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A localized study on the influence of surface preparation on the reactivity of cast Al-7Si-1Fe and Al-7Si-2Cu-1Fe alloys and their effect on cerium conversion coating deposition
Jönköping University, School of Engineering, JTH, Materials and Manufacturing.ORCID iD: 0000-0002-6245-7693
Jönköping University, School of Engineering, JTH, Materials and Manufacturing.ORCID iD: 0000-0003-2924-137X
2022 (English)In: Applied Surface Science, ISSN 0169-4332, E-ISSN 1873-5584, Vol. 585, article id 152730Article in journal (Refereed) Published
Abstract [en]

The spontaneous triggering of cerium-based conversion coating deposition occurs due to active micro-galvanic couple induced local rise in pH at cathodic sites. Since surface preparations prior to conversion coating treatment modify the surface reactivity or the extent of micro-galvanic coupling between the anodic and cathodic phases, they are of crucial importance. While many past works have studied the effect of various preparations including alkaline etching and/or acid pickling and their parameters on the resulting conversion coating, very few studies focus on the local surface reactivity changes from the surface preparation. Moreover, most of the studies use high Cu containing AA2024, and related work on other alloys are scarce. In this study, two model cast alloys Al-7Si-1Fe and Al-7Si-2Cu-1Fe have been created to obtain a relatively homogeneous microstructure containing two different cathodic activity intermetallics Fe-rich IM β-Al5FeSi and Cu-rich IM θ-Al2Cu. Changes in the surface state of each alloy are monitored with AFM-SKPFM after subjecting them to four specific surface preparations: (1) Mechanical polishing (2) NaOH etching (3) NaOH etching + HNO3 Pickling (4) NaOH etching + H2SO4 pickling and correlations have been made with localized conversion coating deposit observations.

Place, publisher, year, edition, pages
Elsevier, 2022. Vol. 585, article id 152730
Keywords [en]
AFM-SKPFM, Cerium, Conversion coating, Localized deposition, SEM, Volta potential, Alkalinity, Aluminum coatings, Copper alloys, Etching, Iron alloys, Silicon alloys, Sodium hydroxide, AFM, Cerium conversion coatings, Coatings deposition, Conversion coatings, Localised, Surface preparation, Surface reactivity, Deposition
National Category
Manufacturing, Surface and Joining Technology
Identifiers
URN: urn:nbn:se:hj:diva-55961DOI: 10.1016/j.apsusc.2022.152730ISI: 000776625500004Scopus ID: 2-s2.0-85124178334Local ID: HOA;;798115OAI: oai:DiVA.org:hj-55961DiVA, id: diva2:1641519
Funder
EU, Horizon 2020, 764977Available from: 2022-03-02 Created: 2022-03-02 Last updated: 2022-10-07Bibliographically approved
In thesis
1. An Insight into the Critical Role of Microstructure and Surface Preparation on Localized Conversion Coating Deposition on Cast Al Alloys
Open this publication in new window or tab >>An Insight into the Critical Role of Microstructure and Surface Preparation on Localized Conversion Coating Deposition on Cast Al Alloys
2022 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The spontaneous cerium conversion coating formation over aluminium alloys is affected by the chemical process conditions, the surface preparation prior to conversion treatment and the microstructure of the underlying alloy. Most research performed until now focus only on the former whereas the influence of the latter two is poorly understood. The microstructure of aluminium alloys contains cathodic intermetallic particles that have a crucial role in conversion coating formation as they are responsible for the increase in pH to a critical level where chemical precipitation reaction of conversion coating compounds becomes possible. There are many different types of intermetallics in the aluminium alloys’ microstructure whose cathodic potential depends on their chemical composition, but no systematic study exists on the influence of their geometric properties on the reactivity of intermetallics. The surface preparation also critically affects reactivity of intermetallics, and a consensus exists regarding which preparation procedure makes an alloy surface most conducive for conversion coating deposition, but little is known about the topographical and volta potential changes occurring around the intermetallics from the surface preparations. An insight into both the role of microstructure and surface preparation is thus crucial as they can help design better treatments.

