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1-ethyl-3-methylimidazolium chloride ionic liquid
Electrochimica Acta 103 (2013) 211–218
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Electrodeposition and corrosion characterisation of micro- and nano-crystalline aluminium from AlCl3 /1-ethyl-3-methylimidazolium chloride ionic liquid Ashraf Bakkar a,b,∗ , Volkmar Neubert a,∗∗ a b
Institut für Materialprüfung und Werkstofftechnik (Dr. Neubert GmbH), Freiberger Strasse 1, 38678 Clausthal-Zellerfeld, Germany Department of Metallurgy and Materials Engineering, Suez University, P.O. Box 43721, Suez, Egypt
a r t i c l e
i n f o
Article history: Received 11 January 2013 Received in revised form 26 March 2013 Accepted 29 March 2013 Available online xxx Keywords: Electrodeposition Aluminium Ionic liquids Mechanism Nanocrystalline Corrosion X-ray diffraction
a b s t r a c t This study reports on the electrodeposition of Al from AlCl3 /1-ethyl-3-methylimidazolium chloride (EMIC) ionic liquid, with the aim at determination of the mechanism applied and correlation of deposition kinetics and coulombic current efficiency with the microstructure and corrosion behaviour of Al deposits. Cyclic voltammetry studies were conducted on both platinum and steel substrates with various scanning rates and reversal potentials. Potentiostatic electrodeposition experiments were applied to obtain functional Al layers on low carbon steel substrates and the effect of deposition potential on the deposit morphology and crystal size was investigated using SEM-EDX and XRD. Nano-crystalline Al deposits were obtained on low carbon steel substrates using polarisation potentials more negative to −600 mVvs Al , and the Al crystallite size was scaled down to 28 nm at −700 mVvs Al . Nano-crystalline deposits, with brighter and flatter surfaces, showed higher corrosion resistance in aqueous chloride solutions. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction There are several methods applicable for coating with aluminium, such as hot dipping, spraying, cladding, physical vapour deposition (PVD), and chemical vapour deposition (CVD). However, each of these techniques has its own disadvantages. The high temperature may cause problems such as thermal stresses in coating or substrate materials, and undesirable intermetallics arising at metal/coat interfaces. PVD and CVD techniques although offer the possibility to deposit thin films, they are rather expensive [1]. The electrodeposition of aluminium has long been utilized for its extraction with the well known Hall-Heroult process, but this method is not suitable to coat other metals by aluminium because the electrolysis is carried out at about 1000 ◦ C, a temperature at which cryolite (Na3 AlF6 )/alumina (Al2 O3 ) mixture, as well as, aluminium is in the liquid state [2]. The electrodeposition of aluminium must be carried out in non-aqueous electrolytes because
∗ Corresponding author at: Institut für Materialprüfung und Werkstofftechnik (Dr. Neubert GmbH), Freiberger Strasse 1, 38678 Clausthal-Zellerfeld, Germany. Tel.: +49 5323 989890; fax: +49 5323 989899. ∗∗ Corresponding author. Tel.: +49 5323 989890; fax: +49 5323 989899. E-mail addresses: [email protected] (A. Bakkar), [email protected] (V. Neubert). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.03.198
hydrogen is evolved in aqueous solutions before aluminium can be deposited. One possible way for electrodeposition of aluminium is based on electrolysis of its molten salts at elevated temperatures. Aluminium can be electrodeposited in inert gas atmosphere from molten salts, composed of binary aluminium chloride–alkali chloride (AlCl3 /MCl) mixtures: M can be Na, Li, or mixture of K and Na; and chlorides can also be replaced by bromides or iodides [3,4]. The drawbacks of these electrolytes are ascribed to their highly corrosive nature and high AlCl3 vapour pressure, which may result in explosions at high temperatures [5,6]. Moreover, elevated temperatures may result in formation of brittle intermetallic compounds in the underlayer. Ionic liquids were the appropriate alternative for electrodeposition of aluminium at ambient temperatures. Electrochemical studies of aluminium and its alloys plating from low temperature and room temperature ionic liquids (RTILs) has been the subject of a number of studies over the years [e.g. 1,7–21]. In these studies, several ionic liquids were used as electrolytes for the electrodeposition of aluminium. One of the most frequently studied RTILs, namely chloroaluminates, is the system formed by mixing aluminium chloride AlCl3 with 1-ethyl-3-methylimidazolium chloride (EMIC). This system is liquid at 8 ◦ C for 1:1 molar ratio composition of AlCl3 to EMIC and at −98 ◦ C for 2:1 molar ratio composition [1]. Moreover, this ionic liquid system has proven to be excellent for electrodeposition of dense and adherent Al layers at room temperature
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[14,17]. Some of previous studies concerning the electrodeposition mechanism reported that functional aluminium layers are straightforwardly depositable when the molar ratio of AlCl3 /EMIC is 2:1 [1,5,14,22], and the electroactive species “Al2 Cl7 − ” are responsible for aluminium deposition according to the following reaction [5,10,14,22–24]: 4 Al2 Cl7 – + 3 e– = Al + 7 AlCl4 –
(1)
Nanocrystalline aluminium electrodeposition was the subject of some recent studies [20,25,26]. It was feasible without any additives from the ionic liquids with pyrrolidinium cations [25,26]. Concerning the chloroaluminate ionic liquids, incorporation of a methoxy group into the side chain of the imidazolium cation led to electrodeposition of nanocrystalline aluminium layers [20]. Pulsed current electrodeposition, as an experienced technique for modifying the nucleation and growth of discharged metallic cations [27–30], reduced the crystal size of aluminium deposits from AlCl3 /EMIC to be about 300 nm [31]. Higher corrosion resistance of nanocrystalline electrodeposits compared to microcrystalline ones has been reported. Nanocrystalline zinc produced by pulsed current electrodeposition had corrosion rate of about 60% lower than that of electro-galvanized steel in 0.5 M NaOH. The nanocrystalline structure enhanced both kinetics of passivation and stability of the passive film formed [32]. In a comparison study between corrosion of nanocrystalline nickel electrodeposits and that of the bulk nickel in 1 M H2 SO4 , corrosion current density and tendency for localised corrosion were found to be lower in the case of nanocrystalline nickel, but the passive film formed on nanocrystalline nickel recorded higher passive current density [33]. As a mismatched result, nanocrystalline electrodeposited cobalt exhibited higher corrosion rates than microcrystalline cobalt [34]. Numerous studies on the corrosion behaviour of electrodeposited nanocrystalline metals and alloys have been reported in recent review articles [35–37]. The present study aimed to show the feasibility of modification of microstructure of aluminium layers electrodeposited in AlCl3 /EMIC to obtain nano-crystals through potentiostatic deposition at different potentials determined with the reference to cathodic peak obtained from cyclic voltammogram. The mechanism of electrodeposition was also investigated using cyclic voltammetry (CV) analysis in terms of the scan rates and reversing potentials. The CV results obtained were compared with that reported in literature and tried to be tied up with the characteristics of deposited aluminium layers. Also, the microstructure and corrosion behaviour of Al coats in aqueous chloride media were described. 2. Experimental The preparation of ionic liquid as electrodeposition electrolyte involved mixing of 40 mol% of 1-ethyl-3-methylimidazolium chloride “EMIC” (Fluka, ≥95%) with chemical formula C6 H11 ClN2 , and 60 mol% of aluminium chloride anhydrous “AlCl3 ” (Fluka, ≥99%) in argon-filled glove box. Continuous stirring was successively conducted until a forest honey-like solution was obtained. The ionic liquids formed were used without further purification, in order to investigate the electrodeposition of aluminium in a condition, which could be transferred to industry. The cyclic voltammogram measurements and electrodeposition experiments were carried out in a conventional three electrode cell using the potentiostat model “Wenking LB 94 L (Auto Range) Laboratory Potentiostat”, controlled by a PC via the software program CPCDAU 41. Al wire and Al sheet (both 99.99%) were used as reference electrode and counter electrode, respectively. Working electrode used was either platinum or low carbon steel.
