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The Influence of Al Alloy Microstructure on Conversion Coating Formation

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The Influence of Al Alloy Microstructure on Conversion Coating Formation Licentiate Thesis The Influence of Al Alloy Microstructure on Conversion Coating Formation Salil Sainis Jönköping University School of Engineering Dissertation Series No. 059 • 2021 Licentiate Thesis The Influence of Al Alloy Microstructure on Conversion Coating Formation Salil Sainis Jönköping University School of Engineering Dissertation Series No. 059 • 2021 Licentiate Thesis in Materials and Manufacturing The Influence of Al Alloy Microstructure on Conversion Coating Formation Dissertation Series No. 059 © 2021 Salil Sainis Published by School of Engineering, Jönköping University P.O. Box 1026 SE-551 11 Jönköping NMÄ Tel. +46 36 10 10 00 NE RK A E V T S www.ju.se Trycksak Printed by Stema Specialtryck AB 2021 3041 0234 ISBN 978-91-87289-63-7 ABSTRACT Keywords: aluminium alloys, cast Al-Si alloys, conversion coating treatment, cerium, localized deposition, surface pre-treatments, microstructure, SEM, FIB-SEM, AFM- SKPFM The formation of conversion coatings based on Ce, trivalent Cr and Ti/Zr is triggered by the local pH increase at cathodic IM sites of the aluminium alloy microstructure. The pH gradient is created over the cathodic intermetallic (IM) sites of the microstructure and its intensity is influenced by their activity which depends on their chemical composition, their size, and spatial distribution. Furthermore, the pre-treatment applied also affects the surface reactivity. The role of each of the microstructural features on the increased pH gradient and the subsequent triggering of local conversion coating deposition remains to be understood. To address the knowledge gap, model cast Al-Si-Fe and Al-Si-Cu-Fe alloys have been designed. Cerium based conversion coating treatment with standard parameters is applied to investigate the microstructure’s influence. Furthermore, four different surface pre-treatments’ effect on the topographical and electrochemical properties have been investigated by localized techniques and have been correlated with deposition experiment observations to prove surface reactivity. In this study, it was found that the four surface pre-treatments – polishing, NaOH, NaOH- HNO3, NaOH-H2SO4 activate the surface of alloys containing Fe-rich IM and Cu-rich IM differently. The surface pre-treatment NaOH-HNO3 was found most detrimental to the surface reactivity as the pre-treatment resulted in passivation of the IM and a drastic reduction in its volta potential. The best pre-treatment for the alloy Al-Si-Fe was found to be one with NaOH etching. In the case of Al-Si-Cu-Fe alloy, pre-treatments where a pickling step (with either H2SO4 or HNO3) was applied followed a NaOH etching step, the surface of the IM was activated more than other pre-treatments due to selective Al dealloying and Cu-redeposition. The extent of Cu-redeposition was observed to be the most when surfaces were pickled with HNO3 solution and with the NaOH- HNO3 pre- treatment, fastest deposition kinetics were observed. In the cast Al-Si-Cu-Fe alloy, the localized deposits were preferentially observed to form on only strong cathodic Cu-rich IM. The size (surface area) of the Cu-rich IM correlated linearly with the lateral deposition area as well as z-direction spread. It was found that the pH gradient resulting from the oxygen reduction reaction near an IM is very local and does not affect pH gradients of a neighbouring Cu-rich IM. The size did not have a profound impact on the extent of deposition occurring on a Cu-rich IM, but it was found that big Cu-rich IM activated faster for deposition reaction than small Cu-rich IM. When the progression of deposition on both coarse and fine microstructure cast Al-Si-Cu-Fe was quantitatively monitored at increasing conversion coating times 0.5h, 1h and 2h, it was observed that big Cu-rich IM in the coarse alloy triggered deposition faster than small Cu- rich IM. Deposition mechanism on Fe-rich IM was found to be composition specific. In the cast of big Fe- -Al5FeSi IM, localized deposition initiated at the border on the IM and is explained based on Si content in the composition of the IM, which has very high resistivity. In anotherrich Feβ -rich IM, although of a much smaller size, which had lower Si content and was richer in Fe, a localized deposition was observed on the entire IM. SAMMANFATTNING Nyckelord: aluminiumlegeringar gjutna l-Si legeringar, omvandlingselggningar, cerium loaliserad deposition tfrbehandlingar microstruktur EM, IB-S FM- SKPFM ildning av omvandlingselggningar aserade p Ce trivalent r och ir initieras av loalt frhd pH vid atodisa intermetallisa platser I aluminiumlegeringens mirostruktur. tbredningen av den starka pH-gradientens uppkomst ver katodisa intermetallisa (IM platser av mirostrukturen pverkas av dess ativitet som i sin tur beror p dess kemisa sammansttning storle, och rumslig frdelning. tans reaktivitet pverkas dessutom av dess frbehandling. ollen av mirostruturens