Investigation of the corrosion behaviour of a bilayer cerium-silanepre-treatment on Al 2024-T3 in 0.1 M NaCl
Luis E.M. Palomino a, Patricia H. Suegama a, Idalina V. Aoki a,Zoltan Paszti b, Hercılio G. de Melo a,∗
a Polytechnic School of the Sao Paulo University, Brazilb Institute of Surface Chemistry and Catalysis, Chemical Research Center, Hungarian Academy of Sciences,
P.O. Box 1XXX, Budapest, Hungary
Received 7 July 2006; received in revised form 9 February 2007; accepted 1 March 2007Available online 7 March 2007
bstract
In the last few years great efforts have been made in order to find environmentally friendly substitutes for Cr6+ pre-treatments applied to aluminiumlloys used in the aircraft industry. In this work we have investigated the electrochemical response of a bilayer pre-treatment consisting of a Ceonversion bottom layer and a non-functional silane (bis-1,2-(triethoxysilyl) ethane (BTSE)) top layer applied on Al 2024-T3, and compared itsehaviour with monolayer coated samples. The investigation was carried out in 0.1 M NaCl solution, and the electrochemical techniques employedere anodic polarization curves and electrochemical impedance spectroscopy (EIS). EIS experiments performed with bilayer coated samplesave shown a continuous increase of the impedance response during the whole test period, which was interpreted on the basis of a pore blockingechanism supported by scanning electron microscopy (SEM) images and equivalent circuit fitting. Moreover, the impedance of the bilayer coated
amples was approximately one order of magnitude higher than that presented by monolayer coated ones. On the other hand, mechanical tests havevidenced the good adhesion of the silane layer to the Ce conversion layer, which can be likely attributed to a better linking between the silaneolecules and the cerium bottom layer.2007 Elsevier Ltd. All rights reserved.
at
ewahos
eywords: EIS; Al 2024-T3; Cerium-silane layers; XPS
. Introduction
Al 2024-T3 is an Al–Cu alloy extensively used to manufac-ure aircrafts. This alloy presents a complex microstructure, withhe presence of intermetallics of different composition, render-ng it very prone to localized corrosion [1]. Due to this fact,or general-purpose use, it is coated with organic layers. Dueo security reasons a complete coating system of an aircraft isomposed of three layers: a conversion coating, a primer and a
opcoat [2]. The first layer is aimed to improve adhesion prop-rties between the alloy and the primer, as well as to increasets corrosion resistance by hindering the direct contact of the
∗ Corresponding author at: Universidade de Sao Paulo, Depto. de Eng.uımica, Av. Prof. Luciano Gualberto, Cidade Universitaria “Armando de Sallesliveir” Travessa 3, n◦ 380, CEP 05508-900 Sao Paulo, SP, Brazil.el.: +55 1130912231; fax: +55 1130313020.
lloy surface with aggressive specimens that penetrate throughhe organic coating porosities and/or defects.
In the aircraft industry the pre-treatment used to impart adher-nce between the primer and the Al substrate is chromatizing,hich is extremely efficient, and, in addition, increases the wear
nd corrosion resistance of the material [3–5]. Nevertheless,ealth and environmental restrictions, as well as the high costsf wastage treatment, have led researchers and the industries toearch for non-toxic substitutes for this treatment. One of theselternatives treatments is based on the use of Ce salts [6–18].
Ce conversion layers on Al alloys started to be investigatedn the mid 1980s by Hinton and co-workers [9,10]. From theserst works many others have been published using different con-itions to obtain Ce conversion layers on several Al substrates
f interest to the aircraft industry, and their microstructure andorrosion behaviour have been investigated [6–8,11–18]. In allhe tested conditions these conversion layers have shown goodorrosion protection towards Al 2024 alloy; however, improved
onditions seem to be still needed for the use of the treatment oncommercial basis. Shortly, the conversion coating is formed
ue to galvanic activity between the matrix alloy and cathodicntermetallic particles present in its microstructure. The pHncrease in the solution, above and in the vicinity of the cathodicites, provokes the precipitation of an insoluble protective Cexide/hydroxide layer. Nevertheless, the irregular distribution ofhese particles on the alloy surface has been claimed to produceoatings with uneven characteristics [11,12]. However, recentorks have shown that a fine distribution of Cu particles on the
lloy surface can generate more uniform Ce conversion coatings6,7,13–15]. Another drawback of the Ce conversion layers ishe presence of cracks that can cross the whole cross-sectionf the layer, representing preferential pathways for penetrationf aggressive species, however the layer seems to exhibit self-ealing ability, which helps their long-term protection properties19].