Low carbon steel strips (grade A 516 with nominal wt.% composition: 0.21 C, 0.13–0.45 Si, 0.55–0.98 Mn, 0.035 P and 0.040 S) with dimensions of 100 × 20 × 2 mm were used as substrate specimens. Each specimen was ground down to 2400 grit SiC paper and polished with diamond paste to have a mirror-like surface. The specimen was then subjected to acidic pickling for 5 min in 10% HCl, followed by tap water washing, rinsing in acetone and drying. The specimen was finally coated by an appropriate lacquer exposing free surface of 10 × 10 mm for CV measurements or 15 × 15 mm for electrodeposition experiments. Pt sheet, used also as a working electrode for CV measurements, was prepared through treating it with dilute HCl, ultrasonic cleaning in acetone bath, rinsing in distilled water, and then drying. Thereafter, it was coated with lacquer exposing a free surface of 10 × 10 mm to be the effective electrode area. All cyclic voltammetry measurements and electrodeposition experiments were performed at room temperature in about 80 ml static ionic liquid, using a conventional three-electrode cell which consisted of the ionic liquid-contained beaker, in which Al sheet (80 × 15 × 1 mm) is immersed and connected as anode. Low carbon steel sheet was immersed and connected as cathode. The cell was covered with a PVC sheet with openings for holding the electrodes, with keeping a constant distance of 3 cm between the working and counter electrodes, as will as keeping the Al wire, connected as reference electrode, about 2 cm far from both other electrodes. After electrodeposition experiments, Al-plated steel specimens were rinsed in ethanol, dried and stored in desiccator for next microstructure investigations and corrosion testing. Potentiodynamic polarisation experiments for characterisation of corrosion behaviour of Al deposits were carried out in 0.1 M NaCl aqueous solution at 25 ◦ C using the same potentiostat aforementioned above with an Avesta-cell [38]. As-deposited surface of specimen, with area of 1 cm2 , was exposed to 500 ml solution for 15 min prior to polarisation, by which time a stable potential (open circuit potential “OCP”) was monitored. With reference to saturated calomel electrode (SCE), the polarisation was obtained by scanning from −1100vs. SCE at a rate of 20 mV/min. Scanning continued in the noble direction until a sharp rise in the current, indicating the onset of pitting, was obtained. The scan was then reversed at the current density of 0.2 mA/cm2 to obtain cyclic polarisation measurements. The reverse scan continued until the curve intersected the forward scan curve, thereby forming a loop. The intersection determines the protection potential (Eprot ). The area enclosed by the cyclic polarisation loop is commonly known as the hysteresis area. Each test was repeated two times and the average value of corrosion potential (Ecorr ), pitting potential (Ep ), corrosion current density (Icorr ), protection potential (Eprot ) and the hysteresis area were determined by using the software program CPCDAU 41. The Icorr was determined by extrapolation of the Tafel lines of each polarisation curve. Microstructural investigations of Al layers, as deposited and as corroded surfaces, were conducted using an optical microscope and a scanning electron microscope (model CamScan Series 4) coupled with an energy dispersive X-ray analyser (EDX). X-ray diffraction patterns of Al deposits were recorded by D5000 powder diffractometer using Cu K␣ radiation (wavelength = 0.15406 nm) with a nickel filter at 40 kV and 30 mA. Diffraction signal intensity throughout the scan was monitored and processed with DIFFRACplus software. 3. Results and discussion 3.1. Cyclic voltammetry The cyclic voltammograms of platinum and low carbon steel in AlCl3 /EMIC ionic liquid (IL) shown in this section revealed the shape of peaks well known for aluminium elctrodeposition reported for
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(a) 1200
40 -1
Epa
-1
30
50 mV.S
800
-1
20 mV.S 20
-1
10 mV.S
Ep vs. Al / mV
Current density / mA cm-2
100 mV.S
-1
5 mV.S
10 0
400 0 1
2
3
4
5
6
7
8
9
10
11
12
10
11
12
-400
-10
Epc
-800
-20 -1200
-30
Scanning rate, v
1/2
-1500
-1000
-500
0
500
1000
1500
2000
Potential vs. Al / mV
/ mV
(b) 40
-1/2
s
Ipa
30
Fig. 1. Cyclic voltammograms recorded on Pt sheet electrode in AlCl3 /EMIC (60/40 mol%) at room temperature with different scan rates.
