Surface & Coatings Technology 215 (2013) 260–265

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Surface & Coatings Technology

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Influence of pretreatments and aging on the adhesion performance ofepoxy-coated aluminum

Ö. Özkanat a,b,⁎, F.M. de Wit c, J.H.W. de Wit b, H. Terryn b,d, J.M.C. Mol b

a Materials Innovation Institute (M2i), Mekelweg 2, 2628 CD Delft, The Netherlandsb Delft University of Technology, Department of Materials Science and Engineering, Mekelweg 2, 2628 CD Delft, The Netherlandsc Delft University of Technology, Adhesion Institute, Kluyverweg 1, 2629 HS Delft, The Netherlandsd VrijeUniversiteitBrussel, Research Group Electrochemical and Surface Engineering, Pleinlaan 2, B-1050 Brussels, Belgium

⁎ Corresponding author at: Materials Innovation InstiCD Delft, The Netherlands. Tel.: +31 1527 83723; fax:

E-mail address: o.ozkanat@tudelft.nl (Ö. Özkanat).

0257-8972/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.surfcoat.2012.07.096

a b s t r a c t

a r t i c l e i n f o

Available online 5 November 2012

Keywords:Surface energyHydroxyl fractionShear testingPseudoboehmite

In this work, we investigate how the adhesion of epoxy coatings to aluminum surfaces is influenced by sur-face pretreatments and by aging. First, aluminum substrates were electropolished or grinded as a preliminarystep; then different pretreatments (acid, alkaline and immersion in boiling water respectively) were appliedin order to create variations in the surface properties of aluminum substrate. Differently pretreated surfaceswere then characterized by means of contact angle measurements. The surface energy for each pretreatedsurface was estimated from the contact angle values obtained with 3 different liquids. Pseudoboehmiteoxide, created after immersion in boiling deionized water, with the highest hydroxyl fraction and oxidethickness exhibited the lowest contact angle, so as the highest surface energy, compared to the other surfaces(i.e., reference, acid pretreated, alkaline pretreated). It was observed that the polar component of the surfaceenergy was surface dependent and determined the total surface energy. A direct correlation between polarenergy and hydroxyl fraction was then shown for the aged electropolished samples. This study was followedby the application of an epoxy-based coating on the pretreated surfaces. Epoxy/aluminum joints werecharacterized by means of lap shear testing and adhesive failure was observed for all cases, except thepseudoboehmite surfaces. It was shown that all differently pretreated surfaces along with the reference sur-face were influenced by aging, i.e., 4 h after pretreatment no considerable pretreatment effect on contactangle nor on shear stress was observed, indicating the importance of limiting the time between pretreatmentand application of an organic coating. Furthermore, it was observed that surface pretreatments influenced theadhesion performance. The pseudoboehmite surface exhibited a lower adhesion performance, despite havingthe highest surface energy. This was explained by cohesive failure of the pseudoboehmite oxide layer. Thisresearch presents a relation between surface pretreatments and adhesion performance of the aluminumsubstrates.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Organic coatings are generally used to protect aluminum alloys fromcorrosive environments [1,2]. Coated metals are frequently expected towithstand prolonged exposure to aqueous and corrosive environments.Under such conditions they are susceptible to premature failure andtheir degree of susceptibility is directly correlated with the type ofsurface pretreatment employed to the substrate [3], besides chemicalcomposition and the functional groups of the organic coating.

In previous studies [4–6], influence of pretreatments on the oxidefilmproperties (e.g., hydroxyl fraction, oxide thickness, and morphology) ofhigh purity aluminum was extensively studied. Three different pretreat-ments (i.e., acid pretreatment, alkaline pretreatment and immersion in

tute (M2i), Mekelweg 2, 2628+31 1527 86730.

rights reserved.

