anodisation of aluminium alloys by micro-capillary technique as a

Research ArticleAnodisation of Aluminium Alloys by Micro-CapillaryTechnique as a Tool for Reliable Cost-Efficient and QuickProcess Parameter Determination

Daniela Nickel12 Dagmar Dietrich1 Roy Morgenstern1 Ingolf Scharf1

Harry Podlesak3 and Thomas Lampke1

1Materials and Surface Engineering Group Technische Universitat Chemnitz 09107 Chemnitz Germany2Center of Engineering Materials State Materials Testing Institute Darmstadt (MPA) Chair and Institute for Materials Science (IfW)Technische Universitat Darmstadt 64283 Darmstadt Germany3Composite Materials Group Technische Universitat Chemnitz 09107 Chemnitz Germany

Correspondence should be addressed to Dagmar Dietrich dagmardietrichmbtu-chemnitzde

Received 20 September 2015 Revised 25 January 2016 Accepted 7 February 2016

Academic Editor Gianluca Percoco

Copyright copy 2016 Daniela Nickel et al This is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

Anodisation is essential for improving surface properties of aluminium alloys and composites regarding wear and corrosionbehaviour Optimisation of the anodising process depends on microstructural constituents contained in aluminium alloys andrepresents a key task consisting of the control of process parameters and electrolyte formulation We applied the micro-capillarytechnique known from corrosion studies and modified it to form anodic aluminium oxide films on high-strength aluminiumalloys in comparison to pure aluminium in sulphuric acid A glass capillary with an opening of 800 120583m in diameter was utilizedCorresponding electrochemical measurements during potentiodynamic and potentiostatic anodisation revealed anodic currentresponses similar to conventional anodisation The measurement of film thickness was adapted to the thin anodised spots usingellipsometry and energy dispersive X-ray analysis Cross sections prepared by focused ion beam milling confirm the thicknessresults and show the behaviour of intermetallic phases depending on the anodising potential Consequently micro-capillaryanodising proved to be an effective tool for developing appropriate anodisation conditions for aluminium alloys and compositesbecause it allows quick variation of electrolyte composition by applying low electrolyte volumes and rapid film formation due toshort process durations at small areas and more flexible variation of process parameters due to the used set-up

1 Introduction

Thegeneration ofmicrostructural constituents in tailor-madealuminium based materials resulted in substantial progressachieved in development and application of this materialgroup Kinds of microstructural constituents can be dislo-cation arrangements and grain boundaries (high- and low-angle boundaries) solid solution formation and arrange-ments of intermetallic compound (IC) particles of varyingsizes As regards the latter these second-phase particles inaluminium alloys affect the material behaviour dependingon their size constituent particles of fairly large size disper-soids of mostly intermediate size or fine-scaled hardeningprecipitates For example the dominant constituent phase in

AlZn55MgCu alloys is Al7Cu2Fe additionally existent are

(Al Cu)6(Fe Cu) A1

6Fe Mg

2Si 120572-Al

12Fe3Si and SiO

2[1]

Most of the second-phase particles possess complex chem-istry crystal structure and morphology Dispersoids in theAlZn55MgCu alloy show a variety of rod-like triangular orglobular shapes [1] The general properties of this alloy aremainly determined by equilibrium phases like MgZn

2(M-

phase 120578 phase) Al2Mg3Zn3(T-phase) and Al

2CuMg (S-

phase) Responsible for high strength are the 120578 phase its pre-cursors GP zones and intermediate precipitates (1205781015840 phase)The transition phases are semi-coherent and mostly locatedinside thematrix grains ondislocations and low-angle bound-aries The incoherent equilibrium phases nucleate on high-angle boundaries and in the grain interior [2]Thedispersoids

Hindawi Publishing CorporationAdvances in Materials Science and EngineeringVolume 2016 Article ID 1374897 12 pageshttpdxdoiorg10115520161374897

2 Advances in Materials Science and Engineering

Table 1 Chemical composition of the studied materials according to the specification of the producer

Alloy Si (wt) Fe (wt) Cu (wt) Mn (wt) Mg (wt) Cr (wt) Zn (wt) Ti (wt) AlAl 5N 000079 000098 lt00002 000004 000103 0000059 0 0000098 Bal1050 lt025 lt04 lt005 lt005 0ndash005 lt003 lt007 lt005 Bal2024 050 050 35ndash49 03ndash09 12ndash18 010 025 015 Bal7075 040 050 12ndash20 030 21ndash29 018ndash028 51ndash61 020 Bal

Al2Mg3Zn3andAl

2CuMg are known to affect hotworkability

and fracture properties Compared to T561 temper 1205781015840 phaseand 120578 phase are coarsened by artificial overaging in temperT73 which is beneficial to increase the resistance to stress-corrosion cracking [2ndash4]

Automotive and aerospace applications require enhancedcorrosion resistance improved tribological properties ora simply improved decorative appearance This is usuallyaccomplished by the formation of an anodised aluminiumoxide (AAO) due to the conversion of the surface region ofthe aluminium alloy [5] AAO films on pure aluminium arefairly homogeneous showing hexagonal arranged pores per-pendicular to the surface which results from the dynamicequilibrium between film growth at the metaloxide interfaceandfield-assisted dissolution at the pore bases [6] In contrastthe microstructure and the resulting properties of AAOfilms formed on aluminium alloys are severely influenceddue to the conversion of solid solutions and the behaviourof second phases during anodisation [7 8] In particularanodising of copper-containing alloys is remarkably limiteddue to inefficient film growth and enhanced porosity of theproduced AAO film [9 10] Dissolution of second-phase par-ticles results in voids and gas-pockets [11 12] Second-phaseparticles with low oxidation rates can be incorporated intothe film because of the faster oxidation of the surroundingmatrix [12ndash14]This observation was similarly made for AAOfilms formed on aluminium matrix composites (AMC) withalumina or silicon carbide particles [15 16] Anodic oxidationrates of a variety of ICs have been examined in relation to thatof thematrix [17] Recent studies considered the detailed pro-cesses occurring at ICs by simultaneously recording the ele-ctrochemical data Film formation was performed by eitherpotentiostatic (constant potential) galvanostatic (constantcurrent density) or potentiodynamic anodisation (increasingpotential with defined rate) The current density responseduring potentiodynamic anodisation and the time-voltageresponse during galvanostatic anodisation were discussedwith regard to several commercial alloys different elec-trolytes and occurring growth stages during AAO film for-mation [12ndash14] Potentiodynamic anodising was proposed asan efficient tool for the development of appropriate anodisingparameters since it provides a rapid characterisation of thecomplex electrolytealloy behaviour over a wide potentialrange [18] Such studies comprise the application of newlydesigned electrolyte formulations for anodising of aluminiumalloys and aluminiummatrix composites Nevertheless usingconventional anodising for this task necessitates the applica-tion of large electrolyte volumes in the scale of several litreswith the disadvantage of high expenses for procurement anddisposal

The aim of this work is to transfer the micro-capillarytechnique known from local corrosion research [19ndash23] tolocal anodising studies Recently this technique was sophisti-cated by developing multichannel arrangements for improv-ing scanning droplet cell microscopy [24] We applied themicro-capillary technique onpure aluminiumand three com-mercial aluminium alloys Sulphuric acid was chosen as acommon electrolyte to provide the comparison to conven-tional anodising and to demonstrate the feasibility of the tech-nique The simultaneous electrochemical characterisationduring initiation and progress of the film growth has beenstudied and correlated with microstructural features in par-ticular with the behaviour of second-phase particles and thedevelopment of the film thickness on the example of thehigh-strength and stress-corrosion resistant alloy EN AW-7075 in T73 condition that is artificially aged for providingstress-corrosion resistance Film thickness measurement wasadapted to thin and small AAO areas Finally a discussion onpotentials and limitations of the presented method is given

2 Materials and Methods

Pure aluminium (Al 5N Hydro Aluminium Rolled ProductsGmbH) and three commercial aluminium alloys in extrudedcondition and the given temper (EN AW-1050 EN AW 2024T4 EN AW-7075 T651 and T73 for chemical compositionsee Table 1) were used Samples were etched in 3wt sodiumhydroxide (NaOH) for 60 s at 50∘C and desmutted in 30 volnitric acid (HNO

