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A DFT-Study of Cl Ingress into α-Al2O3(0001) and Al(111) and Its Possible Influence on Localized Corrosion of Al
KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry, Surface and Corrosion Science. University of Science and Technology Beijing, National Center for Materials Service Safety.
University of Science and Technology Beijing, National Center for Materials Service Safety.
KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry, Surface and Corrosion Science.ORCID iD: 0000-0002-9453-1333
KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry, Surface and Corrosion Science.ORCID iD: 0000-0002-4431-0671
2019 (English)In: Journal of the Electrochemical Society, ISSN 0013-4651, E-ISSN 1945-7111, Vol. 166, no 11, p. C3124-C3130Article in journal (Refereed) Published
Abstract [en]

Ingress of chloride into alpha-Al2O3(0001) and Al(111) was calculated by first-principles theory calculations by considering Cl insertion into Al or O vacancies within alpha-Al2O3(0001) or into vacancy or interstitial sites within Al(111). For alpha-Al2O3, the formation of an O vacancy is energetically more favorable than of an Al vacancy. The insertion of Cl into an O vacancy is also more favorable than into an Al vacancy. A high energy-barrier has been derived for Cl transport within the neighboring O vacancies. In addition, the work function decreases with Cl ingress into the interior of the oxide. For Al(111), Cl insertion into either an Al vacancy or an interstitial site is less favorable compared to the insertion into alpha-Al2O3. The work function only changes slightly with Cl insertion into an Al vacancy. Moreover, the pre-inserted Cl reduces the energy-barrier for further Cl ingress into alpha-Al2O3, whereas Cl tends to stay in the sub-surface layer of Al(111), suggesting Cl accumulation at the Al2O3/Al.

Place, publisher, year, edition, pages
Electrochemical Society, 2019. Vol. 166, no 11, p. C3124-C3130
National Category
Physical Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-251275DOI: 10.1149/2.0161911jesISI: 000465526200002Scopus ID: 2-s2.0-85073451731OAI: oai:DiVA.org:kth-251275DiVA, id: diva2:1315860
Note

QC 20191121

Available from: 2019-05-15 Created: 2019-05-15 Last updated: 2019-11-21Bibliographically approved
In thesis
1. DFT calculations of initial localized corrosion of aluminum: Influence of aqueous ad-layer, chloride ions, and intermetallic particles
Open this publication in new window or tab >>DFT calculations of initial localized corrosion of aluminum: Influence of aqueous ad-layer, chloride ions, and intermetallic particles
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Localized corrosion of aluminum (Al, here including Al alloys) involves a series of physico-chemical processes at the interface between the metal and the aqueous ad-layer or the aqueous solution. The mechanisms that govern localized corrosion are quite complex and have been the subject of many experimental studies. Efforts to improve our understanding through computational studies have so far been much more limited. The primary aim of this Doctoral Thesis was to apply Density Functional Theory (DFT), together with some Molecular Dynamics calculations (limited effort), to gain a deeper mechanistic understanding of some of the most influential factors for the initiation of localized corrosion of Al: chloride ions, intermetallic particles (IMPs) and the presence of an aqueous ad-layer on the solid phase.In the scientific literature three scenarios have been proposed for the interaction of chloride ions with an aluminum and/or passive aluminum surface: through adsorption onto the passive layer, through breakdown of the same layer or through migration of chloride ions into the layer. DFT-calculations have been able to explore these scenarios in more detail, and provide evidence that chloride ions induce partial de-passivation in several ways. On the bare Al surface, chloride ions may inhibit the re-passivation through competitive adsorption with oxygen molecules, as suggested by density of state calculations. Chloride ions are also found to migrate via oxygen vacancies into the inner part of the investigated aluminum oxide films (α- and γ-Al2O3), where a critical amount of accumulated chloride can promote meta-stable pitting propagation. γ-Al2O3 exhibits a more open structure than α-Al2O3, resulting in a lower energy barrier for chloride migration.Micro-galvanic effects induced by Volta potential differences between representative intermetallic particles (Mg2Si and Al2Cu) and the surrounding Al matrix were predicted by calculating the work function of the bare surfaces of these phases with DFT. These values vary with crystalline face orientation and with terminal atomic configuration in the outmost surface layer. Calculated Volta potential differences between IMPs and Al show a reasonable agreement with reported experimental data, and suggest the possibility of predicting the nobility of specific IMPs relative to Al. Moreover, both DFT and scanning Kelvin probe force microscopy show evidence of electrochemical nobility inversion of Mg2Si versus Al upon adsorption of pure water ad-layers. This implies that an originally- 2 -cathodic Mg2Si phase becomes anodic compared to Al upon water adsorption, and is attributed to surface relaxation according to DFT calculations. With subsequent introduction of chloride ions into the water ad-layer, the nobility inversion of both Mg2Si and Al2Cu retains. This is due to a strong oxidizing effect of water on Al, while the effect of chloride seems less pronounced.In all, these and other examples presented show that DFT-calculations can provide more detailed atomistic and molecular information on physico-chemical processes governing localized corrosion of Al than experiments alone can do.