Other of these alternative non-toxic treatments is based onhe use of organosilanes coupling agents, hereafter denomi-ated simply silanes. These molecules have been reported inhe literature to afford a good corrosion protection to Al andts alloys [2,20–22]. Their general formula can be written as
3Si(CH2)nY. In the formula, X is a hydrolysable alkoxy group21], and Y can be a CH3 group (being classified as non-unctional silane), an organofunctional group (being classifieds functional silane) or another X3Si group (being classified asis-silane) [23–25]. Another group is the bis-functional silanesaving the general structure X3Si(CH2)nYm(CH2)nSiX3 (theunctional group Y can be for example an amine group or ahain of sulfur atoms and X are hydrolysable alkoxy groups)26].
It is generally accepted in the literature that the hydrolysisf silanes alkoxy groups in water, or water/alcohol mixturesroduces silanol groups (Si–OH). These groups, when in con-act with hydroxyl-covered metallic surfaces, form hydrogenonds. Moreover, in a temperature enhanced condensation reac-ion, these bonds forms metallo-syloxane bonds, increasinghe adherence of the silanes to the metallic substrate, and thexcess Si–OH groups present in the adsorbed structure formiloxane (Si–O–Si) bonds, originating a highly cross-linkedetwork, which hinders the penetration of aggressive species.ven though silanes have proved to be effective to protect Algainst corrosion, the degree of protection afforded to the coatedetal depends on the specific silane structure [21] that means
ilanes with different functionalities provide different protectionegrees. In addition, processing parameters like silane concen-ration, pH of the silane hydrolysing solution and hydrolysisime [27], the curing temperature [20], and the nature of theubstrate [21] also influence the performance of the formedayer. Moreover, it has been recently shown by SIMS and XPS
easurements that the pre-treatment applied to the metal sub-trate, particularly Al, prior to its immersion in the hydrolysedilane containing solution can dramatically affect the adsorp-
ion degree, and the strength of the Si–O–Me bonds (adherence)28–30]. Even though promising results for the protection ofl alloys have been obtained for silane protected samples theirerformance is still below that presented by the chromate con-
whro
ica Acta 52 (2007) 7496–7505 7497
ersion layers, and these coatings cannot offer an adequate longerm protection due to the presence of micro-pores, cracks andreas with low cross-link density. These zones favour the diffu-ion of aggressive species to the coating/substrate interface andre preferential sites for corrosion initiation [31].
Especially concerning silane coatings, in recent yearsesearchers have turned their attention to more complex systems.n this sense some studies have been devoted to investigate theynergistic effect among silanes themselves [32], and betweenilanes and other types of corrosion protecting structures, likeorrosion inhibitors and silica nanoparticles [3,22,31,33–35]. Inseries of recent works Montemor and co-workers have inves-
igated the microstructure and the corrosion protection affordedy silane coatings doped with rare-earth salts applied on differentetallic substrates, particularly galvanised steel [34,36–40] andl 2024-T3 [36]. The authors verified that the doping procedure
ncreased substantially the impedance response of the sampleshen compared with undoped ones. However, this feature wasore relevant for Ce(NO3)3 doped layers, which the authors
scribed to the self-healing properties of Ce ions [36,37,39,40].In a closer approach to the one employed in this
aper, Montemor et al. [41,42] have analyzed and investi-ated the electrochemical behaviour of a functional silanebis-[triethoxysilylpropyl]tetrasulphide) BTESPT [41,42] and
non-functional bis-1,2-[triethoxysilyl] ethane-BTSE) [42]ilane layers deposited on galvanised steel previously coatedith a rare earth conversion coating (Ce [41] or La [41,42]),hich was denominated a two-step pre-treatment. The authors
ound that the samples treated with this treatment have betterorrosion behaviour than those treated with single ones, and theest response was found for the couple La + BTESPT, whichas attributed to interactions between the sulphur atoms and then layer underneath [41,42]. However in the case of Al substrate
hese interactions must not be present since sulphur atoms haveo special affinity for Al, instead, increased protection affordedy BTESPT to Al substrate has been attributed to hydrophobicityf sulphur atoms [43].
