Active corrosion protection of AA2024 by sol–gel coatings with cerium molybdate nanowires

Electrochimica Acta 112 (2013) 236–246

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Active corrosion protection of AA2024 by sol–gel coatings with cerium molybdate nanowires K.A. Yasakau ∗ , S. Kallip, M.L. Zheludkevich, M.G.S. Ferreira CICECO-Department of Materials and Ceramic Engineering, University of Aveiro, Campus Universitario de Santiago, 3810-193, Aveiro, Portugal

a r t i c l e

i n f o

Article history: Received 31 May 2013 Received in revised form 19 August 2013 Accepted 19 August 2013 Available online xxx Keywords: Sol–gel AA2024 Inhibitor Cerium molybdate nanowires Active corrosion protection

a b s t r a c t Amorphous cerium molybdate nanowires (CMN) have been used to impart active corrosion protective properties to hybrid sol–gel coatings on AA2024. Inhibitive ability of the novel sol–gel coatings against corrosion has been studied in detail by electrochemical impedance spectroscopy (EIS) and scanning vibrating electrode technique (SVET). Localized electrochemical measurements demonstrate significant suppression of corrosion activity in micro-scale defects made in sol–gel coatings loaded with CMN when compared to blank systems. The mechanism of active protection involves release of cerium and molybdate ions from the sol–gel coating during immersion in NaCl electrolyte and their inhibiting action in corroding defects. Crown Copyright © 2013 Published by Elsevier Ltd. All rights reserved.

1. Introduction The aerospace industry employs high strength aluminum alloys as structural materials for airplanes because of combination of valuable mechanical properties and strength to weight ratio [1]. Nevertheless, aluminum alloys are highly susceptible to corrosion, especially in chloride containing environments, which is mainly associated to the presence of numerous intermetallic phases [2–6]. Corrosion mechanisms of 2024 aluminum alloy, which is widely used in aerospace industry, have been actively investigated [7–9]. High corrosion susceptibility of such alloy is attributed to the presence of high content of copper as a main alloying element. Dealloying of S-phase intermetallics and copper redeposition has been identified as first steps in corrosion of AA2024 [2,3,5,7–12]. The role of intermetallic clusters in localized corrosion progress and pit initiation was emphasized by Boag et al. and Hughes et al., who suggested that local clustering plays an important role in cooperative corrosion and pit development [7,9]. Uncontrollable corrosion can lead to unpredictable failure of metallic structures. In order to reduce corrosion impact and increase service life of metallic structures protective coatings are applied. Typical protection system consists of several layers such as pre-treatment, primer and topcoat [13]. Pre-treatment plays an essential role in a protection system increasing adhesion between the metal and the organic coating [14,15] and very often providing barrier and active

∗ Corresponding author. Tel.: +351 234378146; fax: +351 234378146. E-mail address: [email protected] (K.A. Yasakau).

protection [16]. Traditional chromate based conversion coatings have been the most effective pre-treatments used in aeronautics [16]. However, due to toxicity of Cr(VI)-containing pre-treatments, alternative environmentally friendly systems have been developed such as boric–sulphuric acid anodized coatings, rare earth and inorganic conversion coatings, sol–gel coatings etc. [16–18]. Sol–gel coatings have been extensively studied as a potential alternative pre-treatment [19–22]. Sol–gel coatings properties such as adhesion, flexibility, durability and hardness can be tailored. Although sol–gel layers have some advantages compared to other protection coatings, they can offer only passive corrosion protection. Defects and pores present in the sol–gel coatings or developed during the service life create pathways for corrosive species initiating corrosion processes on the metal surface. Therefore, the addition of corrosion inhibitors to the coatings in order to provide active protection is of great interest. Among different inorganic inhibitors such as vanadates, molybdates, permanganates [23–26] cerium salts when added to sol–gel coatings could impart active corrosion protection [25–28]. However, in spite of being potential inhibitors for aluminum alloys [29–31] cerium salts can chemically react with the sol–gel matrix causing changes of its properties such as described in Ref. [32]. Moreover, the addition of highly soluble inhibitors can destroy the coating matrix by forming crystalline aggregates after curing or causing excessive blistering [25,33]. In order to surpass the negative effects of direct inhibitor addition different strategies have been recently developed. These strategies allow to control inhibitor release by immobilizing the inhibiting compound on a carrier such as oxide nanoparticles [34,35] or zeolite fillers [36].

0013-4686/$ – see front matter. Crown Copyright © 2013 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.08.126

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The incorporation of inhibitive compounds with low solubility product such as strontium polyphosphates [37] in sol–gel network is another way. More advanced strategies of inhibitor controlled release employ cation- or anion-exchange solids [38–40] and layerby-layer polyelectrolyte coated nanoparticles [41] for inhibitor delivery on demand. Although such approaches of inhibitor incorporation partially solve the issue of sol–gel stability, the overall corrosion inhibiting performance depends mainly on the efficiency of the used inhibitors. There is a number of works reported in literature suggesting that a combination of different inhibiting species such as rare earth elements with organophosphates or inorganic anions can provide a synergistic inhibition effect, conferring a superior corrosion protection for alloys and metals [42–46]. For example a mixture of soluble rare earth salts and molybdates demonstrates a synergistic effect in corrosion inhibition of AA2024 [44,45]. Although coating matrix enriched with both potentially cooperating inhibitors may not provide synergism due to lack of inhibitors timely release, the concept is still attractive. Recently amorphous cerium molybdate nanowires were suggested for corrosion inhibition of AA2024 in NaCl solution [47]. Corrosion protection of AA2024 has been improved because both cerium and molybdate species are released and actively acting as inhibitors. The use of amorphous nanowire pigments is attractive because of the effect of pigment crystallinity and size on solubility [48]. The main goal of the present work is to confer active synergistic inhibition functionality to hybrid sol–gel coating by its modification with cerium molybdate nanowires. The concept is based on cooperation of one dedicated cathodic (Ce3+ ) [29] and another potentially anodic inhibitor (MoO4 2− ) [49] when released from the sol–gel matrix on demand. Amorphous cerium molybdate nanowires were introduced during sol–gel synthesis to ensure their compatibility with the components of the hybrid formulation. Active corrosion protection offered by the sol–gel coatings was characterized by using electrochemical methods such as EIS and SVET. Transmission electron microscopy (TEM) and scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) were used to provide complementary microstructural information.

