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 Fe β -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 gutna li legeringar omvandlingselggningar cerium loaliserad deposition tfrehandlingar microstrutur

ildning av omvandlingselggningar aserade p e trivalent r och ir initieras av loalt frhd p vid atodisa intermetallisa platser aluminiumlegeringens mirostrutur. tredningen av den stara pgradientens uppomst ver atodisa intermetallisa platser av mirostruturen pveras av dess ativitet som i sin tur eror p dess emisa sammansttning storle och rumslig frdelning. tans reativitet pveras dessutom av dess frehandling. ollen av mirostruturens egensaper p den frhda pgradienten och efterflande initiering av loal omvandlingselggning terstr att frst. r att omplettera detta unsapsgap har lie och liue modellgutlegeringar utformats. eriumaserade ehandlingar fr omvandlingselggning med standardparametrar tillmpas fr undersning av mirostruturens pveran. ffeten av fra olia tfrehandlingar p topografisa och eletroemisa egensaper har dessutom understs med loaliserade metoder och har orrelerats med oservationer vid depositionseperient fr att evisa treativitet. denna studie har det visats att de fra tfrehandlingarna – polering a a a – ativerar tan hos legeringar innehllande eria intermetaller och u ria intermetaller p olia vis. tfrehandlingen med a visades vara sadligast fr tans reativitet eftersom frehandlingen resulterade i passivering av och en drastis snning av dess voltapotential. en sta frehandlingen fr lie legeringen visades vara med aetsning. liuelegeringens fall var frehandling dr ett etningssteg antingen med eller tillmpades flt av etsning med a ativerades tan av mer n med andra frehandlingar p grund av seletiv orrosion av l och uterdeposition. tredningen av uterdeposition oserverades vara strst nr tor var etade med lsning och depositionens ineti var som hastigast vid frehandling med a. den gutna liuelegeringen oserverades loaliserade depositionerna att ildas endast p start atodisa uria . torleen tarea p de uria orrelerade linrt med den laterala depositionstan samt med spridning i ritning. et visade sig att pgradienten som uppom via sreredutionsreationen nra r vldigt loal och pverar e p gradienter vid nrliggande uri . torleen hade ingen anmrningsvrd veran p utredningen av deposition p uri men stora uria visade sig ativeras snaare fr depositionsreation n sm uria . r depositionens framfart ver grov och fin mirostrutur i guten liue fldes vantitativt vid ande omvandlingselggningstid h h och h s oserverades att stora uria i den grova legeringen prade depositionen snaare n sm uria . epositionsmeanismen p eria visades vara specifi fr sammansttning. gutgodset med stora eria lei visades loaliserad deposition initieras vid grnsen av vilet frlaras aserat p ihalt i sammansttning som har vldigt hg resistivitet. en annan eri β doc av mcet mindre storle som hade lgre ihalt och riare med e en loaliserad deposition var oserverad ver hela .

ACKNOWLEDGEMENTS

European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska

SUPPLEMENTS

he ooing suppements constitute the asis o this thesis

Supplement I ainis E hassemai and anea The role of microstructure and cathodic intermetallics in localized, S. deposition Roșoiu, mechanism of conversion compounds on Al (Si, Fe, Cu) alloy

urace and oating echnoog o 02 pp 202 2020

httpsdoiorg00surcoat2020202 S. Sainis is the main author and perormed maorit o eperiments and invoved in the conception design and anasis o the stud S. Roșoiu assisted ith anasis E. Ghassemali contriuted ith conception and design o ao used in the stud and assisted ith E anasis C. Zanella contriuted ith conception design and anasis o the stud assisted ith characterization and invoved in manuscript preparation and revision

art o this or has een presented ainis at EUROCORR 2020, Virtual Congress on eptemer 2020 and then ater on upgraded to a u ourna artice

Supplement II ainis and anea A study on the localized cerium coating deposition on Fe-rich intermetallics in an Al- Si-Fe cast alloy umitted to aterias ourna

S. Sainis is the main author perormed maorit 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 ith characterization and invoved in manuscript preparation and revision

art o this or has een presented at Electrochem2019, Glasgow on 2 ugust 20 and ater upgraded to a u ourna artice

Supplement III ainis 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 maorit 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 ith characterization and invoved in manuscript preparation and revision

TABLE OF CONTENTS

CHAPTER 1. INTRODUCTION ...... 1 U 2 UUE 2 H EU E EHE EE 0 CHAPTER 2. RESEARCH METHODOLOGY...... 13 2 UE HE 22 EEH E esearch perspective esearch uestions esearch approach 2 EEH EH U 2 E EH CHAPTER 3 SUMMARY OF RESULTS AND DISCUSSION ...... 21 HE UUE E 2 ast iue and ie ao microstructure 22 2 EEHE EE HE 2 HE UUE E EE UE 2 onversion coated cast iue 2 onversion coated ie sustrates CHAPTER 4. CONCLUSIONS ...... 43 CHAPTER 5. FUTURE WORK ...... 45 REFERENCES………………………………………………………………………………………………………………………….47 APPENDED PAPERS ...... 57

CHAPTER 1

.

he impact o human activities on the ecoogica aance has een negative and there is no a signiicant shit toards adopting more ecoriend poicies societ governments and industr uminium and its aos are essentia engineering materias used or various appications and ui environmenta sustainaiit goas he auminium aos sho good properties such as high strengthtoeight ratio high eectrica and therma conductivit eading to its use in the transportation sector uiding and construction eectrica eements and pacaging ts ighteight strength and high reccaiit 2 can oer energ costs and 2 emissions

uraiit is another important eature o materias that can mae a materia sustainae o ensure a ongterm duraiit o auminium aos it must e protected rom corrosion so that its service ie is increased ure auminium shos eceent corrosion resistance in near to neutra environments as it orms a e nanometrethic aer o auminium oide that ocs the contact o aueous media ith the undering auminium sustrate Hoever or engineering appications pure auminium is rare used due to o mechanica strength and it is oten aoed ith a comination o eements he reuired properties are achieved the ormation o heterogeneous microstructures designed careu addition o aoing eements and heat treatments he second phases in the heterogeneous microstructure are rich in aoing eements and thus demonstrate dierent eectrochemica properties compared to the auminium matri oreover the continuit o the protective native auminium oide is roen due to the presence o second phases Eectrochemica heterogeneit and non continuous oide im mae the ao susceptie to corrosion

o protect auminium aos rom corrosion a mutiaered coating sstem is oten empoed he dierent aers o such a sstem are eucidated in igure the irst aer o hich is a pretreatment aer his aer is in direct contact ith the auminium matri and is artiicia ormed either anodic oidation o auminium anodizing 0 or conversion coating treatment he atter is ormed a chemica pretreatment here the native aer o oide is repaced another meta oide that is resistant to aggressive environments onversion coating are ased on ut not imited to heavaent chromium trivaent chromium vanadium titanium zirconium cerium anthanum modenum and phosphate 2