In the present work we have used electrochemical impedancepectroscopy (EIS) to compare the corrosion responses exhib-ted by Al 2024-T3 alloy protected either with a Ce conversionayer or with a BTSE (bis-1,2-(triethoxysilyl) ethane) layer, withhe one presented by a sample protected with a bilayer systemonsisting of a Ce conversion bottom layer and a BTSE top layer.he electrochemical investigations were carried out by follow-
ng the EIS response up to 72 h of immersion in 0.1 M NaClolution, and the experiments were terminated by an anodicolarization curve. Moreover a mechanical test was employed inrder to test the adhesion of the different layers to the alloy sub-trate prior and after the electrochemical tests. SEM images andDS semi-quantitative analysis were also used to assess someicrostructural features of the layers, which had their compo-
itions checked by X-ray photoelectrons spectroscopy (XPS).TSE was chosen for this work for many reasons: the first one
as that it is a non-functional silane that can form up to sixydrolysable silanol groups, facilitating, as a consequence, theeaction of the silanol groups with the metallic surface, the sec-nd reason was that the use of a non-functional silane avoids the
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ossibility of upside-down bonding that can occur with func-ional ones, and finally because, as a first step investigation,e want to avoid any interference of functional groups in the
esponse of the system, which, ultimately, could complicate thenalysis of the electrochemical tests.
. Experimental procedure
The AA2024-T3 alloy used in this work was sup-lied by a Brazilian industry as sheets with dimensions of6 cm × 10 cm × 0.1 cm. This alloy has a high Cu content, typ-cally between 3.8 and 4.9 wt% [44].
To obtain the Ce conversion layer the material was cut inamples of 4 cm × 5 cm, which were vigorously washed withoap and tap water before being thoroughly washed with dis-illed water, alcohol and acetone, and dried in hot air stream.rior to any coating procedure the samples were immersed formin in 0.5 M NaOH solution, followed by 5 min treatment in.5 M acetic acid solution, with a washing step in-between, andnally washed with distilled water, alcohol and dried in hot airtream. For the sake of simplicity, this pre-treatment sequenceill be denominated standard pre-treatment in the text below.revious works [6,7,19] have shown that the HAc pre-treatmenttep leaves a Cu-rich smut at the alloy surface which favours therecipitation of the Ce conversion layer. Following this sequencehe samples were immersed for 10 min in the Ce conversionolution, which composition is given in Table 1.
The silane solution was prepared by adding 4% (w/w) ofTSE molecules to a 50%/50% (w/w) ethanol/water solution,hich, afterwards, had its pH adjusted to a value between 3.5
nd 4 by the addition of acetic acid, and was left to hydrolyseuring 30 min, according to the procedure described by Oliveira45]. To obtain the silane coatings, the samples submitted to thetandard pre-treatment were vigorously washed to remove theu-rich smut, and then immersed during 5 min in the hydrolysed
ilane solution at 25 ◦C. Next, the treated samples were eitherried on air or left at rest for 2 min at room temperature andubsequently cured at 100 ◦C for 10 min. The same procedureas adopted to obtain the top silane layer in the bilayer coating,ith the difference that the samples had been previously coatedith the Ce conversion coating.All the electrochemical tests were carried out using a classical
hree electrodes cell with Ag/AgCl (KCl sat.) and a platinumrid as reference and counter electrode, respectively, which werelways positioned at the same place relatively to the working
lectrode. Details of the cell construction can be found elsewhere19].