1/2
(i) Epc becomes more cathodic as the scan rate (v) increases, Fig. 2a. (ii) Ep (|Epc −Epa |) increases as v increases, Fig. 2a. (iii) The cathodic peak current Ipc , as well as the anodic peak current Ipa , increases in a linear relationship with the square root of scan rate (v½ ), Fig. 2b. (iv) The ratio Ipa /Ipc is greater than unity and decreases towards unity as v increases, Table 1. (v) The current efficiency increases as v increases, Fig. 2c. The first four relationships are in agreement with the criteria for quasi-reversible charge transfer mechanism according to Brown and Large [39]. In general, kinetics of an electrochemical reaction, expressed quantitatively as the current, is determined by two processes; charge transfer process (O + ne− = R), and mass transfer (diffusion) of electroactive species. If the rate of charge transfer process is very fast and the mass transfer is the only process that determines the flux of electroactive species at the electrode
Ip / mA cm
10 0 1
2
3
4
5
6
7
8
9
-10
Ipc
-20 -30 -40
Scanning rate, v / mV 1/2
1/2
-1/2
s
(c) 100 95 50 mV.s-1
90
100 mV.s-1
20 mV.s-1 85
Efficiency (%)
this IL system [10,17,20]. The onset potential of Al deposition was about −200 mVvs Al , indicating that relatively large overpotential is necessary for nucleation. Fig. 1 shows a typical set of cyclic voltammograms, obtained at various scan rates, on Pt electrode. The data obtained were characterised using the following parameters normally employed [14,22,30,39–41]: the cathodic peak current (Ipc ), the anodic peak current (Ipa ), the cathodic peak potential (Epc ), the anodic peak potential (Epa ), and the peak potential difference (Ep ) between the cathodic and anodic peaks, as well as the coulombic current efficiency () of Al deposition. is the ratio of anodic charge consumed by Al dissolution (determined as the area enclosed by anodic peak) against the cathodic charge during the cathodic polarisation (determined as the area enclosed by the cathodic peak). All of these parameters were analysed as a function of scan rate (v), or square root of scan rate (v½ ), and shown in Table 1 and Fig. 2. The precise assessments of these voltammograms and the parameters characterised can lead to the following observations:
-2
20
10 mV.s-1
80 75
5 mV.s-1
70 65 60 55 -10
-15
-20
-25
-30
I pc / mA cm
-2
Fig. 2. Relationships between the electrochemical parameters deduced from cyclic voltammograms depicted in Fig. 1, at different scan rates; (a) variations of cathodic (Epc ) and anodic (Epa ) peak potentials with the square root of scan rate (v), (b) variations of cathodic (Ipc ) and anodic (Ipa ) peak currents with the square root of scan rate (v), (c) variations of coulombic current efficiency of Al deposition on Pt, as a function of cathodic current density peak (Ipc ), with the scan rates.
Table 1 Typical cyclic voltammetry data for Pt electrode in AlCl3 /EMIC (0.6/0.4 mol%) at different scan rates. Scan rate (mV s−1 )
Ep (mV)
Ipc (mA cm−2 )
Ipa (mA cm−2 )
Ipa /Ipc
Ipc /v½ (mA cm−2 s½ mV−½ )
Ipa /v½ (mA cm−2 s½ mV−½ )
5 10 20 50 100
1239 1317 1457 1896 2196
10.01 12.90 15.93 21.79 27.40
14.79 16.73 19.38 25.80 34.66
1.48 1.30 1.22 1.18 1.26
4.48 4.08 3.56 3.08 2.74
6.61 5.29 4.33 3.65 3.47
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(a)
20 till -1200 mV till -1000 mV
Current density / mA cm
-2
15 till -1500 mV
10 5 0
till -800 mV till -600 mV
-5
till -400 mV
-10 -15 -1500
-1000
-500
0
500
1000
1500
Potential vs. Al / mV
(b) 100 90 80
Efficiency (%)
surface, as well as there is no coupled chemical reaction involved in the total electrochemical reaction, then the reaction is called a reversible charge transfer reaction. On the other hand, irreversible reaction is characterised by the slower charge transfer process to be the rate determining step. A quasi- reversible reaction is one in which the current, is controlled partly by diffusion of electroactive species and partly by charge transfer kinetics [39,40]. In the deposition process studied here the cathodic charge transfer reaction may be typically represented by the Eq. (1), although the actual process may be more or less complex. A quasi-reversible process is therefore one in which this reaction proceeds at a rate comparable to that of diffusion process. The current function Ip /v½ has to be virtually independent on v for the formal quasi-reversible process [39]. This prediction is not fulfilled by our data in Table 1. However, the clear linear relationship of cathodic current peak with the square root of the scan rate (the plot Ipc vs. v½ ), Fig. 2b, indicates that the deposition rate is determined by the diffusion process [10,14,22,30,40,41]. It is seen that the representative line does not intercept with the origin as expected for a simple diffusion controlled process. This is in agreement with Refs. [14,22,41], in which the ILs were used without purification as the same circumstance of our ILs, and in disagreement with Refs. [10,30,40], in which the ILs were previously purified. This non-interception indicates a contribution of a side reaction with the deposition reaction [14,22,40]. The side reaction has been assumed to be the reduction of some electroactive impurities, probably including water, in the ionic liquid ingredients [40]. In the same manner, the plot Ipa vs. v½ does not pass through the origin, indicating that the dissolution kinetics can also be influenced by a chemical reaction, which is mostly likely to be the chemical corrosion of Al film deposited by the ionic liquid or its impurities. High corrosion impact of AlCl3 /EMIC IL on metallic materials has been reported, so that titanium was severely corroded with absence of its well known passive behaviour in aqueous solutions [42]. Fig. 2c illustrates the dependence of the coulombic efficiency (ratio of the stripping charge to the deposition charge) on the cathodic peak current density Ipc estimated from various scan rates. The higher efficiency at higher current densities Ipc indicates that the contribution of deposition reaction, rather than side reactions, is maximising. Also, relatively higher scan rates minimise the contribution of chemical corrosion reaction of Al during the striping peak [22,30]. Fig. 3a shows a set of cyclic voltammograms monitored by low carbon steel substrate with different reversal potentials. It is clear that the more the negative sweep is, i.e. the higher the reductive charge is, the higher is the oxidative charge and the wider is the stripping peak. This indicates that the bulk deposition of Al on steel continues with increasing the negative charge regardless the presence of side reactions at far negative potentials. However, polarisation to negative potentials behind the Al deposition peak, namely −650 mV, presumes the higher contribution of side reactions, which are likely to be the dissociation of ionic liquid ingredients. Cathodic decomposition of the imidazolium ions to a considerable extent has been reported after exposure of the Lewis acidic IL AlCl3 /EMIC to an electrode potential of −700 mVvs Al [43]. This is clear for extreme negative polarisation to −1500 mV, where the CV plot depicts lower anodic current peak with appearance of a side peak. Note that the successive increase of current, followed its gradual decrease after oxidation peaks for some CV plots, is due to the corrosion of steel alloy substrate. Also, narrower stripping peaks monitored for CV plots with less negative reversal potential (e.g.−400 mV) show a secondary side anodic peak, that can be ascribed to dissolution of iron aluminide deposited at steel/aluminium interface due to presence of Fe++ ions arisen during free immersion of steel substrate [17].