boilingwater), besides reference surface, were applied in order tomodifythe oxide film. Immersion in boiling water resulted in the formation ofa so-called pseudoboehmite oxide layer, which is a thick aluminumhydroxide layerwith excesswater (AlOOH·H2O) [7,8]. In a Fourier Trans-form Infrared (FTIR) study [4], among all the pretreated surfaces, thepseudoboehmite surface exhibited the strongest band at 1070 cm−1

which was attributed to the monohydrate Al\OH bending. This wasconsistent with the highest hydroxyl fraction of the pseudoboehmitesurface obtained from XPS analysis [4,5] in which the hydroxyl fractionof the differently pretreated samples was found as a function ofpretreatment and it was in the order of acid-pretreatedbalkaline-pretreatedbpseudoboehmite. In the study of Brand et al. [6], SEMimage of the pseudoboehmite surface showed a continuous oxide layerand Visible Spectroscopic Ellipsometry (VISSE) measurements [4]showed that the thickness of the pseudoboehmite oxide layer was ap-proximately 210 nm with about 80% void due to its intrinsic porousstructure. Moreover, it was shown [4] that the reference surface

Table 1List of samples tested and the corresponding testing techniques.

Samples

Fresh Aged Fresh Aged

reference reference

Electropolished Grinded

reference reference

acid-pretreated acid pretreated acid pretreated acid pretreated

alkaline

pretreated

alkaline

pretreated

alkaline

pretreated

alkaline

pretreated

pseudoboehmite pseudoboehmite pseudoboehmite pseudoboehmite

XPS Lap Shear Testing

Contact Angle Measurements / Surface Energy Estimation

Testing Techniques

261Ö. Özkanat et al. / Surface & Coatings Technology 215 (2013) 260–265

exhibited bands in the 1380–1560 cm−1 region, ascribed to symmetric/asymmetric carboxylate stretching species vibrations, which can be orig-inated from contamination in the case of freshly electropolished baresurfaces.

In order to investigate the interactions at the polymer/(oxyhydr)oxide/aluminum interface, bonding of representative functional groupswas studied and it was found [9–11] that most types of organic func-tional groups bond through hydroxyls on the oxide surface; however,the bonding mechanism itself with organic functional groups is notdifferent for pseudoboehmite as compared to other aluminum oxides(created after acid pretreatment or alkaline pretreatment). Interactionsat the buried polymer/(oxyhydr)oxide/aluminum interfaces were pre-viously studied by using Scanning Kelvin Probe [12–15]. The potentialshift after the application of an epoxy coating correlated to the degree ofinteractions as a function of pretreatment in which the pseudoboehmitesurface exhibited the highest potential shift which was attributed to thissurface having the highest interaction at the polymer/(oxyhydr)oxide/aluminum interface [15]. TEMcross section image analysis after the appli-cation of an epoxy coating onto pseudoboehmite surface showed that theepoxy coating fully penetrated into the layer [6].

A correlation was previously shown between the above mentionedpretreatments and adhesion of PETG coatings via asymmetrical doublecantilever beam (ADCB) experiments [16,17], in which the acidpretreated samples showed significantly lower adhesion energies thanalkaline and boiling water pretreatments. Lap shear testing as amacroscopical testing method is used to investigate the adhesion ofcoated metal systems, which is primarily aimed at studying the adhe-sion of adhesives, but can also be applied to coatings [18,19]. In general,three types of failure are recognized: adhesive, cohesive and mixedfailure. In this study, epoxy coating with a high enough cohesivestrength was used to prevent cohesive andmixed failuremodes; there-fore the failure stress values can be directly attributed to the variationscreated by the surface pretreatments.

The information currently available in literature regarding the re-lation between the oxide film properties along with aging of alumi-num surface and adhesion performance is segmented. In this paperwe focused on the aluminum surfaces with known oxide film proper-ties (e.g., hydroxyl fraction, oxide thickness, and morphology) formedafter surface pretreatments [4,5,20,21]. We then presented the resultsof investigating the relation between these surface pretreatmentsand adhesion performance. First, contact angle measurements wereperformed on modified oxide surfaces, which were pretreated aftera pre-step of electropolishing or grinding. In order to study the influ-ence of aging, samples were also tested immediately after preparation(fresh) and 4 h after preparation (aged). Surface energy values werethen estimated from the contact angle measurements obtained for 3liquids (i.e., deionized water, ethylene glycol and diiodomethane).The correlation between surface energy and hydroxyl fraction [4](as an oxide film property) was shown for the electropolished sur-faces. Since X-Ray Photoelectron Spectroscopy (XPS) has a limitedcapability in determining the hydroxyl fraction on rough grinded sur-face, this correlation was shown only for smooth electropolished sur-face. Surface energy values of grinded surfaces were then correlated tothe adhesion performance studied by lap shear testing. The effect oftime between pretreatment and coating application was also shown.