For the EIS experiments a Solartron® 1260 frequencyesponse analyser coupled to a Solartron® 1287 electrochem-
able 1omposition of the Ce conversion solution
eCl3 (g/L) 2.46
2O2 (g/L) 0.3
3BO3 (g/L) 0.02ime (min) 10
ddcetfold
v
ica Acta 52 (2007) 7496–7505
cal interface was used. The measurements were performed inhe 10 kHz to 5 mHz frequency range, with an acquisition rate of0 points per decade. The ac signal amplitude was 15 mV. All thexperiments were performed at the open circuit potential (OCP),hich was recorded prior and after the measurement in order toerify that the system was stationary. The impedance behaviouras accompanied up to 72 h of immersion, and afterwards an
nodic potentiodynamic polarization curve was obtained at a.5 mV s−1 scan rate. In both case the software Corrware® con-rolled the experiments.
SEM/EDS analyses were performed in all the coated samples.he equipment used was a Philips XL-30 scanning microscopequipped with an EDS spectrometer. The acceleration voltagesed to perform EDS analyses was 20 keV, giving a penetrationepth of approximately 1 �m. Semi-quantitative elemental anal-ses were performed using internal standards of the equipment.he adhesion test consisted in immersing the coated sampleuring 30 min in DI water containing ultrasonic bath. It must bemphasized that due to the semi-quantitative nature of the EDSechnique all the analyses reported were performed using low
agnification SEM images (200×) in an attempt to minimizehe effects of particular microstructural features in the overallesults.
X-ray photoelectron spectroscopy measurements were car-ied out using an electron spectrometer manufactured byMICRON Nanotechnology GmbH (Germany). The photoelec-
rons were excited by Al K� (1486.6 eV) radiation. Spectra wereecorded at normal emission in the Constant Analyser Energyode: for survey spectra 50 eV pass energy was used, while
etailed scans of the substrate and protective layer related peaksere measured at 30 eV pass energy, with resolution better thaneV. In order to collect information about the subsurface ele-ental composition, the samples were etched by 4 keV Ar+ ions
t 10 �A/cm2 current density. Spectra were evaluated by fittingith Gaussian–Lorentzian sum peaks after removing a Shirley or
inear background. Elemental concentrations were determinedsing the XPS MultiQuant software package.
. Results and discussion
Fig. 1 presents SEM micrographs of a bare pre-treated Al024-T3 sample from which the Cu-rich smut was removed, andf samples coated according to four different procedures. Theare sample, Fig. 1(A), exhibits numerous irregularly shapedntermetallics, the rolling marks, and also a large number ofoles, this latter feature likely caused by the detachment and/orissolution of the intermetallics during the pre-treatment proce-ure. In its turn Fig. 1(B) shows the surface of a sample onlyovered with the silane layer where is possible to see the pres-nce of the intermetallics underneath the coating. Although inhis particular image silane distribution appears to be quite uni-orm, lower magnification images shows that silane adsorptionn the electrode surface is quite irregular, with zones where the
ayer seems to be almost absent. This issue was further confirmeduring XPS analyses.
In Fig. 1(C) an image of a sample submitted to the Ce con-ersion treatment is shown where its usual dry mud aspect is
ig. 1. SEM micrographs of the AA 2024-T3 alloy (A) bare (after the standanon-cured); (E) cerium-silane coated (cured).
videnced. However, this feature is no longer present in theample that was further coated with the silane layer, and whichas not submitted to the curing procedure, Fig. 1(D). It isenerally accepted that the cracked microstructure of the Ceonversion layers is due to their drying [5]; however, someuthors have also proposed that this feature could be due tofast precipitation rate [11], or either to stress release dur-
ng layer growth [46]. Nonetheless, in the previously publishedorks, no proofs were offered that the vacuum inside the SEM
hamber did not actually provoke the cracking of the Ce con-ersion layer. The fact that after the curing step the dry-mudspect of the Ce conversion layer is again evidenced in theEM image, Fig. 1(E), seems to definitively prove that dry-
ng is the real driving force for the superficial cracking of theayer. Moreover, in this latter sample, the cracked network is less
ntense and the trenches seem to be narrower when compared toig. 1(C). This is probably due to the introduction of compressivetress during the cross-linking of the silane layer in the curingtep.