Abstract [sv]

Kvantmekaniska metoder såsom täthetsfunktional-teori (Eng. Density functional theory, DFT) har under de senaste årtiondena börjat tillämpas på allt mer komplicerade modellsystem. I denna doktorsavhandling har det primära syftet varit att tillämpa DFT för att kunna uppnå en djupare förståelse för mekanismerna bakom initiering av lokala korrosionsangrepp på aluminium. I möjligaste mån har de teoretiskt framtagna resultaten även jämförts med experimentella data. Lokalkorrosion är en komplicerad fysikalisk-kemisk process och en utmaning har varit att definiera frågeställningen så att förenklade men ändå relevanta modellsystem går att beräkna med DFT. Tre faktorer, som alla har stor betydelse för den lokala korrosionsinitieringen, har studerats mer ingående: kloridjoners inverkan, mikrogalvaniska effekter orsakade av intermetalliska sekundärfaser i aluminium-matrisen, och närvaron av en tunn adsorberad vattenfilm. 

Baserat på tidigare experimentella studier finns tre möjliga sätt på vilka kloridjoner kan påverka initieringen av lokalkorrosion: genom adsorption på aluminiumets passivfilm, genom strukturell nedbrytning av passivfilmen eller genom migration av kloridjoner genom passivfilmen. DFT-beräkningarna har kunnat belysa dessa processer mer ingående. Adsorberat klorid kan deformera ett monolager adsorberat syre, som täcker den rena aluminiumytan, och därvid åstadkomma en minskad strukturell stabilitet och passiverande förmåga hos monolagret syre. Bindningen mellan aluminium- och syreatomer är mycket stark och klorid kan inte bryta upp den bindningen. Däremot kan adsorberat klorid hämma repassiveringsförmågan hos monolagret syre genom att konkurrera med syre om möjliga adsorptionsplatser på aluminiumytan. DFT-beräkningarna visar även att kloridtransport sker via syrevakanser i den undersöka aluminiumoxidfilmen (α-Al2O3), som uppvisar en relativt liten energibarriär för klorid-migration. Energibarriären minskar ytterligare genom dopning med andra legeringselement eller genom klorid som redan placerats i oxidgittret. Transport av klorid genom aluminiumoxiden försämrar dess skyddande egenskaper. Det sker genom att oxidstrukturen relaxerar men också genom en kraftig sänkning av oxidens utträdesarbete (eng. Work function). När tillräckligt med klorid, i form av reaktionsintermediärer, samlats i fasgränsen mellan oxid och aluminium uppstår förutsättningar för en stabil tillväxt av det lokala korrosionsangreppet. Jämfört med α-Al2O3 uppvisar en annan tänkbar oxidfas, γ-Al2O3, en mer öppen struktur och lägre energibarriär, vilket underlättar kloridtransporten genom γ-Al2O3.

Mikrogalvaniska effekter mellan två representativa intermetalliska sekundärfaser (Mg2Si respektive Al2Cu) och den omkringliggande aluminium-matrisen har kunnat uppskattas genom uträkningar av skillnaden i Volta-potential mellan de intilliggande faserna. De beräknade värdena på Volta-potential korrelerar utmärkt med motsvarande uppmätta värden för såväl rena som vattenadsorberade ytor. Potentialskillnaderna visar sig vara beroende av de ingående fasernas orientering, av strukturen hos den allra yttersta ytans atomära konfiguration, samt av adsorption av olika species såsom klorid eller vatten. På en ren aluminiumyta kommer adsorberade species med högt utträdesarbete att resultera i en höjning av systemets totala utträdesarbete, och tvärt om. Beräkningarna visar att adsorberat vatten har en starkt oxiderande effekt på aluminium, oavsett närvaron av klorid. I överensstämmelse med experiment kan de undersökta systemen genomgå så kallad elektrokemisk inversion. Det innebär att en intermetallisk sekundärfas, som från början var katodisk relativt aluminiummatrisen, blir anodisk när mängden adsorberat vatten på ytan ökar. Fenomenet tillskrivs förändringar i atomär ytkonfiguration hos de ingående faserna i samband med adsorption. 

Sammantaget visar dessa och andra exempel i avhandlingen hur DFT-beräkningar av fysikaliskt-kemiska förutsättningar under initiering av lokalkorrosion på aluminium kan leda till mer detaljerad molekylär information än vad som vore möjligt med enbart experimentella studier.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2019. p. 59
Series
TRITA-CBH-FOU ; 2019:66
Keywords
aluminum alloys, alumina, chloride, surface adsorption, aqueous ad-layer, localized corrosion, corrosion initiation, work function, micro-galvanic effect, density-functional theory, Volta potential, scanning kelvin probe force microscopy
National Category
Physical Chemistry
Research subject
Chemistry
Identifiers
urn:nbn:se:kth:diva-264028 (URN)978-91-7873-372-9 (ISBN)
Public defence
2019-12-13, Lecture Hall K1, Teknikringen 56, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 2019-11-21

Available from: 2019-11-21 Created: 2019-11-21 Last updated: 2019-11-21Bibliographically approved

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