3) for 30 s at room temperature After ano-

dising the surface was rinsed carefully with deionized waterand ethanol

The samples were anodised in 04M sulphuric acid(H2SO4) at room temperature (22 plusmn 05) ∘C using a micro

corrosion cell (Swiss Microcell System) based on a glass cap-illary technique Parts of the three-electrode set-up are aglass capillary with a ground tip of 800 120583m in diameter anda seal of a silicone rubber layer to prevent leakage actingas anodising electrode a 05mm platinum wire acting ascounter electrode and a silversilver chloride (3MKCl) elec-trode acting as reference electrode An exact positioning ofthe capillary was permitted by mounting the electrode set-upat the nosepiece of an optical microscope (OM) and fixingthe samples at the stage of the OM A detailed descriptionof the microcell system has been published elsewhere [23][BOH95] Each test was performed at least twice Anodisingwas accompanied by micro-electrochemical measurementsusing a high precision voltage source (DIGISTANT Model4462 Burster Prazisionsmesstechnik) a potentiostat (IMP83PCT-BC Jaissle Elektronik) and a digital multimeter (Model

Advances in Materials Science and Engineering 3

2000 Keithley Instruments) providing a noiseless data acqui-sition with a rate of 91 Hz matching one point every 185mVduring the used sweep rate Potentiodynamic anodising wasdone from the open circuit potential to 10V at a sweep rateof 17mVs Potentiostatic anodising was performed for 600 susing potentials between 03 V and 10V For comparisonwith conventional anodising a three-electrode corrosioncell with the working electrode (exposed area 10mm indiameter) Pt counter electrode and AgAgCl (3MKCl)reference electrode was used Macro-scaled anodising wasperformed in stirred electrolyte at controlled temperature(22 plusmn 05) ∘C at an anodising potential of 10V for 1200 sand 3600 s The ratio of electrolyte volume to sample surfacearea was around 3mLmm2 in themicro-capillary set-up and50mLmm2 in the macroscopic set-up All potentials desig-nated in this paper refer to the AgAgCl (3MKCl) electrodeif not indicated differently

Microstructural studies were done using a scanning elec-tron microscope (SEM Zeiss NEON 40EsB crossbeam)equipped with a focused ion beam mill (FIB Orsay) and anenergy dispersive X-ray spectrometer (EDS EDAX) Alloyswere examined by low-voltage scanning transmission elec-tron microscopy (LV-STEM) at 30 kV in combination withEDS Electron transparent samples were prepared by grind-ing electropolishing and finally ion-polishing on both sidesusing 3 kV Ar ions with 6∘ incidence angle AAO films werestudied by scanning electron microscopy Surface imagingwith secondary electrons (SE) was done without carbon coat-ing at 2 kV using Everhart-Thornley detector EDS spectrawere acquired at 5 kV Cross sections of the 800 120583m circularAAO film areas were prepared by focused ion beam (FIB)milling after deposition of a tungsten protective coating30 kVGa ions with a current of 10 pA for the last polishingstep were used Imaging of the cross sections was accom-plished at 3 kV using the InLens detector of the SEM

Ellipsometry data for film thickness and porosity eval-uation were collected with a rotating analyser ellipsometer(M-2000 JA Woollam Co) in the spectral range from 1690to 245 nm with incidence angles of 45 50 55 and 60∘ Thediameter of the polarised light beam was 1mm resulting inan elliptical measuring area comparable to the anodised areaModelling regression analysis and parameter fit was doneusing the software CompleteEASE (JA Woollam Co) B-spline was used as polynomial function for the substrate andan effective medium approximation for the films Modellingof the pores in the layer was done by a Cauchy layer of (100x)vol alumina and x vol air

3 Results and Discussion

Potentiodynamic linear polarisation curves were recordedduring anodisation in sulphuric acid in a micro-capillarycell arrangement (Figure 1) The curves of the alloys consistof four stages (i) starting with a current peak around 0Vwhich is most pronounced for EN AW-2024 (ii) followedby monotone rising up to 3V (iii) a shallow peak between 3and 6V for ENAW-2024 and correspondingly between 4 and95 V for ENAW-7075 (iv) further risingwith different slopes

0

1

2

3

4

5

Curr

ent d

ensit

y (m

Ac

m2)

0 1 2 3 4 5 6 7 8 9 10minus1

Potential (V) (AgAgCl (3M KCl))

EN AW-1050EN AW-2024 T4

Al 5N EN AW-7075 T73EN AW-7075 T651

Figure 1 IV behaviour of Al 5N EN AW-1050 EN AW-2024 T4EN AW-7075 T73 and EN AW-7075 T651 during potentiodynamicanodising in 04M sulphuric acid using the micro-capillary tech-nique vertical lines mark the potentials selected for potentiostaticanodising (as shown in Figures 4 and 5)

depending on thematerial Different temper states of the alloyEN AW-7075 slightly affect the overall current density leveland the peak width in (i)

As discussed in the Introduction the alloy EN AW-7075was selected for further experiments since it includes a largevariety of Zn Mg Cu Fe and Si containing IC particleswith high influence on the microstructure of the AAO filmThe film after potentiodynamic anodisation is characterisedby numerous voids due to the oxidation and dissolution ofsecond-phase particles Using secondary electrons the arra-ngement of the hardening IC particles in the alloy is shownon the alloy surface before anodisation (Figure 2(a)) A crosssection of the alloy with the AAO film on top is shown inFigure 2(c) after Using backscattered electrons grain bound-aries of partially elongated grains of the alloy parallel to theextrusion direction are visible due to the orientation contrastThe coarsened hardening precipitates are located on the grainboundaries finer intermetallic particles are located inside thegrains A rather corresponding arrangement of voids in theAAO film can be observed in Figure 2(b) in planar viewLarger voids seem to mirror former grain boundaries andfiner voids are located inside the thus bordered areas A largeconstituent particle appears mostly unchanged embedded inthe film A detailed image of the film morphology at highermagnification (Figure 2(d)) clearly reveals shape and size ofthe voids originating from the IC particles and the pore struc-ture in between respectively The pores originally straightperpendicular directed to the alloy surface show a frequentchange of the direction as a result of the wavy interface whichevolves between the alloy and the anodised film Howeverthe pore structure is not as complex as it develops on ENAW-2024 The branching of pores is associated with copperenrichment during anodisation of the alloys which is delayed


5120583m

lowast

(a)

AAO film

5120583mlowast

(b)

5120583m

lowast

(c)

500nm

(d)

Figure 2 The microstructure of EN AW-7075 T73 in (a) planar view and (b) cross section (with the AAO film on top) and the AAO filmformed by potentiodynamic anodising of this alloy in 04M sulphuric acid in (c) planar view and (d) cross section showing the behaviour ofcoarse constituent particles resistant to anodising (marked by asterisks) and IC particles along grain boundaries (some ones are marked bywhite arrows) and in the grain interior resulting in voids in the AAO film (some ones are marked by black arrows)

due to the lower concentration of copper in ENAW-7075 andthus permits the development of straight pores to a largerextent

TEM imaging of the grain interior of the alloy(Figure 3(a)) reveals a variety of dispersoids Additionallyuniformly arranged hardening precipitates are shown Thegrain boundary decorating particles were confirmed as 120578phase (MgZn

2) EDS analyses verified also the existence of

a range of other ICs Mg2Si MgZn

2 Mg32Zn49 Mg(AlCu)

Al2CuMg and Al

7Cu2Fe However it should be noted that

it was not possible to identify all particles exactly by EDSanalysis due to the excitation volumewhichmostly comprisesthematrix tooMoreover it was not possible tomake a certaincorrelation between shape and stoichiometry The AAO filmis shown in the same magnification in Figure 3(b) givingevidence of amultitude of poremouths typical for AAOfilmslarger voids as a result of complete dissolution of IC particlesand embeddedparticles probablyoriginating fromchemicallytransformed ICs