The electron microscopic studies of the silane or Ce-silaneovered Al 2024-T3 samples were completed by electron spec-roscopic investigations by which compositional data on a fewanometer (nm) depth scale can be obtained. The samples werexposed to various amounts of 4 keV Ar+ ion bombardment inrder to determine any depth-dependent compositional changesn the subsurface regions.
XPS analysis of the cured silane layer on Al sample revealed,n agreement with the SEM images, that the thickness distribu-ion of the silane film is very inhomogeneous. Clearly observablel signals were found on the sample, indicating that even when
reshly prepared small regions of the substrate still remainsncovered. Accordingly, during subsequent data evaluation atructural model consisting of an infinitely thick but not nec-ssarily continuous silane layer on an alloy substrate was used.
able 2 summarises the compositional data after certain amountsf ion etching.
As the data in Table 2 indicate, the C content significantlyrops after a few minutes of ion etching due to the removal of a
Table 2Sputtering time dependence of the nominal composition of the sample of curedsilane on Al 2024-T3 (in at.%)
Sputtering time (min) C O Si Al Cu
0 54 33 12 1 –6 33 42 21 4 –
1
fnccTscrii
Table 3Sputtering time dependence of the nominal composition of the sample of curedsilane on cerium-oxide covered Al 2024-T3 (in at.%)
Sputtering time (min) C O Si Ce Al Cu
0 29 50 18 1 1 –6 13 58 19 6 4 –
1
tttTmt
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60 23 43 21 11 250 20 40 18 19 3
ew nanometer thick hydrocarbon contamination layer, which isaturally present on almost all materials exposed to atmosphericonditions. The O and Si percentage initially increases as theontamination on the top of the silane film is sputtered away.hen, after a certain amount of sputtering, both the C and the Siignal decreases, while their ratio remains constant. This stage
orresponds to the gradual removal of the silane layer from theough substrate surface, which is also evidenced by the increas-ng signals of Al. The amount of C and Si atoms at this stages practically the same, which corresponds to the stoichiome-
t
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ig. 2. SEM micrographs of the AA 2024-T3 alloy after 72 h of immersion in 0.1 MC) cerium coated; (D) cerium-silane coated (non-cured); (E) cerium-silane coated (c
60 5 53 14 22 5 150 4 48 10 31 6 1
ry of the fully hydrolysed silane compound. The assignment ofhe O signal is not straightforward, since both the substrate (inhe form of Al-oxides) and the silane layer gives contributions.he Cu content of the substrate is around 10–15 at.%, which isuch larger than the nominal Cu percentage. It indicates that
he substrate surface clearly remains enriched in copper even if
he copper smut was removed.
A similar experiment was carried out on a sample with curedilane layer on a cerium-oxide treated Al 2024-T3 substrate.elected composition results are presented in Table 3.
NaCl solution: (A) bare (after the standard pre-treatment); (B) silane coated;ured).
chimica Acta 52 (2007) 7496–7505 7501
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Fig. 4. Si and Ce peaks intensity in EDS spectra of double-layer coated Al2Ac
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The silane coverage, similarly to the case of the Al substrateithout the cerium-oxide layer, is again uneven. Interestingly,owever, the C to Si ratio after the removal of the hydrocar-on contamination is around 0.4, very far from the expectedtoichiometry of the silane film. It suggests that a carbon lossrocess took place either during the silane layer formation and/oruring, which obviously influences the stability and the proper-ies of the film. The small but almost constant Al signal indicateshat a small portion of the sample is not covered by the cerium-xide, which slowly increases during the sputtering. Recentlyontemor et al. [47] have suggested that the presence of Ce ionsay catalyse the reticulation of silane layers. This would be at
he basis of the increased corrosion resistance verified by theseuthors for samples coated with Ce doped silane layers whenompared to undoped ones, and may also explain the increasedmounts of Si found in the silane layer on the top of the Ceoating in the present work.