2. Experimental 2.1. Materials Unclad AA2024-T3 with the following composition wt.% (Cu 3.8–4.9; Fe 0.5; Cr 0.1; Mg 1.2–1.8; Mn 0.3–0.9; Si 0.5; Ti 0.15; Zn 0.25; other 0.15; Al balance) was used as a metallic substrate. 3 × 4.5 × 0.1 cm panels were chemically treated according to procedures typically used in the aerospace industry including: alkaline cleaning in Metaclean T2001 at 68 ◦ C for 25 min, alkaline etching in TurcoTM Liquid Aluminetch N2 at 65 ◦ C for 45 sec and acid etching in TurcoTM Liquid Smutgo NC at 30 ◦ C for 7 min. Rinsing with distilled water was used between treatment steps and at the end of cleaning procedure for removal of different soluble chemicals from the metal surface. To synthesize the cerium molybdate nanowires, sodium molybdate solution (19 g/L) cooled down to 10 ◦ C was pumped using a gear pump with a flow of ca. 275 mL min−1 into the reactor containing cerium sulphate solution (7.3 g/L) cooled down to 10 ◦ C. Following the mixing process the resulting mixture was stirred for 1 h at 10 ◦ C. Then, the obtained material was centrifuged at 1000–1500 rpm during 30 min and washed with 8 L of deionized water. Finally, the water remaining in the precipitate was exchanged by addition of 2 L of pure ethanol. A gray-yellowish paste was obtained [47]. The X-ray studies [47] demonstrate a

237

Table 1 Sol–gel systems with and without cerium molybdate nanowires. Cerium molybdate mass in coating

Thickness, ␮m

Reference name

– 0.8 wt.% 1.6 wt.%

1.8 ± 0.2 1.7 ± 0.1 1.7 ± 0.3

Blank Sg CMN Sg CMNx2

non-defined material and the broad reflexes detected may be related to the initiation of crystallization processes in the amorphous material. The obtained XRD pattern has not been associated to any known cerium molybdate species, implying that the assynthesized cerium molybdate material is structurally different from conventional cerium molybdate crystals. 2.2. Sol–gel synthesis and coating application Hybrid sols were prepared by a controllable hydrolysis of metalorganic (zirconium(IV) propoxide (TPOZ)) and organosiloxane (3-glycidoxypropyltrimethoxysilane (GPTMS)) precursors in 2-propanol solution with aqueous nitric acid solution. The detailed procedure [28] is the following. The first solution was obtained by mixing TPOZ precursor (70 wt.% in n-propanol) with ethylacetoacetate in 1:1 volume ratio during 20 min. Then the solution was hydrolyzed during 1.5 h in the presence of 0.316 M nitric acid aqueous solution in the ratio 3:1 of acidified water to TPOZ. During hydrolysis the solution was ultrasonically agitated. The second solution was obtained by mixing GPTMS in 2-propanol in 1:1 volume ratio. The solution was hydrolyzed by 0.316 M nitric acid in 2:1 molar ratio of water to GPTMS under constant mechanical stirring during 1 h. Both TPOZ and GPTMS containing solutions were mixed and ultrasonically agitated during 1 h followed by aging for 1 h after which the final sol–gel solution was ready for coating application. A slurry of CMN material in ethanol containing 14 wt.% of solid content was introduced into hybrid sol–gel solution at the step of mixing both TPOZ and GPTMS containing solutions. A stable dispersion of nanowires in sol–gel solution was obtained. The etched AA2024 plates were immersed in the final hybrid sol–gel systems during 100 s and withdrawn with the speed of 18 cm/min. The coated plates were held in open air during 1 h at room temperature (∼22 ◦ C) and then cured at 120 ◦ C during 80 min. The concentration of inhibitor in the dry sol–gel coating was 0.8 wt.% and 1.6 wt.% and the samples have been denominated respectively as Sg CMN and Sg CMNx2 (Table 1). The unmodified coating (Blank) was prepared according to the sol–gel preparation and application procedures without CMN addition. Average thickness of the prepared sol–gel film is within the range of 1.7–1.8 ␮m as demonstrated by SEM analysis of samples cross-sections (5 different measurements, not shown) (Table 1). 2.3. Experimental methods Electrochemical impedance spectroscopy was employed to assess the corrosion performance of the hybrid sol–gel films at immersion conditions (0.5 M NaCl) using a Gamry FAS2 Femtostat with a PCI4 controller. A conventional three-electrode cell setup, which comprises a saturated calomel reference electrode (SCE), a platinum foil as a counter electrode and the coated AA2024 samples with an exposed surface area of 3.35 cm2 as a working electrode, was used. The measurements were carried out at temperature 22 ± 1 ◦ C in a Faraday cage to avoid external electromagnetic interference. The impedance measurements were done at open circuit potential with applied 10 mV RMS sinusoidal perturbation in the frequency range of 2.6 × 10−3 to 1 × 105 Hz with 7 data points per decade. The impedance plots were fitted using commercially available Echem Analyst (Gamry Inc.) software.

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Fig. 1. SEM micrographs of three sol–gel coatings before immersion Blank (a), Sg CMN (b) and Sg CMNx2 (c); bare cerium molybdate nanowires are presented in figure (d).

