Theoretical Study On The Mechanism Of H₂O And Cl On The Stainless Steel Cr₂O₃ Surface Doped With Alloy Elements

The service performance and corrosion resistance of stainless steel are determined by its corrosion behavior in corrosive environment.The excellent corrosion resistance depends on the passivation film formed on the surface which only has a thickness of several nanometers and includes Cr₂O₃as the main composition.The passive film effectively protects the metal surface from the erosion of aggressive medium.Therefore it is very important to clarify its composition,structure and corrosion mechanism in the corrosive medium.In this paper,the density functional theory was used to construct the surface structure model of Cr₂O₃ passivation film of austenitic stainless steel from an atomic level.When corrosion media(H₂O and Cl)adsorbed on the passivation film,the surface stability of Cr₂O₃was analyzed and the corresponding corrosion mechanism were discussed.High-performance austenitic stainless steel contains a variety of alloy elements.For example,austenitic heat resistant steel and super austenitic stainless steel contain alloy elements of Fe,Ni,Mn,Co,Cu,Si,Nb,W,Mo,Ce and Y.Upon that,the influence of H₂O and Cl on composite structure of these alloying elements and Cr₂O₃ in order to guide for the design of austenitic stainless steel with high corrosion resistance.The main research contents and the corresponding results are as follows:(1)The micro-mechanisms of H₂O and Cl on the surface structure of Cr₂O₃(0001)were studied respectively by using of First-principles calculations.The results indicated that the surface structure of Cr-terminal was easier to be formed,and the surface structure of the formed oxide film was more stable.The stable adsorption positions of corrosive medium(H₂O and Cl)on Cr₂O₃(0001)surface were different.For H₂O,the most stable adsorption position was at fcc,while it is at top position for Cl.Moreover,the surface layer spacing of Cr₂O₃(0001)increased under the action of corrosive medium of H₂O and Cl.The bond length between Cr-O atoms in the surface layer was elongated and the bonding strength between the atoms decreased,accompanied with a decrease of the charge density of surface atoms,which caused an instability of the oxide film in the vicinity.So it can be concluded that corrosive medium destroyed the structural stability of the passive film and Cl was more aggressive.(2)The effect of H₂O on the surface structure stability of alloying elements doped Cr₂O₃was studied.When alloying elements Fe,Ni,Co and Cu were combined with the surface of Cr₂O₃(0001),they had no significant effect on the surface adsorption position of H₂O.However,doped surface could weaken the erosion of H₂O.The corrosion resistance of the oxide film was improved.The composite system involved rare earth elements of Ce and Y changed the stable adsorption position of H₂O,improved the surface electrochemical stability,and thus enhanced the corrosion resistance of the oxide film.In the interstitial element N doped model,N could weaken the destructive effect of H₂O on Cr-O and improve the electrochemical stability of the surface and increase the corrosion resistance of the oxide film.N-Y composite system showed smaller adsorption energy and lower surface electrochemical activity than that of single element doped system.Therefore,the effect of heat-resistant steel on H₂O can be further improved under the synergistic effects of N and Y.(3)The effect of Cl on the surface structure stability of alloying elements doped Cr₂O₃ was investigated.When single alloy element doped on the surface,Mo or Y formed composite system could reduce the aggressiveness of Cl on the surface,meanwhile,improve the stability of the surface structure,and thus improve the corrosion resistance of the passive film.For N doped system,N could change the adsorption position of Cl on the surface,purify the surrounding environment of the surface.As Mo coverage on the surface of Cr₂O₃(0001)increased,the interactions between composite system and Cl became weaker.The structural stability of the composite system was better,and the corrosion resistance of the passivation film was improved.For Cr-Mo-N and Cr-Y-N composite system,the stable adsorption site of Cl on the surface of the two composite systems changed.The surface electrochemical stability of Cr-Y-N composite system was better.Thereby,the effect of super austenitic stainless steel on Cl can be further improved under the synergistic effects of N and Y.