Potentiostatic anodisation experiments using the capil-lary technique were conducted to get a deeper insight into thefilm formation and the behaviour of second-phase particlesin EN AW-7075 Four anodisation potentials were selectedas indicated in Figure 1 The corresponding current densityresponse as a function of the duration of anodisation isshown for anodising small spots under the micro-capillary(Figure 4) and for the first 600 s of the larger areas 10mm indiameter (Figure 5) Both experiments show a fairly similar

behaviour Differences in the absolute current density arenoticed only for both lower potentials which is attributed tothe influence of electrolyte alteration in the micro-capillaryset-up without a continuous electrolyte flow and stirringThe overall behaviour shows increasing current density withincreasing anodising potential Generally an initial currentdensity drop is followed by a plateau This behaviour isassociated with an uneven film growth in the initiation phasefollowed by a rather uniform thickening in the steady currentdensity regionCurioni et al [13] founda similarbutmore pro-nounced transient region between the initial current densitydrop and the plateau region for the alloy 2024-T3 above 3V(SCE)

The AAO films formed by potentiostatic anodisation areshown in Figure 6Their circular areas correspond to the cap-illary opening The spots show distinct interference colourswith regard to thickness of the formed films as expected fromthe variation of the applied charge transfer Naturally this isthe case for all studied alloys as well as for pure aluminiumAccording to Faradayrsquos law the theoretically achievable thick-ness of alumina films on pure aluminiumwas calculated firstSeveral methods were used to determine the actual thicknesson the alloy because themeasurement of submicrometre filmsat fairly small areas with inhomogeneous composition andvarying porosity is a nontrivial task An estimation basedon the interference colours was detailed by ellipsometry Theresults were proved by SEM imaging on FIB cross sectionsand confirmed by EDS analysis in planar view


500nm

(a)

500nm

(b)

Figure 3 (a) LV-STEM image of IC particles in the grain interior of ENAW-7075 T73 and (b) SE image of a corresponding AAO film formedby potentiodynamic anodising of this alloy in 04M sulphuric acid showing typical pore mouths voids caused by IC particle dissolution and(transformed) IC particles embedded in the film

100 200 300 400 500 6000Time (s)

10V (AgAgCl (3M KCl))

38V (AgAgCl (3M KCl))18V (AgAgCl (3M KCl))

03V (AgAgCl (3M KCl))0

1

2

3

4

Curr

ent d

ensit

y (m

Ac

m2)

Figure 4 Current density-time behaviour during potentiostaticanodising in 04M sulphuric acid on EN AW-7075 T73 using themicro-capillary technique to form circular oxide areas 800 120583m indiameter

The thickness of an anodic oxide film (119889) formed onpure aluminium can be estimated according to Faradayrsquos lawof electrolysis using the quantity of charge transfer at theelectrode (119876) the molar mass (119872) of the formed film thenumber of electrons transferred per ion (119899) the Faradayconstant (119865) and the film density (120588) Consider

119889 =119876119872

119899119865120588 (1)

Using the charge transfer according to the averaged currentdensities (Figure 4) and the anodisation time of 600 s119872 =0102 kgmol 119899 = 6 transferred electrons per reaction(2Al3++ 3O2minus rarr Al

2O3) and 120588 = 3940 kgm3 for bulk alu-

mina a film thickness 119889119865is calculated (Table 2 column 3)

100 200 300 400 500 6000Time (s)


38V (AgAgCl (3M KCl))18V (AgAgCl (3M KCl))03V (AgAgCl (3M KCl))

0

1

2

3

4Cu

rren

t den

sity

(mA

cm

2)

Figure 5 Current density-time behaviour during potentiostaticanodising in 04M sulphuric acid on EN AW-7075 T73 using theconventional technique to form circular areas 10mm in diameter

The figures obtained are a rough approximation Calculatedfor pure aluminium they represent the upper limit of aluminaformation on one side Otherwise the density of the aluminafilm is overestimated in particular due to the inherent poreformation in AAO films

The interference colours of the AAO films vary from tanto blue and green under broadband incident light (Figure 6)The refractive index of alumina (corundum) is around 18in the visible wavelength range which is between the cor-responding refractive indices of silicon dioxide (sim15) andsilicon nitride (sim20) Hence it is reasonable to refer to corre-sponding colourthickness charts for SiO

2and Si

3N4related

to microfabrication processes [26] Accordingly the tancolour indicates a thickness 119889

119862of around 50 nm the blue to

bluish colour corresponds to values between 100 and 200 nm


10V

38V

18V

03

V(a)

800120583m

(b)

Figure 6 Photograph of the circular areas formed by micro-capillary anodising of EN AW-7075 T73 in the potentiostatic regime in 04Msulphuric acid at 03 V 18 V 38 V and 10V for 600 s showing interference colours from tan to blue and green under broadband incident lightdue to their film thickness (a) and an example in SEM imaging (b)

Table 2 Film thickness 119889119865calculated by Faradayrsquos law (on pure Al) film thickness 119889

119888according to colour film thickness 119889

119864estimated by

ellipsometry film thickness119889SEM measured on cross sections (all on ENAW-7075T73)OAl ratio according to the filmsubstrate contributioncalculated using the EDS spectra andOAl ratio derived from EDS analysis as a function of the anodisation potential119880 (AgAgCl (3MKCl))

Film 119880 (V) 119889119865(nm) 119889

119888(nm) 119889

119864(nm) 119889SEM (nm) OAl ratio calculated OAl ratio measured

1 (Figure 7(a)) 03 150 sim50 49 plusmn 15 25 plusmn 15 005 008 plusmn 005

2 (Figure 7(c)) 18 205 sim100 92 plusmn 15 85 plusmn 20 046 045 plusmn 005

3 (Figure 7(e)) 38 275 sim200 165 plusmn 15 145 plusmn 40 112 102 plusmn 005

4 (Figure 7(g)) 100 745 sim500 599 plusmn 15 590 plusmn 50 mdash 173 plusmn 005

and the green colour approximates 500 nm Undoubtedly theassumption of a uniform refractive index for all AAO filmsregardless of the anodising potential is not reliable becauseof differences in composition and porosity influencing theiroptical properties This became apparent during the proce-dure of ellipsometry data analysis which iteratively convertsthe complex reflectance ratio as the primary data into thewavelength dependent optical constant and thickness In thevisible wave length region (ie 600 nm) the refractive indexrises from 165 for the thinnest film formed at 03 V to 19for the other films Correspondingly the mean film porosityrises from 43 for films formed between 03 V and 38V to49 for the film formed at 10V porosity gradients were notconsideredThe corresponding thickness 119889

119864derived from the

iterative procedure is shown in Table 2 column 5For verification cross sections located in the centre of

the anodised spots were prepared by FIB The achieved crosssections are shown in Figures 7(a) 7(c) 7(e) and 7(g) Itshould be noted that prior to ionmilling theAAOfilmswerecovered by a protection layer on top This layer is a tungstencoating which was deposited to avoid the influence of thehigh-energy 30 kVGa ions Additionally and for furthercharacterisation of the filmmicrostructure the surface of theAAO films is shown in Figures 7(b) 7(d) 7(f) and 7(h) TheSEM images of the cross sections reveal the thickness 119889SEM(Table 2 column 6) Differences between ellipsometry andSEM values are low and suspected to originate mainly from

optical anisotropy Nevertheless variations in the film thick-ness cannot be excluded because the cross sections preparedby FIB show only a limited part of the centre of the film

EDS analysis of a 500 times 500120583m2 large area in planar viewwas used to evaluate the homogeneity of the film thickness(Figure 8) An increasing oxygen signal in the EDS spectrawith increasing film thickness is expected because the exci-tation volume as the source of the characteristic of O K120572 andAl K120572X-rays includes the thin AAOfilms and the aluminiumalloy below as well Figure 8 shows to which extent film andsubstrate have to be considered in the case of the electronenergy of 5 keV which is for certain high enough to excite AlK120572 X-rays Considering the paths of 500 electrons using theMonte Carlo (MC) simulation method the mean excitationdepth is around 300 nm for aluminiumoxide and aluminiumas well The calculated depth is nearly equal for both thefilm and the substrate because of the opposing influences ofthe mean atomic number and the density of the materialsgiving a spherical excitation volume of 8120583m3Thismeans thatexcitation occurs completely in the film in the case of thethickest film formed at 10V The nominal OAl ratio ofstoichiometric alumina is 15 but the measured OAl ratioof this film is 173 plusmn 005 This is not unexpected since AAOfilms are known to contain additional OH-groups at the porewallsTheOAl ratio is reduced due to the rising contributionof the base material to the excitation volume in the case ofthe thinner films Basing on the film thickness derived from


