas for convenience.The test results of hydrogen permeation and slow strain rate tension?SSRT?shown that the hydrogen diffusion coefficient?D?of X80 steel didn't vary obviously with increase of hydrogen pressure within 12 MPa,but the steady hydrogen penetration current density?I_??and the adsorption of hydrogen concentration?C₀?increased with pressure.The plastic properties,such as the deformation amount,the percentage elongation after fracture,the shrinkage on cross section and fracture energy,decreased with increase of hydrogen pressure,which result the increase in HE index.Analysis suggested that 12 MPa hydrogen pressure only led to a certain elastic deformation,which had no effect on D but caused the increase in C₀ as well as hydrogen-induced ductility loss.The test results of hydrogen permeation and SSRT for bare X80 steel and X80 steel with corrosion product film under different hydrogen pressures shown that corrosion product film could significantly increase hydrogen penetration parameters,such as I_?,D,C₀,which indicated that corrosion product film promoted the hydrogen permeation process.The data obtained from SSRT,such as deformation amount,the percentage elongation after fracture,the shrinkage on cross section and fracture energy of sample,shown that corrosion product film could decrease plastic properties of X80 steel and increase HE index,leading to the conclusion that the corrosion product film increased the HE sensitivity of X80 steel.Microstructure analysis of corrosion product film analysis demonstrated that although this film was uniform and dense in general,the surface showed a bit of roughness,some defects such as cracks and holes were distributed in local sites.These defects increased the paths for hydrogen adsorption and diffusion on the surface area,and facilitated the process of adsorption,entry and penetration of hydrogen.The increase of adsorbed hydrogen concentration contributed to the increase of hydrogen embrittlement sensitivity.