nanotecnologia

Electrochimica Acta 51 (2005) 208–217

Nanostructured sol–gel coatings doped with cerium nitrate aspre-treatments for AA2024-T3

Corrosion protection performance

M.L. Zheludkevicha,∗, R. Serraa, M.F. Montemorb, K.A. Yasakaua,I.M. Miranda Salvadoc, M.G.S. Ferreiraa,b

a University of Aveiro, Department of Ceramics and Glass Engineering, 3810-193 Aveiro, Portugalb Instituto Superior Tecnico, ICEMS, Dep. Chem. Eng., Av. Rovisco Pais 1049-001 Lisboa, Portugal

c University of Aveiro, CICECO, Department of Ceramics and Glass Engineering, 3810-193 Aveiro, Portugal

Received 21 December 2004; received in revised form 12 February 2005; accepted 16 April 2005Available online 17 May 2005

Abstract

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Nanostructured hybrid sol–gel coatings doped with cerium ions were investigated in the present work as pre-treatments for th3 alloy. The sol–gel films have been synthesized from tetraethylorthosilicate (TEOS) and 3-glycidoxypropyltrimethoxysilane (recursors. Additionally the hybrid sol was doped with zirconia nanoparticles prepared from hydrolyzed tetra-n-propoxyzirconium (TPOZerium nitrate, as corrosion inhibitor, was added into the hybrid matrix or into the oxide nanoparticles.The chemical composition and the structure of the hybrid sol–gel films were studied by XPS (X-ray photoelectron spectros

FM (atomic force microscopy), respectively. The evolution of the corrosion protection properties of the sol–gel films was studieelectrochemical impedance spectroscopy), which can provide quantitative information on the role of the different pre-treatmentsquivalent circuits, for different stages of the corrosion processes, were used in order to model the coating degradation. The mupported by SEM (scanning electron microscopy) measurements.The results show that the sol–gel films containing zirconia nanoparticles present improved barrier properties. Doping the hyb

ructured sol–gel coatings with cerium nitrate leads to additional improvement of the corrosion protection. The zirconia particleshe sol–gel matrix seem to act as nanoreservoirs providing a prolonged release of cerium ions. The nanostructured sol–gel filmserium nitrate can be proposed as a potential candidate for substitution of the chromate pre-treatments for AA2024-T3.2005 Elsevier Ltd. All rights reserved.

eywords: AA2024; EIS; Corrosion; Nanostructured coating; Sol–gel; Inhibitor; Cerium

. Introduction

The development of environmentally friendly pre-reatments for metallic substrates is a filed of growingnterest due to banning the use of chromates as protectivere-treatments[1].

Among the possible candidates for environmentallyriendly pre-treatments for aluminium alloys are the silica-ased sol–gel ones[2]. Such pre-treatments are able to form a

∗ Corresponding author. Tel.: +351 234 378146; fax: +351 234 425300.E-mail address: [email protected] (M.L. Zheludkevich).

Si O Al conversion layer providing a stable alumina/sol–film interface, which impairs the onset of corrosion[3].Schmidt et al.[4] reports that the alumina–silica mixcompounds present Gibbs energy lower than the boe(AlO(OH)), which is the product of the first stage of aminium oxidation in the presence of moisture. The lovalue of the Gibbs energy leads to thermodynamical staof the silica/aluminium interface in corrosive environmeThe chemical bonding of the sol–gel films to the aluminsubstrate also contributes to improved adhesion. Moresol–gel coatings, especially the hybrid films, provide a dbarrier against electrolyte uptake[2]. However, the coating

013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2005.04.021

M.L. Zheludkevich et al. / Electrochimica Acta 51 (2005) 208–217 209

by itself, is not sufficient to effectively protect for long termthe metal substrate against corrosion. The sol–gel coatingscontain micro-pores, cracks and areas of low cross-link den-sity that provides pathways for diffusion of corrosive speciesto the coating/metal interface[2]. The corrosion processesstart in these zones. The crack ability and porosity of thesol–gel films can be decreased by incorporation of oxidenanoparticles into the hybrid matrix[5]. However, even suchfilms cannot offer an adequate protection when the coating isdamaged due to the lack of self-healing properties.