AAO

500nm

(a)

2x

500nm

(b)

AAO

500nm

(c)

2x

500nm

(d)

AAO

500nm

(e)

2x

500nm

(f)

AAO

500nm

(g)

2x

500nm

(h)

Figure 7 Cross section (left) and surface (right) of AAO films on EN AW-7075 T73 after potentiostatic anodising in 04M sulphuric acid at(a b) 03 V (c d) 18 V (e f) 38 V and (g h) 10V the AAO films are marked in the cross sections to clearly distinguish the protective coatingon top (bars indicate 500 nm referring to identical magnification except for the twice enlarged inserts)

SEM imaging the expected OAl ratios are calculated andcompared to the measured values (Table 2) Apart from thethinnest film whose thickness estimation remains uncertaina satisfactory agreement in the limits of the accuracy ofthe method is stated This in turn verifies that the films oflocalised and conventional anodisation have a fairly uniformthickness and the values obtained from cross sections by SEMare representative for the entire film

The correlation between the transferred charge per unitarea and the film thickness on potentiostatic anodised ENAW-7075 T73 is shown in Figure 9The results of anodising amacroscopic area at 10V for durations of 1200 s and 3600 sare added to demonstrate that the results achieved by the

capillary technique are comparable to anodising in a thermo-statically controlled conventional process at room tempera-ture Macroscopic anodising at lower anodisation potentialsresulted in thin films comparable to the correspondingmicro-capillary AAO films However because of the longduration times needed for macroscopic anodisation at lowanodisation potentials preponderance of film redissolutionover film formation compromised the thickness evaluation

The main purpose of the cross section preparation wasthe thickness evaluation of the potentiostatic anodised filmsAn additional benefit is provided by revealing the behaviourof the IC particles in dependence of the applied potential forfilm formationwhich is shown exemplarily for the AAOfilms


1

3

Surface

Al substrate

2

4

500 paths

Al2O3 film

5kV electrons

500

400

300

200

(nm

)Ex

cita

tion

dept

hfil

m th

ickn

ess

(a)

O Al

Zn

1

2

3

4

09 1503E (keV)

(b)

Figure 8Monte Carlo simulated electron paths showing the excitation volume in the alumina films on aluminium (a) and the correspondingEDS spectra (referred to as numbered in Table 2) representing the dependence of O K120572 and Al K120572 intensities on film thickness (b)

0

1

2

3

4

Thic

knes

s (120583

m)

2 4 6 8 100

Anodising charge (As cmminus2)

Figure 9 Thickness of the AAO films on EN AW-7075 T73 afterpotentiostatic anodising in 04M sulphuric acid formed by themicro-capillary technique at 03 V 18 V 38 V and 10V (solid circles)for 600 s and by conventional anodising of circular oxide areas10mm in diameter at 10V for 1200 s and 3600 s (open circles) as afunction of the transferred charge per area unit

on EN AW-7075 T73 For further characterisation of the filmmicrostructure the surface of the AAO films is shown inFigures 7(b) 7(d) 7(f) and 7(h)

The AAO film formed at 03 V (Figure 7(a)) is difficult tonotice because of its low thickness The dominating feature isa large iron-rich IC particle (most likely Al

3Fe or Al

7Cu2Fe)

in the alloywhich shows a bright layerwith a thickness similarto the AAO film thicknessThe SE image of the surface showsshallow pores (Figure 7(b))

As expected the AAO film formed at 18 V (Figure 7(c))is thicker and shows some voids MgZn

2particles in the alloy

are supposed to decorate the grain boundaries correspondingto the appearance in Figure 2(a) Clearly visible is a largerparticle located at a high-angle grain boundary near a triplejunction in the right of the figure At the site (marked by anarrow) where the corresponding vertical grain boundary cutsthe surface the AAO film formation proceeded concurrentlywith the chemical dissolution of the MgZn

2phase Similar

particle dissolution that occurred simultaneously with theprogress of AAO formation is observed in two other positionsin the figure At such locations the film starts to becomeuneven around the developed voids Similarly those voids arealso observed at the film surface together with a multitude ofpore mouths (Figure 7(d))

The AAO film formed at 38 V (Figure 7(e)) shows pro-gressing film thickening and dissolution of a former largerod-like particle in the centre of the image Interestingly notonly enclosed the segment of the former particle now by theformedAAOfilm but also the other half is already completelydissolved Varying chemical behaviour of diverse IC particlescan be observed Notably some globular second-phase par-ticles located at the film formation front and near the rod-like void appear unaltered They will become embedded intheAAO layer supposedly in unchanged or transformed stateif the film formation would proceed The surface of the filmreveals evidence of such embedded particles additionally topore mouths and voids (Figure 7(f)) The pore number isslightly reduced

The AAO film formed at 10V (Figure 7(g)) shows allmorphological features known from conversion layers of thestudied alloy Apart from voids and transformed second-phase particles irregularities of the pore morphology canbe noticed A pronounced uneven front of film growth hasbeen developed due to the fast dissolution of second-phase


Table 3 IC particles in Al alloys and their behaviour during sulphuric acid anodisation

Phase Evidence (this work) Behaviour during sulphuric acid anodisation (published)Mg2Si (120573) Verified Dissolution cavity formation [17]

MgZn2(120578 M 120590) Verified Passivation above 5V (SCE) [25]

Mg32Zn49(T1015840) Verified Partial dissolution partial oxidation [17]

Al2CuMg (S) Verified Preferential oxidation above 0V (SCE) [12]

Al6Mn Partial dissolution embedment [17]

Al-2Cu (120572) Yellow coating enhanced porosity [17] copper enrichment at interface below 3V (SCE) [9 13]Al2Cu (120579) Fast oxidation [17] preferential oxidation above 18 V (SCE) [12]

Al7Cu2Fe Verified Formation of Cu sponge [25] preferential oxidation above 18 V (SCE) [12]

Al3Fe (120573) Partial dissolution embedment [17] preferential oxidation above 22 V [12]

AlFeSi phases (120572) Partial dissolution partial oxidation [17]

particles at the first contact with the electrolyte at the wavyinterface Branching of pores has proceeded with growingfilm thickness Pore mouths at the surface are remarkablyreduced in number and enhanced in diameter (Figure 7(h))

4 Discussion

The pronounced peaks in the polarisation curve duringpotentiodynamic anodising (Figure 1) are associated withthe oxidation and dissolution of IC particles in the alu-minium alloys Conventional ldquomacro-areardquo anodising studiesby Saenz de Miera et al [12] and Curioni et al [13] revealsimilar potentiodynamic behaviour This can be stated forall studied alloys but is particularly obvious in the case ofEN AW-2024 According to [13] the striking peak around0V (SCE) is expected for EN AW-2024 and EN AW-7075attributed to the S-phase (Al

2CuMg) with the peak intensity

indicating the higher concentration in EN AW-2024Similarly shown for high purity aluminium in conven-

tional experiments [12] no peaks appear in the curves ofthe micro-capillary cell experiment of Al 5N and EN AW-1050 due to the absence of copper-containing second-phaseparticles The thickening of the natural oxide on aluminiumstarts around 035V (SCE) [12] and the following region ofmonotone current rise is connected with further oxide for-mation and film thickening

According to the results of Keller et al [6] and OrsquoSullivanand Wood [27] the thickness of the barrier layer and thediameter of the pores theoretically extend from 05 to 12 nmwith rising anodising potential from 03 to 10V Correspond-ingly the cell diameter enhances from 06 to 22 nm Althoughthese findings relate to pure aluminium a qualitatively simi-lar behaviour is observed for the pore and cell dimensions ofthe alloy ENAW-7075 T73 (Figures 7(b) 7(d) 7(f) and 7(h))Corresponding to the pore and cell sizes in pure aluminiumthe calculated pore volume decreases from 55 to 23 [27]In practice pore volume is generally greater than calculatedbecause dissolution occurs at inner and outer surfaces ofthe coating during the time of anodisation Additionally theporosity in alloys is enhanced by voids formed due to the ICparticle dissolutionTherefore considering the rising quantityof IC particles which underwent dissolution and leave addi-tional voids in the film porosity values between 43 and 49

derived from the ellipsometry study are sound and supportthe thickness results of ellipsometry