Fig. 2 exhibits the SEM micrographs of the samples surfacefter 72 h of immersion in the 0.1 M NaCl test electrolyte. Themages show that for the bare alloy, Fig. 2(A), the corrosionrocess is located mainly at the vicinity of the intermetallics.n the other hand, for the silane protected sample, Fig. 2(B),
t is possible to observe the formation of a great quantity oforrosion products indicating that the treatment conditions didot form a compact silane layer. In its turn in Fig. 2(C), obtainedrom a Ce-coated sample, it is possible to observe the build-upf a small quantity of corrosion products at the sample surface.n the other hand, for the bilayer protected samples, Fig. 2(D)
nd (E), there is a precipitation of corrosion products, which isess intense in the latter sample, confirming the beneficial effectf the cross-linking process in the corrosion protection affordedy the silane layers.
The intensities of the Si peaks in EDX spectra of Al 2024-3 samples coated either with silane or with the Ce–Si layer areresented in Fig. 3. No significant difference was found betweenhe analyses, indicating that the silane molecules have equivalentffinity both for the Al surface and for the Ce conversion layer.
Fig. 4 shows the intensity of the Si and Ce peaks for bilayer
oated samples prior and after the adhesion test consisting inhe immersion in an ultrasonic bath during 30 min. From thepectra, it is evident that the mechanical test removes almost all
ig. 3. Si peak intensity in EDS spectra of coated Al 2024-T3 samples: (A)l–Si (cured); (B) Al–Ce–Si (non-cured); (C) Al–Ce–Si (cured).
toietgdadiacqititbl
024-T3 samples prior and after the adhesion test—(A) Al–Ce–Si: cured; (B)l–Ce–Si: non-cured; (C) Al–Ce–Si: cured and sonicated; (D) Al–Ce–Si: non-
ured and sonicated.
he silane molecules from the samples surface; however a weaki peak is still visible in the cured sample, indicating that theuring procedure increases the adhesion of the silane moleculeso the sample surface. Regarding the intensity of the Ce peaks,or the cured sample no change was verified after the mechanicalest, on the other hand, a slight diminution was observed for theon-cured one. These results put in evidence two features: firstf all the excellent adhesion of the Ce layer to the alloy surface,ven though it was precipitated on a sample covered with a Cu-ich smut, and secondly the interaction between the Ce and theilane layer, since the presence of only a little amount of thisatter on the top on the former seems to completely avoid theetachment of the Ce from the alloy surface. Finally it must beaid that identical tests performed with samples coated only withhe silane layer have shown similar results for silane adherenceo the Al surface.
Fig. 5 shows the impedance response, up to 72 h of immersionn the 0.1 M NaCl solution, of an Al 2024-T3 sample coated withhe Ce-silane layer. The diagrams show that the impedance ofhe sample increases throughout the whole test period, indicatinghat its corrosion resistance is enhanced. Moreover, at the endf the test, the impedance modulus is greater than 105 � cm2,ndicating a good corrosion protection ability of the coating,ven though some corrosion products were already present onhe sample surface at the end of the test, as shown in the micro-raphs of Fig. 2. Two capacitive time constants can be easilyistinguished in the Nyquist diagrams; however, Bode phasengle diagrams show that, actually, the HF phase angle is veryeformed indicating the superposition of two time constantsn this frequency region. For silane coated Al samples severaluthors [36,43,48–51] have verified the existence of two timeonstants in the HF domain, the one appearing in the higher fre-uency region was attributed to the presence of the silane layertself while the one in the lower frequency region was ascribedo the existence of an interfacial layer. The comparison of the
mpedance response obtained for the bilayer coated sample withhose obtained in other works [36,43,48–51] shows that, proba-ly, silanes also form an intermediate layer on the Ce conversionayer treated sample, indicating that the structure of the layer
Fig. 5. Evolution of the impedance response in 0.1 M NaCl solution f
btained in the present work must be similar to that formed onhe naked Al matrix.