The scanning vibrating electrode technique measurements were carried out using an Applicable Electronics (USA) system. The technique is based on measurement of potential gradients in solution associated to ionic fluxes originated from localized oxidation and reduction reactions. The microprobe had a spherical black platinum tip of 10 ␮m in diameter and vibrated in two directions (normal and parallel to the surface) with amplitude of 20 ␮m. The frequencies of vibration were Y 398 Hz and X 122 Hz, respectively. Only signals from the field normal to the surface (Z axis) were considered in the present study. The measured voltage differences were converted to current densities (normalized for 1 cm2 ) by a calibration routine performed with a point current source (microelectrode with a tip of ∼3 ␮m) driving a current of 60 nA at 150 ␮m of the vibrating probe. Maps of 40 × 40 data points were obtained with the probe vibrating at a distance of 100 ␮m above the sample during immersion. Coupons of coated samples (1 × 1 cm) were glued to an epoxy holder in such a way to leave open only a window of sol–gel coating with dimensions approximately of 3 × 3 mm. Active corrosion protection provided by the sol–gel coatings has been tested in place with intentionally created artificial defects. Defects were made to induce a local corrosion activity and monitor its progress during immersion in 0.5 M NaCl corrosive solution. Two defects with a size of 130 (±10) ␮m separated by a distance of 1 mm were created using a home-made micro-indenter. Scanning electron microscopy/energy dispersive spectroscopy using Hitachi S-4100 system with electron beam energy of 25 keV has been used to study the microstructure and elemental composition of the samples. Cross-sectional observations of the sol–gel coatings were performed using TEM Hitachi H-9000 with electron beam energy 300 kV. X-ray diffraction (XRD) measurements were performed with a Philips X’Pert MPD diffractometer (Nifiltered Cu K␣ radiation, tube power 40 kV, 50 mA; X’celerator detector, and the exposition corresponded to 2 s per step of 0.02). The release of cerium and molybdate species from sol–gel coatings after immersion in 0.5 M NaCl solution was studied by Horiba Jobin Yvon model M inductively coupled plasma optical emission

spectrometry (ICP-OES). Detached sol–gel films having mass of 100 ± 5 mg were immersed in 0.5 M NaCl solutions with pH 3, 6.5 and 10 adjusted either by HCl or NaOH. The concentration of Ce and Mo elements was measured after different immersion time in the respective electrolytes and expressed in ppm (mg of species per 1 kg of solution). 3. Results and discussion 3.1. Characterization of sol–gel coatings The blank sol–gel films deposited on alloy substrate are transparent and colorless, though cerium molybdate containing coatings are slightly yellowish. SEM investigation showed that the unmodified hybrid films are uniform, defect- and crack-free (Fig. 1a). Due to the cleaning and etching of AA2024 craters are formed on the alloy surface. Darker places on SEM micrographs of sol–gel coatings are associated with a slightly thicker sol–gel layer formed in such craters. The micrographs obtained on sol–gel coatings containing cerium molybdate nanowires Sg CMN and Sg CMNx2 (Fig. 1b and c) show well defined white filaments attributed to the presence of cerium molybdate nanowires. Fig. 1d shows an SEM image of bare CMN powder. The nanowires are uniformly distributed inside the sol–gel coating matrix without provoking formation of any additional defects in the hybrid layer. These results demonstrate a good compatibility between the hybrid sol–gel matrix and CMN. The detailed study of the sol–gel coating morphology was performed employing TEM. Fig. 2a presents a cross-section transmission micrograph of blank sol–gel layer. The coating is dense and adherent to the alloy surface. In contrast to the blank hybrid layer the appearance of sol–gel coatings with cerium molybdate additives (Sg CMN and Sg CMNx2) is different (Fig. 2b and c). Cerium molybdate inclusions are visible in the TEM micrograph as dark inclusions due to the presence of heavy elements. It is important to notice that the nanowires are homogeneously distributed in the sol–gel matrix without formation of agglomerates larger than

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239

Fig. 2. TEM micrographs of blank sol–gel coating (a) and coating modified with cerium molybdate nanowires Sg CMNx2 (b); EDS analysis (c) has been performed on the dark inclusions showed in picture (b).

200 nm, which is in accordance with SEM observations presented above. An example of EDS spectrum made on the dark black particle with a diameter around 20–60 nm shows a clear signal of Ce, whereas the signal from Mo is overlapped with the signal of Zr present in the sol–gel matrix (Fig. 2c). Usually oxide nanoparticles have a good compatibility with sol–gel matrix and do not cause deterioration of barrier properties of the sol–gel coatings [50,51]. On the other hand the incorporation of microparticles in sol–gel matrix can lead to defect formation due to large particle agglomerates formed during the coating preparation. Further examination of alloy sol–gel interface reveals the presence of a thin region (not presented) corresponding to the alumina film formed after the etching process of the bare alloy plates. Studies reported in literature confirm the existence of such a layer after chemical etching of aluminum alloys [47,51].

3.2. Electrochemical investigations 3.2.1. EIS study Electrochemical impedance spectroscopy is a useful tool which can provide important information on the physicochemical processes occurring on the coated alloy during immersion [22,41,52]. The evolution of different parameters, such as barrier properties or polarization resistance, during immersion provides essential information on active corrosion protection properties and long term performance.