The correlation between the transferred charge and thefilm thickness is similar comparing the results of micro-capillary anodising in a stagnant electrolyte and of macro-scopic anodising in a thermostatically controlled and agitatedelectrolyte (Figure 9) A rise in temperature during themicro-capillary process might be expected due to the heatreleased above the spots inside the capillary However thetransferred charge remains very low because of the small areato be anodised and the small duration (600 s) consequentlyneeded Nevertheless parameter control by temperaturestabilisation and composition balance of the electrolyte hasto be kept in view for further development

The anodising behaviour of second-phase particles wasreviewed by Jelinek [17] Recent detailed studies were pub-lished by Paez et al [9] Saenz de Miera et al [12 14] Curioniet al [13] and Tardelli and Rocca [25] The phases withrelevance to the studied alloys are listed in Table 3 Theirverification by EDS on electron transparent samples of thealloy we focused on in this work is also indicated in the tableAlthough the micro-capillary cell technique has been appliedfor corrosion studies either on synthesised analogues ofintermetallic compounds by Birbilis and Buchheit [22] mod-elling the second phases present in aluminium alloys or ona 7075 alloy in solution heat-treated condition and in T6temper by Andreatta et al [21] the applicability for localanodising was missing even after several modifications inthe technical equipment As presented for corrosion studies[21 22] single precipitates could not be examined but theirgalvanic coupling revealed changes from anodic to cathodicbehaviour with respect to the temper state of the alloymatrixComparing our local results of anodising with the dissolutionbehaviour in chloride containing medium by Cavanaugh etal [28] and Wloka and Virtanen [29] for example similarbehaviour can be stated Cavanaugh et al found anodicbehaviour of the phases MgZn

2and Mg

2Si and cathodic

behaviour of the phases Al7Cu2Fe Al

3Fe and Al

2CuMg with

respect to the matrix of AA7075 T6Wloka and Virtanen [29] confirmed anodic behaviour of

MgZn2and cathodic behaviour of120572-Al

7Cu2Fewith respect to

thematrix and evidenced thatMgZn2is first dissolved due the

high dissolution rate Mg2Si undergoes a selective dissolution


100nm

(a)

250nm

(b)

250nm

(c)

Figure 10 Comparison of a (a) coarsened MgZn2particle (120578-phase) surrounded by Mg

2Si or Mg

32Zn49

particles in the alloy EN AW-7075 with the corresponding appearance in the AAO film on that alloy after anodisation at 38 V showing (b) dissolution of MgZn

2and

(c) transformation of Mg2Si to SiO

119909

of Mg leaving SiO119909particles behind oxygen reduction takes

place at the cathodic Al7Cu2Fe phase with pitting corrosion

of the matrix The behaviour of the second phases duringanodisation in sulphuric acid shows similarities Tardelli andRocca [25] anodised model phases in sulphuric acid andevidenced selective dissolution of Al

7Cu2Fe accompanied by

formation of a nanometric sponge of copper at the surfaceBulk MgZn

2formed an oxidic passivation layer above 5V

(SCE) with a high redissolution rate Saenz deMiera et al [1214] prepared intermetallic compound layers as macroscopicmodel specimens The oxidation of the S-phase (Al

2CuMg)

was found to start around 0V (SCE) with the magnesiumhindering the formation of a stable oxide Oxidation of cop-per and iron containing particles like Al

7Cu2Fe started above

2V (SCE) but at reduced rate Depending on their initialsize corresponding particles were suspected to be partiallyor completely oxidised supporting a relatively stable oxide[12] We found a thin metallic film formation on the largeconstituent phase (apparently Al

7Cu2Fe) after anodisation at

03 V which is correlated to the selective dissolution foundby Tardelli and Rocca [25] We observed fast and completedissolution of MgZn

2particles resulting in voids of corre-

sponding shape in agreement with Tardelli and Rocca [25]and Cavanaugh et al [28] Transformed particles embeddedin the AAO film are supposed to be mainly SiO

119909as a result of

selective Mg dissolution of Mg2Si particles [17 22 28]

To illustrate thisMgZn2particles coarsened by overaging

and surrounded byMg2Si orMg

32Zn49particles were studied

before and after anodisation by TEM and SEM imagingrespectively (Figure 10)The behaviour of the IC particles andtheir influence on the morphology of the anodised film isshown on the cross section after FIB milling (Figures 10(b)and 10(c)) Because of the combination of milling and imag-ing the SE image quality is influenced by the compromisesconcerning the sample position with respect to the objectivelens and moreover limited due to the beam sensitivity of theAAO film and the residual curtaining left after preparationHowever in contrast to metallographic preparation of cross


sections ion beam milling provides more detail concerningembedded particles inside of voids Such particles regularlydisappear during the grinding and polishing processes forsample preparation as can be seen in Figure 2(d) The TEMimage in Figure 10(a) shows a coarse MgZn

2particle with

surrounding Mg2Si or Mg

32Zn49

particles in the alloy Theshape of a corresponding void in the oxide film after particledissolution is evidenced in Figure 10(b) a magnified detailof Figure 7(e) The void is surrounded by relatively stableparticles which seem to get anodised or transformed andembedded in the film (Figure 10(c)) This appears to be fairlysimilar to the behaviour of the IC particles observed undercorrosive attack [28]

Themicroscopic study of the cross sections combined thestudy of the oxidemorphology with themicrostructure of thealloy below the formed AAOfilm As a result of the presentedwork above 18 V the dissolution of the hardening phase inENAW-7075 T73 during anodisation produces a coatingwithnumerous voids of the size of the IC particles and moreoverwith local thickness variations due to the dissolution of thesemicrostructural constituents at the anodisation frontier

One advantage of the used method is that electrochem-istry and microstructure of the IC particles contained in thealloy and transformed by anodising can be studied corre-spondingly Unfortunately the micro-capillary method can-not offer the possibility to size down the anodised area to thesize of single secondary particles Using a potential minimalcapillary diameter around 15 120583m would at least enable thestudy of small areas with different phases side by side Thesmaller the diameter of the capillary the more necessary acontinuous exchange of the electrolyte to avoid alteration andto control temperatureThis requires a continuous electrolyteflow in the micro-capillary cell as already presented for amulti-scanning droplet cell presented in [30] used as examplefor anodising titanium between 11 and 18V

5 Conclusions

The successful development of electrolyte formulations foranodising of aluminium alloys and aluminium matrix com-posites requiresmeasurement of electrochemical parametersthickness monitoring and visualising the formation of thefilm and the transformation of the containing IC particlesUsing conventional anodising for this task necessitates theapplication of large electrolyte volumes in the scale of severallitres with the drawback of high expenses for procurementand disposal

The micro-capillary technique provides an advantageousapproach for a more efficient investigation of the anodisingprocess The influence of the electrolyte composition andanodisation regime on the electrochemical response of alloyswith different composition and condition can be investigatedby applying low electrolyte volumes and short anodisationdurations at small areas Methods of film evaluation (thick-ness microstructure and morphology) were adapted to theanodised spots of limited thickness (ie below 1120583m) and area(ie 05mm2) This was accomplished by using ellipsometryenergy dispersiveX-ray analysis and preparation of cross sec-tions by focused ion beammilling and exemplarily shown for

AlZn55MgCuThus the method provides an efficient tool tostudy different electrolyte formulationsAdditionally insightsinto film formation and transformation of IC particles areprovided by the applied techniques and can be extended tostudy particle strengthened aluminium matrix composites

A comparison of the behaviour of the studied alloy duringconventional anodising with a large circulated electrolytevolume at controlled temperature demonstrated the generalutility and the limitations of themicro-capillarymethod Fur-ther work will include the improvement of the microcell set-up In particular a continuous electrolyte flow should beprovided to avoid electrolyte alteration and heating