Incorporation of corrosion inhibitors into the sol–gel filmscan improve the protective ability of the coatings, suppressingthe corrosion process in the defects. Phosphates, vanadates,borates, cerium and molybdenum compounds were found tohave inhibiting action on the corrosion processes[6–8]. It hasbeen reported[6,9–11] that some of the most effective andenvironmentally friendly corrosion inhibitors for aluminiumalloys are derived from cerium salts. This seems to resultfrom deposition of hydrated cerium oxide on the cathodicintermetallic particles, existing in the aluminium alloy,thus suppressing the cathodic reaction[12]. Voevodin et al.[13] investigated the corrosion protection properties of theepoxy-zirconia sol–gel coatings containing inhibitor ionssalts such as Ce(NO3)3, NaVO3, and Na2MoO4. The sol–gelfilms with NaVO3 and Na2MoO4 did not provide adequatecorrosion protection due to the decrease of the sol–geln iumd poxy-z np pre-t ating[

thes tionop haseP zolea The� ings.T wass anicite gedr m-p torsa ancet canp byt

The present work aims at producing and assessing thecorrosion protection conferred to the aluminium alloyAA2024-T3 by hybrid sol–gel films with incorporatedzirconia nanoparticles and doped with cerium nitrate ascorrosion inhibitor. The cerium inhibitor was added to thehybrid matrix or to the oxide nanoparticles at the respectivehydrolysis steps. The hybrid film and the oxide nanopar-ticles were prepared by the controllable hydrolysis of 3-glycidoxypropyltrimethoxysilane (GPTMS)/tetraethylortho-silicate (TEOS) and tetra-n-propoxyzirconium (TPOZ),respectively. Electrochemical impedance spectroscopy (EIS)was used to assess the corrosion performance of the devel-oped pre-treatments. The surface morphology and the evo-lution of the coatings structure were investigated by atomicforce microscopy (AFM) and scanning electron microscopy(SEM) techniques. The chemical composition of the pre-treated surfaces was assessed by X-ray photoelectron spec-troscopy (XPS) and energy dispersive spectroscopy (EDS).

2. Experimental

2.1. Sol–gel pre-treatments

The aluminium alloy 2024-T3 was used as substrate. ThenT eouss t6 urei on.

singt osolw erec .

ningG andt thea eo them

waso aredb inn redw per-a fC gin-n terw ified

TC

E MC lance

etwork stability. However, the sol–gel coatings with ceropants performed at least as good as the undoped eirconia films[13]. Additional improvement of the corrosiorotection was also revealed in the case of silane

reatments when cerium nitrate was introduced in the co14].

Organic inhibitors can be also incorporated inol–gel matrix in order to improve the corrosion protecf metallic substrates. Khramov et al.[15] studied therotection properties of SNAP (Self-assembled NAnoparticles) films with incorporated mercaptobenzothiand mercaptobenzimidazole as corrosion inhibitors.-cyclodextrin was also included in some of these coathe corrosion protection of sol–gel derived coatingsufficiently improved when the encapsulation of the orgnhibitors in the presence or absence of�-cyclodextrinook place. However, the SNAP films with�-cyclodextrinxhibited superior corrosion protection due to the prolonelease of the inhibitor from the cyclodextrin–inhibitor colexes. As a result the incorporation of corrosion inhibibsorbed in nanoparticles could be a good way to enh

he protection properties of these films. This procedurerovide an additional reinforcement of the hybrid matrix

he nanoparticles[5] and a prolonged release of inhibitor.

able 1omposition of aluminium alloy 2024-T3

lement Cu Cr Fe Mgoncentration 3.8–4.9 0.1 0.5 1.2–1.8

ominal composition of the used alloy is given inTable 1.he metallic samples were immersed in an alkaline aquolution containing 60 g/l of TURCO