. chematic representation o the mutiaer coating sstem

Heavaent chromiumased conversion coatings r have een ide used or protecting auminium aos rom corrosion heir prevaence as the most successu conversion treatment is attriuted to the eceent corrosion resistant properties it imparts to the materia surace and compementari oers added eneits ie reative ease o appication promoting paint or primer aer adhesion and oer seheaing properties in the event o damage to coating Heavaent chromium hoever is deeterious to the environment and have a carcinogenic eect on humans rocesses ased on heavaent chromium are eing so phased out and there are various edera mandates and environmenta agenc recommendations in the U and EU to heavi imit its use he Environmenta rotection genc E is the main reguator imiting chromate use through acts ie the ean ater ct the omprehensive Environmenta esponse ompensation and iaiit ct and oic ustances ontro ct EU counterpart is the European hemica genc EH that paces heavaent chromium under restricted ist in the rameor o egistration Evauation uthorisation and estriction o hemicas EH

ith the need or producing more environmenta riend and compiant coatings severa aternatives to r or corrosion protection have een studied onversion coatings ased on trivaent chromium process coatings cerium zirconium andor titanium have shon promise

.

i cast aos are important casting aos ide used or severa appications 20 he addition o i increases uidit and decreases soidiication shrinage aoing the casting o compeshaped components and thus resuts in increased castaiit 2 he i addition can var eteen and 2 raction eight in commercia casting aos and depending on the proportion used a hpoeutectic eutectic and hpereutectic microstructure ma orm 22 he phase diagram o the i sstem is shon in igure 2 mong the cast i aos hpoeutectic compositions are most used ith tpica i concentration range eteen t and 0 t n the automotive industr the tpica appications o hpoeutectic i aos incude engine ocs cinder heads and hees 22

. i phase diagram 2

Hpoeutectic i aos soidi ith a dendritic structure orms the dendritic arms that aso deveop secondar arms igure a n the region eteen the arms and secondar arms orms the iwhere eutectic primary consisting α he i eutectic consists o sot and i partices ith a pateie morphoog he distance eteen the to secondar arms aso caed econdar endritic rm pacing igure has een shon to e sensitive to the soidiication rate 2 ith a higher soidiication rate a iner microstructure is ormed ith a o and vice versa or oer soidiication rate he intra eutectic i region is rich in aoing eements that have een reected soidiied epending on the concentration and cominations o aoing eements present in the ao dierent intermetaics ma orm α

a . a pica microstructure o a cast hpoeutectic i ao ater soidiication as oserved under optica microscop deinition o

he properties o auminium aos are usua designed seecting appropriate additions o aoing eements and heat treatments he intermetaic partices are cassiied into three road categories precipitates constituent partices and dispersoids recipitates genera sma ma orm rom supersaturated

soution during natura or otemperature artiicia ageing nuceation and groth oing eements additions ie u g i n i eads to the ormation o tpica precipitates such as gn2 g 2u he second tpe constituent partices are reative arger and are ormed during ao soidiication hese partices are rich in aoing eements evera dierent aoing eements are ound in constituent partices most common eing u e n i g and orming compounds ie e 2u u2e uen and ueni ispersoid partices are smaer and contain eements that are high soue in auminium ie r i r n

mong the ide variet o that can orm in auminium aos the most ound in cast i incude 2u g2i 2eni2 ei he soidiication rates inuence their size morphoog and distriution in the microstructure u and g are common aoing eementsα that are addedand toβ increase strength and the presence o these aoing eements in the composition o the ao resut in the ormation o some o these ie 2u and g2i ron is an inevitae impurit in casting aos arising rom the ore or rom the casting processes herein the moud as are made o iron Usua present in ver o proportions 0 t ei that have a pateie morphoog 2 their low solubility in Al allows the formation of β .

orrosion is an eectrochemica phenomenon hich taes paces hen the ooing conditions are met eposed meta surace 2 eectron acceptor continuous eectrica contact eteen the to components continuous eectrote gavanic circuit is estaished upon uiment o the our conditions and the reative anodic component undergoes an oidationdissoution reaction hie the cathode supports a reduction reaction he dierent phases o the auminium ao microstructure are eectrochemica heterogeneous and thus operate oca as reative anodes and cathodes 2

n auminium aos a discontinuit in the native oide im eists due to the presence o in the microstructure his eposes the sustrate and hen the ao sustrate is eposed to an aueous environment the microgavanic coupe gets activated ue to most o the in aos eing reative cathodic a micro gavanic ce is estaished igure s can e seen rom the igure auminium in the near vicinit o the cathodic phase undergoes an oidationdissoution reaction ater the spontaneous oide is roen

n untreated meta surace i ui the our reuirements mentioned aove and cause corrosion to occur o protect it one o the conditions must e cured t is impossie to separate the eectrica contact eteen the to phases thus the route most oten ooed at in corrosion inhiition invoves the ormation o a arrier aer hich phsica prevents contact o the eectrote ith the meta he irst aer o a mutiaer coating sstem pretreatment or conversion coating

is a thin aer ith dimensions in the range o severa nanometres hich in addition to providing arrier protection aso gives the sustrate good adhesive properties or the appication o suseuent primer aer he to most common pretreatments are anodization and chemica conversion o hich the atter is the ocus in this thesis

. chematic description o the ocaized microgavanic corrosion ce

mong the man options o conversion coatings those ased on the erium e trivaent hromium itanium and irconium orm the same undering mechanism 2 he everage the eectrochemica corrosion reaction rom the gavanic couping eteen the dierent phases to orm a oca region o high pH rom the cathodic reduction o ogen that urther comines ith cations rom ath to orm insoue products that inturn inhiit the corrosion reaction ecause ogen reduction occurs at the oca cathodic sites o the microstructure their roe is crucia in the deposition process 2 hese hoever are on a part o the stor as there are other actors inuencing the deposition process ie the immersion ath chemistr concentration pH additives and immersion time among others 2 n this research cerium conversion coatings are empoed to understand the roe o microstructure in ormations driven ocaized pH gradient over intermetaics ommon chemistr has een appied and no optimization o deposition process parameters is perormed e coatings ere chosen ecause there are man resuts in the iterature to compare ith and chemistr o conversion coating soutions is optimized

..

earearth ased oides have shon promise since the 0s as corrosion inhiiting conversion coating repacements or the conventiona used ut toic r he demonstrate the corrosion inhiition aiit as e as environmenta compiance and phsioogica nontoic nature 0 hese coatings ere pioneered at eronautica esearch aorator in ustraia here Hinton et al. 2– demonstrated that immersion o 0 in eectrote aths containing rear earth meta E choride and nitrate sats increased corrosion inhiition properties onversion coatings utiizing various Es ie a e r

Mechanism of corrosion protection – cathodic inhibition et al. icorr

Deposition process – –– – et al.

’ et al. ,

Deposition mechanism

Al → Al

→ 4OH → H

→ Ce(OH) ↓

→ 2Ce(OH) O → 4Ce(OH)

→ CeO ↓ + H

→ CeO↓ + 2H

et al. et al. “ ”

.

