Protective ability of hybrid nano-composite coatings with cerium sulphate as inhibitor against corrosion of AA2024 aluminium alloy

Progress in Organic Coatings 73 (2012) 95–103

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Protective ability of hybrid nano-composite coatings with cerium sulphate as inhibitor against corrosion of AA2024 aluminium alloy S. Kozhukharov a,∗ , V. Kozhukharov a , M. Schem b , M. Aslan b , M. Wittmar b , A. Wittmar b , M. Veith b a b

University of Chemical Technology and Metallurgy, Sofia 1756, Bulgaria INM – Leibniz Institute for New Materials, Campus D2 2, 66123 Saarbruecken, Germany

a r t i c l e

i n f o

Article history: Received 6 January 2010 Received in revised form 1 June 2011 Accepted 9 September 2011 Keywords: Corrosion AA2024 aluminium alloy Hybrid nano-composite coatings Linear voltammetry Electrochemical impedance spectroscopy Superficial morphology

a b s t r a c t The corrosion protective ability of hybrid oxy silane nano-composite coatings deposited on AA2024 by sol–gel technique was studied. The coatings are developed as an environmentally friendly alternative of the toxic chromium containing coatings on aluminium. A cerium salt, Ce2 (SO4 )3 , was used as inhibitor of the corrosion process. Two methods were applied to introduce the salt in the hybrid matrix: directly in the matrix, or by porous Al2 O3 nano-particles preliminary loaded by the salt. Atomic force microscopy (AFM) was used to evaluate the superficial morphology of the coatings, while their layer structure was studied by means of scanning electron microscopy (SEM). Linear voltammetry (LVA) and electrochemical impedance spectroscopy (EIS) were used for assessment of the barrier ability. The hybrid matrix was found to possess remarkable barrier ability which was preserved even after prolonged exposure of the coatings to a model corrosive medium of 0.05 M NaCl. In all cases, the cerium salt involved either directly or by Al2 O3 nano-particles proved to deteriorate the protective properties of the coatings and to accelerate pitting nucleation. The experimental results have shown that cerium sulphate, introduced in the by the both manners in the hybrid matrix did not efficiently inhibit the corrosion of AA2024, unlike the reported inhibiting properties of other cerium salts. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Aluminium alloys are known to be among the basic materials used in the aircraft industry for production of various metal parts and constructions. The alloy AA2024 is the object of special attention, due to its remarkable mechanical strength [1]. Nevertheless, this alloy possesses a relatively high susceptibility to local corrosion. The reason for this corrosion susceptibility is the presence of alloying elements as Cu, Mn, Mg, etc., which form intermetallics with different compositions [2]. The latter occupy a negligible part of the geometrical alloy’s surface [3], and initiate centers for local corrosion [4–6]. Thus, the remarkable mechanical strength of the alloy becomes useful only after application of different methods for corrosion protection. The experience so far has shown that such protection can be achieved by coatings of various types and compositions. A high protective efficiency has been attained by using

∗ Corresponding author. Tel.: +359 899 83 72 82; fax: +359 2 8685 488. E-mail addresses: [email protected] (S. Kozhukharov), [email protected] (V. Kozhukharov), [email protected] (M. Schem), [email protected] (M. Aslan), [email protected] (M. Wittmar), [email protected] (A. Wittmar), [email protected] (M. Veith). 0300-9440/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2011.09.005

chromium (VI) compounds [7]. Due to their toxicity, however, the compounds were prohibited in the electrical and electronic industry, since 2006, by the Restriction of Hazardous Substances Directive (RoHS) of EU. One year later, chromium (VI) coatings have been banned from the automotive industry according to the End of Live Vehicles Directive (ELV). Both directives have stimulated the investigation of new types of coatings free of Cr (VI) and other harmful metals [8–10]. The hybrid oxy-silane coatings obtained by sol–gel technique have proven to be a reliable alternative. The coating procedure enables the formation of stable Al–O–Si conversion layers between the native oxide layer and the oxy-silane coating [11]. It has been established, however, that the sol–gel coatings themselves do not provide sufficient corrosion protection. The coatings usually possess a micro-porous structure, and micro-cracks and ruptures appear after annealing (an inevitable technological process), as well as areas with non-uniform density and thickness. Considerable improvement of the protective performance is achieved by incorporation of nanoparticles of the oxides of rare-earth elements, like ZrO2 and CeO2 [8,12–15]. The efficiency has been even further improved by loading of the nanoparticles with corrosion inhibitors. The compounds of rare-earth elements are now considered to be among the best inhibitors. Hence, they could be used as an environmentally friendly alternative of chromium. Their inhibiting activity against corrosion of various aluminium–magnesium [16], aluminium–silicon

