First-principle Calculations Of The Influences Of Environmental Media And Micro-galvanic Effect On Aluminum Localized Corrosion

This work focuses on understanding several mechanisms for the pitting initiation of aluminum(Al):"chloride(Cl)adsorption","film breakdown","Cl"migration,as well as metastable pitting from an atomic view by first-principle calculations.Structural and electronic characteristics including the rupture of single-layer oxide film,inhibition of re-passivation,breakdown of Al2O3 film and pitting propagation caused by Cl are studied specifically.Additionally,the exact atomic configurations for four common intermetallic particles(IMP)within A1 are built based on literature.The work function(WF)is calculated for various crystalline faces as well as terminal atoms by first-principles theory.Relationship between WF,Volta potential difference and corrosion potential is derived directly and used for a better understanding of micro-galvanic effect.The effect of materials and aqueous environment are specially studied.Results show that:(1)One monolayer of oxygen atoms(O)adsorb as a "O-Al3"film covered on Al.Film stability is deteriorated by Cl introduction by a vertical relaxation,and thus a decreased binding energy from-1.22eV to-0.35eV between first and inner Al layers.(2)"O-Al-Cl2"structure forms on bare Al surface,as a result of O2-Cl competitive interaction with the increasing amount of both two species.Density of state shows that the hybridization peak of Al-O drops whereas Al-Cl rises,indicating the inhibition of Al re-passivation.(3)Energetics shows a favorable Cl ingress through O vacancy over Al vacancy within Al2O3,with a high energy barrier;whereas Cl tends to stay in vacancy other than interstitial sites of Al.Cl insertion leads to work function decrease in both Al2O3 and Al,explained by structural relaxations.Cl transport rate in Al2O3 calculated by Arrhenius equation indicates the thicker the oxide,the larger the band gap,the higher the energy barrier,the slower the Cl transport.Moreover,pre-inserted Cl promotes more Cl transport.(4)A weak hybridization peak is observed between Al and Cl when Cl coverage is relatively low.While active dissolution of the Al surface occurs when a critical Cl coverage(2/3ML)is reached.Products such as AlCl3 and Al2Cl5 form and readily leave the surface.It proves that high Cl concentration(the so called "salt-film")is a necessity for pitting propagation.(5)The calculated WF agrees well with measured Volta potential difference in literature.Moreover,different crystalline orientations and terminations correspond to distinct WF,which may explain the scattered data reported in literature.It also proves that"nobility inversion"observed in IMP is attributed to that atom terminations bearing low WF are prone to easier or faster dissolution,exposing terminations with higher WF.Volta potential difference is capable of predicting the corrosion sequence of two metallic phases(M1,M2)within an aqueous solution(aq.)only in case that the contact potential difference between M1/aq.and M2/aq.interfaces is negligible.(6)The influence of pure water ad-layer on the Volta potential difference is studied combining first-principles theory and scanning Kelvin probe force microscopy,showing that even water adsorption can cause nobility inversion within IMP.When Cl is introduced into the water ad-layer,Mg2Si-Si and Al2Cu show inversion from cathodic to anodic relative to Al as water coverage increases to one full monolayer.Mg2Si-Mg attains its anodic character as coverage increases.