70 60 50 40 30 -200
-400
-600
-800
-1000
-1200
-1400
-1600
Reversing Potential vs. Al / mV Fig. 3. (a) A set of cyclic voltammograms for low carbon steel sheet electrode in AlCl3 /EMIC (60/40 mol%) at room temperature with different reversal potentials. (b) Variation of coulombic current efficiency () of Al electrodeposition with the CV reversal potentials; is ratio of the stripping charge to the deposition charge obtained from CV diagrams in (a).
In a comparison observation between CV on Pt substrate and that on steel substrate, it can be stated that there is no significant difference, and the reduction peak started at about −200 mV on each substrate, indicating that the Al reduction kinetics on Pt and steel substrates are similar in AlCl3 /EMIC ionic liquid. In contrast,
-11 -10
Current density / mA cm-2
214
-9 -8
- 700 mV
-7
- 600 mV
-6 -5
- 500 mV
-4 -3
- 400 mV
-2
- 300 mV
-1 0 1
10
100
1000
Time / s Fig. 4. Potentiostatic current transients for Al deposition on low carbon steel substrate in AlCl3 /EMIC (60/40 mol%) at room temperature.
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Fig. 5. SEM micrograph Al layers electrodeposited at (a) −400 mV for 90 min, (b) −600 mV for 60 min, (c) −700 mV for 50 min, (d) EDX spectrum of the area shown in (a), and (e) EDX spectrum of the area shown in (c).
significant discrepancies in reduction kinetics in other IL systems depending on the substrate materials have been reported [29,44]. Fig. 3b shows the variation of coulombic efficiency with the reversing potential reached by cathodic polarisation. The efficiency initially increases with the increase of reversing potential but decreases at higher reversing potential. Relatively low efficiency at low reversing potential indicates that a side reaction other than deposition occurs. The fall of efficiency at high reversing potential is attributed to more significant contribution of side reactions, which are most likely to comprise partial dissociation of IL ingredients.
3.2. Potentiostatic elctrodeposition Fig. 4 shows the typical current-time transients resulting from potentiostatic electrodeposition experiments on low carbon steel substrate. Studying of the initial stage of electrodeposition, named as chronoamperometry, has been extensively reported with the aim at understanding useful information about nucleation and growth processes [10,14,22,30,45]. The current-time transients presented here are not as informative as that reported in literature. The time step of the potentiostat used was limited to 1 s as the fastest measurement. However, it can be observed from initial
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stage in Fig. 4 that the current increased, to be maximum at the 2nd second, due to sudden deposition of Al to form nuclei. The following decrease in current is suggested to be caused by formation of depletion layer of Al ions. For higher applied potential the current increases significantly at longer times to be 2000 s for −600 mV and 800 s for −700 mV. This subsequent increase in current density is presumed to be due to arising of side reactions accompanying the depletion of Al ion in the electrolyte, where a partial dissociation of the ionic liquid could be expected. 3.3. Microstructure
3.4. X-ray diffraction A further crystal structure assessment was conducted using X-ray diffraction (XRD) pattern analysis. Fig. 6 shows the XRD patterns of Al layers deposited at different potentials. All diffraction peaks detected are attributed to pure Al with FCC structure. The Al deposits at higher potentials are preferentially textured taking the plane orientation (200). A close analysis of the (200) peak showed observable broadening, indicating rather small crystalline size of Al electrodeposits at higher potentials. The full width at half maximum (FWHM) of the (200) peak was determined in radians and made up in the Scherrer equation [47] for estimation of the average crystallite size to be 177 nm and 28 nm for Al layers deposited at −600 mV and at −700 mV, respectively. It is notable that the estimated crystallite size by XRD is in a good consensus with the SEM micrographs in Fig. 5. In a comparison observation of the d-value for the (200) representing peaks, it decreased from 2.0238 for Al layer deposited at −400 mV to be 2.0082 and 2.0078 for deposits at–600 mV and at −700 mV, respectively. This indicates the presence of uniform elastic strain in deposits at high potentials, expressed by maximum d/d = 0.0079. Because the d-value decreased, i.e. obviously, the peak positions for high potential deposits shifted to the right side of that for low potential deposit, the indicated strain is compressive [47]. Stressed Al deposits at high potential were reported to
Fig. 6. XRD patterns of Al layers deposited at different potentials on low carbon steel substrate from AlCl3 /EMIC (60/40 mol%).