MgCu et al and. θ et al.

et al. et al. . et al. as “chemical dealloying” during the corrosion process upon

et al.

et al. et al. et al. et al. et al. deposits were observed to occur on β rich IM β et al.

deposition reaction do not start ery rapidly upon suecting the alloy to e treatment n their study they ound that initial stages inoled the dissolution o l dealloying and suseuent u redeposition and only ater this initial process as the rapid e deposition osered on the actie rich sites

or actiating the alloy surace to mae it conducie or spontaneous e deposition reactions to occur the alloy sustrate is commonly suected to one or more pretreatments lie alaline etching andor acid picling he alaline etching is ale to dissole the natie oide layer and the picling step is applied suseuently to “desmut” the surface off any hydroides or oides let ehind y the etching step he commonly applied a alaline etching and ring out dierent changes in the surace properties and hae a aried eect in a range o alloys on the e deposition inetics

or high ucontaining alloys lie the acid picling step as shon to e an important pretreatment step as it results in u redeposition proiding actie sites or the reduction o oygen posure o suraces to acids promote the local attac on l in the icinity o dealloying l and suseuently redepositing u “smut” on the surface. tudies y angel et al hae reported rapid deposition inetics hen suected to tostep pretreatment inoling a etching and picling inc et al shoed that a dierent acid picling solution lie is ale to also hae a positie eect on the surace that is conducie or rapid e depositions

ther lo u containing alloys seem to e actiated to a greater etent hen suected to only a etching ecolry et al. hae shoed that deposition reaction as aster hen a a etched surace as conersion coated than a a etched and picled surace here the coating deposits ere non eistent een ater prolonged eposure oreoer ater inestigating the dierent seuence o steps they ound that the pretreatment procedure hich ends ith a etching step prepares the surace est or e deposition ndreea et al. hae also shon that deposition on alloy occurs est hen the surace is pretreated ith a etching than ith a etching and picling

.

hile ughes et al. hae shon that the e precipitate initiates on highly cathodic due to rise in local p and that the localied deposit etends eyond the oundary o the the inluence o IM’s sie on the local p gradient remains yet to e understood n addition it is not non i the local p gradients oer an is inluenced y the gradient oer its neighouring cathodic

ost studies hae studied the deposition on high u containing alloys lie that hae urich he high cathodic actiity o these hile can lead to rapid e deposition the presence o u in the alloy is not an essential criterion to

achee deoston. ocaed deoston has een osered on nonucontann IM e Mner n IM n e IM n and heocast aoys . hese studes do not nestate the deoston mechansm on the nonu contann IM ecet for theβ ones y sam et al. that addresses t to a mted etent. he deoston mechansm on these o cathodc actty IM remans to e fuy understood.

urface retreatments are dey aed on aumnum aoys efore conerson coatn treatments ut ther effect on the ocaed surface chanes s not etensey nestated. ocaed aroach to charactere surface reactty after dfferent retreatments as done y hou et al. . hey eaned that the surface retreatments hhht chanes n surface toorahy and ota otenta that creates condtons for ocaed deoston to tae ace. her or hoeer focused on the secfc IM of and there s st a ac of smar nestate aroaches to charactern surface chanes n other aoys and ther effect on surface reactty for conerson coatn deoston.

CHAPTER 2

.

onerson coatns tycay are the frst ayer of a mutayer coatn system and amon the many otons aaae conerson coatns ased on e traent r and r form sontaneousy on aumnum aoy surfaces th the same mechansm. ere an nterfaca chane at certan cathodc stes of the mcrostructure from the reducton of oyen trers the chemca rectaton reacton of conerson comounds. onerson coatn deoston s nfuenced y three man factors rocess arameters of the conerson coatn ath concentraton and tye of sat souton tme of mmerson temerature and use of addtes the surface retreatment and the mcrostructure of the aoy. ny the former has een dey nestated and as st e n understandn the nfuence of the atter to on conerson coatn deoston. hs thess adots the use of cerumased conerson coatn treatment to understand the nfuence of mcrostructure and retreatments due to ast terature aaae comared to other conerson coatn treatments.

he underyn mcrostructure of the dfferent aumnum aoys s derse contann seera dfferent ntermetac artces th a de rane of chemca morhooca and eectrochemca trats. Most terature nestatn conerson coatn deoston choose the hh u contann hose IM are ery acte. he hhy cathodc nature of u enhances deoston ther resence s not essenta for the deoston reacton to tae ace as s shon n studes th nonu contann cathodc ntermetacs. urthermore seera deoston mechansms hyothesed n terature are many reated to urch ntermetacs and do not consder eometrca roertes and sata dstruton. urthermore the deoston mechansm on IM that s not as acte on u s not addressed n the terature. he am then of ths study s to understand ho the roertes of ntermetacs oth eectrochemca and eometrca affect the ntaton of cerum conerson comound deosts on them. retreatments’ effect on the chanes n surface reactty and conseuent deoston aso needs to e understood. hese ams hae een addressed n the frameor of the secfc research uestons defned for the study ecton ...

.

hs research dees nto fundamenta asects of the mcrostructure that dre ocaed ncreases the most necessary condton for the conerson coatn

’ the localized precipitation reaction, relatively “clean”

reaction on cathodic intermetallic, IM’s size as and spatial distribution

’

interest to characterizecharacterie these chaneschanges throuhthrough local electrochemical methods and study their inluenceinfluence on the deposition

sac aac

his research is eperimentallyexperimentally based and adopts the use oof cerium conversion coatincoating treatments to understand the local suracesurface reactivity oof as a unctionfunction oof pre treatment as ell as the cathodic IM’s chemical composition and geometrical properties or such a study,study model alloys ith controlled microstructures ill be created and subected to the dierentdifferent combination oof suracesurface pretreatments and subseuent conversion coatincoating treatments

cn a csn an cssn – ann scc csc “canvas” he obective to control the microstructures variable as approached by castincasting li alloys ith alloyinalloying elements u and e and solidiyinsolidifying them at dierentdifferent rates for obtaining the desired “microstructure canvases” for deposition experiments lti is chosen as the base alloy as it is one oof the most common commercially used cast l alloy lloyinlloying elements e and u have been added to allo the formationormation ofo intermetallics ith the differentdierent cathodic property he alloy compositions are selected so that they closely resemble the engineeringenineerin alloys in applications and yet are sufficiently simple to produce ‘clean and uniform’ microstructures

y varyingvaryin the solidificationsolidiication raterate, IM ith differentdierent siesize and spacingspacin ere obtained schematic is shon in igureiure he fourour microstructure canvases obtained fromrom the tailoringtailorin ofo microstructures are used to investigateinvestiate the variables mentioned in ection and to anser the uestions posed in ection

. chematic representation oof the strateystrategy used orfor developindeveloping microstructure canvases foror deposition experimentseperiments

epending on the research uestion being ansered the microstructure canvases are subected to specific surface pretreatments and subseuent conversion coating treatment hen the role of underlying cathodic IM’s geometry and spatial distribution is studied all the parameters except for the microstructure are fixed and therefore for such a study the same surface pretreatments and conversion coating treatment is applied on all the different microstructure canvases or the study involving the influence of pretreatment on the reactivity of the surface different pretreatment is applied on different surfaces but the conversion coating treatment is kept uniform for all “canvases”

sac cs an a

ne of the main ethical aspects that ill be taen into considerations concerns data validity and reliability o ensure reliability repeatability of the results