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[17] and other alloys has been studied. A number of works is dedicated to the AA2024 aluminium–copper alloy, because of its excellent mechanical strength [18,20,21]. Among all rare-earth elements, cerium has the highest inhibiting effect on the corrosion of the AA2024 alloy. The mechanism of inhibition can shortly be described as follows [19]: the partial reaction of oxygen reduction, O2 + 2H2 O + 4e− = 4OH− , occurs at cathodic areas of the alloy. The OH− -ions thus obtained, interact with the Ce-ions, producing weakly soluble hydroxides and oxides. These products precipitate over the cathodic areas and strongly and hinder the cathodic partial reaction. As a result, the entire corrosion process is blocked. This mechanism is described in detail in Ref. [2]. Chlorides and nitrates are the Ce-salts which are mostly used as inhibitors of Al-corrosion. The inhibiting effect of cerium sulphates is reported to be masked by the oxidizing ability of sulphate ions [20]. There is no information, however, about the inhibitive behaviour of these salts when incorporated in hybrid oxy-silane coatings. The aim of the present work was to investigate the inhibiting activity of Ce2 (SO4 )3 against corrosion of the AA2024 aluminium–copper alloy in 0.05 M NaCl solution. Protective coatings were obtained by introducing the Ce-salt in the hybrid matrix either directly or by preliminary loaded nano-particles of porous Al2 O3 . The protective or barrier ability of the coatings was evaluated with respect to their structure and composition by means of AFM, SEM and electrochemical methods (linear voltammetry, LVA and electrochemical impedance spectroscopy, EIS).

2. Experimental 2.1. Sol preparation A hybrid inorganic–organic sol was obtained by mixing three previously prepared solutions. First, the organic substructure was obtained by dissolving of 2,2 -bis-(4-hydroxyphenyl)propane (BPA, 97%, Aldrich, Germany) in isopropoxy-ethanol (p.a. 99.0%, Fluka, Germany). The second sol, used to create cross-links between organic and inorganic substructures, was prepared by hydrolysis of 3-glycidoxypropyl trimethoxysilane (GPTS, 98%, ABCR, Germany) with 0.1 M hydrochloric acid (NeoLab, Germany). The inhibitors were added to this sol in the concentrations shown in Table 1. Third, an inorganic sol was made by co-hydrolysis of methyltriethoxysilane (MTEOS, 98%, ABCR, Germany) and tetraethoxysilane (TEOS, 98%, ABCR, Germany) in the presence of SiO2 nano-particles (Levasil 300, aqueous, colloid-dispersed solution of amorphous SiO2 , Bayer AG, Germany). The hydrolysis was initiated by concentrated hydrochloric acid. After the completion of the hydrolysis, the three solutions were mixed together and the cross-linking agent 1-methylimidazol (MI, 98%, ABCR, Germany) was added 10 min before deposition of the coating. The pot life of the sol without addition of MI lasts several months, when the sol is stored in a cool place [21].

2.2. Substrate pre-treatment and coating procedure Single sol–gel layer coatings were deposited over samples of the AA2024 alloy having an area of 40 mm × 100 mm. The samples were first degreased with acetone, then cleaned with Metaclean T2001 (Chemie Vertrieb Hannover, Germany), etched in an alkaline cleaner (Turco Liquid Aluminetch Nr. 2 from Turco Chemie, Germany) and finally desmutted in Turco Liquid Smutgo NC. After each of the steps mentioned above, the substrates were rinsed with water. The coatings were deposited by a dip-coating procedure with a withdrawal speed of 9 mm/s, and subsequently annealed for 4 h at 120 ◦ C.