be so high that led to peeling off of Al films from gold substrate [20]. However, in the present study, the resultant residual stresses are significantly lower than the adhesion strength between the Al deposits and steel substrate, where the Al coats were still adherent as long with time. Nanocrystalline electrodeposits have been reported to contain considerable levels of residual stresses. When the individual nanocrystals grow, attractive forces tend to bridge the gap between them, but since the interaction with the substrate or the underlying deposit hinders this approach, stress originates. Conversely, stress relief can be provided by grain growth in the coarse-grained deposits [37]. 3.5. Corrosion behaviour The corrosion behaviour of micro- and nanocrystalline Al layers, deposited at different potentials, were obtained by potentiodynamic polarisation in 0.1 M NaCl solution (Fig. 7 and Table 2). The results illustrated that the nanocrystalline Al coats deposited at −600 mV and −700 mV have higher corrosion resistance, monitoring lower corrosion current densities (Icorr ) to be about one order of magnitude lower than that of the microcrystalline Al layer deposited at −400 mV. Also, Nanocrystalline Al layers showed passivation behaviour, and the Al layer with grain size = 28 nm, 1 0.1
Current density / mA cm-2
Fig. 5 shows the microstructure of the outer surface of Al layers deposited at different potentials. It is clear that higher overpotentials (resulting higher current densities) improved deposit compactness and reduced the grain size to reach down to nano scale, which was out of the resolution ability of the SEM employed. However, it can be observed from Al layer deposited at −600 mV (Fig. 5b) that the grain size is roughly <200 nm, which was ascertained by XRD assessments shown below. Visually, the Al deposits were ranged from dull, satiny white, to bright with increase of the applied potential, and the nanocrystalline Al layer deposited at −700 mV showed a goldish shining surface, as the same appearance reported in Refs. [20,43]. Fig. 5d and Fig. 5e depict the typical EDX spectra of the deposited layers revealed in Fig. 5a and Fig. 5c respectively. A notably small Cl peak in layers deposited at lower potential (Fig. 5d) can be attributed to entrapment of electrolyte impurities in the deposit bumps and fissures. The EDX analysis of nanocrystalline Al layers deposited at high potentials (e.g. Fig. 5e), on the other hand, show pure aluminium with no evidence of Cl or other impurities. The small oxygen peak is due to atmospheric oxidation of Al surface during exposure to air. In conclusion, nanocrystalline Al can be deposited at high over potentials. A large overpotential brings about a large current density and thus a large rate of nucleation at the expense of rate of growth [46]. Also, depletion of electroactive Al species as a result of high discharge rate may allow some organic species to be adsorbed on the Al nuclei and suppress their growth [43,46]. Application of higher potential have been reported to lead to reduction of imidazolium cation ([EMIm]+ ) in AlCl3 /EMIC IL and result in nanocrystalline Al deposits [43].
0.01 1E-3 1E-4 1E-5 1E-6
Al deposit at -400 mV Al deposit at -600 mV Al deposit at -700 mV -1100 -1000
-900
-800
-700
-600
-500
-400
Potentialvs. SCE / mV Fig. 7. Potentiodynamic polarisation diagrams in 0.1 M NaCl aqueous solution for microcrystalline, and 177 nm- and 28 nm-crystalline Al layers deposited, at −400 mV, −600 mV, and −700 mV, respectively, from AlCl3 /EMIC (60/40 mol%).
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Table 2 Corrosion characteristics, obtained by polarisation in 0.1 M NaCl solution, of microcrstalline and nanocrystalline Al electrodeposits, compared with that of bulk hp Al in Ref. [48]. Material
Icorr (A/cm2 )
Ecorr (mVvs.
Micro-crystalline Al deposited at −400 mVvs. Al 177 nm-Crystalline Al deposited at −600 mVvs. Al 28 nm-Crystalline Al deposited at −700 mVvs. Al Bulk hp Al [48]
0.313 0.075 0.039 0.253
−722 −680 −652 −828
SCE )
Ep (mVvs. −617 −581 −477 −615
SCE )
Eprot (mVvs. −907 −675 −626 −695
SCE )
Hysteresis area (mA s/cm2 ) 36.43 18.71 22.49 27.50
deposited at −700 mV, showed a noticeable ability to re-passivate after the polarisation was swept in the reverse direction when pitting clearly began in the cyclic polarisation procedure. Its Eprot lies in the passivation range and is nobler than its Ecorr . This signifies a good pitting performance owing to the re-passivation possibility. In contrast, microcrystalline deposits showed Eprot more negative than Ecorr . Also, there was a clear shift of Ecorr towards noble direction for the finer grain size deposits. This is in agreement with the results of nanocrystalline nickel electrodeposits [33]. The progressive shift of Ecorr towards noble direction for the finer nanocrystalline deposits was related to the modification in cathodic reaction processes [33]. Fig. 8 shows the microscopic observation of the corroded surfaces after cyclic polarisation to Eprot . It is clear that nanocrystalline deposits have shallow pits overshadowed with corrosion products. In contrast, the micro-crystalline deposits showed deleterious defects. Consequently, it can be stated that the nanocrystalline Al layers electrodeposited at high potentials have higher corrosion resistance than that of the microcrystalline deposits. This higher corrosion resistance appeared in clearly lower Icorr values with showing passivation behaviour and a noticeable tendency for re-passivation. Higher corrosion resistance of nanocrystalline electrodeposits compared with microcrystalline ones has been reported [32,33,35,36]. In a comparison observation of corrosion behaviour of micro- and nanocrystalline Al electrodeposits in the present study with that of polished surfaces of highly pure (hp) bulk Al studied in the same electrolyte with the same procedure and reported in a previous study by the authors [48], Al electrodeposits monitored more noble Ecorr values than that of bulk Al. This indicates lower surface activity of Al deposits compared to polished surface of bulk Al. Micro-crystalline Al electrodeposits recorded higher Icorr values than that of bulk Al. This can be attributed to Cl inclusions in deposits at low potentials, as shown in Fig. 5d. On the other hand, nanocrystalline Al deposits showed lower Icorr values than that of bulk Al, indicating a higher resistance of nanocrystalline Al to anodic dissolution.
4. Conclusions The following conclusions have emerged from a detailed experimental study on the electrodeposition of aluminium from AlCl3 /EMIC (60/40 mol%) ionic liquid:
Fig. 8. Photo micrographs of corrosion surfaces following potentiodynamic cyclic polarisation, depicted in Fig. 7, in 0.1 M NaCl aqueous solution for: (a) microcrystalline Al deposited at −400 mV, (b) nanocrystalline Al deposited at −600 mV, and (c) nanocrystalline Al deposited at −700 mV.
