First-principles Study Of Surface Characteristics Of Ni And Ni-Cr Alloy

Because of excellent mechanical properties and corrosion resistance,nickel-based alloys are widely used in PWR and BWR nuclear power plant as important structural components of primary loop.In the aggressive service environment,severe corrosion can attack these alloys,which is an electrochemical reaction process in nature.Because the surface of nickel-based alloys directly contact with electrolyte solution,the surface characteristics play a decisive role in the study of the whole system and the interaction mechanism with the surrounding environment.In this paper,the surface characteristics of Ni and Ni-Cr alloys,including surface energy,work function,adsorption and dissociation,are studied by first principle calculations based on density functional theory.It can help us get a deeper understanding of nickel-based alloy surface corrosion mechanism.?1?The surface energies and work functions of three low Miller indices?111?,?110?and?100?surfaces of Ni are calculated.The results show that the surface energy of Ni?111?is the least which is the most stable surface,and that of Ni?110?is the largest which is the most unstable surface.However,the work function is the opposite,the work function of Ni?111?is the largest whose surface activity is the lowest,and the work function of Ni?110?is the smallest whose surface activity is the largest.With the increase of the doping amount of Cr atoms,the surface energies of the three surfaces have been increased,mainly because the surface energy of Cr is higher than Ni.For?111?and?100?surfaces,the work function of the system decreases first and then increases with the doped Cr atoms increasing.However,for?110?surface,the work function is decreasing gradually,which is caused by the change of the dipole moment resulting from charge transfer and polarization.?2?The most stable?111?surfaces of Ni and Ni-Cr alloys are selected to calculate the adsorption energies of four adsorbates,namely O atom,OH group,H atom and H₂O molecule,respectively.By comparing the adsorption energies of FCC,HCP,bridge and top sites,it is found that the most favorable adsorption site is top site for H₂O and FCC site for the other three kinds of particles.The order of adsorption energies of each adsorption site is O>OH>H>H₂O.Compared with the Ni?111?surface,the adsorption energies on Ni-Cr?111?surface increased greatly,indicating that the Cr atom have a stronger affinity with the adsorbates enhancing the chemical activity of the surface.Bader charge analysis clearly shows that the number of electrons transferred between adsorbates and substrate:the largest for O adsorption and the smallest for H₂O adsorption.It is consistent with the cases of adsorption energies mentioned above.Through PDOS analysis,we can see that the hybridization between Cr and Ni 3d orbitals and O 2p and 2s orbitals occurs,which helps the formation of the bond in system.?3?The dissociation of H₂O molecule on the Ni?111?and Ni-Cr?111?surfaces is calculated by NEB transition state search.It is found that the dissociation energy barrier of the second step of the H₂O dissociation?OH + H->O + 2H?is higher than that of first step?H₂O->OH + H?,indicating that the second step dissociation of H₂O is more difficult.The doping of Cr atoms increases the chemical reaction activity of the metal surface and reduces the energy barrier of H₂O dissociation,making the dissociation of the H₂O and the formation of oxide film much easier.By comparing the dissociation of H₂O on Cl-preadsorbed and O-preadsorbed surfaces,it is found that the presence of Cl atom has a certain inhibitory effect on the dissociation of H₂Omolecule on the Ni-Cr?111?surface,while the inhibitory effect is not obvious on the Ni?111?surface.This can be explained by the fact that the electronegativity difference between Cl and Cr atoms is greater making it easier to form ionic bonds,therefore the O-H bond is not easily broken in H₂O adsorbed on the top of Cr atom.However,the presence of O atom accelerates the dissociation of H₂O molecule.This is because the preadsorbed O atom is more likely to produce hydrogen bond and weakens the O-H bond.