First-principles Study On The Initial Processes Of Oxide Film Formation And Degradation On Iron-based Alloys

Iron-based alloys are the most widely used structural material for their low cost,excellent mechanical and processing properties,along with good oxidation and corrosion resistance.However,when they are exposed to harsh environments with high chloride concentration,serious corrosion phenomenon will occur,limiting their application range.The corrosion resistance of a material is highly dependent on the physical and chemical properties of the oxide film formed on the surface that acts as a protective barrier to ion transport,metal dissolution,and degradation.Study on the oxide film formation and degradation mechanism on the atomic and electronic scale as well as the alloying effects takes an important role on in-depth understanding of the corrosion mechanism of iron-based alloys.Moreover,it has an important guiding role for the design and improvement of corrosion-resistant alloys.In this work,the following researches are carried out within the framework of first-principles calculations with the aims of understanding the initial stage of the oxide film formation and degradation on iron-based alloys:(1)The adsorption characteristics of H,O,OH and H2O on Fe(110)surface,along with the effects of solvation and applied potential were systematically studied.The study found the order of adsorption strength and the most stable adsorption site of four adsorbates are OH(LB site)>O(3FH site)>H(3FH site)>H2O(T site).The implicit solvation model does not change the optimal adsorption site,and has little effect on the value of the adsorption energy.The applied electrode potential has a great influence on the adsorption energy,charge transfer and geometry structure.(2)Alloying effects on the adsorption and co-adsorption characteristics of O and Cl on Fe(110)surface was studied.The role of alloying elements in the initial stage of oxide film formation was revealed.The study found that Mn,Ti,Cr,Mo,W,and Nb are beneficial elements for passivation film formation,while Co,Cu,and Zn doping have negative effects.Most alloying elements(except Ti)hinder Cl adsorption.Competitive adsorption between O and Cl was found.which reflects the hindering effect of Cl on the initial stage of oxide film formation.Cr,Mn,Mo,W,and Nb can significantly alleviate the negative effects of Cl on the oxide film formation,improving the service performance of iron-based alloys in harsh environments.(3)Alloying effects on the adhesion strength and stability of Fe(110)/Cr2O3(0001)interface was studied.The role of alloying elements in the initial stage of oxide film degradation was revealed.By calculating the adhesion work,the study found that Cr and W significantly improve the bonding strength,while doping Nb,Ti,Zn,Cu,Al,and Co have negative effects.The vacancy formation energy at the interface region,the reaction energy and energy barrier for vacancy diffusion from interior to the interface were compared.It was found that Ti,Mn,and Al can effectively hinder the formation and aggregation of vacancies at the interface and further improve the interface stability;on the contrary,Cu,Co,and Zn reduce the interface stability.(4)The interaction between Cl and the double-layer oxide film of iron-based alloys were studied to explain the theoretical mechanism of Cl-induced corrosion.The underlying reason why the inner oxide film Cr2O3 is more protective was revealed.It was found that oxygen vacancy are easier to form than cation vacancy;the vacancy formation energy in α-Cr2O3 is much greater than that in α-Fe2O3.The interface region has high reactivity and the lowest vacancy formation energy.The penetration of Cl into the perfect oxide film through octahedral interstitial site is an endothermic process,while vacancy can provide favorable channels for Cl penetration into the interior of the oxide film.Among them,Cl insertion into O vacancy is an exothermic process in thermodynamics,which confirms that the depassivation mechanism of iron-based alloys conforms to the ion exchange model.By comparing the in-plane and inter-plane diffusion of O and Cl,it was found that the order of the diffusion barriers is α-Cr2O3>α-Fe2O3>interface.The interface provides favorable channels for the diffusion of Cl ions and O vacancies,which may lead to the accumulation of Cl in the interface region and eventually local breakdown.The result shows that the interface region is the weak point of corrosion.