• Results obtained from cyclic voltammetry (CV) measurements indicated that electrodeposition reaction of aluminium is a quasireversible process, which is controlled partly by diffusion of electroactive species and partly by charge transfer kinetics. In addition, the deposition kinetics appear to be influenced by a side chemical reaction preceding charge transfer, probably dissociation of water contaminated in the ionic liquid. • Coulombic current efficiency estimated from CV diagrams increased with increasing the farthest negative potential reached in CV, namely reversal potential, and then decreased significantly, indicating co-deposition of organic species at high negative potentials. These organic species are suggested to inhibit the
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growth of Al crystallites deposited with higher nucleation rates at high negative potentials, resulting in nano-structured Al deposits. • Nanocrystalline, bright, dense and adherent Al layers were electrodeposited at high negative potentials. XRD patterns showed that the nanocrystalline Al deposits are compressively strained and preferentially textured taking the plane orientation (200) with average crystallite size of 177 nm and 28 nm at −600 mV and at −700 mV, respectively. Nanocrystalline Al deposits showed superior corrosion resistance than microcrystalline Al deposits, and lower corrosion rate than bulk Al. Acknowledgements The authors would like to express their thanks to Dr. Ahmed Hannora, Suez University, for XRD analysis and related discussions. Also, thanks are due to the Duetscher Akademischer Austausch Dienst (DAAD) for partial funding provided to Dr. Ashraf Bakkar. The revision of English by Prof. Samir Abdel Hakeem, Suez University, is also greatly appreciated. References [1] M. Zhang, V. Kamavaram, R.G. Reddy, New electrolytes for aluminum production: ionic liquids, JOM 55 (2003) 54. [2] U. Grjotheim, H. Kvande, Introduction to Aluminum Electrolysis. Understanding the Hall-Heroult Process, Aluminum Verlag GmbH, Germany, 1993, pp. 260. [3] G.R. Stafford, The Electrodeposition of an aluminum–manganese metallic glass from molten salts, Journal of the Electrochemical Society 136 (1989) 635. [4] M. Jafarian, M.G. Mahjani, F. Gobal, I. Danaee, Electrodeposition of aluminum from AlCl3 –NaCl–KCl mixture, Journal of Applied Electrochemistry 36 (2006) 1169. [5] E.M. Moustafa, Ph.D. Thesis, Electrodeposition of aluminium in different air and water s table ionic liquids, TU, Clausthal, Papierflieger, Clausthal-Zellerfeld, 2007. [6] J. Fransaer, E. Leunis, T. Hirato, J.-P. Celis, Aluminium composite coatings containing micrometre and nanometre-sized particles electroplated from a non-aqueous electrolyte, Journal of Applied Electrochemistry 32 (2002) 123. [7] F.H. Hurley, T.P. Wier, The electrodeposition of aluminum from nonaqueous solutions at room temperature, Journal of the Electrochemical Society 98 (1951) 207. [8] J. Robinson, R.A. Osteryoung, The electrochemical behavior of aluminum in the low temperature molten salt system n butyl pyridinium chloride: aluminum chloride and mixtures of this molten salt with benzene, Journal of the Electrochemical Society 127 (1980) 122. [9] R.T. Carlin, W. Crawford, M. Berch, Nucleation and morphology studies of aluminum deposited from an ambient-temperature chloroaluminate molten salt, Journal of the Electrochemical Society 139 (1992) 2720. [10] Y. Zhao, T.J. VanderNoot, Electrodeposition of aluminium from room temperature AlCl3 –TMPAC molten salts, Electrochimica Acta 42 (1997) 1639. [11] Q. Liao, W.R. Pitner, G. Stewart, C.L. Hussey, G.R. Stafford, Electrodeposition of aluminum from the aluminum chloride-1-methyl-3-ethylimidazolium chloride room temperature molten salt + benzene, Journal of the Electrochemical Society 144 (3) (1997) 936. [12] Q. Zhu, C.L. Hussey, G.R. Stafford, Electrodeposition of silver–aluminum alloys from a room-temperature chloroaluminate molten salt, Journal of the Electrochemical Society 148 (2) (2001) C88. [13] Q. Zhu, C.L. Hussey, Galvanostatic pulse plating of Cu–Al alloy in a roomtemperature chloroaluminate molten salt. Rotating ring-disk electrode studies, Journal of the Electrochemical Society 148 (5) (2001) C395. [14] T. Jiang, M.J. Chollier Brym, G. Dube, A. Lasia, G.M. Brisard, Electrodeposition of aluminium from ionic liquids: Part I—electrodeposition and surface morphology of aluminium from aluminium chloride (AlCl3 )–1-ethyl3-methylimidazolium chloride ([EMIm]Cl) ionic liquids, Surface and Coatings Technology 201 (2006) 1. [15] T. Tsuda, C.L. Hussey, G.R. Stafford, Electrochemistry of titanium and the electrodeposition of Al–Ti alloys in the lewis acidic aluminum chloride–1ethyl-3-methylimidazolium chloride melt, Journal of the Electrochemical Society 150 (2003) C234. [16] T. Tsuda, C.L. Hussey, G.R. Stafford, Electrodeposition of Al–Mo alloys from the lewis acidic aluminum chloride–1-ethyl-3-methylimidazolium chloride molten salt, Journal of the Electrochemical Society 151 (2004) C379. [17] Q.X. Liu, S. Zein El Abdeen, F. Endres, Electroplating of mild steel by aluminium in a first generation ionic liquid: a green alternative to commercial Al-plating in organic solvents, Surface and Coatings Technology 201 (2006) 1352.