The modification of the shape of the impedance diagrams inig. 5 shows that, during the test period, there is an evolutionf the interfacial phenomena, which can be better explained anduantified using equivalent circuit fitting. We have employed theame three time constants arrangement proposed by Zhu et al.43,48] for silane coated Al and later modified by Montemort al. [38] to take into account diffusive phenomena, which, forhe sake of simplicity, is being reproduced in Fig. 6 togetherith the physical model of the interface. In the circuit Re, Rpo,il account for the electrolyte resistance, pore resistance of theilane layer and resistance of the intermediate layer, respectively,hereas C(Si) and C(il) represent the capacities of the silane
nd of the intermediate layer, respectively. W is the Warburglement which accounts for diffusion controlled processes. Inddition, to justify the now homogeneity of the electrochemical
ystems sometimes pure capacitors were substituted for constanthase elements (CPE). It must be emphasized that other similarrrangements fitted equally well the experimental data, however
ig. 6. (A) Equivalent circuit used to fit the impedance data for Al 2024-T3oated with the Ce-silane bilayer. (B) Physical model of the interface.
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2024-T3 alloy coated with the double Ce-silane layer. Cured sample.
e have chose the one that married the most logical parametersariation with a reasonable physical meaning.
Table 4 presents the values of the equivalent circuit parame-ers obtained by adjusting the impedance data presented in Fig. 5ith the circuits of Fig. 6. In all the cases the errors involved in
he fitting procedure were inferior to 10% (below 5% in most ofhe cases), and when CPEs were used the exponent value (n) isiven in the table. To better evaluate the variation of the capacityssociated to the intermediate layer, the CPE value associatedith this layer was corrected using the formula proposed by Hsu
nd Mansfeld [52], and rewritten below:
= Y0(ωnm)(n−1) (1)
In the equation, the term ωnm corresponds to the frequency
here the imaginary part of the particular time constant haseached its maximum value and Y0 is the CPE value.
The results presented in Table 4 show that during the testeriod there is an increase of one order of magnitude of bothPE(Si) and Rpo as a result of water uptake and pore blocking,
espectively. As a consequence, the high frequency (HF) timeonstant is displaced to lower frequencies and an increase in itshase angle maximum is observed.
The evolution of the process associated with the intermediateayer is somehow more intricate, and is linked to the complextructure of the bilayer. In the first 24 h its capacity is rep-esented by a CPE (CPE(il)) with an exponent close to 0.6,ndicating an important contribution of diffusive phenomena.he silane curing conditions used in the present work can beonsidered as quite mild, in their work with silanes Montemort al. [36,38–41], Van Ooij et al. [21] and Terryn et al. [26,51,53]ave frequently employed curing temperatures above 100 ◦C foronger periods, so it is expected that the layer will not be fullyross-linked and that it exhibits a high porosity degree. The dif-usion of aggressive species through these preferential pathways
ould account for this time constant, which must also include a
ontribution of the Ce conversion layer underneath. For longermmersion periods (48 and 72 h) C(il) behaves like a pure capac-ty, indicating an improvement of its protective properties with
esults obtained by fitting the circuit to the diagrams of Fig. 5.
mmersion time. Using XPS and FT-IR as analytical tools Mon-emor et al. [54] have shown the presence of some esthers andydroxyl groups in the structure of cured silane layers depositedn zinc substrates. These authors have also verified that theydrolysis and cross-linking process can occur in the aggressiveolution as the hydration of the layer proceeds, improving itsrotective ability [50,54]. It is likely that this same mechanismould explain the results obtained in the present work.