To address this knowledge gap, model microstructures of hypoeutectic cast Al-7Si alloys have been created in the study with the desired intermetallic composition and geometry. By addition of Cu and Fe to the hypoeutectic cast alloy, two types of intermetallics, namely θ-Al2Cu and β-Al5FeSi form and are the main objects of investigation in the study. The geometrical dimensions of the intermetallics were modified by directionally solidifying the cast alloys at different rates. Different surface preparation procedures, namely mechanical polishing, NaOH, NaOH-HNO3 and NaOH-H2SO4 have been tried in the study. Furthermore, parameters such as etching time have been varied to understand their influence. The microstructural features, particularly intermetallic geometry and cathodic potential have been systematically compared with localized deposition on them through conversion coating treatment. Experimentally derived data-based analyses have been conducted to come to conclusions in the study.

Firstly, the study found differences in the initiation of localized deposits on θ-Al2Cu and β-Al5FeSi attributed this to the conductivity variation due to compositional difference. Among the three differently sized θ-IM investigated in the study, namely fine, coarse and bulky θ, increasing the size from fine to coarse led to increase in reactivity of the IM for localized deposition. But increasing the size even further from coarse to bulky θ decreased the reactivity. Such a decrease in reactivity was found to be due to a combination of factors such as volta potential difference relative to the matrix and geometry.

Surface preparation affected localized deposition and the choice of procedure applied critically depends on the intermetallics present in the microstructure. The β-Al5FeSi is most reactive for conversion coating deposition when prepared with NaOH etching solution but becomes passive when further pickled with HNO3 solution. The θ-Al2Cu, on the other hand, becomes most active when subjected to multi-step NaOH- HNO3 procedure. Such observations make the choice of a surface preparation procedure difficult when the alloy microstructure contains both θ-Al2Cu and β-Al5FeSi intermetallics. The localized deposition of cerium conversion compounds after surface preparation are triggered due to a combination of surface factors like including volta potential and presence of Al(OH)3 smut, with varying degrees of dominance depending on the type of IM. Al(OH)3 smut content was found sensitive to the NaOH etching time. A further HNO3 pickling step cleans the surface. A “cleaner” surface was found to be associated with more consistent coverage numbers, while the presence of Al(OH)3 smut, on the one hand provides additional source of alkalinity, does not consistently result in good coverage.

Abstract [sv]

Den spontana formationen av Ce-omvandlingsbeläggningar på aluminiumlegeringar påverkar förhållandet i den kemiska processen, behandlingen av ytan före beläggning samt legeringens mikrostruktur. Den största delen av den forskning som bedrivits inom detta område har, fram tills nu, framför allt fokuserat på förståelsen kring den kemiska processen, medan inverkan av de två sistnämnda nästintill saknas. Mikrostrukturen i aluminiumlegeringar innehåller katodiska, intermetalliska partiklar vilka har en avgörande roll för omvandlingsbeläggningar, då de är orsaken till ökandet av pH till den kritiska nivå där den kemiska utfällningsreaktionen för omvandlingsbeläggningssammansättningar blir möjlig. Det finns ett flertal olika typer av intermetalliska föreningar i en aluminiumlegerings mikrostruktur, där dess katodiska potential beror på legeringens kemiska sammansättning, men inga tidigare studier av systematisk karaktär finns gällande hur reaktiviteten påverkas av intermetallernas geometriska egenskaper. Även förbehandlingen av ytan påverkar reaktiviteten hos intermetalliska föreningar, och det råder en allmän konsensus gällande vilken typ av behandling som gör en legeringsyta mest gynnsam för omvandlingsbeläggning, men kunskapen om de topografiska och voltapotentialförändringar som sker kring intermetallerna från ett ytbehanlingsperspektiv är inte allmänt känd. Insikt om hur mikrostruktur och ytbehandlingsmetod påverkar är därför avgörande då de kan hjälpa till att designa förbättrade behandlingar.