Four kinds of coatings were deposited and studied for the purpose of the present work. Selected characteristics of these coatings are presented in Table 1. The additives introduced in the hybrid matrix are Ce2 (SO4 )3 (97%, Aldrich, Germany), Al2 O3 (AluC, specific surface area 105 m2 /g, Degussa, Germany), and alumina nanoparticles loaded by Ce2 (SO4 )3 . 2.3. Preparation of alumina containers and loading by cerium sulphate Aluminium oxide was washed with deionised water until a conductivity of the filtrate of 20 ␮S/cm was reached, and freeze-dried afterwards. The desired amount of cerium sulphate was solved in deionised water. Alumina powder was added up to a concentration of 10 wt.%. The slurry was stirred for 10 h and then freeze-dried at 15 ◦ C. The dried particles were re-dispersed in ethanol for 10 h by stirring with a magnetic stirrer at room temperature, followed by a 30 min treatment with ultrasonic agitation (Sonifier 450; Branson, Danbury, USA). Alumina powder was used as dispersant for this redispersing step and 5 wt.% polyvinylbutyral with respect to the dry amount (Mowital B30T from KSE, Frankfurt/Main) was added [22]. The alumina content of the ethanol slurries of Al2 O3 loaded with Ce2 (SO4 )3 was 17 wt.% with respect to the total amount. 2.4. Methods and conditions for testing The observation and characterization of the sample surface was done by atomic force microscopy. The observations were performed by AFM EasyScan 2 produced by Nanosurf (Switzerland), supported by cantilever TAP190G, produced by BUDGETSENSORS. The measurements were done over a square area with a linear size of 49.5 ␮m. The resolution was 256 points per line; the imaging rate was 2 s per line. The images were obtained at “scan forward” mode and scanning was carried out from down to up in static regime. In addition, cross-sections of the coated samples were examined by a scanning electron microscope JOEL – GSM 35 in order to observe interfaces. The corrosion protective ability of the coatings was examined by potentiodynamic linear voltammetry and by EIS. An electrochemical three-electrode cell with 100 ml in volume was used. The coated samples served as working electrodes. The working area had the form of a circle with defined area, equal to 0.64 cm2 . The counter electrode was platinum net. It had more than two orders of magnitude larger surface area than the working electrode in order to eliminate the influence of its capacitance on the measurements. A standard silver chloride electrode, Ag–AgCl/3 M KCl, model-6.0733.100, produced by Metrohm, was used as reference electrode (RE). The polarization curves were obtained by a Potentiostat/Galvanostat PGstat 30/2 – Autolab, produced by Ecochemie (Netherlands). They were acquired in potentiodynamic regime, with a linear sweep rate of 5 mV/s. The measurements were carried out in the potential interval from −0.600 to + 0.600 V with respect to the open circuit potential (OCP), since a very good reproducibility of results was ascertained in this voltage interval. At least two samples from each coating type underwent testing procedures. All measurements were accomplished at room temperature in 0.05 M NaCl solution. This value of the concentration is enough high to provoke corrosion activity for relatively short exposure time, and simultaneously – enough low to enable the observation of the inhibitor’s effect. In order to avoid external electromagnetic influence, the cell was enclosed in a Faraday cage. A special analytical block FRA-2, product of Ecochemie, was used for the impedance measurements. All spectra were obtained at OCP in the frequency range from 105 down to 2 × 10−3 Hz at seven frequency steps per decade with signal amplitude of 20 mV. Due to the

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Table 1 Overview of thickness and composition of the coatings studied. Coating code

A B C D

Ingredients

Thickness (␮m)

GMT (matrix)

Al2 O3 nano-particles (wt.%)

Ce2 (SO4 )3 (wt.%)