Regarding the resistance Ril, data evolution shows that it
trongly decreases during the first 48 h of test and than slightlyncreases. Interestingly this parameter presents a much higher
bcd
Fig. 7. Impedance response in 0.1 M NaCl solution for Al 202
alue in the initially non-fully cross-linked situation than inhe final period, when the layer behaves like a pure capacity.his variation leads us to suppose that, during the first 24 h, theorous structure comprises both the intermediate layer and thee conversion layer, which have already been show to exhibit
ome cracks along its cross-section [19]. Afterwards, for theully cross-linked system, these contributions should be sepa-ated and Ril would correspond solely to the resistance offered
y the silane intermediate layer, explaining its lower value. Theomparison of the values of Rpo and Ril shows that the interme-iate layer is the main responsible for the protection afforded
4-T3 alloy with different coatings. Immersion time 72 h.
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y the silane layer, in agreement with data published by otheruthors [36,43,50].
The resistive component associated with the Warburg ele-ent (W-R) increases during the whole test period, and must be
ssociated to the diffusion of species through porous corrosionroducts formed during the test. This component is the mainesponsible for the resistive part of the impedance response,or the proposed interface model, porous corrosion productsntrapped below the intermediate layer would be the responsibleor this time constant.
Fig. 7 shows impedance diagrams of a bare Al 2024-T3 sam-le and of samples coated according to three of the proceduresresented in Fig. 1. All the diagrams were obtained after 72 h ofmmersion in the 0.1 M NaCl test solution. The results show thathe samples coated with the Ce-silane layer exhibited the besterformance in the corrosion test, presenting impedances val-es almost one order of magnitude higher than the single layeroated ones. Comparing the Bode phase angle diagrams of allhe coated samples it is likely that the HF responses of the bilayeroated samples are due to a mixed response of the Ce and theilane layer, and would be the result of the incomplete coveragef the substrate by the two layers as demonstrated in the XPSeasurements (see Tables 2 and 3) and of the porous nature
f the silane layer, as discussed in the fitting procedure. On thether hand, in the LF domain, the fact that in all the diagrams theime constant associated with the interfacial phenomena occurst the same frequency range confirms that the bilayer coatingoes not retard the rate of the interfacial reaction but, instead,t seems to diminish the corrosion intensity, in accordance withhe blocking mechanism previously proposed.
Fig. 8 presents anodic polarisation curves obtained imme-iately after the end of the impedance experiments presentedn Fig. 7. The sample treated with the bilayer process presentshe lowest passive current and also the highest pitting poten-ial, confirming the better corrosion resistance of this sample.
he sample protected with the Ce conversion layer presented
he second best performance, followed by the silane protectedample and by the bare alloy.
ig. 8. Anodic polarization curves in 0.1 M NaCl solution for Al 2024-T3 alloyith different Ce and silane coatings. Curves obtained after the impedance
xperiments presented in Fig. 7.
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ica Acta 52 (2007) 7496–7505
. Conclusions
The electrochemical behaviour of Al alloy 2024-T3 pro-ected with a bilayer consisting of a Ce conversion layer and ailane (BTSE) top layer was investigated in 0.1 M NaCl solution.he EIS response has shown that the impedance of the bilayerrotected sample increased during the whole 72 h test period,pparently due to the build-up of corrosion products at the sam-le surface. The comparison of the impedance response of theilayer coated sample with silane coated one has shown that theature of the interaction between the silane layer and the Ce con-ersion layer seems to be the same as that presented between theilane layer and the bare alloy. Moreover, the impedance resultsave shown that bilayer protected samples present impedancealues approximately one order of magnitude higher than thatxhibited by samples protected by only a single layer of eithererium conversion layer or silane.
In addition, the experiments performed here have also provedhat drying is the reason for the cracking of the Ce conversionayer and have also evidenced the excellent adhesion propertiesf the Ce conversion layer to the Al matrix.
cknowledgements
The author H.G. de Melo and P.H. Suegama (05/51851-4) arehankful to FAPESP, Sao Paulo state (Brazil) research-financinggency, for the financial support to this project.
Luis E.M. Palomino is thankful to CNPq, Brazilian researchnancing agency, for the grant.
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