För att belysa denna kunskapslucka har modellmikrostrukturer av hypoeutektiska gjutna Al-7Si-legeringar, med den önskade intermetalliska sammansättningen och geometrin skapats till studien. Genom tillsats av Cu och Fe till den hypoeutektiska legeringen bildas två typer av intermetallföreningar, nämligen θ-Al2Cu och β-Al5FeSi, och är de huvudsakliga undersökningsobjekten i studien. De geometriska dimensionerna hos de intermetalliska materialen modifierades genom riktningsstelning, med olika hastigheter, av de gjutna legeringarna. Olika ytbehandlingsmetoder, nämligen mekanisk polering, NaOH, NaOH-HNO3 och NaOH-H2SO4 provades i studien. Dessutom har parametrar, så som etsningstid, varierats för att förstå dess inverkan. De mikrostrukturella egenskaperna, särskilt intermetallisk geometri och katodpotential, har systematiskt jämförts med lokal deposition på dem genom omvandlingsbeläggningsbehandling. Experimentellt härledda databaserade analyser har genomförts för slutsatserna i studien.

Primärt fann studien skillnader i initieringen av lokala depositioner på θ-Al2Cu och β-Al5FeSi och tillskrev detta till konduktivitetsvariationen orsakad av skillnad i sammansättningen. Bland de tre olika stora θ-IM som undersöktes i studien, nämligen fina, grova samt skrymmande θ, ledde ökning av storleken från fin till grov till ökad reaktivitet hos IM för lokal deposition. Men genom att öka storleken ytterligare från grov till skrymmande θ minskade reaktiviteten. En sådan minskning i reaktivitet visade sig bero på en kombination av faktorer så som voltapotentialskillnad i förhållande till matrisen och geometrin.

Förbehandling av ytan påverkade lokal deposition och valet av tillvägagångssätt som tillämpas beror på de intermetalliska föreningar i mikrostrukturen. Mest reaktiv för omvandlingsbeläggningsavsättning är β-Al5FeSi när den framställs med NaOH-etsningslösning men blir passiv när den betas ytterligare med HNO3-lösning. När det gäller θ-Al2Cu, blir ytan som mest aktiv när den utsätts för en flerstegs NaOH-HNO3-behandling. Dessa observationer gjorde valet av en ytbehandlingsprocedur svår när legeringens mikrostruktur innehåller både θ-Al2Cu och β-Al5FeSi intermetalliska föreningar. Den lokala depositionen av Ce-omvandlingssammansättningar efter ytbehandling utlöses på grund av en kombination av faktorer som, inklusive voltapotential och närvaro av Al(OH)3 ”smut”, med varierande grader av dominans beroende på typen av IM. Al(OH)3- ”smut”- innehåll befanns vara känsligt för etsningstiden med NaOH. Ytterligare ett HNO3-betningssteg rengör ytan. En "renare" yta visade sig vara associerad med mer konsekventa täckningstal, medan närvaron av Al(OH)3- smut ger ytterligare en källa till alkalinitet, inte konsekvent resulterar i god täckning.

Place, publisher, year, edition, pages
Jönköping: Jönköping University, School of Engineering, 2022. p. 58
Series
JTH Dissertation Series ; 073
Keywords
aluminium alloys, cast Al-Si alloys, conversion coating treatment, cerium, localized deposition, surface preparation, microstructure, SEM, FIB-SEM, AFM-SKPFM, Aluminiumlegeringar, gjutna Al-Si legeringar, Cerium, lokal deposition, omvandlingsbeläggningsbehandling, förbehandling av yta, mikrostruktur, SEM, FIB-SEM, AFM-SKPFM
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-58601 (URN)978-91-87289-79-8 (ISBN)
Public defence
2022-11-09, E3231, School of Engineering, Jönköping, 10:00 (English)
Opponent
Supervisors
Available from: 2022-10-07 Created: 2022-10-07 Last updated: 2022-10-07Bibliographically approved

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