First-principle Study On Diffusion And Aggregation Behavior Of Hydrogen Atoms In Austenite Steel

With the increasing service conditions of oil pipeline,the corrosion caused by high sulfur medium is the key factor affecting the reliability and service life of pipeline transportation.The main failure modes are hydrogen induced cracking and stress corrosion cracking,so the hydrogen embrittlement has always been an important issue of concern.So far,a variety of mechanisms were put forward to try to explain this hydrogen embrittlement phenomenon.However,from the fundamental poin of view,the hydrogen embrittlement phenomenon is due to the interaction between hydrogen and metal,which includes the adsorption and dissociation behavior of hydrogen source molecules in the interface,the diffusion behavior of hydrogen atoms in metal,and the effect of hydrogen on metal deformation.The first principle calculation method provides an effective and feasible method for studying the microscopic mechanism of the interaction between hydrogen and metal matrix.In this paper,water adsorption behavior on Pt?111?and Fe?111?surfaces were studied by using the first principle calculation method and the adsorption and dissociation mechanism of H₂S at water/Fe?111?interface was investigated;secondly,H migration paths in bcc,fcc and hcp crystallic lattiecs were investigated and the diffusion coefficient of H in these three lattices were calculated according to the Arrhenius equation;finally,the effect of H atom on stacking fault energy of fcc Fe was investigated.The main results are as follows.?1?For water/Pt?111?interface,at low temperatures?10 and 150 K?,the water layer retains an ice-like network with a 1:1 ratio of the H-up and H-down molecules.At 300 and450 K,the water molecules are still located on top of the Pt atoms but exhibit increased disorder in their heights.In addition,the OH bonds of more water molecules point towards the Pt surface with increase in temperature to form Pt–H covalent bonds,which induced work function increasing of interface.On the basis,we find that OOH species will dissociate to O_ad and OH_add from top-hcp-bridge site instead of top-bridge-top site in aqueous enviroment,which is significant for understanding oxygen reduction reaction mechanism.?2?Water/Fe?111?interface,the water molecules are more likely to adsorb on the Fe?111?surface in the form of water cluster,and the water molecules prefer to form six-ring network adsorbed on the Fe?111?surface when the coverage increases.With the increase of temperature,the movement ability of water molecules increased and the effective adsorption distance of water adsorbed layer decreased.The H₂S molecule is dissociated in two steps on the clean Fe?111?surface,where the first dissociation?ie,dissociation from H₂S to HS and H atoms?is the controlling step.The presence of water molecules can increases the equilibrium bond length of the H₂S molecule,weakening the H-S bond,wihch reduced the energy barrier to dissociation.Therefore,it is more favorable for the dissociation of the H₂S molecule in water/Fe?111?interface.?3?H migration path in bcc,fcc and hcp crystallic lattiecs and the diffusion coefficient of H in these three lattices were determined.The hydrogen atom diffused in bcc Fe crystal through migrating from tetrahedral interstice to its nearest tetrahedral site;hydrogen atom diffused in fcc Fe along octahedral site-tetrahedral site-octahedral site,alternately;the diffusion path of hydrogen atom in hcp Fe is from octahedral site to its nearest octahedral site.The order of the diffusion coefficient of hydrogen in the three crystal structures is D_bcc>D_fcc>D_hcp.In addition,the energy barrier difference between hydrogen atom in perfect lattice site and vacancy in bcc crystal is higher than that in fcc crystal,indicating that hydrogen in bcc Fe crystal can be more sensitive to vacancy than that in fcc Fe crystal.?4?The stacking fault energy can determin the deformation mechanism of fcc metals.In fcc Fe,H atom may increase the unstable stacking fault energy in perfect crystal,inhibiting the formation of stacking fault;The unstable stacking fault energy decreased11%due to the hydrogen atoms in the divacancy cluster,which is beneficial to the formation of?martensite,and then lead to embrittlement of marterials.