Bode and Nyquist plots of sol–gel coatings with and without inhibitors after 3 days of immersion in 0.5 M NaCl solution are presented in Fig. 3. The impedance spectra show two well defined time constants that are typically observed in the impedance spectra of the sol–gel coated aluminum alloy [22,28,41,53]. The one at high frequency around 104 Hz can be attributed to sol–gel coating response and the time constant at lower frequencies is associated to mixed oxide layer. The resistive plateau at around 103 Hz corresponds to the pore resistance of the hybrid layer. The resistance of sol–gel coatings with cerium molybdate pigment is slightly higher than in the case of blank sol–gel layer. The low frequency impedance is also higher for sol–gel coatings loaded with CMN demonstrating lower defectiveness of the oxide layer (Fig. 3). The behavior of the mixed oxide after 3 days is still mainly capacitive (having phase angle below −70◦ ) and the resistive response from the mixed oxide is not yet visible especially for inhibited samples. The corrosion activity is not yet detectable at this period and becomes evident only after longer immersion time. Fig. 4 presents Bode and Nyquist plots of Blank and Sg CMNx2 samples and their replicas (as an example) after 7 days of immersion in 0.5 M NaCl solution. It is clearly seen that the inhibited samples confer higher impedance compared to Blank samples. For the purpose of discussion of impedance results and fitting procedure only representative samples will be shown. However, parameters obtained after impedance fitting will be presented as averaged values obtained for samples and their replicas.

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|Z|, Ωcm

2

5

10

Mixed Oxide

4

10

3

10

2

10 -3 -2 -1 0 1 2 3 4 10 10 10 10 10 10 10 10 Frequency, Hz

ZrSi ZrSi_CMN ZrSi_CMNx2

Coating -10 -20 6 10 -30 Corrosion 5 -40 10 -50 4 Mixed 10 -60 Oxide 3 -70 10 -80 2 10 -90 -3 -2 -1 0 1 2 3 4 5 10 10 10 10 10 10 10 10 10 Frequency, Hz

a) 10

7

2

6

10

-10 -20 -30 -40 -50 -60 -70 -80 -90 5 10

θ, deg.

a) 10

Coating

θ, deg.

ZrSi ZrSi_CMN ZrSi_CMNx2

7

|Z|, Ωcm

240

0.002

-Zimag/MΩ

8

b)

0.004

3.7 mHz

6

3727 Hz 0.002

0.000 0.000

0.002

0.004

5.1 mHz

4 Blank Sg_CMN Sg_CMNx2

2 0

2

4

6 8 2 Zreal/ΜΩ cm

10

6

|Z|/Ω cm

2

10

5

10

4

10

3

10

2

10 -3 -2 -1 0 1 2 3 10 10 10 10 10 10 10 f/Hz

-Z

/ΜΩ cm2 imag

b)

10 8 2.6 mHz

6

-10 -20 -30 -40 -50 -60 -70 -80 -90 4 5 10 10

o

7

10

θ/

Blank #1 Blank #2 Blank #3 Sg_CMNx2 #1 Sg_CMNx2 #2 Sg_CMNx2 #3

Blank #1 Blank #2 Blank #3 Sg_CMNx2 #1 Sg_CMNx2 #2 Sg_CMNx2 #3

4 2 0

0

2

4 6 8 2 Zreal/ΜΩ cm

10

Fig. 4. Bode (a) and Nyquist (b) plots of Blank and Sg CMNx2 samples and their replicas after 7 days immersion in 0.5 M NaCl.

Fig. 5 presents impedance spectra of coated AA2024 after 21 days of immersion in 0.5 M NaCl solution. As it can be seen the coating resistance decreases for all the systems indicating weakening of sol–gel barrier properties with time (Fig. 5). The difference in the performance of the coatings is also visible at low frequencies. After about 7 days of immersion signs of a third time constant appear at low frequency. This relaxation process is associated with the

0.001

8 6

2.6 mHz

9998 Hz

0.000 0.000

0.001

0.002

4 2.6 mHz

2 0

0

Fig. 3. Bode (a) and Nyquist (b) plots after 3 days immersion in 0.5 M NaCl.

a)

10

cm2

cm2

b)

2.6 mHz

-Zimag/MΩ

10

0

2

4 6 8 2 Zreal/ΜΩ cm

Blank Sg_CMN Sg_CMNx2 10

Fig. 5. Bode (a) and Nyquist (b) plots after 21 days immersion in 0.5 M NaCl.

corrosion activity and cannot be clearly distinguished up to a couple of weeks because it stays very close to the frequency at which the second time constant associated to oxide layer is located. At longer immersion time, for example after 21 days of immersion, the difference between the two relaxation processes increases as it can be seen in Fig. 5. Nyquist plots provide a better comparison of low frequency between the inhibited and uninhibited systems (Fig. 5). It can be clearly seen that inhibited systems demonstrate higher impedance at low frequency and consequently slower corrosion processes in comparison to the blank hybrid coating. The evolution of the coating system and the corrosion processes were quantified by fitting the impedance spectra using appropriate equivalent circuits. The applicability of different models has been verified by comparing Chi square parameters which have typical values of around 0.6 × 10−3 and less representing a good fitting. However, the selection of equivalent circuit model should also account for the estimated errors for the fit values. Typical error of fitting corresponding to sol–gel coating was less than 5%. The fitting errors for mixed oxide layer resistance (Rmix ) and polarization resistance (Rpolar ) were around 10–20% and 30% respectively. Higher errors are attributed to the fact that at low frequency the resistive part corresponding to Rmix and Rpolar is not fully visible on EIS spectra and both time constants are overlapping. At the beginning of immersion the impedance spectra can be adequately fitted by the equivalent circuit that includes only two time constants the first one representing the sol–gel coating capacitance (Ccoat ) and sol–gel film pore resistance (Rcoat ) and the second one associated to mixed oxide layer capacitance (Cmix ) and resistance (Rmix ) (Fig. 6a). The mixed oxide layer parameter accounts for the properties of native aluminum oxide layer and passive oxide layer formed due to possible action of the inhibitive pigment (CMN). The interpretation of impedance spectra attributed to aluminum oxide layer for AA2024 has been discussed in previous papers [41,47]. For the purpose of fitting the constant phase elements (CPE) were used instead of capacitances in order to take into account the dispersive character of the time constants originating from the nonuniformity of the dielectric layers [54]. Bode

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241

Rcoat/Ω cm

2

a)

Blank Sg_CMN Sg_CMNx2

4

10

3

10

0

100

200

300

400

500

t/h

b)

Fig. 6. Equivalent circuit models used for EIS data fitting. Rsol (electrolyte solution resistance) parameter has been placed inside the dotted case indicating its experimental inaccessibility in the used frequency range.