egensaper p den frhjda pH-gradienten och efterflande initiering av loal omvandlingselggning terstr att frst. r att komplettera detta unsapsgap har l-i-Fe och l-i-Cu-Fe modellgjutlegeringar utformats. erium-baserade behandlingar fr omvandlingselggning med standardparametrar tillmpas fr underskning av mirostrukturens pverkan. ffeten av fra olia tfrbehandlingar p topografisa och elektrokemisa egensaper har dessutom understs med loaliserade metoder och har korrelerats med oservationer vid depositionsexperient fr att bevisa ytreativitet. denna studie har det visats att de fra ytfrbehandlingarna – polering, a a-HNO3 a-H2SO4 – ativerar tan hos legeringar innehllande Fe-ria intermetaller och u- ria intermetaller p olia vis. tfrbehandlingen med a-HNO3 visades vara sadligast fr tans reaktivitet eftersom frbehandlingen resulterade i passivering av IM och en drastis snning av dess volta-potential. en sta frbehandlingen fr l-i-e- legeringen visades vara med a-etsning. I l-i-Cu-e-legeringens fall, var frbehandling dr ett etningssteg antingen med 2SO4 eller NO3 tillmpades flt av etsning med a ativerades ytan av IM mer n med andra frbehandlingar på grund av seletiv korrosion av l och Cu-terdeposition. tredningen av Cu-terdeposition oserverades vara strst nr tor var betade med NO3-lsning och depositionens ineti var som hastigast vid frbehandling med a-HNO3. den gjutna l-i-Cu-e-legeringen oserverades loaliserade depositionerna att ildas endast p starkt atodisa Cu-ria IM. torleen tarea p de Cu-ria IM orrelerade linrt med den laterala depositionstan samt med spridning i z-ritning. et visade sig att pH-gradienten som uppkom via sreredutionsreaktionen nra IM r vldigt loal och pverkar e pH- gradienter vid nrliggande Cu-ri IM. torleen hade ingen anmrkningsvrd veran p utredningen av deposition p Cu-rik , men stora u-ria M visade sig ativeras snabbare fr depositionsreation n sm u-ria M. r depositionens framfart ver grov och fin mirostrutur i gjuten l-i-Cu-Fe fldes vantitativt vid ökande omvandlingselggningstid ,5h h och h, s oserverades att stora Cu-ria IM i den grova legeringen påbrjade depositionen snabbare n sm u-ria M. epositionsmeanismen p e-ria M visades vara specifi fr sammansttning. gjutgodset med stora Fe-ria -l5Fei IM visades loaliserad deposition initieras vid grnsen av IM vilet frklaras baserat p Si-halt i IM sammansttning, som har vldigt hg resistivitet. en annan Fe-ri IM,β doc av mcet mindre storle, som hade lgre Si-halt och riare med e en lokaliserad deposition var oserverad ver hela M. ACKNOWLEDGEMENTS xpress y ncere titude to h following: European Union’s Horizon 2020 research and innovation programme under the -Curie Actions ra agreement o. 764977) funding his research Marie Skłodowska mCBEEs ovativ Training etwork o invaluab ofessiona riences P ter her upport, guidance, ienc throughout thi research journey know h elped me immensely, constant motivatio o goo science has a driving my mproveme researcher. Asst Ehsa assemali is competenc y developme and his extensiv insight working SEM. mCBEEs mbers goo trav companionsh cheerfu times ring training ools. Colleagues iends at erials Manufacturing th great ompany constant otivation. Patrick ant Jorg iv Juan and exandra great iendship morable moments. My ents brother heir lov constan support, ienc throughout this ourney. SUPPLEMENTS he oowing suppements constitute the asis o this thesis: Supplement I S ainis E hassemai and anea The role of microstructure and cathodic intermetallics in localized, S. depositionRoșoiu, mechanism of conversion compounds on Al (Si, Fe, Cu) alloy Surace and oating echnology ol. 02 pp 26502 2020) https:doiorg0016/jsurcoat202026502 S. Sainis is the main author and perormed maority o eperiments and invoved in the conception design and anasis o the stud S. Roșoiu assisted ith RD anasis E. Ghassemali contriuted ith conception and design o aoy used in the stud and assisted ith SE analysis C. Zanella contriuted ith conception, design and anasis o the stud assisted with -S characterization and invoved in manuscript preparation and revision Part o this or has een presented Sainis at EUROCORR 2020, Virtual Congress on Septemer 2020 and then ater on upgraded to a ull ourna artice Supplement II S ainis and anella A study on the localized cerium coating deposition on Fe-rich intermetallics in an Al- Si-Fe cast alloy Sumitted to aterias ourna S. Sainis is the main author, perormed maority o eperiments and invoved in the conception design and anasis o the stud C. Zanella contriuted ith conception design and anasis o the stud assisted with -S characterization and invoved in manuscript preparation and revision Part o this or has een presented at Electrochem2019, Glasgow on 2 ugust 209 and later upgraded to a ul ourna artice Supplement III S Sainis and anea A localized study on the influence of pre-treatments on the surface condition of cast Al-Si-Fe and Al-Si-Cu-Fe alloys and their effect on cerium conversion coating deposition anuscript under preparation S. Sainis is the main author perormed maority o eperiments and invoved in the conception design and anasis o the stud C.
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