• ach sample produced and test case are uniuely indexed and a code designated a register to eep trac of single specimens and each measurements parameter ill be created • ests on the same type of sample are repeated multiple times to consider measurement error and ensure results soundness pecimens from different samples ill be tested for repetition of measurement to chec the repeatability of the results herever necessary and feasible • he characterisation euipment is checed for calibration periodically before maing any observations tandard analysis methods are folloed during characteriations • Microstructure observations are reported ith the same magnification for good comparisons

aas an s

aas

Model cast li alloys ere created for this study for tailored microstructures to be used as a canvas for deposition experiments he composition selected is presented in able ure aluminium purity ith e impurity along ith alloying elements i e and copper ere melted in a abtherm ilting or ensuring that the elements are added in the right proportions before the metal samples are cast “coins”Furnace are at 800cast ⁰Cfirst and that cast are in analy cylindricaled under graphite a spectrometer moulds. for determining the eight fraction of the different alloying elements in the melt nce confirmed the melt as cast into cylindrical graphite moulds and left to cool in air he cast cylinders ere then subected to remelting at and directional solidification in the ridgeman furnace at starly different rates of mms and mms his solidification rate expressed here in mms is710 the ⁰C speed ith hich the furnace is raised aay from fixed cylindrical rods he melted aluminium is

solidified along the length of the cylinder under state conditions to otain homogeneous microstructure

a hemical composition of model alloys a a n Model alloy lie alance Model alloy liu alance e

sa aan

he cylindrical rods ere cut into smaller coinshaped pieces of approimate mm mm and mounted in either a nonconductive epoy resin or a conductive olyast resin he samples ere ground ith papers of grit sie ⌀ and then polished ith a diamond suspension of colloidal particle sie m m and m hey ere then rinsed in acetone ultrasonic ath for minutes to remove the residues of polishing

ac aan

his ork implements four different types of surface preparation procedures hich include

• Mechanical polishing don to m sie This surface is used as a benchmark and hereon referred to as polished surface preparation • Mechanical polishing folloed y alkaline etching ith M a solution for minutes Hereon referred to as NaOH surface preparation • Mechanical polishing folloed y alkaline etching ith M a solution for minutes and suseuent acid pickling in vol solution for seconds Hereon referred to as NaOH-HNO3 surface preparation • Mechanical polishing folloed y alkaline etching ith M a solution for minutes and suseuent acid pickling in vol solution for seconds Hereon referred to as NaOH-H2SO4 surface preparation

eteen each step in a multistep surface preparation procedure the samples ere rinsed ith distilled ater

nvsn can an

Immediately after surface etching the samples ere immersed in a conversion coating solution o different cerium salts el and e have een used in this study ach salt as used to prepare to different conversion coating solutions M el and M e M al he different sustrates ere suected to conversion coating treatment for various times ranging from h and h

o different mounting resins ere used a conductive one and a nonconductive one hen samples mounted in a conductive resin ere suected to conversion coating treatment they ere suected to conversion coating treatment in a home uilt cell elucidated in upplement II he samples mounted in a nonconductive resin ere simply immersed in a ulk conversion coating solution

na cns an caacan

csc an csn haracteriation involving the morphology and chemical composition of microstructure containing l matri li eutectic intermetallics as ell as cerium conversion coating ere done using canning lectron Microscopy and nergy ispersive ray pectroscopy M he M system primarily can “see” microscopic features of surfaces ith a focussed electron eam traversing the sustrate surface in a raster hen the eam of incident electrons interacts ith a sustrate three different types of rays are generated depending on the interaction volume he econdary lectrons are generated from relatively superficial interaction of incidence electrons ith the sustrate and are thus useful for characteriing features such as the morphology of the surface he contrast in M images is achieved from different phases interacting differently ith incidence electrons ue to the short avelength of incidence electrons very highresolution images of surface morphology can e acuired and this is used to the enefit of oserving the microstructural features of surfaces in this study rays originating from deep interacting ith the sustrate are scattered at different avelengths from different elements rom a spectrum of rays detected y the detector spectroscopic analysis can e done hich allos analysis of the chemical composition of the sustrate ecause the interaction volume of the incident electron eam is relatively higher to e ale to e scattered as rays the elemental composition acuired is not very accurate especially so for fine microstructural features he scanning electron micrographs reported in this thesis ere acuired using at accelerating voltage k urthermore for characteriing the elemental composition and distriution across the microstructure an inuilt nergy ispersive ray pectroscopy pollo detector as utilied he accelerating voltage in case of characteriations as k analysis has its limitations due to the interaction volume of the electron eam eing higher than the volume of the microscopic particle so it has een complemented ith ray iffraction Intermetallic phases in the microstructure are thus characteried also using an ay iffraction mart ystem –

ac a

anodimensional topography of are microstructure and deposits is characteried using ark ystems atomic force microscopy M he

se esees e e ee e s ee es ee e esees ee e ee e esees

ca ccca s

eee ees e e ese sse e ee s ses e e e s e esees ee s se ee e e s e e ee e s se ese e e ee es e se e e se e e e ee es e ee ses es e se e sess e ess e e es ee se e se e e e e ee ses e se

CHAPTER 3

csc a cas as

eirectional ssolidification eeeseeriments esedescribed in ehater eallowed ethe sesssynthesis of etailored semicrostructure containin eeeither a bi and ssesarsely sesaced iureses a and a or smalls and closelyse sacedse iureses b and b ereafteeeer, slow and fast solidified microstructures are referred to as ‘coarse’ and ‘fine’ microstructures,ses resectivelyesee

chematice reresentationeese of a ridemane furnacee

To obtain desired microstructure “canvases”, the cylindrical cast Ali secimenssees eewere edirectionally ssoleidified in a erideman efurnace eiure at a econtrolled erate to obtain a eerelatively uniform semicrostructure on eeremeltin and coolin scast eehyoeutectic Ali salloys, it ssolidifesies with a edendritic sestructure eThe Al formin ethe edendritic arm and ethe einteredendritic esreions eare Ali eeeutectic eentermetallic esarticles eenucleate and row in ethe intraeeeutectic ereion αeThe sesecondary edendritic arm ssacin A sis sesesensitive to ethe edirectional coolin erate eiure seaster coolin esentails fasester esinetics of formation eleadin to elower A and sesmaller eeintermetallic sessies eviceesversa for seslower coolin eewhere ehiher A and elarer eesintermetallics eare eeachieved And sthus, by controllin ethe ssolidification erate in ethe erideman efurnace, it sis sseossible to eecreate semicrostructure eewhere eesintermetallics are of esedesired sessies and ssatial sdistribution eThe m m window in eeach semicrostructure of eiure and eiure ssshows how coolin erate esaffects ethe sessies and sdistribution of eesintermetallics in for ethe sesie and sdistribution for Alie and Aliue scast salloys, eseeresectively