Blank Sg_CMN Sg_CMNx2

8

7

10

0

100

200

300

400

500

t/h 8

10

Blank Sg_CMN Sg_CMNx2

2

c)

Rpolar/Ω cm

plots presented in Fig. 3 show solid lines representing the fitted impedance spectra using the described model (Fig. 6a). A new time constant related to the corrosion activity starting in the defects is added to the equivalent circuit used for fitting the spectra obtained after longer immersion times (Fig. 5b). The main corrosion process of AA2024-T3 in chloride-containing medium is the formation and growth of pits. Usually they appear on zones around the intermetallic particles which have a nobler potential than the potential of the aluminum alloy matrix. The intermetallic zones enriched in copper promote anodic dissolution of alloy matrix and lead to the development and growth of pits [7–9]. Solid lines shown in Fig. 5 represent fitting results according to the described equivalent circuit model (Fig. 6b). The graphics presented in Fig. 7 show average values of Rcoat , Rmix and Rpolar which were obtained by fitting impedance spectra of samples and their replicas. Evolution of the sol–gel coating resistance (Rcoat ) during immersion in 0.5 M NaCl solution is presented in Fig. 7a. The film resistance is almost equal for all the coating systems at the beginning of tests, ca. 30 k cm2 . A fast drop of the resistance occurs during the first day followed by a monotonous decrease during longer immersion time. Although all sol–gel coatings show clear decrease of barrier properties, sol–gel coatings modified with cerium molybdate demonstrate somewhat higher sol–gel film resistance compared to unmodified sample (Blank) (Fig. 8a). Higher sol–gel film resistance can be caused by a slightly denser sol–gel matrix formed in the presence of cerium molybdate pigment. During sol–gel preparation the cerium molybdate can release some cerium species in liquid sol–gel at low pH, which can act as a sol–gel matrix modifier. Several studies show an improvement of sol–gel matrix properties due to incorporation of cerium based inhibitors [55–57]. Cambon et al. related the increase of sol–gel barrier properties with the improved polymerization of the organic matrix [32]. On the other hand the nanowires present in sol–gel matrix can reinforce the coating, which can explain slightly higher barrier effect in the case of doped samples. The reported sol–gel coatings do not provide a very high barrier effect. However, the incorporated inhibitors can stabilize the metal/oxide interface providing an additional active corrosion protection. The evolution of mixed oxide layer resistance (Rmix ) for different sol–gel coatings is presented in Fig. 7b. The best performance was demonstrated for Sg CMNx2 system that shows the highest initial resistance. After one day of immersion the resistance was around 7 × 107  cm2 . After 5 days of immersion a stepwise decrease of Rmix until 1.7 × 107  cm2 took place, followed by a monotonous drop down to 9 × 106  cm2 by the end of immersion (Fig. 7b). Sg CMN system initially shows the resistance around 6 × 107  cm2 which is still higher compared to unmodified

Rmix/Ω cm

2

10

7

10

0

100

200

300

400

500

t/h Fig. 7. Average values of fitting parameters indicating trends in evolution of (Rcoat ) (a), mixed oxide film resistance (Rmix ) (b) and polarization resistance (Rpolar ) (c) obtained after fitting EIS spectra.

coating (Blank) 3.5 × 107  cm2 . During immersion Rmix monotonously decreases to about 6 × 106  cm2 by the end of corrosion testing (Fig. 7b). The evolution of oxide layer resistance demonstrates that there is an enhanced corrosion protection of aluminum alloy 2024 during the first 200 h of immersion in NaCl electrolyte provided by inhibitor containing coatings. Such active protection is clearly conferred by cerium molybdate additive. Nevertheless, at longer immersion time the inhibiting effect decreases as shown by the continuous decrease of impedance. The development of corrosion after longer immersion is most probably caused by exhausting of the inhibitive species from the coating adjacent to the corrosion induced defects. The depletion of inhibiting action does not seem to be very fast taking into account the fact that sol–gel coatings on aeronautic aluminum alloys are considered as a pre-treatment layer rather than a standalone coating. The application of primer and top coats will drastically reduce the rate of release of the inhibiting species. Moreover such systems usually are not employed in full immersion conditions.

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Fig. 8. Optical photographs, SVET maps and selected profiles of ionic currents representing the maximal anodic and cathodic activities at artificial defects during 7 days of immersion in 0.5 M NaCl corrosive media. Blank sol–gel coating (a) and sol–gel coating doped with CMN (Sg CMNx2) (b).

Fig. 7c presents evolution of polarization resistance (Rpolar ). The time constant related to corrosion activity becomes distinguishable after a day of immersion on blank hybrid coating. Rpolar for Blank system during immersion drops to around 1 × 107  cm2 and then fluctuates around 5–9 × 106  cm2 . Polarization resistance for samples coated with inhibitor-containing sol–gel films can be adequately resolved in impedance spectra only after longer immersion time demonstrating values around 2 × 107  cm2 after one week of immersion and then falling down to 1 × 107  cm2 indicating a delay of the corrosion activity. The experimental results suggest that the addition of cerium molybdate imparts active corrosion protective properties to the coatings reducing the corrosion progress during immersion in NaCl solution. It is well known that imperfections, pores and defects that exist in sol–gel coatings can assist the ingress of electrolyte during immersion in corrosive solution [20,22,28]. When reaching the metal surface the aggressive species can initiate local corrosion processes which can develop into pitting if no countermeasures are taken to suppress corrosion. In order to clarify better the corrosion protection mechanism additional

experiments have been performed using localized electrochemical microprobe method, namely SVET.