+ + + +

– 8 – 18

– – 2.0 2.0

extraordinary low currents through the working electrodes (of the order of nano Amperes), a considerable scattering of the phase shift (ϕ) values, especially in the low frequency range, was observed. The experimental points in Bode plots looked like a “summer night sky”, and, consequently, no relationship between the phase shift (ϕ) and the frequency (f) could be established. For that reason, it was necessary to increase the amplitude up to 100 mV during some of the experiments. The same approach has been used by other authors [23] as well. A high reproducibility of the results would mean that they correspond to the basic requirements for EIS measurements: linearity, stability and absence of scattering signals [24]. The results were supplementary checked according to the test of Kramers–Kroenig. 3. Results and discussion 3.1. Superficial morphology, structure and composition Superficial morphology reflects the internal structure of the coatings and thus enables to assess the influence of their composition on the structure. Fig. 1 represents AFM photographs of the superficial topography of the four types of coatings studied. It can be seen that the surface of the referent coating A is smooth, without clearly expressed nodules, cavities, etc. This topography indicates a completely homogeneous structure. The surface of coating B, prepared by addition of 8% Al2 O3 nano-particles, reveals the presence of oval mounts of aggregates, a sort of clusters formed by the particles. These clusters are not equally distributed in the bulk of the coating and they probably deteriorate its barrier ability. The surface of the sample with coating C, which contains 2% Ce2 (SO4 )3 , is very rough and significantly differs from that of the referent coating. The superficial effect is probably due to deterioration of the bulk homogeneity, in comparison with the referent coating A. This detrimental influence is expectable to reflect on the barrier ability of the samples. The sample with coating D seems to unify the effects of both additives. Due to the incorporation of the Ce-salt in the alumina nano-particles, however, the structural changes in the bulk of the hybrid matrix are more weakly expressed than those in the bulk of coating C. The above observations indicate a possible deterioration of the protective properties of the coatings in the case when cerium sulphate is directly introduced in the hybrid matrix. This assumption was checked on comparing the values of two basic parameters, and namely the mean value of the roughness, Sm , and the peak-tovalley height, Sy . Numeric values of the parameters are shown in Fig. 2. The diagrams shown in Fig. 2 undoubtedly confirm the conclusions regarding the influence of the additives on the structure and the superficial morphology of the coatings. The values of both parameters, Sm (Fig. 2a) and Sy (Fig. 2b), indicate that the referent coating A has the smoothest surface. Its mean Sm value is more than two orders of magnitude lower than that of the rest coatings. As seen in Fig. 2a, the highest roughness is observed for coatings C with direct incorporation of Ce2 (SO4 )3 . The addition of the Ce-salt to the sol–gel system during the deposition of the coatings probably leads to agglomeration, within the gel matrix, of the SiO2 nano-particles from the inorganic sol. After the

8.87 ± 0.50 8.74 ± 0.45 8.82 ± 0.41 –

enclosure of Ce2 (SO4 )3 in the Al2 O3 nano-particles (coating D), the impact of the salt on roughness proved to be less significant than that established for coating C. Coating B exhibits almost the same roughness as coating D, obviously due to agglomerating alumina nano-particles. The diagram b in Fig. 2 shows that the highest value of the peak-to-valley height Sy is measured for coating D containing 18% Al2 O3 nano-particles impregnated with 2% Ce-sulphate. This value is more than three times higher than the corresponding value of coating B, which does not contain Ce-sulphate but includes 8% alumina nano-particles. On the other hand, all coatings studied contained almost the same amount of SiO2 nano-particles in the hybrid matrix. The results let us assume that the presence of nano-particles always enhances agglomeration and cluster formation; with larger amounts of nano-particles, higher mean values of Sy are observed. Moreover, the addition of Ce2 (SO4 )3 further promotes the agglomeration processes between the particles. Cerium sulphate affects not only the morphology and respectively the structure of the coatings; it seems to have some impact on the intermediate oxide layer formed during the deposition process. Fig. 3 shows photographs performed by scanning electron microscopy of cross-sections of samples covered with coatings A and C. It can be seen that the intermediate oxide layer of the cerium-containing coating C exhibits a larger number of defects and a higher degree of nonuniformities than the respective under layer of the referent coating A. A larger number of defects certainly deteriorate the barrier ability of coatings. In spite of the ascertained negative effect of Ce2 (SO4 )3 on superficial morphology, some inhibitive action of the salt may be present. Voltammetric measurements were undertaken in order to assess such an action. 3.2. Voltammetric measurements Voltammetric measurements are widely used for investigations of the corrosion behaviour of metals. They provide valuable information about corrosion rate, inhibition efficiency towards the corrosion process, passivation of the metal surface, etc. In the presence of a protective film on the metal surface, a difference between the corrosion potential at the metal/film and the potential occurring on the film/electrolyte interface can be measured. This potential difference originates from the ohmic drop in the bulk of the protective film. With the coatings studied in the present work, this ohmic drop might reach relatively high values in the range of 1 V, thus leading to deformations in the shape of the polarization curves, and, consequently, to incorrect interpretation of the experimental data. That is the reason to use linear voltammetry rather as a comparative method for qualitative assessment of the protective ability. Fig. 4 summarizes potentiodynamic polarization curves recorded for the investigated coatings after 1 h of exposure of the samples to the corrosive medium of 0.05 M NaCl. The curves are compared with that of the bare AA2024 alloy. The analysis of the polarization curves enables to clarify the influence of both the alumina nano-particles and the cerium salt on the barrier ability of the corresponding coatings. As seen in the

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Fig. 1. Superficial images of different types of coatings: (a) coating A; (b) coating B; (c) coating C; (d) coating D.