d e

A measurements for the two solidification rates in cast alloy, a Al iue c Alie

a b omarison of intermetallics formed in Alie alloy after subect to a slow solidification and b fast solidification usin a rideman furnace

a b

omarison of intermetallics formed in Aliue alloy after subect to a slow solidification and b fast solidification usin a rideman furnace

as an a csc

ntermetallic articles nucleate and row in the Ali eutectic reions between the secondary dendritic arms hoshorus is a common imurity eistin in trace

amounts in aluminium alloys, and its comound Al acts as a nucleatin site for ecause A is sensitive to solidification rate iure , for each comosition, two different microstructures can be roduced iure The of Alie and Alieu, as observed under with the same manification, are shown in iure and iure , resectively

a b escrition of seen in the a coarse and b fine microstructures of Alie cast alloy

a b escrition of seen in the a coarse and b fine microstructures of Aliue cast alloy rom comarison with microstructures in literature, the different in the two microstructures of alloys Alie and Alieu were identified The latelie in both the microstructures is Alei , n the Alieu alloy, the Alu , and the small formin on the border Aluβe owever, the cannot always be roundedaccurately IM identified with a eutectic by their structure morholoy is θ and thus sulementary and characteriationsof β were have ω been done The averae atomic fraction of in the coarse and fine Alie are listed in Table The same characteriation for in coarse and fine of Aliue are reorted in ulementintermetallics βTable The roortion of elements obtained usin oint analysis does not match eactly with the stoichiometry of the intermetallic comounds, as the beam has a certain volume interaction that may eceed the

volumeolume of the intermetallicallic. This is eseciallyespeciall so in the case of fine microstructure withith small intermetallicsintermetallics. techniue is a owerfulpoerful analyticalanaltical techniue that can accuratelaccurately identifidentify phaseshases in a crystallinecrstalline material.material nAn incident raray eambeam on the samlesample scatters it at secificpecific anlesangles from the lattice lanesplanes in the samlesample. Because each phase has distinct atomic positions and lattice planes, “fingerprint” atternspatterns in the sectrumspectrum are observedosered for the different hasesphases. A standard databasedataase eists withith these finerrintfingerprint patterns for tpicaltyical phaseshases and has eenbeen used to confirm the presenceresence of lAl and i lAlei ,– lAluCu ,– lAluCue , haehave eenbeen confirmed by comarisoncomparison of sectrumspectrum withith those in. Otherthe literature IM like TaleTable β . igureiure shosshows, θ the indeed peaseas ofand the ωcoarse and fine IM of lAlie alloalloy. imilarl,imilarly, the indeed peaseas for the IM in lAliCuue haehave eenbeen reportedreorted in upplementulement igiure .

a spotsot analsisanalysis of different intermetallics in coarse and fine microstructure of lAlie cast alloalloy csc nac a a a oarseCoarse e .. .. .. ine e .. .. .. rich β

rich β

a b spectrumsectrum and indeed phaseshases in a coarse and b fine microstructures lAlie alloyallo The igbi IM phaseshases of platelate in the coarse microstructure erewere measured to ebe eteenbetween m lonlong and m wide,ide, m in euivalenteuialent diameter and .like m β, roundedwidth,idth, resectivelyrespectiel. θ and ω nIn the fine microstructure, on the other hand, the dimensions of platelate microstructures erewere in the rangerane m in lengthlenth and .m in width,idth, m in euialenteuivalent diameter and . m in diameter,like β, rounded respectiel.resectively θ and ωIM in particlesarticles the fine forth referred to as “Curich” IM. The distances to nearest Cuurich IM from indiidualindividual Cuurich IM wereere measured and wereerecontaining found Cu, to beethat on were averaeaerage θ and . ω are . hence m and . . m resectivelyrespectiel for coarse and fine microstructuresmicrostructures.

a eas in the characteried spectrum identified using literature stud for the coarse and fine microstructures of liCue and lie allo as 2θ as caac a cas n cas n α ,,– ., ., ., ., ., ., ,,, , ., ., ., ., ., ,,–,, ., ., . . ,–,, ., . ,,,, , , ., ., ., ., ., ., ,,,,, ., . ., ., ., ., ,,, ., . ., . ,,, , , ,, ,

β ,,, ., ., ., ., ., ., ,, , ., ., ., ., . ,, . ., . ,,, ., , , ., .

θ ,, ., ., ot ot ,, , ., , applicale applicale , ., ., ,, , ., ., , , ., ., , ., ., ,, ., . . ω ., ., ot ot ., ., applicale applicale , ., ., ., ., ., . . nn ot applicale ., ., . ., ., ., . .

ca ccca s ass

Volta potential difference with respect to Al is denoted hereon as ΔV. β FeSi, θ Cu and ω θ are highly cathodic. β cathodic as θ to be lower than θ but higher than α IM phase ω ΔV value, in between that of θ and ω. This is expected as the IM ω

θ and Fe rich β.

csc cnvsn can a sacs

nvsn ca cas

nnc sac aan n cnvsn can an “”

θ

morphology like “leopard skin”. This morphology is also The θ eutectic θ f the eutectic Al from the θ throughout the eutectic θ

eutectic θ

nnc s n ca s nan The mosm rapidd deposiiond kineis ere oseredd hen he sraeu as pre reaedd ih a dand or he olloing udsdy i has een ddadoped. ll ussraes drepored in his usseion ere udseed o he msame usrae preparaion dand onersion oaing mreamen. The usrae as daiaed y ehing ih . a usolion or o mumines dolloed y daid pikling in ol. usolion. The onersion oaing reamenm inldedudd immersionmm o as liue ssraesu in a solionu o . e . al or hree dierend immersionmm imesm .h h andd h respeiely. romm eah speimenm o samplesm eah ere seedud o sraeu preparaion andd onersion oaing mreamen o gaher saisis or umnmerial analysis. umrhermore he saisial upoplaion uddinlded mrom ddieren regions in eah msample.