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

Financial support was granted by Deutsche Forschungsge-meinschaft (German Research Foundation) in the collab-orative research centre ldquoHigh-Strength Aluminium-BasedLightweight Materials for Safety Componentsrdquo (DFG SFB692) and in the cluster of excellence ldquoMERGE Technolo-gies for Multifunctional Lightweight Structuresrdquo (DFG EXC1075) The authors wish to thank Peter Richter and ProfessorD R T Zahn Semiconductor Physics TU Chemnitz forproviding ellipsometry measurements and analyses TEMpreparation and study of the alloy were performed by AnneSchulzeMaterials EngineeringGroup TUChemnitzThanksto Dagobert Spieler Materials and Surface EngineeringGroup TU Chemnitz comprehensive electrochemical mea-surements careful anodisation and competent service of themicrocell equipment have been accomplished

References

[1] R Ayer J Y Koo J W Steeds and B K Park ldquoMicroanalyticalstudy of the heterogeneous phases in commercial Al-Zn-Mg-Cu alloysrdquoMetallurgical Transactions A vol 16 no 11 pp 1925ndash1936 1985

[2] S T Lim I S Eun and S W Nam ldquoControl of equilibriumphases (MTS) in the modified aluminum alloy 7175 for thickforging applicationsrdquo Materials Transactions vol 44 no 1 pp181ndash187 2003

[3] D J Lloyd and M C Chaturvedi ldquoA calorimetric study ofaluminium alloy AA-7075rdquo Journal of Materials Science vol 17no 6 pp 1819ndash1824 1982

[4] J K Park and A J Ardell ldquoMicrostructures of the commercial7075 Al alloy in the T651 and T7 tempersrdquo MetallurgicalTransactions A vol 14 no 10 pp 1957ndash1965 1983

[5] PG Sheasby andR PinnerTheSurface Treatment and Finishingof Aluminum and Its Alloys vol 2 ASM International MaterialsPark Ohio USA 2001

[6] F Keller M S Hunter and D L Robinson ldquoStructural featuresof oxide coatings on aluminumrdquo Journal of the ElectrochemicalSociety vol 100 no 9 pp 411ndash419 1953


[7] G E Thompson H Habazaki K Shimizu et al ldquoAnodizing ofaluminium alloysrdquo Aircraft Engineering and Aerospace Technol-ogy vol 71 no 3 pp 228ndash238 1999

[8] D Nickel T Lampke G Alisch and S Steinhauser ldquoMicro-structural effect on the wear behaviour of the hard-anodisedaluminium alloys ENAW-6082 and ENAW-7075rdquoMaterialwis-senschaft und Werkstofftechnik vol 40 no 7 pp 523ndash531 2009

[9] M A Paez O Bustos G EThompson P Skeldon K Shimizuand G CWood ldquoPorous anodic film formation on an Al-35 wt Cu alloyrdquo Journal of the Electrochemical Society vol 147 no3 pp 1015ndash1020 2000

[10] S Feliu Jr M J Bartolome J A Gonzalez V Lopez and SFeliu ldquoPassivating oxide film and growing characteristics ofanodic coatings on aluminium alloysrdquo Applied Surface Sciencevol 254 no 9 pp 2755ndash2762 2008

[11] Y-S Huang T-S Shih and J-H Chou ldquoElectrochemicalbehavior of anodized AA7075-T73 alloys as affected by thematrix structurerdquoApplied Surface Science vol 283 pp 249ndash2572013

[12] M Saenz deMieraMCurioni P Skeldon andG EThompsonldquoModelling the anodizing behaviour of aluminium alloys insulphuric acid through alloy analoguesrdquo Corrosion Science vol50 no 12 pp 3410ndash3415 2008

[13] M Curioni M Saenz de Miera P Skeldon G E Thompsonand J Ferguson ldquoMacroscopic and local filming behavior ofAA2024 T3 aluminum alloy during anodizing in sulfuric acidelectrolyterdquo Journal of the Electrochemical Society vol 155 no8 pp C387ndashC395 2008

[14] M Saenz deMieraMCurioni P Skeldon andG EThompsonldquoThe behaviour of second phase particles during anodizing ofaluminium alloysrdquo Corrosion Science vol 52 no 7 pp 2489ndash2497 2010

[15] D Dietrich S Nehrkorn M Handel et al ldquoA hardnessndashmicro-structure correlation study of anodised powder-metallurgicalAlndashCu alloy compositesrdquo Surface and Coatings Technology vol242 pp 118ndash124 2014

[16] RMorgensternDNickel DDietrich I Scharf andT LampkeldquoAnodic oxidation of AMCs influence of process parameters oncoating formationrdquo Materials Science Forum vol 825-826 pp636ndash644 2015

[17] TW JelinekOberflachenbehandlung von Aluminium Eugen GLeuze Saulgau Germany 1997

[18] M Curioni T Gionfini A Vicenzo P Skeldon and G EThompson ldquoOptimization of anodizing cycles for enhancedperformancerdquo Surface and Interface Analysis vol 45 no 10 pp1485ndash1489 2013

[19] H Bohni T Suter and A Schreyer ldquoMicro- and nanotech-niques to study localized corrosionrdquo Electrochimica Acta vol40 no 10 pp 1361ndash1368 1995

[20] M M Lohrengel ldquoElectrochemical capillary cellsrdquo CorrosionEngineering Science and Technology vol 39 no 1 pp 53ndash582004

[21] F Andreatta M M Lohrengel H Terryn and J H W deWit ldquoElectrochemical characterisation of aluminium AA7075-T6 and solution heat treated AA7075 using a micro-capillarycellrdquo Electrochimica Acta vol 48 no 20-22 pp 3239ndash32472003

[22] N Birbilis and R G Buchheit ldquoElectrochemical characteristicsof intermetallic phases in aluminum alloysmdashan experimentalsurvey and discussionrdquo Journal of the Electrochemical Societyvol 152 no 4 pp B140ndashB151 2005

[23] T Suter and H Bohni ldquoA newmicroelectrochemical method tostudy pit initiation on stainless steelsrdquo Electrochimica Acta vol42 no 20ndash22 pp 3275ndash3280 1997

[24] J P Kollender M Voith S Schneiderbauer A I Mardareand A W Hassel ldquoHighly customisable scanning droplet cellmicroscopes using 3D-printingrdquo Journal of ElectroanalyticalChemistry vol 740 pp 53ndash60 2015

[25] J Tardelli and E Rocca ldquoElectrochemical behaviour of inter-metallic phases in sulfuric acid at high voltagemdashanodizationof Al

7Cu2Fe and MgZn

2phaserdquo httpsitespoliuspbrorg

emcr2012CDPDFOral2026 Roccapdf[26] J Henrie S Kellis S M Schultz and A Hawkins ldquoElectronic

color charts for dielectric films on siliconrdquo Optics Express vol12 no 7 pp 1464ndash1469 2004

[27] J P OrsquoSullivan and G C Wood ldquoThe morphology and mecha-nism of formation of porous anodic films on aluminiumrdquo Pro-ceedings of the Royal Society of London Series A Mathematicaland Physical Sciences vol 317 no 1 pp 511ndash543 1970

[28] MK Cavanaugh R G Buchheit andN Birbilis ldquoEvaluation ofa simple microstructural-electrochemical model for corrosiondamage accumulation in microstructurally complex aluminumalloysrdquo Engineering Fracture Mechanics vol 76 no 5 pp 641ndash650 2009

[29] J Wloka and S Virtanen ldquoDetection of nanoscale 120578-MgZn2phase dissolution from an Al-Zn-Mg-Cu alloy by electrochem-ical microtransientsrdquo Surface and Interface Analysis vol 40 no8 pp 1219ndash1225 2008

[30] J P Kollender A IMardare andAWHassel ldquoMulti-ScanningDroplet Cell Microscopy (multi-SDCM) for truly parallel highthroughput electrochemical experimentationrdquo ElectrochimicaActa vol 179 pp 32ndash37 2015

Submit your manuscripts athttpwwwhindawicom

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CorrosionInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Polymer ScienceInternational Journal of



CeramicsJournal of


CompositesJournal of

NanoparticlesJournal of



International Journal of

Biomaterials


NanoscienceJournal of

TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Journal of

NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

CrystallographyJournal of


The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014


CoatingsJournal of

Advances in

Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Smart Materials Research



MetallurgyJournal of


BioMed Research International

MaterialsJournal of


Nano

materials


Journal ofNanomaterials


Table 1 Chemical composition of the studied materials according to the specification of the producer