TM4215 for 15 min a

0◦C followed by immersion for 15 min at room temperatn a 20 wt.% solution of nitric acid before coating applicati

The nanostructured hybrid films were synthesized uhe controllable sol–gel route. An organosiloxane alcith another alcosol containing the zirconia precursor wombined in order to provide the hybrid sol–gel solution

The first organosiloxane sol was prepared by combiPTMS, TEOS and 2-propanol in a 1:1:2 volume ratios

hen stirring for 30 min at room temperature; 5 vol.% ofcidified water or the solution of Ce(NO3)3·6H2O (in the casf the hybrid matrix doped with inhibitor) was added toixture at the beginning of the stirring.The second sol containing the zirconia nanoparticles

btained hydrolyzing TPOZ precursor. The sol was prepy the addition of ethylacetoacetate to the TPOZ, 70%-propanol, in a 1:1 volume ratio. This solution was stirith the synchronous ultrasonic agitation at room temture for 80 min. Acidified water (pH∼ 1) or solution oe(NO3)3·6H2O was added to the sol 20 min after the being of the solution stirring. The amount of acidified waas about 10 vol.%. Concentration of the nitrate in acid

n Si Ti Zn Other Al0.3–0.9 0.5 0.15 0.25 0.15 Ba

210 M.L. Zheludkevich et al. / Electrochimica Acta 51 (2005) 208–217

Table 2Types of sol–gel coatings

Coating reference A B C D EZrO2 nanoparticles + + + + −Cerium in the hybrid matrix − + − − −Cerium in the nanoparticles + − + − −Concentration of cerium in the

final coating (wt.%)0.5 0.5 1 0 0

water was varied in order to achieve 0.5 wt.% or 1 wt.% ofcerium ions in final sol–gel coating.

The two prepared sol–gel solutions were mixed in 2:1 vol-ume ratio, respectively. The final sol–gel system was stirredand ultrasonically agitated for 60 min and then aged for 1 h atroom temperature before the pre-treatment of the substrates.

A reference coating using only the first sol stirred for90 min was prepared in order to compare its performancewith that of the doped coatings. The different types of coat-ings used in this work are shown inTable 2.

The sol–gel films were produced by a dip-coating proce-dure conducted by immersion of the clean substrate in thefinal sol–gel solution for 100 s, followed by controlled with-drawal with a speed of 18 cm/min. After coating application,the samples were cured at 130◦C for 1 h in an oven.

2.2. Experimental techniques

Electrochemical impedance spectroscopy (EIS) was usedto estimate the electrochemical parameters associated withthe corrosion process occurring on the AA2024-T3 pre-treated with the different hybrid sol–gel coatings during im-mersion in 0.005 M NaCl solution. Low concentration ofthe chloride ions was used in order to decrease rate of thecorrosion processes since the sol–gel film is not a completec . Thed esti-m er, ass ctlye atedA tive

pre-treatments. Use of such corrosive environments to studyearly stages of pre-treated metallic substrates is describedelsewhere[16,17]. All the measurements were carried outat room temperature in a Faraday cage. A three-electrodearrangement was used, consisting of a saturated calomel ref-erence electrode, a platinum foil as counter electrode andthe exposed sample as a working electrode. The workingarea was 3.4 cm2. The EIS measurements were performedusing a Gamry FAS2 Femtostat with a PCI4 Controller. Theimpedance measurements were carried out in a 100 kHz downto 10 mHz frequency range with 10 steps per decade. All thespectra were recorded at open circuit potential with applied10 mV sinusoidal perturbation. At least two samples preparedfor the same conditions were tested in order to ensure re-producibility of the results. The impedance plots were fittedusing different equivalent circuits in order to simulate thedifferent stages of the pre-treated system during immersion.