2. Experimental

2.1. Material and sample preparation

AA1050 (min. 99.5 wt.% Al, max. 0.4 wt.% Fe, max. 0.25 wt.% Si)(Salomon's Metalen, Groningen, Netherlands), a commercially purealuminum alloy, was used as the substrate. One set of samples waselectropolished, while another was grinded. The electropolished alumi-num surfaces were prepared as follows: a samplewas first immersed ina 25 g/l aqueous solution of NaOH at 70 °C for 1 min, followed by

rinsing for 10 s with deionized water. The sample was then ultrasoni-cally cleaned in deionized water for 2 min. After drying, the degreasedsurface was electropolished for 6 min in an 80 vol.% ethanol–20 vol.%perchloric acid solution (current density of 70 mA/cm2) and thenrinsed for 10 s with deionized water. Thereafter the sample was ultra-sonically cleaned in deionized water for 2 min and dried with com-pressed nitrogen. The grinded surfaces were prepared by rubbing thealuminum substrate with Scotch Brite 3 M Clean N Finish grade A-VFNunder flowing water. The sample was then rinsed with ethanol anddried with compressed nitrogen.

A set of samples, referred to as the reference samples, was ultra-sonically rinsed in ethanol for 10 min and tested immediately afterthe above-mentioned substrate preparation, without any furtherpretreatment given. In order to prepare the modified oxide films,samples were further pretreated in one of the three following ways:acid, alkaline and immersion in boiling water. For acid pretreatment,the sample was immersed for 30 s at room temperature into a30 vol.% HNO3 solution (chemically pure, Sigma Aldrich), preparedusing deionized water. This was followed by 3 min rinsing underrunning deionized water and drying with compressed nitrogen. Thealkaline pretreated surface was prepared by immersing the samplesfor 30 s at room temperature in a NaOH solution (pH 12.5), whichwas prepared by dissolving NaOH pellets (97+% pure) in deionizedwater. This was followed by 3 min rinsing with running deionizedwater and drying with pressurized nitrogen. A set of samples wasprepared by immersion into boiling deionized water for 60 s, resultingin approximately 210 nm of oxide film thickness [4]. This set wasallowed to dry for a few minutes in an upright position, resulting inthe formation of a pseudoboehmite oxide layer.

A set of samples was measured right after preparation (referred toas “fresh”) and another set was measured after 4 h exposure to 40%RH (referred to as “aged”). Table 1 exhibits the list of the samplestested along with the corresponding experimental technique studied.

2.2. Coating system

The solvent-free epoxy coating was prepared by mixing two compo-nents: Epikote 816 (a lowviscosity epoxy resin produced frombisphenolA and epichlorohydrin) and Epikure 3155 (a modified polyamide epoxycuring agent based on dimerized fatty acid and polyamines), as suppliedby Momentive Specialty Chemicals Inc. The curing agent was added tothe epoxy resin at a stoichiometric ratio and mixed for 10 min with amagnetic stirrer. The epoxy coating was then applied on specimensand was cured at 60 °C for 24 h.

2.3. Contact angle measurements

Liquid contact angles were measured at room ambient using aCAM 200 optical contact angle meter system (KSV Instruments Ltd.,

262 Ö. Özkanat et al. / Surface & Coatings Technology 215 (2013) 260–265

Finland). The total dosing volume of the droplet was 5 μl and thecontact angles were measured for 3 different liquids, i.e., deionizedwater, ethylene glycol and diiodomethane. For each sample, contactangles were measured for at least 5 drops from both sides and thevalues of each side were found within error range. Surface energieswere deduced by fitting the contact angle values of each liquid intoOwens–Wendt–Kaeble equation [22,23].