Fig. 2. Numeric values of the parameters Sm (a) and Sy (b) for the investigated coatings.

figure, the polarization curve of the bare alloy differs remarkably from those for the coated samples. The difference appears so according the current densities for the cathodic and anodic branches of the curves, so regarding the corrosion potential value. A sharp increase of the current density in the anodic branch of

the polarization curve of the bare alloy is observed. According to some authors [25], this increase is related to the initiation of pitting nucleation, and intergranular corrosion. Deep pits were actually observed over the surface of the bare alloy after its exposure to 0.05 M NaCl.

Fig. 3. SEM-images of cross sections of two coated AA2024 samples: (a) coating A and (b) coating C.

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Fig. 4. Potentiodynamic polarization curves of the four types of coatings and of the bare alloy, recorded after 1 h of exposure to the model corrosive medium of 0.05 M NaCl: (A) referent coating; (B) coating with 8% Al2 O3 nano-particles; (C) coating with 2% Ce2 (SO4 )3 ; (D) coating with 18% Al2 O3 nano-particles impregnated with 2% Ce2 (SO4 )3 .

The lack of such rise of the current density in the anodic branch for the polarization curves of the coated samples (samples A and B) reveals that the process of anodic dissolution is blocked. The currents in the anodic branches for the coated samples are with orders

99

of magnitude lower than this of the bare metal. The referent coating A, although without additives, revealed the best barrier ability among the coatings studied, the currents in the anodic branch were at least 5 orders of magnitude lower than those of the bare alloy. Similar, but slightly deteriorated protective properties exhibited coating B with 8% Al2 O3 nano-particles added. Both coatings proved to rend the alloy nobler, showing much more positive values of the corrosion potential (Ecorr ) than that of the bare alloy. The addition of Ce2 (SO4 )3 to the composition of the coating leads to remarkable changes in the shape of the polarization curves. As mentioned in the beginning, two strategies were used for introduction of the salt: either directly in the hybrid matrix (coating C) or indirectly through Al2 O3 nano-particles previously loaded by Ce2 (SO4 )3 (coating D). In the latter case, the nano-particles act as containers for the Ce-salt and support its gradual release. The shapes of the polarization curves of coatings C and D reveal that cerium sulphate significantly deteriorates the barrier ability of these coatings with respect to the referent one; the corresponding currents in the anodic branches of the polarization curves are several orders of magnitude higher than those of coating A. The deterioration is more significant after direct introduction of Ce2 (SO4 )3 in the matrix. At the same time, Ecorr shifts towards more negative values and the “noble” character of the pure sol–gel matrix vanishes. The detrimental impact of the inhibitors introduced to the matrix is reported by other authors, as well [26,27]. They explain it

Fig. 5. Evolution of impedance spectra of (a) coating A and (b) coating C in dependence of their time of exposure to the corrosive medium in both Nyquist and Bode plots.

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Fig. 6. Schematic presentation of equivalent circuits which fit the experimental impedance spectra of coating D for the initial exposure period (a) and after an exposure of 24 h (b).

either by changes in the coating’s structure, or deactivation of the inhibitor. Furthermore, sharp rises of the currents in the anodic branches of the polarization curves were observed for both coatings, being quite similar to the current rise established for the bare alloy and related to local corrosion processes. Hence, the introduction of Ce2 (SO4 )3 besides to deteriorate the barrier ability of the coating but also seems to promote the local corrosion of the alloy. 3.3. Impedance measurements The voltammetric measurements by themselves cannot clarify the physicochemical processes which occur in the bulk or at the interfaces of the phases in the system metal/coating/electrolyte. In order to elucidate the nature of these processes and their evolution,