3.2.2. SVET study Localized electrochemical techniques have demonstrated high potential for the quantification of localized corrosion processes and self-healing ability of protective coatings. Usually the localized activity appears as pitting, intergranular, crevice or micro-galvanic corrosion on alloy intermetallics. Localized processes can be examined by scanning reference electrode technique (SRET) or scanning vibrating electrode technique (SVET) demonstrated in early works of Isaacs and Ogle [58,59]. More recently other groups have successfully applied SVET [60–64]. SVET technique is based on the detection of ionic fluxes close to active substrates. In the case of corrosion processes such current gradients are originated from localized anodic and cathodic reactions occurring at different zones of metal surface. The cathodic current usually originates in oxygen reduction or

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hydrogen evolution reactions with the formation of hydroxyl ions:

−

O2 + 2H2 O + 4e → 4OH ,

(1) (2)

The respective anodic current (flux of cations) is created by the dissolution of metal according to the following reaction: Me → Men+ + ne−

Al → Al

−

+ 3e ,

Mg → Mg2+ + 2e− .

Blank Cathodic Sg_CMNx2 Anodic

(3)

In the case of AA2024 the anodic activity is attributed mainly to anodic dissolution of aluminum and magnesium from the solid solution and dissolution of Mg and Al from intermetallics such as the S-phase (Al2 CuMg), according to reactions (4) and (5). 3+

Sg_CMNx2 Cathodic

Intensity/a.u.

−

Al Si Zr

(4)

Mo Cl 2

(5)

The measured SVET signal can originate from ionic fluxes by diffusion of ions generated by electrochemical processes at the metal surface and from migration of ions of electrolytic solution. Depending on the distance between the probe and the metal surface one or another component will dominate. The measurements were performed at a distance of 100 ␮m. At such distance the probe is likely to be in the diffusion layer. Therefore the contribution to the measured signal by diffusion of ionic fluxes exists. However, given a high concentration of electrolyte the effects of Na+ and Cl− migration cannot be excluded. The measured sum of both components is proportional to the localized electrochemical activities at the interface and can be converted to the current density in the active zone. Measurement of localized currents can provide information on kinetics of electrochemical processes in coatings defects as well useful insights on the self-healing properties. Modification of the coating with inhibitive compounds such as CMN (reported herein) can offer additional protection, reducing the kinetics of the electrochemical processes in the local defects. The active corrosion protection ability has been studied by SVET on the sol–gel coatings with two intentionally created well-defined microdefects. Two defects are required to give a chance for the self-corroding AA2024 system to form a natural microgalvanic system with separated anodic and cathodic activities, taking place on the different artificial defects. The concept has been successfully applied in previous works [62,65] and allows SVET to distinguish between cathodic and anodic ionic currents. In the case of only one defect, the two corrosion processes would provide the fluxes of OH− and Men+ at the same location and due to the equalization (cross-cancelation) of ionic fields the signals might not be detected adequately by SVET. On the other hand, when both cathodic and anodic reactions are too close, the ionic current fluxes generated by both processes may not cross the scan plane of the SVET probe and, consequently, will not be detected [64]. The SVET results for blank (Blank) film and the one with cerium molybdate nanowires (Sg CMNx2) are presented in Fig. 8a and b respectively. The samples were immersed in 0.5 M NaCl for up to one week. SVET maps for 1 h, 24 h, 48 h and one week of immersion are presented showing the evolution of ionic current profiles across the defects. The optical photographs taken in the beginning and final stage of immersion are also shown. It can be seen that corrosion has evolved faster in the case of blank coating. The anodic defect in Fig. 8a remarkably expanded and became surrounded with corrosion products. The SVET measurements performed on this coating demonstrate high anodic and cathodic activities attributed respectively to dissolution of active elements such as Al and Mg due to corrosion of intermetallics and surrounding alloy matrix

3

Ce 4

5 keV

Cu 6

7

8

Fig. 9. EDS analysis performed on samples after SVET measurements in the anodic and cathodic defects.

Concentration/ppm

2H2 O + 2e− → 2OH− + H2 ↑,

243

Ce 1 day Ce 7 days Mo 1 day Mo 7 days

17 16 15 5 4 3 2 1 0 pH 3

pH 6.5

pH 10

Fig. 10. Concentration of Ce and Mo detected after 1 and 7 days of immersion of sol–gel films Sg CMNx2 in 0.5 M NaCl solutions with pH 3, 6.5 and 10.

and oxygen reduction on copper rich zones. The maximal ionic currents observed by SVET (when scanning at 100 ␮m above the sample) tend to show some slight increase with time in the case of blank sol–gel coating (Fig. 8a) reaching the values of 38 ␮A cm−2 on anodic defect and −32 ␮A cm−2 at cathodic one. The blank coating shows clearly higher ionic currents (Fig. 8a) than the sample with CMN doped coating, Fig. 8b. The local corrosion activity on CMN containing coating is very low and the defects are still nearly passive after 1 week of immersion in corrosive medium (Fig. 8b). The initial very low activity observed during the first day of exposure diminishes and becomes undetectable after 48 h. The observed results demonstrate an active corrosion protection effect conferred by the addition of cerium molybdate nanowires. In order to understand the mechanism of active protection the corroded defects after SVET measurements have been analyzed by EDS (Fig. 9). In the cathodic defect of CMN-containing coating apart from the elements representing the sol–gel coating (Si and Zr) and alloy substrate (Al, Cu) signals of cerium and molybdenum along with Cl can be identified. Mo peak is partially overlapped with the signal from Zr. On the other hand only one signal of molybdenum is present in the anodic defect. The EDS spectrum taken at the cathodic defect of unmodified (Blank) coating is presented for comparison. The fact of detection of Ce and Mo in the defects of the CMN containing coating suggests an important role of these elements on the observed protection effect. 3.3. Inhibitor release studies Inhibitor release studies have been performed to supplement experimental findings presented above. Fig. 10 presents concentration of cerium and molybdenum species released from the Sg CMNx2 sol–gel film to the electrolyte at different pH values. The highest amount of released cerium can be detected in 0.5 M

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3.4. Inhibiting mechanism

intensity/a.u.