[18] T. Tsuda, C.L. Hussey, G.R. Stafford, Progress in surface finishing with lewis acidic room-temperature chloroaluminate ionic liquids electrodeposition from molten salts and ionic liquids, ECS Transactions 3 (2007) 217. [19] M.T. Carter, G.R. Bourgon, Practical aluminum plating from a room temperature ionic liquid, ECS Transactions 3 (35) (2007) 233. [20] S. Zein El Abedin, P. Giridhar, P. Schwab, F. Endres, Electrodeposition of nanocrystalline aluminium from a chloroaluminate ionic liquid, Electrochemistry Communications 12 (2010) 1084. [21] V. Neubert, A. Bakkar, German Patent DE102011055911.6. [22] P.K. Lai, M. Skyllas-Kazacos, Electrodeposition of aluminium in aluminium chloride/1-methyl-3-ethylimidazolium chloride, Journal of Electroanalytical Chemistry 248 (1988) 431. [23] S. Schaltin, M. Ganapathi, K. Binnemans, J. Fransaer, Modeling of aluminium deposition from chloroaluminate ionic liquids, Journal of the Electrochemical Society 158 (2011) D634. [24] Y. Zhao, T.J. VanderNoot, Electrodeposition of aluminium from nonaqueous organic electrolytic systems and room temperature molten salts, Electrochimica Acta 42 (1997) 3. [25] S. Zein El Abedin, E.M. Moustafa, R. Hempelmann, H. Natter, F. Endres, Additive free electrodeposition of nanocrystalline aluminium in a water and air stable ionic liquid, Electrochemistry Communications 7 (2005) 1111. [26] S. Zein El Abedin, E.M. Moustafa, R. Hempelmann, H. Natter, F. Endres, Electrodeposition of nano- and microcrystalline aluminium in three different air and water stable ionic liquids, ChemPhysChem 7 (2006) 1535. [27] A.M. Alfantazi, U. Erb, Corrosion properties of pulse-plated zinc–nickel alloy coating, Corrosion 52 (1996) 880. [28] Kh. Saber, C.C. Koch, P.S. Fedkiw, Pulse current electrodeposition of nanocrystalline zinc, Materials Science and Engineering A 341 (2003) 174. [29] A. Bakkar, V. Neubert, Electrodeposition onto magnesium in air and water stable ionic liquids: from corrosion to successful plating, Electrochemistry Communications 9 (2007) 2428. [30] J. Tang, K. Azumi, Optimization of pulsed electrodeposition of aluminum from AlCl3 –1-ethyl-3-methylimidazolium chloride ionic liquid, Electrochimica Acta 56 (2011) 1130. [31] B. Li, C. Fan, Y. Chen, J. Lou, L. Yan, Pulse current electrodeposition of Al from an AlCl3 –EMIC ionic liquid, Electrochimica Acta 56 (2011) 5478. [32] Kh.M.S. Youssef, C.C. Koch, P.S. Fedkiw, Improved corrosion behavior of nanocrystalline zinc produced by pulse-current electrodeposition, Corrosion Science 46 (2004) 51. [33] R. Mishra, R. Balasubramaniam, Effect of nanocrystalline grain size on the electrochemical and corrosion behavior of nickel, Corrosion Science 46 (2004) 3019. [34] A. Aledresse, A. Alfantazi, A study on the corrosion behaviour of nanostructured electrodeposited cobalt, Journal of Materials Science 39 (2004) 1523. [35] V.S. Saji, J. Thomas, Nanomaterials for corrosion control, Current Science 92 (1) (2007) 51. [36] N.R.N. Masdek, A. Alfantazi, Review of studies on corrosion of electrodeposited nanocrystalline metals and alloys, ECS Transactions 28 (24) (2010) 249. [37] L.P. Bicelli, B. Bozzini, C. Mele, L. D’Urzo, A review of nanostructural aspects of metal deposition, International Journal of Electrochemical Science 3 (2008) 356. [38] R. Qvarfort, New electrochemical cell for pitting corrosion testing, Corrosion Science 28 (1988) 135. [39] E.R. Brown, R.F. Large, in: A. Weissberger, B. Rossiter (Eds.), Techniques of Chemistry, Vol. 1, Part IIA, Electrochemical Methods, Wiley, New York, 1971, pp. 423–530. [40] D. Elwell, G.M. Rao, Mechanism of electrodeposition of silicon from K2 SiF6 flinak, Electrochimica Acta 27 (1982) 673. [41] I. Irdhawati, H. Tatsumi, I. Noviaandri, B. Buchari, S. Ibrahim, Cyclic voltammetry of ion transfer for phenylpropanolamine hydrochloride at water|nitrobenzene interface, Journal of the Chinese Chemical Society 59 (2012) 40. [42] C.H. Tseng, J.K. Chang, J.R. Chen, W.T. Tsai, M.J. Deng, I.W. Sun, Corrosion behaviors of materials in aluminum chloride–1-ethyl-3-methylimidazolium chloride ionic liquid, Electrochemistry Communications 12 (2010) 1091. [43] O.X. Liu, S. Zein El Abedin, F. Endres, Electrodeposition of nanocrystalline aluminum: Breakdown of imidazolium cations modifies the crystal size, Journal of the Electrochemical Society 155 (2008) D357. [44] A.P. Abbott, C.A. Eardley, N.R.S. Farley, G.A. Griffith, A. Pratt, Electrodeposition of aluminium and aluminium/platinum alloys from AlCl3 /benzyltrimethylammonium chloride room temperature ionic liquids, Journal of Applied Electrochemistry 31 (2001) 1345. [45] P. Rolland, G. Mamantov, Electrochemical reduction of Al2 Cl7 -ions in chloroaluminate melts, Journal of the Electrochemical Society 123 (1976) 1299. [46] R. Hempelmann, H. Natter, in: F. Endres, D. MacFarlane, A. Abbott (Eds.), Electrodeposition from Ionic Liquids, Wiley-VCH Verlag GmbH & Co. KgaA, Weinheim, Germany, 2008, p. 213-288. [47] B.D. Cullity, Elements of X-ray Diffraction, Addison-Wesley Publishing Company, USA, 1956, pp. 99, 262–269. [48] V. Neubert, B. Smola, I. Stulikova, A. Bakkar, J. Reuter, Microstructure, mechanical properties and corrosion behaviour of dilute Al–Sc–Zr alloy prepared by powder metallurgy, Materials Science and Engineering A 464 (2007) 358.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electrodeposition and corrosion characterisation of micro- and nano-crystalline aluminium from AlCl3 /1-ethyl-3-methylimidazolium chloride ionic liquid Ashraf Bakkar a,b,∗ , Volkmar Neubert a,∗∗ a b
Institut für Materialprüfung und Werkstofftechnik (Dr. Neubert GmbH), Freiberger Strasse 1, 38678 Clausthal-Zellerfeld, Germany Department of Metallurgy and Materials Engineering, Suez University, P.O. Box 43721, Suez, Egypt
a r t i c l e
i n f o
Article history: Received 11 January 2013 Received in revised form 26 March 2013 Accepted 29 March 2013 Available online xxx Keywords: Electrodeposition Aluminium Ionic liquids Mechanism Nanocrystalline Corrosion X-ray diffraction