as well as to characterize them quantitatively, the EIS method was applied. Fig. 5a depicts, in Bode and Nyquist plots, impedance spectra of the referent coating A and their evolution with the time of exposure to the corrosive medium. The Bode plot reveals that the response of the system has a predominantly capacitive character in the highest frequency range; at middle range frequencies, the resistive response appears. The impedance in this part of the spectrum can be ascribed to the capacitance Ccoat of the coating and the resistance Rcoat of the electrolyte in the pores of the coating, represented by a RC-unit of Ccoat and Rcoat in parallel. After 24 h of exposure, another time constant appeared in the lowest part of the spectrum, in the frequency range between 0.1 and 0.01 Hz, accompanied by an increase of the total impedance (the impedance modulus). A quite similar increase of the low frequency impedance has been reported by other authors, but only in the cases of presence of corrosion inhibitors [28–30]. Various suggestions were reported for explication of this fact. Some authors relate it substrate corrosion accompanied by the building-up of corrosion products [30,31]. According to others – a repair of the intermediate oxide film takes place, with hampering the corrosion activity within its defects [32]. No experimental evidence has been furnished, however, to confirm the latter suggestion [33]. An important finding of the present work is related to the fact that an increase of the low frequency impedance with exposure time was observed even in the absence of inhibitors. After prolonged exposure of coating A to the corrosive medium, the total impedance begins to decrease, and when on the sample’s surface distinguishable pitting appears, the spectrum changes completely. A new, well shaped time constant appears in the frequency range between 10 and 0.1 Hz after 1400 h of exposure (Fig. 5a, symbols over green continuous line). According to us, this time constant has to be attributed to the capacitance Cdl and the resistance Rct characterizing the substrate’s corrosion. It could not manifest itself before degradation of the coating, because of its overlapping with the time constant of the still undestroyed coating. This assumption proved to be rather constructive for the structural impedance modelling. The impedance spectra of coating B (not shown) with addition of 8% porous Al2 O3 -nanoparticles did not differ in shape from those of coating A. In the low frequency part of these spectra, a similar increase of the total impedance with the exposure time was observed. The addition of Ce2 (SO4 )3 to the gel matrix induces considerable changes in the shape of the impedance spectra in comparison to those of the referent coating A. Fig. 5b presents the Bode and

Fig. 7. Conformity between experimental (symbols) and calculated (continuous lines) spectrum of coating A after 24 h of exposure to the corrosive medium, presented in Nyquist and Bode plots.

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Table 2 Numerical values of the impedance modelling parameters of coatings A and D, obtained after choosing the appropriate equivalent circuits for the experimental spectra. Impedance modelling parameter

Coating A

Coating D

24 h Rcoat (M cm2 ) Y0coat (×10−9 sn / cm2 ) ncoat Roxy (M cm2 ) Y0oxy (×10−9 sn / cm2 ) noxy Rct (M cm2 ) Y0oxy (×10−9 sn / cm2 ) nedl

9.070 0.563 0.949 67.410 1.090 0.738 53.100 47.920 0.770

24 days ± ± ± ± ± ± ± ± ±

0.620 0.009 0.001 1.740 0.060 0.014 2.770 3.520 0.030

201 0.323 0.971 393 0.762 0.634 850 22.30 0.756

Nyquist plots of the impedance spectra and their evolution with exposure time for coating C, which contains 2% Ce2 (SO4 )3 introduced directly into the gel matrix. As seen in the figure, the spectra of coating C exhibit three distinguishable time constants. The time constant in the high frequency range of the spectrum has to be attributed, as it was done above, to the impedance components of the coating Ccoat and Rcoat . The time constant in the middle frequency range from 10 to 1 Hz was ascribed to the capacitance Coxy and the resistance Roxy of the intermediate oxide film, and this one, at lowest frequency range was related the corrosion of the substrate. It is represented by the charge transfer resistance Rct across the interface and the capacitance Cdl of the electric double layer. The presence of the three time constants even after a few hours of exposition would mean that the electrolyte easily penetrates the coating, quickly reaches the substrate and attacks it. It is obvious that a reason for this behaviour is the presence Ce2 (SO4 )3 is in the gel matrix. The deteriorating behaviour of the salt could be accounted for its negative temperature coefficient of its solubility. At the temperature of annealing of the coating, 120 ◦ C, the solubility of Ce2 (SO4 )3 is low enough, and a certain amount of the salt in the bulk of the gel matrix does not dissolve. During the exposure of the samples to the corrosive medium at room temperature, the electrolyte penetrates through the coating and dissolves the solid residuals of the Ce-salt. As a result, plenty of pathways form and facilitate the access of the electrolyte to the oxide interlayer. The spectra of coating D (not shown) with incorporated Ce2 (SO4 )3 in the Al2 O3 nanoparticles, possess similar shape as those of coating C. The unique difference concerned the low frequency time constant, which appeared at a later stage (after 24 h) of the exposure of the samples to the corrosive medium. Structural impedance modelling was performed in order to interpret the experimental data obtained by EIS measurements. Thus, all impedance spectra reflecting the evolution of the investigated systems during exposure to the corrosive medium were fitted to appropriate equivalent circuits, shown in Fig. 6. A constant phase element (CPE) Q instead of the pure capacitance C was employed for both of the equivalent circuits. This substitution is always necessary when the phase shift (phase angle ϕ) is lower than 90◦ . Similarly to the capacitance, Q is a frequency dependent parameter according to the following formula: Q = Y0−1 (jω)

