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Effect of Si content and morphology on corrosion resistance of anodized cast Al-Si alloys
Jönköping University, School of Engineering, JTH, Materials and Manufacturing.ORCID iD: 0000-0002-8098-4788
Department of Industrial Engineering, University of Trento, Trento, Italy.
Jönköping University, School of Engineering, JTH, Materials and Manufacturing.ORCID iD: 0000-0001-6445-6005
Jönköping University, School of Engineering, JTH, Materials and Manufacturing. SP-Technical Research Institute of Sweden, Borås, Sweden.ORCID iD: 0000-0002-7095-1907
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2017 (English)In: Journal of the Electrochemical Society, ISSN 0013-4651, E-ISSN 1945-7111, Vol. 164, no 7, C435-C441 p.Article in journal (Refereed) Published
Abstract [en]

This paper investigates the influence of Si content and Si particle morphology on the corrosion protection of anodized oxide layers on Al-Si alloys. Two Al alloys with low Si concentrations (2.43 wt-% and 5.45 wt-%, respectively) were studied and compared with 6082-T6 via electrochemical impedance spectroscopy (EIS) in 3 wt-% NaCl solution prior to oxide layer sealing. Si particles were also modified by the addition of Sr to study the influence of Si particle morphology on the corrosion protection of the oxide layer. The EIS showed that the corrosion protection provided by the oxide layer on Al-Si alloys is significantly affected by the presence of Si particles. Si particles make the oxide layer locally thinner and more defective in the eutectic region, thereby increasing the ease of substrate corrosion attack. However, the addition of Sr can improve the corrosion protection of anodized Al-Si alloys significantly. Furthermore, it was proved that higher Si level influences negatively the anodized oxide corrosion protection due to the higher amount of cracks and defects, but Sr modification is efficient in preventing this negative effect.

Place, publisher, year, edition, pages
The Electrochemical Society , 2017. Vol. 164, no 7, C435-C441 p.
Keyword [en]
cast Al-Si alloys, anodising, EIS, 3D tomography
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:hj:diva-35095DOI: 10.1149/2.1631707jesISI: 000404397300077Scopus ID: 2-s2.0-85020855659Local ID: JTHMaterialISOAI: oai:DiVA.org:hj-35095DiVA: diva2:1075107
Note

Part of licentiate thesis in its submitted form.

Available from: 2017-02-17 Created: 2017-02-17 Last updated: 2017-09-13Bibliographically approved
In thesis
1. On the influence of Si on anodising and mechanical properties of cast aluminium alloys
Open this publication in new window or tab >>On the influence of Si on anodising and mechanical properties of cast aluminium alloys
2017 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The combination of two cost-effective processes, i.e. casting and anodising, would be an interest for the aluminium component applications. However, there are some obstacles in the application of anodising on cast Al alloys. The challenges mostly relate to the alloying elements especially Si and the surface quality. With the development of casting process, cast aluminium alloys with low Si content can be casted, and a complex geometry component with reasonably good surface finish can be achieved. This study aims to identify the influence of Si on anodising and mechanical properties of Al-Si alloys.

In this study, six Al-Si alloys with three different Si level and two different Sr level were investigated. Sr acts as a modifier to change the morphology of Si particles. The directional solidification technology was used to vary the microstructure coarseness by controlling the cooling rate to study the influence of Si level, Si particle morphology and cooling rate on mechanical properties, oxide layer formation and corrosion protection performance in cast Al-Si alloys.

This study has observed that Si has a significant influence on anodising. During anodising, Si particles are anodised at a lower rate than the Al phase. The presence of Si particles in eutectic phase make the oxide layer locally thinner and more defected due to the low oxide growth rate in eutectic phase. This study observed the presence of residual metallic Al phase beneath or between Si particles. Due to their presence and their geometry, Al can be shielded by Si particles and prevented from oxidation. Si particles also act as a key role in the corrosion protection of oxide layer in Al-Si alloys. The corrosion attack propagates along Si particles as well as oxide defects to the Al substrate.