oalided ddeposis on urih in he oarse liue mirosrremuu aer . e . al onersion oaing reamenm

amem regions in eah samplem ere oseredd nderud in imagingm modemd before ha is an nreaedud sraeu ih are andd after onersion oaing reamen.m s an e seen romm an eemplarm in igreu diserniled onersion oaing deposisd ere loally oseredd o ormm preerenially on high aiiy urih dand hose higher dahodi aiiy as desalished y mmaps shon in uigre . The dloalied onersion oaing ddeposis ere dosered o hae θa ‘mudω he lierareu . haraeriaion shoedd ha erih andd icrack’pariles morphology, also hae a characteristichigher ola poenialfeature also han observed l deemingdm in other hemm studies o ein relaiely ahodid omparedmd o he l marim alei no as high as urih . he deposiiond mehanismmm ha enails p rise a loal ahodid sies andd onseenu deposiiond o onersion ompondsmud on hemm are o e deemeddmd o e he mehanismmm a play deposiiond on all ahodid pariles sholdud or.u oeer

u d d d u d mm m u u β u et al. d d d du d d du d m m ud du d u d du u du dd d u β d d mu d m m uu u u ud d m md du um

d d u u muu m m muu m ud m m u u d d d u u muu u u u d d d u d d d d dd d u u dd d d m u um d u u u u d d d d u d muu u

d d d u m u u d d u u u u u u d d u u u mud u before d after d m u d u d u d d m m m m d m d u mud u m d md m ud d d d u

u mum d d um d d u m d u muu m d m mud m m ud d m d m u m muu u mud d d u ud m u m d ud d d dd ud u d u m m dd d d

hile or both coarse dand ine muuicrostructure urich increasing conversion coating mtie dddid not have any eect on the growth o the dlocalied ddeposit an increasing dtrend in the umnuber o depositeddd was observedd aority o coarse urich were dactivated or ddeposition () ater h conversion coating mtreatent dand ater h o the coarse s dinvestigated dshowed a dlocalied ddeposit n the case o ine urich however a lower proportion o the was dactivated or ddeposition () uupon conversion coating or h dand this umnuber dincreased or h conversion coating mtreatent moparing the dtrends in local ddeposit area etension dand umnuber o dcovered at progressive conversion coating mties it mbecoes clear that with the progression o conversion coating mtreatent the dlocalied ddeposits ddo not necessarily grow once dinitiated on an ubut rather start to mor over those urich that are not dcovered umFurtherore a greater proportion o coarse urich activated or ddeposition mdcopared to the ine urich

(a) (b) drends o average area etension o dlocalied ddeposit over an u(blue) dand proportion o showing preerential heavy ddeposition d(red) with increasing conversion coating mties as dobserved or the (a) coarse dand (b) ine muuicrostructure liuFe alloys

pon investigation o ddeposition on many urich it was udound that the area o the dlocalied ddeposit moring over big dand msall was proportional to the udunderlying usurace area o the with a linear correlation (umuppleent uFigure ) he linear correlation between the udunderlying coarse urich usurace area dand the dlocalied ddeposit area over it dhad slopes o dand with correlation coeicients () dand respectively or the three conversion coating mties h h dand h siilarm linear correlation or the ine urich dhad slopes dand with uvalues dand respectively or conversion coating mties h h dand h respectively o increasing dtrend in slope (denoted by ‘k’ in Figure) with increasing conversion coating mtie between h dand h was udound umFurtherore at the global level the ratio between overall area raction o bare usurace area dand conversion dcoated usurace area (denoted by ‘r’ in Figure) no increasing dtrend with conversion coating mtie between h dand h was udound hese uantitativeu observations indicated that

onceone localiedloalied conversiononersion coatingoatin deposits or on an activeatie the kineticskinetis o growthrowth is rtherurther reducedreded whichwhih preventspreents urtherrther growthrowth o the island ro coveringoerin the entire atri with a siilarlysiilarl heavyhea deposit

(a)a (b) rends in slope ‘k’ and ratio ‘r’ with increasinginreasin conversiononersion coatingoatin tie

eryer ew uantitativeantitatie studiesstdies analysinganalsin the growthrowth o localiedloalied deposit eist in the literatureliteratre and none investigatinginestiatin the inluenceinlene o sie on the localiedloalied deposition studystd by ugheshes et al. has oundond that the deposits etend beyondeond the boundaryondar or short iersion tie o upp to inutesintes n their studystd howeverhoweer peroide accelerantaelerant was usedsed whichwhih beingein an oidier provideproided additional oygenoen speciesspeies or rapid p increaseinrease n this work no accelerantaelerant was usedsed and deposits were observedosered to etend beyondeond the boundaryondar o the or relativelyrelatiel longerloner iersion ties oreoveroreoer it was observedosered that at prolongedproloned conversiononersion coatingoatin tie o h the localiedloalied deposit did not etend to a greaterreater etent as coparedopared to deposition at h n the absenceasene o an accelerantaelerant like p inrincrease is onlonly driendriven by redtionreduction o oenoygen speiesspecies ro soltionsolution eneence slower kinetiskinetics o rowthgrowth are oseredobserved a resltresult and loaliedlocalied deposition on atieactive sites the kinetiskinetics is slowed down eveneen urtherrther hinderining the uniornior distributiondistrition o conversiononersion cooatingatin overoer the entire suracesrae includinginldin the atri

nnc n an saa sn n sn acn

he urichrih investigatedinestiated in this work eisted in dierent “neighbourhoodsneihorhoods” that is theythe presented dierent senariosscenarios o iroicroalanigalvanic oplincoupling he urihrich in dierent neighbourhoodsneihorhoods all present in the latri were indeed as (● ) independentlyindependentl eistingeistin ( ) attachedattahed to a plate (▲) attachedattahed to etetieutectic i partileθparticle (■ ) attaattached to othboth etetieutectic ) he loaliedlocalied onersionconversion oatincoating deposit♦ on independentβ ● ire(Figure (b)) was observedosered to laterallylaterall spread beyondeond the boundaryondar o theSi and ne β arlyarl (Figure euallyeall in all directionsdiretions oweveroweer when the urichrih was attachedattahed to either i ▲ (Figureire (b)) or

θ β ♦

ire or oth ■ ire the lateral spread o the loalied deposit appeared to etend ore aross the partile order ire d and e θ/Al border than across the θ/β border or β/Si

a d

e

esription o the dierent neihorhoods in whih rih was present in the irostrtre a ● independentl eistin ▲ attahed to i partiles attahed to plate and ■ attahed to oth plateθ and i partile he loalied deposits on ♦ e βand respetiel β the different θ IM particles are shown in (d), he loalied deposition drien loal p rise is a reslt o iroalani oplin wherein the athodi sites spport the redtion o oen he eletrons reired or the athodi reation are spplied ro the anodi reation side the rate o eletron transer an e eele to none asin an insiniiant rise inthrough p at itsthe loation MGC interface. hs the When lateral such spread an interface o the loaliedconsists of deposit either wasβ or oseredSi on one to e dependent on the loalit or neihorhood within whih the hihl athodi rih eists

he hihθ p radient oer a rih was er loalied st oer the and did not interat with the p radient oer another rih losest to it his is inerred ro lookin at depositions on seeral and oserin i oalesene o deposits orred t was ond that the oalesene o deposits oer two

neihorin deposits onl orred when the two rih were er lose to eah other his was osered in instanes in the ine irostrtre allo t st also e noted that while the oalesene o deposits oer two er losel sitated rih did or the oerlap was howeer not ased de to ier than aerae p radient oer either o the he oarse rih were relatiel arther apart ro eah other and no oalesene o loalied deposit was osered rtherore lookin at ire no deinitie trend etween nearest neihor distane and deposit area etension is seen ro this it an e inerred that the p radient oer a rih is er loalied and does not aet the p radient o it nearest rih neihor he proiit o rih partiles in the irostrtre does not proide a hiherthanaerae driin ore or deposition reation is also interestin to know how ar the p radient oer a rih etends and i the p radient oer it an aet the p radient oer another nearest rih