Alloy Si (wt) Fe (wt) Cu (wt) Mn (wt) Mg (wt) Cr (wt) Zn (wt) Ti (wt) AlAl 5N 000079 000098 lt00002 000004 000103 0000059 0 0000098 Bal1050 lt025 lt04 lt005 lt005 0ndash005 lt003 lt007 lt005 Bal2024 050 050 35ndash49 03ndash09 12ndash18 010 025 015 Bal7075 040 050 12ndash20 030 21ndash29 018ndash028 51ndash61 020 Bal

Al2Mg3Zn3andAl

2CuMg are known to affect hotworkability

and fracture properties Compared to T561 temper 1205781015840 phaseand 120578 phase are coarsened by artificial overaging in temperT73 which is beneficial to increase the resistance to stress-corrosion cracking [2ndash4]

Automotive and aerospace applications require enhancedcorrosion resistance improved tribological properties ora simply improved decorative appearance This is usuallyaccomplished by the formation of an anodised aluminiumoxide (AAO) due to the conversion of the surface region ofthe aluminium alloy [5] AAO films on pure aluminium arefairly homogeneous showing hexagonal arranged pores per-pendicular to the surface which results from the dynamicequilibrium between film growth at the metaloxide interfaceandfield-assisted dissolution at the pore bases [6] In contrastthe microstructure and the resulting properties of AAOfilms formed on aluminium alloys are severely influenceddue to the conversion of solid solutions and the behaviourof second phases during anodisation [7 8] In particularanodising of copper-containing alloys is remarkably limiteddue to inefficient film growth and enhanced porosity of theproduced AAO film [9 10] Dissolution of second-phase par-ticles results in voids and gas-pockets [11 12] Second-phaseparticles with low oxidation rates can be incorporated intothe film because of the faster oxidation of the surroundingmatrix [12ndash14]This observation was similarly made for AAOfilms formed on aluminium matrix composites (AMC) withalumina or silicon carbide particles [15 16] Anodic oxidationrates of a variety of ICs have been examined in relation to thatof thematrix [17] Recent studies considered the detailed pro-cesses occurring at ICs by simultaneously recording the ele-ctrochemical data Film formation was performed by eitherpotentiostatic (constant potential) galvanostatic (constantcurrent density) or potentiodynamic anodisation (increasingpotential with defined rate) The current density responseduring potentiodynamic anodisation and the time-voltageresponse during galvanostatic anodisation were discussedwith regard to several commercial alloys different elec-trolytes and occurring growth stages during AAO film for-mation [12ndash14] Potentiodynamic anodising was proposed asan efficient tool for the development of appropriate anodisingparameters since it provides a rapid characterisation of thecomplex electrolytealloy behaviour over a wide potentialrange [18] Such studies comprise the application of newlydesigned electrolyte formulations for anodising of aluminiumalloys and aluminiummatrix composites Nevertheless usingconventional anodising for this task necessitates the applica-tion of large electrolyte volumes in the scale of several litreswith the disadvantage of high expenses for procurement anddisposal

The aim of this work is to transfer the micro-capillarytechnique known from local corrosion research [19ndash23] tolocal anodising studies Recently this technique was sophisti-cated by developing multichannel arrangements for improv-ing scanning droplet cell microscopy [24] We applied themicro-capillary technique onpure aluminiumand three com-mercial aluminium alloys Sulphuric acid was chosen as acommon electrolyte to provide the comparison to conven-tional anodising and to demonstrate the feasibility of the tech-nique The simultaneous electrochemical characterisationduring initiation and progress of the film growth has beenstudied and correlated with microstructural features in par-ticular with the behaviour of second-phase particles and thedevelopment of the film thickness on the example of thehigh-strength and stress-corrosion resistant alloy EN AW-7075 in T73 condition that is artificially aged for providingstress-corrosion resistance Film thickness measurement wasadapted to thin and small AAO areas Finally a discussion onpotentials and limitations of the presented method is given

2 Materials and Methods

Pure aluminium (Al 5N Hydro Aluminium Rolled ProductsGmbH) and three commercial aluminium alloys in extrudedcondition and the given temper (EN AW-1050 EN AW 2024T4 EN AW-7075 T651 and T73 for chemical compositionsee Table 1) were used Samples were etched in 3wt sodiumhydroxide (NaOH) for 60 s at 50∘C and desmutted in 30 volnitric acid (HNO

3) for 30 s at room temperature After ano-

dising the surface was rinsed carefully with deionized waterand ethanol

The samples were anodised in 04M sulphuric acid(H2SO4) at room temperature (22 plusmn 05) ∘C using a micro

corrosion cell (Swiss Microcell System) based on a glass cap-illary technique Parts of the three-electrode set-up are aglass capillary with a ground tip of 800 120583m in diameter anda seal of a silicone rubber layer to prevent leakage actingas anodising electrode a 05mm platinum wire acting ascounter electrode and a silversilver chloride (3MKCl) elec-trode acting as reference electrode An exact positioning ofthe capillary was permitted by mounting the electrode set-upat the nosepiece of an optical microscope (OM) and fixingthe samples at the stage of the OM A detailed descriptionof the microcell system has been published elsewhere [23][BOH95] Each test was performed at least twice Anodisingwas accompanied by micro-electrochemical measurementsusing a high precision voltage source (DIGISTANT Model4462 Burster Prazisionsmesstechnik) a potentiostat (IMP83PCT-BC Jaissle Elektronik) and a digital multimeter (Model







0

1

2

3

4

5

Curr

ent d

ensit

y (m

Ac

m2)

0 1 2 3 4 5 6 7 8 9 10minus1


EN AW-1050EN AW-2024 T4

Al 5N EN AW-7075 T73EN AW-7075 T651





5120583m

lowast

(a)

AAO film

5120583mlowast

(b)

5120583m

lowast

(c)

500nm

(d)






2 Mg32Zn49 Mg(AlCu)

Al2CuMg and Al







500nm

(a)

500nm

(b)


100 200 300 400 500 6000Time (s)




1

2

3

4

Curr

ent d

ensit

y (m

Ac

m2)



119889 =119876119872

119899119865120588 (1)




100 200 300 400 500 6000Time (s)



0

1

2

3

4Cu

rren

t den

sity

(mA

cm

2)




2and Si

3N4related





10V

38V

18V

03

V(a)

800120583m

(b)




119864estimated by


Film 119880 (V) 119889119865(nm) 119889

119888(nm) 119889













AAO

500nm

(a)

2x

500nm

(b)

AAO

500nm

(c)

2x

500nm

(d)

AAO

500nm

(e)

2x

500nm

(f)

AAO

500nm

(g)

2x

500nm

(h)







1

3

Surface

Al substrate

2

4

500 paths

Al2O3 film

5kV electrons

500

400

300

200

(nm

)Ex

cita

tion

dept

hfil

m th

ickn

ess

(a)

O Al

Zn

1

2

3

4

09 1503E (keV)

(b)


0

1

2

3

4

Thic

knes

s (120583

m)

2 4 6 8 100





3Fe or Al

7Cu2Fe)





2phase Similar
















4 Discussion









2and Mg

2Si and cathodic


3Fe and Al

2CuMg with







100nm

(a)

250nm

(b)

250nm

(c)


2Si or Mg

32Zn49


2and


119909








2CuMg)
















2particle with


32Zn49




5 Conclusions







Acknowledgments


References



























7Cu2Fe and MgZn















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials









0

1

2

3

4

5

Curr

ent d

ensit

y (m

Ac

m2)

0 1 2 3 4 5 6 7 8 9 10minus1


EN AW-1050EN AW-2024 T4

Al 5N EN AW-7075 T73EN AW-7075 T651





5120583m

lowast

(a)

AAO film

5120583mlowast

(b)

5120583m

lowast

(c)

500nm

(d)






2 Mg32Zn49 Mg(AlCu)

Al2CuMg and Al







500nm

(a)

500nm

(b)


100 200 300 400 500 6000Time (s)




1

2

3

4

Curr

ent d

ensit

y (m

Ac

m2)



119889 =119876119872

119899119865120588 (1)




100 200 300 400 500 6000Time (s)