It is found that the morphology of Si particles has a significant influence on the oxide layer formation and corrosion protection performance of the oxide layer on cast Al-Si alloys. A substantially improvement the corrosion resistance of anodised layer on Al-Si alloys is attributed to the morphology change from interconnected flakes to disconnected Si fibres when Sr is added, with less oxide defects and better oxide distribution.

The Si level governs the mechanical properties of Al-Si based alloys. An increase of Si content in Al alloys improves the mechanical properties such as ultimate tensile and yield strength as well as hardness of the materials, but decreases the ductility. However, an increase of Si level in Al alloys decreases the thickness of oxide layer, and thereby, the corrosion protection of the oxide layer is deteriorated.

Abstract [sv]

Kombinationen av två kostnadseffektiva processer, gjutning och anodisering, är av intresse för tillämpning på aluminiumkomponenter. Det finns dock hinder för tillämpning av anodisering på gjutna aluminiumlegeringar. Utmaningarna relaterar till effekten av legeringselement, i synnerhet Si, och komponentens ytkvalité. Med utvecklingen av gjutprocesser kan aluminiumlegeringar med lågt Si-innehåll gjutas, och komponenter med komplex geometri med förbättrad ytkvalité kan uppnås. Denna studie syftar till att identifiera påverkan av Si på anodisering och mekaniska egenskaper hos Al-Si-baserade legeringar.

I denna studie undersöktes sex Al-Si-legeringar med tre nivåer av Si och två nivåer av Sr. Tillsatser av Sr leder till modifikation av morfologin hos Si-partiklar. Med hjälp av tekniken ”riktad stelning” varierades stelningshastigheten för att studera sambanden mellan halten och morfologin av Si, mikrostrukturens grovlek och dess inverkan på mekaniska egenskaper, samt oxidskiktets bildning och korrosionsbeständighet hos gjutna Al-Si-legeringar.

Denna studie visar att Si har ett betydande inflytande på anodisering r. Under anodisering, anodiseras Si-partiklar i en lägre takt än Al-fasen. Närvaron av Si-partiklar i eutektisk fas bidrar till ett lokalt förtunnat oxidskikt med fler defekter på grund av den låga oxidtillväxthastigheten i eutektisk fas. Denna studie observerade förekomsten av kvarvarande metalliska Al fasen under eller mellan Si-partiklar. På grund av Si-partiklarnas närvaro och geometri, kan Al skärmas från oxidation. Si-partiklar har även en nyckelroll i korrosionsskydd av oxidskiktet i Al-Si-legeringar. Korrosionsangreppet propagerar längs Si-partiklar samt oxiddefekter till Al-substrat.

Morfologin av Si-partiklar har funnits ha en betydande inverkan på oxidskiktets bildning och korrosionsskydd hos gjutna Al-Si-legeringar. En väsentlig förbättring av korrosionsbeständigheten i anodiserat skikt hos Al-Si-legeringar tillskrivs förändring av kiselns morfologi från sammankopplade flingor till osammanhängande fibrer när Sr tillsätts, med mindre oxiddefekter och bättre fördelning av oxid.

Si halten påverkar de mekaniska egenskaperna hos Al-Si-baserade legeringar. En ökning av Si innehålleti Al-legeringar förbättrar de mekaniska egenskaperna såsom brott och sträckgräns samt hårdhet hos materialen, men minskar duktiliteten. En ökning med Si halt i Al-legeringar minskar dessvärre tjockleken hos oxidskiktet, och därigenom, försämrar oxidskiktets korrosionsskydd.

Place, publisher, year, edition, pages
Jönköping: Jönköping University, School of Engineering, 2017. 44 p.
Series
JTH Dissertation Series, 19
Keyword
Al-Si based casting alloys, anodising, corrosion protection, mechanical properties, Al-Si-baserade gjutningslegeringar, anodisering, korrosionsskydd, mekaniska egenskaper
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-35096 (URN)978-91-87289-20-0 (ISBN)
Supervisors
Available from: 2017-02-17 Created: 2017-02-17 Last updated: 2017-02-17Bibliographically approved

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Zhu, BaiweiAndersson, Nils-EricLeisner, PeterZanella, Caterina

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