−n

± ± ± ± ± ± ± ± ±

4h 36 0.006 0.002 66 0.114 0.095 93 2.63 0.059

1.22 ± 11.53 ± 0.780 ± 5.87 ± 63.10 ± 0.723 ± – – –

24 days 0.04 0.32 0.028 0.02 2.61 0.017

0.034 19.15 0.746 0.288 1429 0.769 0.729 3174 0.770

± ± ± ± ± ± ± ± ±

0.001 1.12 0.005 0.007 35 0.007 0.073 3265 0.034

Fig. 6a was used to fit the impedance spectra of coating D for the initial exposure period. At a later stage (after 24 h), when the barrier properties of the coating becomes deteriorate, corrosion activity of the substrate appears, the equivalent scheme was substituted by another circuit shown in Fig. 6b. In the cases when a “tail” appeared in the lowest frequency part of Nyquist plots, probably due to diffusion limitations, an additional Warburg element W was added to the equivalent circuit of coating A, or additional CPEs to the circuits of coatings B and D, respectively. In the cases of coatings A and B, an excellent agreement between the experimental spectra and those calculated on the base of the corresponding equivalent circuits was achieved on the assumption that the high frequency semicircle in the Nyquist plots is a result of overlapping of two time constants  of the coating,  coat = Rcoat Ccoat ,

(1)

Here Y0 and n are frequency independent constants. When the values of n are in the range: 0.8 < n ≤ 1, the respective Q has the meaning of capacitance. The capacitance was calculated from the values of Y0 and n obtained from the impedance modelling according to the equation below: C = Y0 ω0n−1

(2)

Here ω0 is the value of the angular frequency for which the imaginary component of the impedance reaches its maximal value for the respective time constant [33,34]. The equivalent circuit depicted in

Fig. 8. Evolution of the resistance Rcoat (a) and the capacitance Ccoat (b) of the investigated coatings with the time of exposure to 0.05 M NaCl.