0

1

2

3

4Cu

rren

t den

sity

(mA

cm

2)




2and Si

3N4related





10V

38V

18V

03

V(a)

800120583m

(b)




119864estimated by


Film 119880 (V) 119889119865(nm) 119889

119888(nm) 119889













AAO

500nm

(a)

2x

500nm

(b)

AAO

500nm

(c)

2x

500nm

(d)

AAO

500nm

(e)

2x

500nm

(f)

AAO

500nm

(g)

2x

500nm

(h)







1

3

Surface

Al substrate

2

4

500 paths

Al2O3 film

5kV electrons

500

400

300

200

(nm

)Ex

cita

tion

dept

hfil

m th

ickn

ess

(a)

O Al

Zn

1

2

3

4

09 1503E (keV)

(b)


0

1

2

3

4

Thic

knes

s (120583

m)

2 4 6 8 100





3Fe or Al

7Cu2Fe)





2phase Similar
















4 Discussion









2and Mg

2Si and cathodic


3Fe and Al

2CuMg with







100nm

(a)

250nm

(b)

250nm

(c)


2Si or Mg

32Zn49


2and


119909








2CuMg)
















2particle with


32Zn49




5 Conclusions







Acknowledgments


References



























7Cu2Fe and MgZn















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




5120583m

lowast

(a)

AAO film

5120583mlowast

(b)

5120583m

lowast

(c)

500nm

(d)






2 Mg32Zn49 Mg(AlCu)

Al2CuMg and Al







500nm

(a)

500nm

(b)


100 200 300 400 500 6000Time (s)




1

2

3

4

Curr

ent d

ensit

y (m

Ac

m2)



119889 =119876119872

119899119865120588 (1)




100 200 300 400 500 6000Time (s)



0

1

2

3

4Cu

rren

t den

sity

(mA

cm

2)




2and Si

3N4related





10V

38V

18V

03

V(a)

800120583m

(b)




119864estimated by


Film 119880 (V) 119889119865(nm) 119889

119888(nm) 119889













AAO

500nm

(a)

2x

500nm

(b)

AAO

500nm

(c)

2x

500nm

(d)

AAO

500nm

(e)

2x

500nm

(f)

AAO

500nm

(g)

2x

500nm

(h)







1

3

Surface

Al substrate

2

4

500 paths

Al2O3 film

5kV electrons

500

400

300

200

(nm

)Ex

cita

tion

dept

hfil

m th

ickn

ess

(a)

O Al

Zn

1

2

3

4

09 1503E (keV)

(b)


0

1

2

3

4

Thic

knes

s (120583

m)

2 4 6 8 100





3Fe or Al

7Cu2Fe)





2phase Similar
















4 Discussion









2and Mg

2Si and cathodic


3Fe and Al

2CuMg with







100nm

(a)

250nm

(b)

250nm

(c)


2Si or Mg

32Zn49


2and


119909








2CuMg)
















2particle with


32Zn49




5 Conclusions







Acknowledgments


References



























7Cu2Fe and MgZn















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




500nm

(a)

500nm

(b)


100 200 300 400 500 6000Time (s)




1

2

3

4

Curr

ent d

ensit

y (m

Ac

m2)



119889 =119876119872

119899119865120588 (1)




100 200 300 400 500 6000Time (s)



0

1

2

3

4Cu

rren

t den

sity

(mA

cm

2)




2and Si

3N4related





10V

38V

18V

03

V(a)

800120583m

(b)




119864estimated by


Film 119880 (V) 119889119865(nm) 119889

119888(nm) 119889













AAO

500nm

(a)

2x

500nm

(b)

AAO

500nm

(c)

2x

500nm

(d)

AAO

500nm

(e)

2x

500nm

(f)

AAO

500nm

(g)

2x

500nm

(h)







1

3

Surface

Al substrate

2

4

500 paths

Al2O3 film

5kV electrons

500

400

300

200

(nm

)Ex

cita

tion

dept

hfil

m th

ickn

ess

(a)

O Al

Zn

1

2

3

4

09 1503E (keV)

(b)


0

1

2

3

4

Thic

knes

s (120583

m)

2 4 6 8 100





3Fe or Al

7Cu2Fe)





2phase Similar
















4 Discussion









2and Mg

2Si and cathodic


3Fe and Al

2CuMg with







100nm

(a)

250nm

(b)

250nm

(c)


2Si or Mg

32Zn49


2and


119909








2CuMg)
















2particle with


32Zn49




5 Conclusions







Acknowledgments


References



























7Cu2Fe and MgZn















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




10V

38V

18V

03

V(a)

800120583m

(b)




119864estimated by


Film 119880 (V) 119889119865(nm) 119889

119888(nm) 119889













AAO

500nm

(a)

2x

500nm

(b)

AAO

500nm

(c)

2x

500nm

(d)

AAO

500nm

(e)

2x

500nm

(f)

AAO

500nm

(g)

2x

500nm

(h)







1

3

Surface

Al substrate

2

4

500 paths

Al2O3 film

5kV electrons

500

400

300

200

(nm

)Ex

cita

tion

dept

hfil

m th

ickn

ess

(a)

O Al

Zn

1

2

3

4

09 1503E (keV)

(b)


0

1

2

3

4

Thic

knes

s (120583

m)

2 4 6 8 100





3Fe or Al

7Cu2Fe)





2phase Similar
















4 Discussion









2and Mg

2Si and cathodic


3Fe and Al

2CuMg with







100nm

(a)

250nm

(b)

250nm

(c)


2Si or Mg

32Zn49


2and


119909








2CuMg)
















2particle with


32Zn49




5 Conclusions







Acknowledgments


References



























7Cu2Fe and MgZn















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




AAO

500nm

(a)

2x

500nm

(b)

AAO

500nm

(c)

2x

500nm

(d)

AAO

500nm

(e)

2x

500nm

(f)

AAO

500nm

(g)

2x

500nm

(h)







1

3

Surface

Al substrate

2

4

500 paths

Al2O3 film

5kV electrons

500

400

300

200

(nm

)Ex

cita

tion

dept

hfil

m th

ickn

ess

(a)

O Al

Zn

1

2

3

4

09 1503E (keV)

(b)


0

1

2

3

4

Thic

knes

s (120583

m)

2 4 6 8 100





3Fe or Al

7Cu2Fe)





2phase Similar
















4 Discussion









2and Mg

2Si and cathodic


3Fe and Al

2CuMg with







100nm

(a)

250nm

(b)

250nm

(c)


2Si or Mg

32Zn49


2and


119909








2CuMg)
















2particle with


32Zn49




5 Conclusions







Acknowledgments


References



























7Cu2Fe and MgZn















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




1

3

Surface

Al substrate

2

4

500 paths

Al2O3 film

5kV electrons

500

400

300

200

(nm

)Ex

cita

tion

dept

hfil

m th

ickn

ess

(a)

O Al

Zn

1

2

3

4

09 1503E (keV)

(b)


0

1

2

3

4

Thic

knes

s (120583

m)

2 4 6 8 100





3Fe or Al

7Cu2Fe)





2phase Similar
















4 Discussion









2and Mg

2Si and cathodic


3Fe and Al

2CuMg with







100nm

(a)

250nm

(b)

250nm

(c)


2Si or Mg

32Zn49


2and


119909








2CuMg)
















2particle with


32Zn49




5 Conclusions







Acknowledgments


References



























7Cu2Fe and MgZn















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials















4 Discussion









2and Mg

2Si and cathodic


3Fe and Al

2CuMg with







100nm

(a)

250nm

(b)

250nm

(c)


2Si or Mg

32Zn49


2and


119909








2CuMg)
















2particle with


32Zn49




5 Conclusions







Acknowledgments


References



























7Cu2Fe and MgZn















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




100nm

(a)

250nm

(b)

250nm

(c)


2Si or Mg

32Zn49


2and


119909








2CuMg)
















2particle with


32Zn49




5 Conclusions







Acknowledgments


References



























7Cu2Fe and MgZn















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials





2particle with


32Zn49




5 Conclusions







Acknowledgments


References



























7Cu2Fe and MgZn















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials























7Cu2Fe and MgZn















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials










CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials



anodisation of aluminium alloys by micro-capillary technique as a

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