First-principles Calculations Of Surface Design For Hydrogen Embrittlement Resistance

As hydrogen diffuses into the materials,it could lead to hydrogen embrittlement.By surface design for hydrogen embrittlement resistance,one can improve the performance of material surface or passivation film.Hydrogen barrier formed on material surface and subsurface can prevent hydrogen from diffusing into materials or reduce the amount of hydrogen inside materials.As a result,hydrogen embrittlement can be prevented or delayed.In this work,we studied effects of atom doping and surface coating on hydrogen adsorption and hydrogen permeation by first-principles theory.The following results are obtained:(1)We investigated the effect of Cr-doping on the properties of a-Fe2O3(001)thin films with Fe termination using the local-density approximation plus a Hubbard U correction.We found that both the doping site and concentration of Cr atoms dramatically affect the electronic structure and work function(WF)of a-Fe2O3 films.The results demonstrate that it is most energetically favorable for Cr atoms to substitute the Fe atoms in the subsurface of a-Fe2O3 thin films.The doping of Cr atoms in the subsurface not only lowers the band gap of the film but also greatly enhances the work function by 0.9 eV with respect to the pure a-Fe2O3 film.The increase of WF correlates with the reduction of occupied O px and py states below the Fermi level which leads to a decrease of the Fermi energy.As the Cr concentration changes from 4.2%to 16.7%,the WF firstly increases,and then drops down.The WF reaches a maximum of 6.61 eV for the Cr concentration of 8.3%.These results suggest that doping Cr atoms in α-Fe2O3(001)thin film can increase the corrosion potential and benefits to the protection of steel from corrosion.(2)Hydrogen embrittlement is a bottleneck problem limiting the application of high-strength steel,prevention of hydrogen penetration into steels plays a vital role in lowering hydrogen damage.We reported effects of atom(Al,Cr,or Ni)doping on hydrogen adsorption on the α-Fe2O3(001)thin films and permeation into the films based on density functional theory.We found that the H2 molecule prefers to dissociate on the surface of pure α-Fe203 thin film with adsorption energy of-1.18 eV.Doping Al or Cr atoms in the subsurface of α-Fe2O3(001)films can reduce the adsorption energy by 0.03 eV(Al)or 0.09 eV(Cr)for H surface adsorption.In contrast,Ni doping substantially enhances the H adsorption energy by 1.08 eV.As H permeates into the subsurface of the film,H occupies the octahedral interstitial site and forms chemical bond with an O atom.Comparing with H subsurface absorption in the pure film,the absorption energy decreases by 0.01-0.22 eV for the Al-and Cr-doped films,whereas increases by 0.82-0.96 eV for the Ni-doped film.These results suggest that doping A1 or Cr prevents H adsorption on the surface or permeation into the passive film,which effectively reduces the possibility of hydrogen embrittlement of the underlying steel.However,doping Ni has an opposite influence since it promotes H adsorption on the surface or permeation into the passive film.(3)Prevention of hydrogen penetration into steels can effectively protect steels from hydrogen damage.We investigated the effect of monolayer MoS2 coating on hydrogen prevention using first-principles calculations.We found that the monolayer MoS2 can effectively inhibit the dissociative adsorption of hydrogen molecules on Fe(111)surface by forming S-H bond.MoS2 coating acts an energy barrier interrupting the hydrogen penetration and hydrogen damage.Furthermore,comparing with the H-adsorbed Fe(111)film,the work function of MoS2-coated film significantly increases under both equilibrium and strained conditions,indicating that the strained Fe(111)film with MoS2 coating also becomes more corrosion resistant.Further studies have shown that other transition metal chalcogenides also can effectively prevent the dissociative adsorption of hydrogen on iron surface by coating a single layer of MoSe2,WS2 and WSe2.The results reveal that transition metal chalcogenide films are effective coatings to prevent hydrogen damage in steels.