a

atter plot showin the easred distane to nearest neihor and the etensions in deposition area or dierent onersion oatin ties in a oarse lie allo and ine lie allo

nvsn ca ssas

he lie ast allos hae proed diilt to oat ater onersion oatin treatent de to erih ein not as hihl athodi as rih e to the asene o in solid soltion pretreatents with and are also not ale to ase srae hanes like redeposited partiles that ake the srae ondie or deposition n the preios setion it was osered that in a irostrtre o ast lie allo ontainin oth rih and erih preerential deposition is osered to or on onl rih een ater lon onersion oatin ties o p to h t is interestin to see how the asene o rih phases aets deposition on the net ost athodi erih

nnc sac ans n sn

he or dierent srae pretreated saples were seted to onersion oatin treatent in a e al soltion or h he iroraphs are reported in ppleent ire onersion oatin treated ehaniall polished and a pretreated saples did not show an loalied deposition whatsoeer ll the erih in the a srae pretreatent ondition did not show deposition ater onersion oatin treatent t indiidal loalied deposits wereβ osered on the eteti li reion o the irostrtre his will e inestiated in etion t or now it is siient to iner that the a pretreatent did not rin srae hanes in the ast lie allo that ake it ondie or onersion oatin deposition nl one pretreated srae a showed loalied onersion oatin deposits on e

he tpe o srae preparation treatentrich β applied aets the oatin oration nlike with ontainin allos alkaline ethin with a and aidi treatent with does not ield an deposition whatsoeer as osered in a std eolr et al. . n their inestiations into seential alkalineaidi and aidialkaline srae pretreatents the ond that the inal step shold alwas e one with an alkaline soltion to otain an e deposition iilar oserations were ade in works o ndreea et al. on and slai et al. on heo ast lie allo wherein preerentiall thik e oatin oer erih was osered when the srae preparation inoled onl a ethin

o inestiate the loal srae hanes orrin on o the ast lie allo the sae reion o the irostrtre in the sstrate ater the or dierent pre treatent onditions were harateried with ire a shows the irostrtral reion in the enhark ehaniall polished ondition he olta potential proile aross white arrow in ire a was siilar or the three pretreatents polished a and a siniiant redtion in the olta potential or the e was osered in the ase o srae pretreated with a le olta potential proile in ire t is inerred ro these reslts that the prerichtreatent β step with passiates the srae and this is the reason or the diinishent o the olta potential o all the phases

a

a irostrtre reion in the ehaniall polished ondition ro the oarse ast lie speien that was seted to the dierent pre treatents oporaph proile aross white arrow and olta potential proile aross the white arrow

he asene o loalied deposits on e ater onersion oatin a pretreated sraes is related to the drasti redtion in the olta potential o the akin it less athodi and nalerich β to spport stron oen redtion reations apale o inreasin the p to a siientl hih or onersion oatin preipitation reations is not as stron an oidiin aid as and ths did not ase the passiation o the srae to the sae etent rae inatiit to spport deposition o e ater a srae pretreatent has een reported eolr et al. in their std o e oration on loalied eplanation or the eet o piklin on the srae state and onseent inatiit to spport e deposition reains nlear and needs ore work he onl pretreatent that worked was the one inolin a and is eplained ased on an inrease in olta potential o the e reater athodi potential an spport stroner oen redtion reations and in trn rich β

raise the loalied p to a siientl hih enoh ale to allo preipitation reation o onersion oatin oponds to or

sn cans n c acs ot onl is the heial oposition and onseentl the olta potential o e rih dierent t the orpholo is also siniiantl dierent e to these dierenes the ehanis o loalied e deposition a βe aried o frominestiate the eutectic the inlene θ o sie on the loalied onersion oatin deposition ehaior oarse and ine ast lie saples were seted to the sae srae preparation a alkaline ethin and the sae onersion oatin treatent with el soltion or h reerential e deposition on oarse erih was osered with orpholo as shown in ire he deposition appeared to preerentiall or on onl the ondar o erih or ine howeer sh orpholo was not osered t deteted a nior e sinal oer the ine erih sites ppleent ire nlikeβ in the ase o where the deposition was seen on onl the order β coarse β

orpholo and eleental distrition o onersion oated oarse irostrtre ater iersion in el or h he initiation o onersion opond preipitates on onl the order o the an e eplained ased on the low ondtiit o the partile sine it ontains i i has a resistiit o whih are respetiel o the ondtiit o o oposition lei is reded de to the preseneΩm compared o i in to its that oposition of Al and Fehe eletrons ths sppliedΩm and ro 1.0 the × 10 anodiΩm, reation reah at hih onentrationβ onl to those parts o the athodi losest to the iroalani ople interae ppleent ire he hiher onentration o eletrons reahin the reions near the ondar is hihβ enoh to rede oen oleles sh that a p radient siient or the hea preipitation is reahed t st e noted howeer that sh preerential deposition phenoenon was seen on onl a ration o oarse

n the ase o ine erih howeer de to their sall sie the ner o eletrons reahin its srae is low and an spport ewer oen redtion reations his does not ieldβ a p hih enoh to ase preerential hea deposition as seen in the ase o oarse he e orpholo was not osered throh t e deteted ppleent ire indiatin that the weak p radient ased onl a er low antit o deposit to or oer the ine erih

nother tpe o deposit orpholoβ was osered on soe speiens o the oarse irostrtre lie saple and srprisinl on the a srae pretreated lie allo ire a ro harateriation presented in etion it was osered that the olta potential o e is reatl reded ater the pretreatent step and this ases no deposition to or on the opared to the sae saple in the a onl prerichtreated β ondition where loalied deposits were seen on e ire rich β

a

oparison deposit orpholo osered on pretreated sraes a a and a ater onersion oatin treatent in e al soltion or h

oe speiens o the oarse lie saple ontained sall partiles that had the sae ontrast as i e plates ire a and onersion oatin treatent ater a pretreatent led to loalied deposition on the sall t not on rich β std showed that the sall erih partile had a h hiher olta potential ppleent ire opared to easredβ or the partile ppleent ire ater the a pretreatent rtherore analsis o the sall erih partile reealed that its oposition wasβ siniiantl dierent than the e ire and ale he sall erih partile was ond to e riher in e he opositional dierene seeed to haerich ade β the sall reat dierentl to the a pretreatent and poorer in Si than the β and ths led to loalied e deposition leaving it with a higher volta potential than the β

a

a harateried oarse irostrtre o ast lie allo showin the two with sae ontrast and oalied deposition otained on onl the sall erih ater srae a pretreatent and onersion oatin treatent in e al soltion or h