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and of the intermediate oxide layer,  oxy = Roxy Coxy . Such overlapping is possible when the capacitances of the both time constants have comparable values [35]. Fig. 7 shows the conformity between the experimental spectrum of coating A and the spectrum calculated on the base of the equivalent circuit in Fig. 6b. Numerical values and standard deviation of the impedance modelling parameters of coatings A and D are shown in Table 2. The resistance of the electrolyte in the pores of the coating, Rcoat , increases with the exposure time for coating A, from Rcoat = 9.07 × 106  cm2 after the initial 24 h to Rcoat = 2.01 × 108  cm2 after 575 h of exposure. It seems that during the exposure an additive intermediated layer forms. This fact is confirmed by the shape of the impedance spectra of the referent coating A, and their evolution within the exposure time. The nature of this additive film is not enough clear, probably this film it is formed by corrosion products. The data in Table 2 show that coating D, prepared by addition of 18% Al2 O3 nanoparticles filled with 2% Ce2 (SO4 )3 , has a relatively high barrier ability only during the initial 4 h of exposure, with Rcoat = 1.22 × 106  cm2 . After 575 h of exposure, however, the value of the resistance breaks down to Rcoat = 3.4 × 104  cm2 ; the cerium sulphate which is incorporated in the nano-particles, passed to the gel-matrix, and caused destructive changes in its structure. The values of the capacitances Ccoat = 5.0 × 10−10 F/cm2 and Coxy = 4.0 × 10−10 F/cm2 of coating A after 24 h of exposure were calculated from formula (2). The both values are actually of the same order, thus supporting our assumption for overlapping of the time-constant  coat of the coating with the time constant  oxy of the intermediate oxide layer. Fig. 8 presents the time dependence of the values for the electrochemical parameters obtained by the impedance modelling of the coatings studied during their exposure to the 0.05 M NaCl solution. As seen in Fig. 8a, the referent coating A possesses the highest resistance Rcoat among the coatings studied. There is an initial increase of Rcoat with the exposure time up to a value which remains almost constant even after prolonged exposure to the corrosive medium. The hybrid oxy-silane coating without any additives definitely possesses remarkable barrier ability; it can be used as a base for development of advanced composite materials for various applications. The introduction of 8% nano-particles of porous Al2 O3 causes deterioration of the barrier ability, by decrease of the respective parameter Rcoat by about one order of magnitude. Probably, the nano-particles increase the micro-porosity of the coating enhancing the penetration of electrolyte in its bulk. The direct addition of Ce2 (SO4 )3 to the hybrid matrix (coating C) renders notable negative influence over its barrier ability; Rcoat of coating C is more than three orders of magnitude lower, compared to this of the referent coating. As was mentioned during the explications for the shape of the spectra of coating C, this decrease is probably related to the negative thermal coefficient of the solubility of the Ce-salt. The detrimental influence of the Ce-salt on the barrier ability decreases, when the salt is added indirectly, through preliminary loaded by the salt, Al2 O3 -nanoparticles (coating D). The nano-particles act as containers of the corrosion inhibitor, providing its gradual release to the bulk of the hybrid matrix. After a long enough exposure time, when the salt is completely leached and the total amount of the inhibitor is introduced into the hybrid matrix, the resistance Rcoat of coating D decreases, becoming comparable to that of coating C. The capacitance is a relevant feature of hybrid coatings because it depends on their ability to uptake water. As shown in Fig. 7b, the capacitance of the referent coating A has the lowest values among the samples studied. After the initial decrease it remains almost unchanged even after prolonged exposure to the corrosive medium. The addition of 8% Al2 O3 nanoparticles leads to increase of

the capacitance as a consequence of the increased ability for water uptake. The addition of the cerium sulphate, however, proved to affect strongly the capacitance of the coatings. After introduction of 2% Ce2 (SO4 )3 by preliminary loaded alumina particles (coating D), the capacitance increased by at least one order of magnitude with respect to that of the referent coating. It looks that this salt deteriorates the stability of the coating, and accelerates its destruction. No consistent results could be obtained for the capacitance of the coatings with directly introduced Ce2 (SO4 )3 . This lack of stability is probably result of the complicated interactions between the changes of the dielectric permeability and the thickness of the coating during its exposure to the corrosive medium. Here should be mentioned that the capacitance is influenced so by the changes of the dielectric permeability, so by the coating thickness, as result of the water uptake. 4. Conclusions The hybrid oxy-silane coating without additives was found to possess remarkable barrier ability and efficiently to protect the AA2024 alloy against corrosion; the resistance of electrolyte enclosed in the pores of the coating, defined by the EIS data, exceeded 108  cm2 . The addition of porous alumina nanoparticles to the gel matrix deteriorates its barrier ability. This detrimental effect caused by the sulphate much remarkable, especially when the salt is involved directly in the gel-matrix. The salt proved to promote agglomeration of nano-particles in the coatings, to increase their roughness, to accelerate pitting nucleation and to facilitate the penetration of the corrosive agent through the pores of the coatings, and as consequence – their barrier ability decrease by more than three orders of magnitude with respect to these of the referent coating. The experimental data obtained did not confirm that this cerium salt involved in the gel matrix possesses whatever inhibitive effect. The fact that in the literature, other cerium salts are pointed to be efficient inhibitors of the aluminium corrosion, gives the reason to assume to anionic moiety of the Ce-salts involved in the gel-matrix to affect strongly its barrier ability. Acknowledgements The support from EC in the frame of 6fp (IP Contract No. NMP3CT-2005-011783) MULTIPROTECT is greatly acknowledged. The authors thank Assoc. Prof. Dr. Ivan Nenov for numerical fitting procedure and useful discussion carried out.

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