1þ cosθið ÞγL;i

2ffiffiffiffiffiffiffiγDL;i

q ¼ffiffiffiffiffiffiγPS

q ffiffiffiffiffiffiffiγPL;i

γDL;i

vuut þffiffiffiffiffiffiγDS

qð1Þ

in which, θi denotes the contact angle of liquid i, γL,i the total surfacetension of liquid i, γL,i

D and γL,iP the dispersive and polar components of

the surface tension of the liquid i and γSD and γS

P the dispersiveand polar components of the surface tension of the solid substrateS. Total surface tension of each probe liquid and their componentsare listed in Correira et al. [22].

2.4. Shear testing

A4 mmthick sheet of AA1050was cut into 80×20 mmsamples andwas grinded using Scotch Brite™. Two pieces of substrates were thenbonded to each other by epoxy coating in a lap joint of 30×20 mm. Asimilar amount of epoxy was used for each sample and care was takenwhile the epoxy was squeezed out at all sides. Samples were thenclamped to each other at the lap joint area applying about 0.1 MPa,achieved by using foldback clips from 2 sides (Fig. 1a). Samples werethen cured at 60 °C for 24 h. Aluminum tabs of 20×8×4 mm werealso glued to the two ends of the substrates in order to prevent second-ary bending of the lap joint during testing. The bond failure stress wasstudied by lap shear testing in a Universal Tensile Testing Machinewithin a 10 kN cell. The pulling rate was kept constant at 10 mm/min

1

2

3

4

5

6

For

ce (

N)

epoxy coating

metal substrate

a

c

b

Fig. 1. (a) Configuration of the samples used in shear testing (b) typical force versus displacof an alkaline pretreated pair sample after failure in shear testing.

and force values were recorded (Fig. 1b). The bond failure stress valueswere then calculated by dividing the break force value by the area of lapjoint. The failure typewas determined from the optical images after thefailure and a typical pair sample after testing was shown in Fig. 1c. Fiveprobes of each sample were prepared and tested.

3. Results and discussion

Modified aluminum oxide surfaces were characterized by means ofcontact anglemeasurements. Fig. 2 shows the contact angles of ethyleneglycol on themodified oxide films, in which surface pretreatments wereapplied to the electropolished (Fig. 2a) and grinded (Fig. 2b) substrates.A set of samples was tested immediately after preparation (fresh) andanother set was tested 4 h exposure to 40% RH after preparation(aged). For both electropolished and grinded surfaces, contact angleson fresh samples decreased as a function of pretreatment in the orderof reference>acid pretreated>alkaline pretreated>pseudoboehmite(b1°). It should be noted that pseudoboehmite oxide exhibited ex-tremely hydrophilic surfaces on fresh electropolished samples and ongrinded surfaces, resulting in a complete dispersion of a droplet. Thisprevented quantification of the contact angle on these particularsurfaces. The contact angle of as-received samples was also measuredand found to be 95°±3° (not shown here). It can be observed in Fig. 2that aging resulted in an increase in the contact angle for bothelectropolished and grinded samples. This is because aging in the ambi-ent results in the adsorption of water and contaminants like organic ad-sorbates, nitrogen, etc. on the oxide surface [21,24,25]. It was alsoobserved in Fig. 2 that, after 4 h on hold, the influence of pretreatmenton the contact angle decreased, resulting in similar contact angle valuesfor acid and alkaline pretreated surfaces, i.e., approximately 30° forelectropolished and 20° for grinded surfaces. By comparing Fig. 2a andb, it is seen that grinded surfaces exhibit lower contact angle values –

for each modified surface – compared to the electropolished ones.

0 1 2 3 4

Displacement (mm)

0

000

000

000

000

000

000 failure of the joint

epoxy coating

metal substrate

ement curve indicating the failure point of the joint (dashed line) and (c) optical image

0

10

20

30

40

reference acidpretreated

alkalinepretreated

pseudo-boehmite

0

10

20

30

40

reference acidpretreated

alkalinepretreated

pseudo-boehmite

a

b

Fig. 2. Contact angles of ethylene glycol on differently pretreated aluminum (oxyhydr)oxide surface of fresh (bold) and aged (faded) on (a) electropolished and (b) grindedsubstrate.

a

b

Fig. 3. Surface energy with the components of dispersive (bold) and polar (faded)obtained from differently pretreated aluminum (oxyhydr)oxide surface of fresh(solid) and aged (striped) on (a) electropolished and (b) grinded substrate.