rosssetionin and o loalied deposits on three dierent erih pot reported in ale

ale toi rations o dierent eleents in the three dierent erih show in ire

a a a

CHAPTER 4

his thesis inestiates the loalied onersion oatin deposition ehanis alini allos sin a noel approah hile the ritial role o in the onersion oatin deposition has een identiied researhers ew stdies hae osed on the inlene o eoetri and eletroheial properties on the loalied deposition ehaior oreoer ost stdies proin the deposition ehanis se allo whih ontains ostl er atie partiles lso ein a oerial allo has a ople irostrtre ontainin seeral phases o dierent noilities and road sie distrition o takle this isse this work reates a odel allo with ontrolled opositions and solidiiation rates to ahieve “clean” or homogeneously distributed phases of narrow size distribution. n this work two odel ast li allos hae een reated with opositions l wt iwtwt e and lwtiwte and the hae een seted to diretional solidiiation at slow and ast rates to otain hooeneos oarse and ine irostrtre respetiel eneral srae pretreatents and onersion oatin treatents hae een applied and loal orpholoial toporaphial and eletroheial harateriation tehnies hae een applied to inestiate the irostrtral inlene on the deposition ehanis ro the inestiations done the ollowin onlsions an e ade

or dierent srae pretreatents’ effect on surface changes and sseent onersion oatin were inestiated and the inlded ehanial polishin a ethin onl a ethin piklin a ethin and piklin hese srae pretreatents roht speii hanes to the toporaph and olta potential o the phases and in trn deterined their atiit or sseent loalied onersion oatin deposition ehanial polishin did not eetiel atiate the srae as the deposition kinetis een on hihl athodi rih partiles ater this srae pre treatent was er slow eposition kinetis were relatiel aster t still slow when onersion oatin treatent was applied to a ethed sraes id piklin with and ased dealloin o l and sseent redeposition on the rih athodi eteti l and de to this ore rapid deposition kinetis was osered iklin with ased ore aont o redeposition and this reated the θost atie srae opared to other pretreatents tried in this std as loalied deposits started to or een ater h o onersion oatin treatent ater pretreatent

. n a cast liue microstructure containing phases l eutectic i lei eutectic lu and lue a preferential deposition was observed on only the latter two that were urich. venα after a prolongedβ h conversion θcoating treatmentω no localized deposits were observed on the former two phases. referential deposition on urich phases is eplained due to their relatively higher volta potential than other phases. hereas the volta potential measurements of the lei showed a weaer cathodic character and did not participate in the localized deposition reaction when other more active urich were presentβ in the microstructure. utectic i particle while showed a positive volta potential did not show localized deposit after any of the conversion coating treatments tried and this observation is eplained based on it very high electrical resistivity. . uantitative analysis of before and after conversion coating treatment show that the size surface area of urich linearly correlated with the lateral as well as direction spread of conversion coating deposit. his correlation was observed for three conversion coating treatments in . . he p gradient over a urich was observed to be very localized and does not interact with the p gradient over another nearby urich . . urface pretreatment a had a detrimental effect on the reactivity of the coarse cast lie surface as a very drastic reduction in volta potential of the erich was observed. he picling step passivates the cast lie surface and this is the reason for no deposition observed after this pretreatmentβ procedure. . he deposition mechanism on erich was composition specific. he initiation of deposits on erich plate was observed to initiate on the boundaries whereas in other erich with lower i content and richer in e although much smaller in sizeβ the deposition morphology was lie that observed in the case of urich . he deposits forming on only the border of erich plate after a short conversion coating treatment is eplained due to the high resistivity of i which is present in the composition of the . β

CHAPTER 5

he localized approach to investigating the influence of properties lie chemical composition size and distribution is novel. rom the current study it was found that chemical compositional differences between eutectic lu and lue did not significantly affect the deposition mechanism as both were readily deposited upon conversion coating treatment in a .θ e .ω al solution as the deposition is observed even after a short conversion coating treatment for .h after a surface pretreatment. owever chemical compositional difference between the two types of erich in this study was shown in this affect the localized deposition of conversion coating on each. fter a a surface pretreatment a preferential deposition was observed on unindeed erich particles that had higher e content than erich lei upplement able where no deposition was observed. characterization supported these localized deposition observations or theirβ lac of as the surface pretreatment a caused a drastic reduction in the cathodic potential of the erich lei but not for the other compositionally different erich . β urthermore it was also shown that the localized deposits on erich lei initiate at the boundary due to its lower electrical conductivity resulting from the presence of significant content of i after a surface pretreatmentβ followed by conversion coating treatment in . el solution for h. n investigating deposition on the two compositionally different erich some more uestions arise in the contet of the role of chemical composition particularly about the i and e proportion in the on the inetics of deposition on an . hile creating with controlled ei atomic fraction ratios is practically difficult indirect clues about the influence of ei ratio may be obtained by studying deposition on other types of containing different e and i content. or future wor it would be interesting to conduct a similar localized investigation with and characterization on other erich . or this l alloys shall be created using a similar approach as in the current thesis with different containing varying e and i content n the current wor only some very high cathodic activity urich were investigated. here are other which have similarly high cathodic activity for instance phase lug. ome commercially important alloys lie wrought and cast liug contain both very cathodically active and phases. or future wor tailored microstructures with a controlled size of these phases will be created to understand how each’s electrochemical and physicalθ properties

aect the localied deosit initiation seuence and uantit o conersion coatin deosited his aroach should be then etended to the dierent o interest and inestiate ore their initiation when resent siultaneousl in the icrostructure

olear iht llos iht llo httsdoior

thiou cen rolli ustainable aluiniu sstes ustainabilit – httsdoiorsu

errenho oran llwood he iact o reducin car weiht on lobal eissions he uture leet in reat ritain hilos rans oc ath hs n ci httsdoiorrsta

uan en in urace enineered titaniu allos or bioedical deices httsdoior

tenohl luinu

technology, applications, and environment : a profile of a modern irbilis metal : uchheit Aluminum lectrocheical from within, 6thharacteristics editio, Joh Wiley o nteretallic & Sons, 2010. hases in luinu llos lectroche oc httsdoior

uchheit oilation o orrosion otentials eorted or nteretallic hases in luinu llos lectroche oc – httsdoior

ratilaachitei errn eldon hoson usc ateran nluence o substrate icrostructure on the rowth o anodic oide laers lectrochi cta – httsdoiorelectacta

elson uhes alor inton ilson enderson haracterisation o aluiniu allos ater ao retreatent ater ci echnol – httsdoior

erohin han nodisin o liht allos in ur n iht llo lu anes itan llo lseier nc – httsdoior

uhes onersion coatins in nccl nteracial he ur ci lectroche lseier – httsdoior

satulu anocoatins or corrosion rotection o aerosace allos in orros rot ontrol sin anoater lseier – httsdoior

ensteiner enins hroate conersion coatins et inish

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The Influence of Al Alloy Microstructure on Conversion Coating Formation

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-rich β-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 another Fe-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.

SALIL SAINIS is currently a PhD student in the Department of Materials and Manufacturing at the School of Engineering, Jönköping University. He is also currently an Early Stage Researcher (ESR) of the ‘mCBEEs’ Innovative Training Network (ITN) within the Marie Skłodowska-Curie Actions initiative of the European Union’s Horizon 2020 - Research and Innovation Framework Programme. He holds a M.Sc. degree in Aerospace Engineering from Delft University of Technology in The Netherlands. His research interests lie in the field of materials science and surface technology.

ISBN 978-91-87289-63-7