The structure of the sol–gel coatings before and after im-mersion was studied by SEM (Hitachi S-4100 system withelectron beam energy of 25 keV) and EDS (energy disper-sive spectroscopy). The chemical composition of the sol–gelfilms was determined by XPS using a Microlab 310 (VGScientific) equipped with a Mg (non-monochromated) anodeand a concentric hemispherical analyzer. The spectra weretaken at constant analyzer mode (CAE = 30 eV).

The morphology of the sol–gel films was assessed bya stru-m tipc oft

3

3

as-sp witht s to

ted wi

oatings and they are working only as a pre-treatmentecreased rate of the corrosion allows more correctation of the processes on the early stages. Howev

hown below, such concentration of chlorides is perfenough to cause drastic corrosion impact to the untreA2024-T3 substrate or the alloy coated with less protec

Fig. 1. SEM micrographs for AA2024-T3 alloy coa

tomic force microscopy using a nanoscope digital inents fitted with a NanoScope III controller and a silicon

overed with PtIr5 in taping mode to prevent scratchinghe pre-treated surface.

. Results and discussion

.1. Structure and composition of the films

The SEM and AFM techniques were used in order toess porosity and structure of the different coatings.Fig. 1resents the SEM micrograph of the AA2024-T3 treated

he sol–gel film A. The surface of the sol–gel film seem

th film A: (a) plane view, (b) cross-sectional view at 40◦.


Fig. 2. Topography of the hybrid sol–gel film A (AFM).

be crack-free, containing few pinhole-like defects (Fig. 1a).The thickness of the film was estimated by the cross sectionanalysis (Fig. 1b) and is about 400–500 nm.

The AFM scan presented inFig. 2 demonstrates the sur-face morphology of the sol–gel coated substrate. The nano-sized particles incorporated into the film matrix are clearlydefined in this scan. The distribution of the nanoparticles isrelatively uniform and the particles have an average diame-ter around 50 nm. Several particles in the hybrid matrix havea bigger diameter and seem to result from agglomerates ofsmaller ones.

The composition of the hybrid films was investigated us-ing the XPS technique.Fig. 3depicts the XPS photoelectronspectra for Zr 3d and Ce 3d obtained on different coatings.The spectra were corrected for the C 1s at 285.0 eV. The re-sults evidence that the binding energy of the Zr 3d5/2 peaksdepends on the concentration of cerium ions. For the coatingswithout cerium nitrate addition the binding energy of the Zr3d5/2 photoelectron spectra is around 182.8 eV, being char-acteristics of the presence of ZrO2. However, the Zr 3d5/2ionization shifts towards lower binding energy (182.1 eV)for the coatings containing 1% of cerium nitrate. This resultmay be due to adsorption and/or absorption of cerium ionson zirconia nanoparticles. Simultaneously the Ce 3d5 photo-electron spectrum becomes evident. The well-defined ceriumpeak appears on the XPS spectrum only in the case of the sam-pp Cea ies.T theC ablyd ctionl ned9 (IV)c3 nd-i n theo esis

Fig. 3. Zr 3d (a) and Ce 3d (b) photoelectron spectra for different sol–gelcoatings.

[18]. Cerium could not be detected after immersion, showingthat cerium ions are released from the sol–gel coating.

3.2. EIS measurements

3.2.1. Equivalent circuit modelsThe impedance spectra can be used to provide adequate

modeling of the physicochemical processes on the coatedsubstrate during corrosion tests.Fig. 4presents the evolutionof the impedance spectrum of coating D (without cerium) af-ter different periods of immersion in the NaCl solution. After7 h of immersion the impedance spectra reveal the presenceof two time constants at 4× 103 and 0.1 Hz, which can beascribed to the sol–gel film capacitance,Ccoat and than in-termediate layer with capacitance,Coxide, respectively. Theintermediate conversion layer is formed due to interactionof Al OH groups with Si OH forming Al O Si covalentbonds as described elsewhere[4]. This oxide film is intact andalmost does not show a resistive response at low frequenciesafter 7 h of immersion. At this stage only two resistive partsare observed in the impedance plot. These appear at 105 and50 Hz, describing the resistance of the solution (Rsolut) andthe pore resistance of the sol–gel film (Rcoat), respectively.The equivalent circuit used for numerical fitting of the exper-