CMN powder After immersion in 0.5M NaCl

10

20

30 o 2θ/

40

50

60

Fig. 11. XRD diffractogram of amorphous CMN powder and the powder after immersion in 0.5 M NaCl solution.

NaCl solution with pH 3. Concentration of cerium increases to about 16.5 ppm after 7 days of immersion. In neutral NaCl solution the amount of Ce increases from around 2.5 to 5.5 ppm after 1 and 7 days of immersion respectively. The smallest content of Ce (about 3 ppm after 7 days) was detected at higher pH. Unlike for cerium, the release of molybdenum species at low pH is the lowest being about 0.4 ppm. Concentration of Mo after 7 days of immersion increases up to about 0.8 ppm and 1.2 ppm at pH 6.5 and 10 respectively. This tendency can be explained in terms of thermodynamic stability of Ce and Mo species at low and high pH [66]. According to Pourbaix diagrams insoluble cerium hydroxides are formed at high pH. Such behavior is consistent with this study, which indicates lower cerium amount detected at pH 10. In regard to Mo, at low pH the stability of soluble molybdate species decreases which can explain lower amount of Mo found at pH 3 (Fig. 10). At neutral pH the results demonstrate that Ce release was several times higher compared to Mo. The higher concentration of Ce in solution can be related to the release mechanism of inhibiting species in 0.5 M NaCl solution, which has been addressed in our previous work [67]. Upon immersion in 0.5 M NaCl solution amorphous CMN material transforms into the crystalline compound (NaCe)0.5 MoO4 identified by XRD measurements (Fig. 11). It can be seen that structural transformation of amorphous material to crystalline one results in the release of inhibiting cations and substitution of Ce by Na cations in the crystalline lattice. When considering cerium molybdate acting as a sparingly soluble compound having the following formula (Ce2 (MoO4 )3 ) the molar ratio of Ce to Mo will be 1:1.5. Release studies from the sol–gel coatings presented in Fig. 10 give Ce/Mo molar ratio of about 4.7:1 at pH 6.5 in 0.5 M NaCl, which is higher compared to what should be obtained in solution in contact with sparingly soluble compound. A higher concentration of Ce species at low pH can be also associated with higher solubility of cerium molybdate crystalline compound in such conditions. It can be speculated that the enhanced release of cerium species can be attributed to the effect of NaCl concentration. CMN demonstrates “smart” release properties at higher concentration of Na+ in solution, similarly to those observed for bentonite nanoclays in which metal cations located in the gallery can be exchanged for cations in electrolyte solution [39]. However, a direct comparison between the release of cerium and molybdenum species from the sol–gel coating and the solution containing cerium molybdate is not straightforward. Diffusion of Na and Cl ions inside the coating as well as the concentration (or activity) of ions in coating at the vicinity of pigment particles should be considered. Moreover, interaction of Ce or Mo species with the sol–gel coating can reduce the concentration of species measured by analytical methods.

The electrochemical results demonstrate superior protective properties of sol–gel coatings with cerium molybdate nanowires. The release of Ce cations and molybdate anions from the hybrid matrix is controlled by the presence of sodium chloride and the pH of the environment. The inhibiting mechanism for cerium species has been discussed in many works and is related to the formation of highly insoluble cerium oxides and hydroxides preferentially in active cathodic zones [12,29,30]. Compared to high inhibiting power of cerium salts, inhibiting efficiency of molybdate is lower and the most important shift of the pitting potential of AA2024 occurs when the concentration of molybdate ions is sufficiently high [49]. Studies by Jakab et al. also report inhibition of oxygen reduction reaction on AA2024 and intermetallics inclusions in cerium (III) and molybdate containing solutions [68,69]. Critical concentration of Ce(III) representing the inhibitor concentration that suppressed corrosion of AA2024 was found to be around 0.0023 mol L−1 (Ce(III)) in 0.5 M NaCl [69]. Such value was obtained extrapolating from the linear dependence of critical inhibitor concentration and Cl− concentration [69]. Critical inhibitor concentration values for Mo(VI) were much higher compared to Ce(III). Release data presented above indicates that the concentration of Ce(III) in bulk neutral NaCl solution is around 5–6 ppm (Fig. 10) which is lower compared to findings presented by Jakab et al. [69]. However, if inhibitor is released in coating pores the local concentration of inhibiting species might be high enough to suppress corrosion. In the previous works [47,67] the inhibiting mechanism of cerium molybdate nanowires has been related to the formation of thin protective layer on the top of intermetallics and surrounding alloy matrix. The presence of molybdenum containing deposits in both defects presented in Fig. 9 has been attributed to red/ox properties of molybdates which can precipitate on cathodic and anodic sites [67]. Precipitation of cerium hydroxides occurs on cathodic zones where sufficient concentration of hydroxyl is generated. It is important to emphasize the fact that the combination of cerium cations and other inhibiting anions can often lead to synergistic corrosion inhibiting properties [44,45,70]. Forsyth et al. reported high inhibiting efficiency of rare earth diphenyl phosphates for AA2024 in NaCl solutions [70]. Although both cerium and diphenyl phosphate species have been found in inhibiting solutions, it was proposed that the enhanced inhibiting effect was due to the presence of complex or polynuclear species that can effectively interact with substrate leading to protective film formation. These studies demonstrate that the inhibiting process provided by rare earth organo-phosphates can be more complex and may not involve simple release and precipitation of cerium hydroxides. Taylor and Chambers have reported that a combination of Ce and La cations in particularly with molybdate ions has synergistic effect in corrosion inhibition for AA2024 [44,45]. Detailed studies show that signals of Mo are not detected on the alloy surface after immersion in molybdate containing NaCl solution. However, when using a binary inhibitive system consisting of La salt and molybdate salt both La and Mo signals are detectable on intermetallic inclusions and alloy matrix. Synergistic corrosion inhibition has been related to several mechanisms involving chemical reactions of alloy surface with polyoxometallates (POM), preferential or cooperative adsorption of different species and interactions between them [45]. In the context of the present work it was demonstrated that sol–gel corrosion protective properties can be enhanced by using cerium molybdate nanowires. A combined inhibiting effect due to mutual release of Ce and Mo species from the sol–gel coatings with embedded CMN leads to higher corrosion resistance and active corrosion protection. Such effect can be associated with the inhibitive action of cerium and

