Hydrogen Permeation Behavior Of X70 Pipeline Steel Under Simulated Marine Environment And The Study Of Hydrogen Barrier Mechanism

Pipeline steel plays an important role in the transportation of oil and marine resources.However,hydrogen cracking is observed in high-strength steels during engineering practices in the environment containing acidic gases,such as CO2 and H2S,or under conventional cathodic protection.The corrosive marine environment and the bending,squeezing,torsion and other tensile loading during the service of pipeline steel provide the perfect conditions for the occurrence of hydrogen embrittlement.Therefore,the study of cathodic hydrogen evolution reaction and hydrogen permeation of X70 pipeline steel has important theoretical value and practical significance for the development of marine engineering.In this paper,the hydrogen permeation behavior,and the activity of hydrogen evolution reaction of X70 pipeline steel under tensile stress and hydrostatic pressure were characterized by electrochemical tests with self-made stress loading and deep-sea hydrogen permeation devices.The variation of hydrogen behavior during natural corrosion in acidic environment and cathodic protection in alkaline environment were analyzed systematically by IPZ and surface effect modelling,which combined microscopic surface morphology observation and fracture mechanics.Meanwhile,the phenomenon of rare earth salt(La3+)inhibiting the hydrogen penetration of the material and its mechanism were analyzed,and X70 pipeline steel with excellent hydrogen penetration resistance was developed.The results were obtained as follows:Tensile stress contributed significantly to the steady-state hydrogen permeation current of X70 pipeline steel in acidic environment at pH=1.88,the promotion of sub-surface hydrogen concentration was obtained by fitting the hydrogen permeation curves in 0.2 mol/L NaOH solution via cathodic polarization.EIS analysis showed that tensile stress enhanced the hydrogen evolution reaction activity.The calculated results of IPZ and surface effect model illustrated that the Volmer reaction and adsorption process of hydrogen atom were promoted,whereas the Tafel reaction and desorption process were inhibited under the application of tensile stress.The effect of hydrostatic pressure on the hydrogen behavior of X70 pipeline steel in acidic and alkaline environments differed significantly.Under acidic solutions,the hydrostatic pressure reduced the steady-state hydrogen permeation current by suppressing the corrosion rate.However,with further increase in hydrostatic pressure,the steady-state hydrogen permeation current increased paradoxically.Electrochemical tests in alkaline environment revealed that hydrostatic pressure increased the hydrogen evolution reaction activity and promoted hydrogen penetration when the number of hydrogen atoms produced was guaranteed to be consistent.The results of the surface effect model analysis showed that the hydrostatic pressure promoted the adsorption of hydrogen atoms and inhibited the movement of hydrogen atoms on the surface,which limited the complex desorption of hydrogen atoms and thus increased the tendency of hydrogen atoms to diffuse into the substrate.Compared with the conventional hydrogen barrier coating,the corrosion product film that contains La3+ inhibited the hydrogen seepage process of steel significantly regardless of the integrity of the corrosion film,where the optimal addition concentration is 1.5 g/L.Experimental results show that the La3+ in the solution suppress hydrogen permeation by participating in the film formation of the corrosion product.The deposition of La(OH)3 in the corrosion product at the cathodic active site not only reduces the rate of cathodic reaction but also greatly facilitates the desorption process of hydrogen atom and reduces the concentration of hydrogen atoms on the subsurface,thereby reducing the susceptibility of the material to hydrogen embrittlement.Although La3+ in solution has a remarkable effect on the hydrogen permeation inhibitory,the deposition of La-containing corrosion products is difficult to realize in service environments,such as marine or oil fields.Therefore,the La-microalloyed X70 pipeline steel was developed.The microscopic morphology shows that the La element is mainly solid dissolved in X70 pipeline steel,and La2O3 during melting process directs the nucleation of inclusions.Electrochemical tests on X70 pipeline steel with La microalloyed in acidic and alkaline environments show that La microalloying can reduce the corrosion rate efficiently and has a significant facilitating effect on the process of hydrogen atom desorption.Moreover,the increase in the number of irreversible hydrogen traps inside the material after La microalloying is beneficial to the uniform distribution of hydrogen atoms inside the material,which is conducive to the mitigation of hydrogen embrittlement.