le with higher concentration of inhibitor ions. The Ce 3d5/2eak is composed by two components corresponding to3+

nd Ce4+ species, with predominance of the former speche peak at 916 eV, which is normally associated withe4+ oxidation state, was not revealed on spectra probue to the cerium species concentration lower than dete

imit of the XPS technique. The absence of the well-defi16-eV photoelectron peak in the case of several Ceompounds was found and discussed elsewhere[18]. The Ced5/2 peaks are very similar to those from the correspo

ng reference salts. Therefore, no significant changes ixidation state of the cerium occur during sol–gel synth


Fig. 4. Bode plots for AA2024 alloy coated with film D immersed for dif-ferent periods.

imental Bode plots during the initial stages is presented inFig. 5A. The increase of the immersion time leads to the de-crease of the pore resistance of sol–gel film due to formationand growth of new cracks and pores.

A new well-defined resistive part (Roxide) appears at lowfrequencies after approximately 50 h resulting from cracksthat are formed in the intermediate oxide layer as schemat-ically exhibited inFig. 5B. Formation of cracks in the in-termediate oxide layer is clearly demonstrated in the SEMmicrograph (Fig. 6). Defects in the oxide layer appear insidethe cracks of the hybrid coating. The EDS measurements (notshown) prove that cracks in intermediate layer appears in theplaces of intermetallic particles. In result the electrolyte caningress directly to the substrate forming conductive pathwaysand causing the low-frequency resistive response (Roxide).

The first signs of the corrosion process were detected bya developing additional time constant at 0.1 Hz after 60 h ofimmersion. This time constant is clearly shown inFig. 4af-ter 196 h of immersion. Thus, two additional elements wereadded to the equivalent circuit in order to account for thedouble layer capacitance (Cdl) and polarization resistance(Rpolar) of the corroded areas. The main corrosion processof AA2024-T3 in chloride-containing medium is the forma-tion and growth of pits as exhibited inFig. 7a. The pits usuallystart at zones around the intermetallic particles which havea nobler potential than the potential of the aluminium alloym ingt Al.T ag-g ted in

Fig. 7b clearly demonstrate that these metallic aggregates arevery rich in copper. The copper aggregates in the pits havesufficiently nobler potential and can act as effective cathodespromoting pitting initiation and growth. Formation of suchlarge copper aggregates can be achieved only due to copperredeposition processes. This statement is in good accordancewith previous results described the process of pits forma-tion in aluminium alloys[20]. The honeycomb-like structureof copper aggregates (Fig. 7a, insert) provides a diffusion-controlled pathway for the corrosion products and corrosiveagents; thus, a new time constant in the impedance spectrumat low frequency become visible (Fig. 4, 214 h). Addition ofa Warburg element (W) to the equivalent circuit (Fig. 5D)can provide an adequate model for the impedance spectrumat this stage. The mass transfer limitations during pit growthin the case of the aluminium alloys were described elsewhere[21].

3.2.2. Influence of the type of coating on the differentcomponents of the coating/metal interface

The low frequency impedance is dependent on the coat-ing and on the immersion time. During the initial stage ofimmersion all the coatings showed identical impedance val-ues. However the impedance decreases with time. The small-est changes were revealed for the coating with Ce-dopednanoparticles.Fig. 8 presents EIS spectra of these coatingsa lu-t v-a ely.C s wereu tioni rentf

cord-i

w l tot dn ngleac alentc

C

w ryi con-s gsb ood-n s Aa usede per-i . Fit-t iva-l of

atrix[19]. Dealloying of these intermetallics occurs durhe initial corrosion stages due to oxidation of Mg andhe result is the formation of honeycomb-like metallicregates inside the pits. The EDS measurements presen

fter 250 h of continuous immersion in 0.005 M NaCl soion. Impedance spectra inFig. 8were fitted using the equilent circuits 5B and 5D for coatings A and D, respectivonstant phase elements (CPE) instead of capacitancesed in all fittings presented in the work. Such modifica

s obligatory when the phase angle of capacitor is differom −90◦.