263Ö. Özkanat et al. / Surface & Coatings Technology 215 (2013) 260–265

According to the Wenzel's relation, surface roughness decreases thecontact angle for a droplet on a hydrophilic surface [26,27], thereforethe reason for the decrease in the contact angle values upon grindingcan be explained with the increased surface roughness after grinding.

After measuring the contact angle for three different liquids on dif-ferently prepared aluminum surfaces, the surface energies were esti-mated by fitting Eq. (1). Fig. 3 shows the estimated total surfaceenergy values along with dispersive (bold) and polar components(faded). Since no values could be recorded for pseudoboehmite surfacesdue to the dispersion of the droplet, a contact angle of 0° was used inthe estimation of the surface energy for that particular surface. Forthe fresh samples, the total surface energy increased as a functionof pretreatment in the order of referencebacid pretreatedbalkalinepretreatedbpseudoboehmite. The dispersive component of the total sur-face energy remained relatively constant for all studied aluminum oxidesurfaces (~32 mJ/m2 for all electropolished samples and ~33 mJ/m2

for grinded samples), whereas the polar component was surface depen-dent and determined the total surface energy. Specifically, the valuesfor the polar component observed for the fresh grinded surfaces wereapproximately 24 mJ/m2, 29 mJ/m2, 32 mJ/m2 and 34 mJ/m2 for refer-ence, acid pretreated, alkaline pretreated and pseudoboehmite, respec-tively (Fig. 3a — solid bar). A similar trend was also observed for thefresh electropolished surfaces (Fig. 3b — solid bar). Therefore, it can beconcluded that the oxide modification mainly affected the polar compo-nent of the surface energy in the case of freshly prepared oxide films.

The influence of aging on surface energy was not straightforward.Comparing the aged samples (Fig. 3— dotted bars) to the fresh samples(Fig. 3 — solid bars), it was seen that the dispersive componentremained almost constant, while the polar energy of acid pretreated

and alkaline pretreated surface decreased (5 to 7 mJ/m2). It should benoted that the electropolished reference surface was the only case, inwhich the surface energy increased after aging. For pseudoboehmitesurfaces, it was not possible to investigate the aging effect due to thelack of precise measurements. Finally, as-received surfaces were not af-fected by aging, as would be expected from an already contaminatedoxide [21]. It was previously shown for electropolished samples after4 h of exposure to 40% RH [4] that hydroxyl fraction was a function ofsurface pretreatments and was in the order of acid pretreatedbalkalinepretreated~referencebpseudoboehmite. Fig. 4 shows a direct correla-tion between the polar component of the surface energy and the hy-droxyl fraction for the aged electropolished samples, in which thehydroxyl fraction data was taken from Ref. [4]. This direct correlationsuggests that the polar energy, so as the surface energy, is influencedby hydroxyl fraction. Furthermore, similar surface energy values of dif-ferently pretreated samples observed after aging can be explained withthe contribution of the adsorbed contamination species during aging[21,24,25], as explained in the case of contact angle measurements.

Fig. 4. Correlation between polar component of the surface energy and hydroxylfraction [4] of freshly prepared electropolished substrate for differently pretreatedaluminum (oxyhydr)oxide surface: reference (■), acid pretreated (●), alkalinepretreated (▲), and pseudoboehmite (▼).

7

8

9

10

11

12

13

She

ar S

tres

s (M

Pa)

acidpretreated

alkalinepretreated

pseudo-boehmitereference as-received

Fig. 5. Failure stress values of differently pretreated aluminium (oxyhydr)oxide surfaceon fresh (bold) and aged (faded) grinded substrate.