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molybdate species giving a possible synergistic inhibitive cooperation. 4. Conclusions Amorphous cerium molybdate nanowires have been used as corrosion inhibitive pigment in sol–gel coatings on 2024 aluminum alloy. Sol–gel coatings with CMN pigment are clear, defect and crack free and show uniform distribution of nanowires in the sol–gel matrix. The samples coated by sol–gel containing CMN show higher mixed oxide layer resistance and polarization resistance demonstrating an improvement of active corrosion protection during immersion in 0.5 M NaCl solution. The improvement of oxide layer stability is associated with inhibitive action of CMN pigment. The suppression of local corrosion activity in artificial defects of the developed coatings is demonstrated by the localized SVET measurements. The inhibiting activity is based on the combined action of cerium and molybdate ions. Release studies indicate a higher amount of Ce leached out from the sol–gel matrix compared to Mo. This has been associated to chemical and structural transformation of amorphous CMN assisting Ce release in 0.5 M NaCl solution. Acknowledgments K.A. Yasakau and S. Kallip thank FCT for Post-Doctoral grants (ref. SFRH/BPD/80754/2011 and SFRH/BPD/64580/2009). Authors wish to acknowledge Prof. G. Thompson and Dr. I. Molchan for their assistance during the preparation of samples for TEM, MULTIPROTECT project (ref. NMP3-CT-2005-011783) and INM (Germany) for supply of the nanowires. References [1] E.A. Starke Jr., J.T. Staley, Application of modern aluminum alloys to aircraft, Prog. Aerospace Sci. 32 (1996) 131. [2] C. Blanc, B. Lavelle, G. Mankowski, The role of precipitates enriched with copper on the susceptibility to pitting corrosion of the 2024 aluminium alloy, Corros. Sci. 39 (1997) 495. [3] P. Leblanc, G.S. Frankel, A Study of Corrosion and Pitting Initiation of AA2024-T3 Using Atomic Force Microscopy, J. Electrochem. Soc. 149 (2002) B239. [4] Z. Szklarska-Smialowska, Pitting corrosion of aluminum, Corros. Sci. 41 (1999) 1743. [5] R.G. Buchheit, R.P. Grant, P.F. Hlava, B. McKenzie, G.L. Zender, Local dissolution phenomena associated with S phase (AI2CuMg) particles in aluminum alloy 2024-13, J. Electrochem. Soc. 144 (1997) 2621. [6] A. Boag, A.E. Hughes, N.C. Wilson, A. Torpy, C.M. MacRae, A.M. Glenn, T.H. Muster, How complex is the microstructure of AA2024-T3? Corros. Sci. 51 (2009) 1565. [7] A. Boag, R.J. Taylor, T.H. Muster, N. Goodman, D. McCulloch, C. Ryan, B. Rout, D. Jamieson, A.E. Hughes, Stable pit formation on AA2024-T3 in a NaCl environment, Corros. Sci. 52 (2010) 90. [8] A. Boag, A.E. Hughes, A.M. Glenn, T.H. Muster, D. McCulloch, Corrosion of AA2024-T3 Part I: localised corrosion of isolated IM particles, Corros. Sci. 53 (2011) 17. [9] A.E. Hughes, A. Boag, A.M. Glenn, D. McCulloch, T.H. Muster, C. Ryan, C. Luo, X. Zhou, G.E. Thompson, Corrosion of AA2024-T3 Part II: co-operative corrosion, Corros. Sci. 53 (2011) 27. [10] P. Schmutz, G.S. Frankel, Characterization of AA2024-T3 by scanning Kelvin probe force microscopy, J. Electrochem. Soc. 145 (1998) 2285. [11] P. Schmutz, G.S. Frankel, Corrosion study of AA2024-T3 by scanning Kelvin probe force microscopy and in situ atomic force microscopy scratching, J. Electrochem. Soc. 145 (1998) 2295. [12] K.A. Yasakau, M.L. Zheludkevich, S.V. Lamaka, M.G.S. Ferreira, Mechanism of corrosion inhibition of AA2024 by rare-earth compounds, J. Phys. Chem. B 110 (2006) 5515. [13] D. Jones, Principles and Prevention of Corrosion, 2nd ed., Prentice Hall Inc., NJ, 1996. [14] G.W. Critchlow, D.M. Brewis, Review of surface pretreatments for aluminium alloys, Int. J. Adhes. Adhes. 16 (1996) 255. [15] G.W. Critchlow, K.A. Yendall, D. Bahrani, A. Quinn, F. Andrews, Strategies for the replacement of chromic acid anodising for the structural bonding of aluminium alloys, Int. J. Adhes. Adhes. 26 (2006) 419. [16] J.H. Osborne, Observations on chromate conversion coatings from a sol–gel perspective, Prog. Org. Coat. 41 (2001) 280.

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