The impedance of the CPE depends on frequency acng to the following equation:

1

Z= Q(jω)n, (1)

hereZ is the impedance,Q a parameter numerically equahe admittance (1/|Z|) at ω = 1 rad s−1, ω the frequency, an≤ 1 a power coefficient calculated as ratio of phase at maximum of corresponding time constant to−90◦. Theapacitance values for the different elements in the equivircuit were calculated using the following equation:

= Q(ωmax)n−1, (2)

here ωmax is the frequency at which the imaginampedance reaches a maximum for the respective timetant[22]. The equivalent circuits were chosen for fittinasing on number of time constants and analyzing the gess of fits.Table 3presents parameters of the coatingnd D after 5 and 250 h of continuous immersion. Thequivalent circuits provide the adequate fitting of the ex

mental results and the good estimation of parametersing the experimental results with an incompatible equent circuit gives very high value of deviation (last column


Fig. 5. Schematic representation of the different stages of defects formation and respective equivalent circuits.

Table 3) and cannot be used for adequate estimation of sol–gelfilms.

The impedance of the aluminium alloy coated with the hy-brid film containing Ce-doped zirconia nanoparticles (coat-ing A) still shows high impedance value (above 106 � cm2)at low frequencies. Two maxima (two time constants), one

at 5× 104 Hz and another at 1 Hz can be seen in the phaseangle plot, which can be ascribed to the sol–gel film andan intermediate mixed oxide layer formed on the alloy/filminterface (as described above), respectively. Contrasting tothis behaviour the coating without cerium (coating D) showsimpedance values at a frequency of 10−2 Hz about five times


Fig. 6. Micrograph of typical crack in the intermediate layer.

lower. The impedance of the film with the Ce-doped ma-trix (not shown) reveals, at the same frequency, intermedi-ate values of impedance. Additionally in the case of the lasttwo films, the low-frequency (0.1 Hz) time constant appearsdue to starting of the corrosion activity as described above.

Fig. 8. Bode plots with respective fittings for coatings A and D after 250 hof immersion in chloride solution.

These results show that the hybrid sol–gel films prepared withcerium-containing zirconia nanoparticles confer higher cor-rosion protection than the undoped ones and those with thedoped hybrid matrix.

The evolution of parameters of the coated systems wasanalyzed in order to assess the corrosion protection prop-erties of the different pre-treatments. The parameters of theelectrolyte/sol–gel film/substrate system were obtained us-ing the fits of experimental spectra with equivalent circuits

Fig. 7. Micrograph of pit (a) and EDS spectra (b) of two different zones.

Table 3Parameters of the sol–gel film/substrate systems obtained from fitting of the

Coating A

IERRQnRQnRQnWG

mmersion time (h) 5 250quivalent circuit 5B 5B

solut (� cm2) 1135± 17 227± 12

coat. (� cm2) 1424± 256 421± 20

coat (nS cm−2) 371± 30 475± 180

coat 0.755± 0.009 0.745± 0.0362

oxide (k� cm ) 2392± 66 1170± 80

oxide (�S cm−2) 3.95± 0.05 4.30± 0.15

oxide 0.938± 0.005 0.958± 0.006

polar (k� cm2) – –

dl (�S cm−2) – –

dl – –(�S cm−2) – –

oodness 1.896e−3 949.2e−6

experimental impedance spectra with different equivalent circuits

Coating D

5 250 2505B 5D 5C

307± 8 231± 16 256± 914230± 172 375± 25 293± 14253± 13 487± 266 81± 360.743± 0.005 0.744± 0.052 0.903± 0.042
28790± 4800 128± 10 258± 44.30± 0.15 5.01± 0.24 7.76± 0.090.913± 0.036 0.913± 0.008 0.829± 0.003– 95± 32 –– 18.62± 2.88 –– 1.001± 0.150 –– 36± 6 –2.402e−3 327.4e−6 12.46e−3


Fig. 9. Evolution of the capacitance for the sol–gel film for the differentcoatings during immersion.

presented inFig. 5. Only models obtained highest goodnesswere used to fit experimental results and to extract parametersof the investigated systems.