264 Ö. Özkanat et al. / Surface & Coatings Technology 215 (2013) 260–265

This studywas followedby investigating the adhesion performance ofthe fresh (bold) and aged (faded) grinded surfaces bymeans of lap sheartesting. For all samples, except the one prepared by immersion in boilingwater, adhesive failure was observed suggesting that adhesive strengthat the polymer/metal interface is lower than the cohesive strength ofthe coating. Fig. 1c shows an alkaline pretreated pair sample after a fail-ure in shear testing as an example of a surface after an adhesive failure.Fig. 5 shows the failure stress values (MPa) for eachdifferently pretreatedsurface. As expected from the lowest surface energy, as-received samples

c

a

Fig. 6. SEM-EDX analysis of pseudoboehmite surface after lap shear testing: (a) SE

exhibited the lowest failure stress values (7.1 MPa). This is followedby the reference surfaces (9.8 MPa). As for the surface energy values, fail-ure stress values increased as a function of pretreatment in the order ofreference (9.8 MPa)bacid pretreated (11.8 MPa)balkaline pretreated(12.4 MPa) samples, which is also consistent with the measured contactangle values. Such variations in failure stress values, so as the adhesionperformance, reveal that the surface pretreatments significantly affectthe adhesion strength. Nevertheless, pseudoboehmite oxide surfaceseems not to fit into this correlation. Despite having the highest surfaceenergy and highest hydroxyl fraction [4,20], it exhibited significantlylower failure stress values (11.0 MPa) compared to the acid pretreated

b

d

M image and EDX element maps of (b) carbon (c) oxygen and (d) aluminum.

265Ö. Özkanat et al. / Surface & Coatings Technology 215 (2013) 260–265

and the alkaline pretreated surface. This result might have been seencontradictory considering only the chemical adhesion at polymer/metalinterface; however in the case of pseudoboehmite oxide layer, mechani-cal properties of the oxide layer were shown as dominating the chemicaladhesion, resulting in the cracking of the oxide rather than the adhesivefailure at polymer/metal interface. Fig. 6a shows the SEM image of thepseudoboehmite surface after the failure in shear testing. In order to de-termine the composition of the cracked layer observed in the SEM image,Energy Dispersive X-Ray (EDX) analysis was performed to trace the ele-ments of carbon, oxygen and aluminium. From the carbonmap (Fig. 6b),it can be observed that there is no residue of coating present on the alu-minum surface after testing. From the complimentary O and Al maps, itcan be seen that the composition of the layer consists of these two ele-ments. This indicates that the observed cracked layer is the thickpseudoboehmite oxide layer, which is suspected to have failed duringthe testing of joint. It should be noted that the SEM image was acquiredafter the EDX analysis and it can be seen that the cracked oxide layer –after lap shear testing – was so unstable that a part of it (indicated in asquare) disappeared during the EDX analysis.

Aging resulted in a lower failure stress value for acid-pretreated, alka-line pretreated and pseudoboehmite surface, however remained almostconstant for the reference and for the as-received samples. Furthermore,for the measurements performed after aging, the reference (9.8 MPa),acid pretreated (10.4 MPa) and alkaline pretreated (10.4 MPa) surfacesexhibited similar values. This decrease in failure stress and reaching sim-ilar values reveal that the contribution of the surface properties to thefailure stress becomes less dominant upon aging. This is also in agree-ment with the surface energy estimation.

4. Conclusions

The influence of surface pretreatment and aging on adhesion ofepoxy coatings to aluminum surfaces was studied by means of contactangle and surface energy and tensile testing in lap shear configuration.First, the correlation between hydroxyl fraction and surface energywas shown on differently pretreated surfaces and then the correlationbetween surface energy and adhesion performance of epoxy-coatedaluminum system. It was found that aging increased the measured con-tact angles on each studied surface for polar solvents like water, whilelowering the contribution of surface modifications. Moreover, the polarcomponent was observed as the only variable component of the totalsurface energy and that pseudoboehmite surface exhibited the highestvalues. A direct correlation between the hydroxyl fraction and thepolar component of surface energy was shown. Based on lap-shear test-ing, it can be concluded from the comparison of the as-received surfaceto the other pretreated surfaces that any pretreatment increases the ad-hesion of the epoxy coatings to aluminum surfaces. Moreover, adhesionperformance of the pseudoboehmite surface, despite having the lowestcontact angle and thus highest surface energy values, was found to

be lower than the other pretreated surfaces studied. Based on thepost-mortem SEM-EDX study, cracking in the oxide layer originatingduring the failure of the epoxy-metal joint was revealed; this couldbe explained by cohesive failure in the relatively thick and brittlepseudoboehmite.

Acknowledgment

This research was carried out under Project Number M42.6.08314in the framework of the Strategic Research program of Materials In-novation Institute (M2i).

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