3.2.2.1. Sol–gel film. The change of the sol–gel film capac-itance during immersion in the chloride solution is presentedin Fig. 9. Generally the capacitance of the dielectric filmdepends on the amount of absorbed water. Coating D pre-pared without addition of cerium inhibitor shows very stablecapacitance for 350 h. The sol–gel film A with Ce-dopednanoparticles is also relatively stable; however the capac-itance increases after around 200 h probably due to waterabsorption. The capacitance of the coating B (with Ce-dopedmatrix) exhibits significantly faster growth. Increase of thesol–gel film water uptake can be due to higher concentrationof the cerium inhibitor in the sol–gel matrix, since ceriumnitrate was introduced directly into the organosiloxane solin contrast to the film A, where the inhibitor was added tothe sol with zirconia nanoparticles. Probably the cerium ionscan decrease stability of the hybrid film leading to fasterhydrolytic destruction in the aqueous solutions.

Another important parameter is the pore resistance (Rcoat)of the sol–gel layer that characterizes the crack ability andporosity of the hybrid film. The pore resistance consists ofthe resistance of electrolyte in pores, cracks and pits con-n no m Dh mer-s t 25 ht thes pedc es-p tiono re-s itudep revi-o sea dopedc ring.

Fig. 10. Evolution of the pore resistance of the sol–gel film for the differentcoatings during immersion.

Only after 150 h the pore resistance of the coating B (withdoped matrix) is decreasing faster than coatings A and D,that is in a good accordance with the evolution of the capac-itance for film B. The pore resistance of the sol–gel film C isdropping very fast showing that high concentration of ceriumnitrate leads to formation of a fragile film with poor barrierproperties. Therefore the addition of the cerium nitrate seemsto negatively affect the barrier properties of the sol–gel layer.However incorporation of cerium into zirconia nanoparticlescan reduce this negative effect.

3.2.2.2. Intermediate oxide layer. The compactness of theintermediate oxide layer formed between the sol–gel film andthe metallic substrate is also very important from the pointof view of corrosion protection, since breakdown of this filmprovide direct ingress of the corrosive agents to the metallicsurface.Fig. 11presents the evolution with time of the resis-tance of the intermediate mixed oxide layer (Roxide). The ref-erence coating E prepared using only the hybrid organosilox-ane sol without addition of ZrO2 nanoparticles and cerium-based inhibitor shows very fast decrease of the resistance ofthe intermediate oxide layer from 4× 106 to 5× 103 � cm2

F r thed

ected in parallel to each other.Fig. 10shows the evolutiof the pore resistance of sol–gel films. The undoped filas the highest pore resistance for the beginning of imion; however the resistance decreases during the firswo orders of magnitude showing formation of cracks inol–gel layer. The initial pore resistance of the cerium-dooatings A, B and C is lower in comparison to coating Decially in the case of the film C with higher concentraf inhibitor ions. However, short after immersion the poreistances show fast increase of about one order of magnrobably due to blocking of the pores as described in pus works[5,23]. Coatings A and B after the initial increachieve pore resistance values close to the ones for unoating and keep similar values during further weathe

ig. 11. Evolution of the resistance of the intermediate oxide layer foifferent coatings during immersion.


Fig. 12. Evolution of the polarization resistance for the different coatingsduring immersion.

after only 24 h of immersion. The AA2024 samples pre-treated with the sol–gel system C with high concentration ofcerium show slower decrease of the resistance. The maximuminitial resistance of the intermediate layer was found for theCe-free coating D. However the resistance is dropping rela-tively fast and achieves 105 � cm2 after 330 h of immersion.The coating B (with cerium-doped organosiloxane matrix)shows a stable value for the intermediate layer impedanceduring the first 100 h of weathering tests. After the initialstable period the resistance of the oxide conversion layer isdecreasing relatively fast from 107 to 105 � cm2 that is closeto the value of the undoped D coating. The stable resistanceof the intermediate layer for 100 h can be obtained due tothe inhibiting and self-healing action of the cerium ions. Thecerium can suppress the cathodic reaction[12] preventingthe propagation of cracks in the intermediate layer. When thecerium ions are completely released from the sol–gel matrixthe resistance value decreases rapidly.

The resistance of the oxide layer for coating A (withcerium-doped nanoparticles) is about three times lower at thebeginning of immersion than for film B and behaves differ-ently with immersion. The resistance slowly decreases dur-ing 350 h of immersion from 5× 106 to 5× 105 � cm2 andfinally reaches a five times higher value than coatings B andD. This behaviour can be explained in terms of prolongedrelease of the cerium ions from the zirconia nanoparticles.A arti-c theo otec-t ionp

3 o-c resist a-t renth 024-

T3 coated with coating A shows the first signs of third timeconstant ascribed to the corrosion processes only at the finalstage of the immersion tests. Thus, the polarization resistancein this case can be measured only after 275 h of immersion.The value of the polarization resistance is at least one orderof magnitude higher than in the case of the other coatings.The impedance spectra of coating B (cerium in matrix) show ameasurable response of the corrosion processes after 150 h ofimmersion, which is in a good agreement with the evolutionof the resistance of both the intermediate layer and the sol–gelfilm. The polarization resistance for the alloy coated with filmB is around 106 � cm2 at this stage and fast decreases withimmersion. Decrease of the polarization resistance can startwhen the most part of the cerium ions is already released fromthe coating and the inhibition of the corrosion processes isdiscontinued. In the case of the cerium-free coating D thecorrosion processes appear twice faster after 75 h. The coat-ing C shows the first signs of corrosion after only 24 h ofimmersion. The polarization resistance in this case is suffi-ciently lower due to the poor barrier properties of this film.The reference coating E has higher corrosion activity andconsequently low polarization resistance immediately afterthe beginning of the immersion tests.

The above results demonstrate significant improvement ofthe corrosion protection due to incorporation of the cerium-inhibitor ions into the zirconia nanoparticles.

4

atingi oys.T f thec r theA

ve-m brids rredb w-e s tod ix.

iumo idel andi-d pre-t

ofn e in-h

R

01)

s a result the coating A prepared with zirconia nanoples doped with cerium inhibitor shows higher stability ofxide intermediate layer and confers better corrosion pr

ion properties from the point of view of long term corrosrotection.

.2.2.3. Corrosion process. The rate of the corrosion presses can be estimated measuring the polarizationance (Rpolar). Fig. 12 shows the evolution of the polarizion resistance during the immersion tests for the diffeybrid sol–gel coatings. The impedance spectra of AA2

-

. Conclusions

The EIS method can be used to model the metal/conterface of the sol–gel pre-treatments on aluminium allhe corrosion resistance of the coating and the kinetics oorrosion process in chloride solution were evaluated foA2024-T3 coated with hybrid sol–gel films.Incorporation of zirconia nanoparticles leads to impro

ent of the barrier properties of the organosiloxane hyol–gel coatings. Additional corrosion protection is confey doping the sol–gel film with cerium-based inhibitor. Hover the too high concentration of cerium nitrate leadegradation of the barrier properties of the sol–gel matr

The hybrid sol–gel coatings with incorporated zirconxide nanoparticles doped with cerium inhibitor prov

ong term corrosion protection and can be prospective cates for development of new environmentally friendly

reatments.The nanostructured zirconia particles play the role

anoreservoirs for storage and controllable release of thibitor.

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