a b s t r a c t This study reports on the electrodeposition of Al from AlCl3 /1-ethyl-3-methylimidazolium chloride (EMIC) ionic liquid, with the aim at determination of the mechanism applied and correlation of deposition kinetics and coulombic current efficiency with the microstructure and corrosion behaviour of Al deposits. Cyclic voltammetry studies were conducted on both platinum and steel substrates with various scanning rates and reversal potentials. Potentiostatic electrodeposition experiments were applied to obtain functional Al layers on low carbon steel substrates and the effect of deposition potential on the deposit morphology and crystal size was investigated using SEM-EDX and XRD. Nano-crystalline Al deposits were obtained on low carbon steel substrates using polarisation potentials more negative to −600 mVvs Al , and the Al crystallite size was scaled down to 28 nm at −700 mVvs Al . Nano-crystalline deposits, with brighter and flatter surfaces, showed higher corrosion resistance in aqueous chloride solutions. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction There are several methods applicable for coating with aluminium, such as hot dipping, spraying, cladding, physical vapour deposition (PVD), and chemical vapour deposition (CVD). However, each of these techniques has its own disadvantages. The high temperature may cause problems such as thermal stresses in coating or substrate materials, and undesirable intermetallics arising at metal/coat interfaces. PVD and CVD techniques although offer the possibility to deposit thin films, they are rather expensive [1]. The electrodeposition of aluminium has long been utilized for its extraction with the well known Hall-Heroult process, but this method is not suitable to coat other metals by aluminium because the electrolysis is carried out at about 1000 ◦ C, a temperature at which cryolite (Na3 AlF6 )/alumina (Al2 O3 ) mixture, as well as, aluminium is in the liquid state [2]. The electrodeposition of aluminium must be carried out in non-aqueous electrolytes because
∗ Corresponding author at: Institut für Materialprüfung und Werkstofftechnik (Dr. Neubert GmbH), Freiberger Strasse 1, 38678 Clausthal-Zellerfeld, Germany. Tel.: +49 5323 989890; fax: +49 5323 989899. ∗∗ Corresponding author. Tel.: +49 5323 989890; fax: +49 5323 989899. E-mail addresses: [email protected] (A. Bakkar), [email protected] (V. Neubert). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.03.198
hydrogen is evolved in aqueous solutions before aluminium can be deposited. One possible way for electrodeposition of aluminium is based on electrolysis of its molten salts at elevated temperatures. Aluminium can be electrodeposited in inert gas atmosphere from molten salts, composed of binary aluminium chloride–alkali chloride (AlCl3 /MCl) mixtures: M can be Na, Li, or mixture of K and Na; and chlorides can also be replaced by bromides or iodides [3,4]. The drawbacks of these electrolytes are ascribed to their highly corrosive nature and high AlCl3 vapour pressure, which may result in explosions at high temperatures [5,6]. Moreover, elevated temperatures may result in formation of brittle intermetallic compounds in the underlayer. Ionic liquids were the appropriate alternative for electrodeposition of aluminium at ambient temperatures. Electrochemical studies of aluminium and its alloys plating from low temperature and room temperature ionic liquids (RTILs) has been the subject of a number of studies over the years [e.g. 1,7–21]. In these studies, several ionic liquids were used as electrolytes for the electrodeposition of aluminium. One of the most frequently studied RTILs, namely chloroaluminates, is the system formed by mixing aluminium chloride AlCl3 with 1-ethyl-3-methylimidazolium chloride (EMIC). This system is liquid at 8 ◦ C for 1:1 molar ratio composition of AlCl3 to EMIC and at −98 ◦ C for 2:1 molar ratio composition [1]. Moreover, this ionic liquid system has proven to be excellent for electrodeposition of dense and adherent Al layers at room temperature
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[14,17]. Some of previous studies concerning the electrodeposition mechanism reported that functional aluminium layers are straightforwardly depositable when the molar ratio of AlCl3 /EMIC is 2:1 [1,5,14,22], and the electroactive species “Al2 Cl7 − ” are responsible for aluminium deposition according to the following reaction [5,10,14,22–24]: 4 Al2 Cl7 – + 3 e– = Al + 7 AlCl4 –
(1)
Nanocrystalline aluminium electrodeposition was the subject of some recent studies [20,25,26]. It was feasible without any additives from the ionic liquids with pyrrolidinium cations [25,26]. Concerning the chloroaluminate ionic liquids, incorporation of a methoxy group into the side chain of the imidazolium cation led to electrodeposition of nanocrystalline aluminium layers [20]. Pulsed current electrodeposition, as an experienced technique for modifying the nucleation and growth of discharged metallic cations [27–30], reduced the crystal size of aluminium deposits from AlCl3 /EMIC to be about 300 nm [31]. Higher corrosion resistance of nanocrystalline electrodeposits compared to microcrystalline ones has been reported. Nanocrystalline zinc produced by pulsed current electrodeposition had corrosion rate of about 60% lower than that of electro-galvanized steel in 0.5 M NaOH. The nanocrystalline structure enhanced both kinetics of passivation and stability of the passive film formed [32]. In a comparison study between corrosion of nanocrystalline nickel electrodeposits and that of the bulk nickel in 1 M H2 SO4 , corrosion current density and tendency for localised corrosion were found to be lower in the case of nanocrystalline nickel, but the passive film formed on nanocrystalline nickel recorded higher passive current density [33]. As a mismatched result, nanocrystalline electrodeposited cobalt exhibited higher corrosion rates than microcrystalline cobalt [34]. Numerous studies on the corrosion behaviour of electrodeposited nanocrystalline metals and alloys have been reported in recent review articles [35–37]. The present study aimed to show the feasibility of modification of microstructure of aluminium layers electrodeposited in AlCl3 /EMIC to obtain nano-crystals through potentiostatic deposition at different potentials determined with the reference to cathodic peak obtained from cyclic voltammogram. The mechanism of electrodeposition was also investigated using cyclic voltammetry (CV) analysis in terms of the scan rates and reversing potentials. The CV results obtained were compared with that reported in literature and tried to be tied up with the characteristics of deposited aluminium layers. Also, the microstructure and corrosion behaviour of Al coats in aqueous chloride media were described. 2. Experimental The preparation of ionic liquid as electrodeposition electrolyte involved mixing of 40 mol% of 1-ethyl-3-methylimidazolium chloride “EMIC” (Fluka, ≥95%) with chemical formula C6 H11 ClN2 , and 60 mol% of aluminium chloride anhydrous “AlCl3 ” (Fluka, ≥99%) in argon-filled glove box. Continuous stirring was successively conducted until a forest honey-like solution was obtained. The ionic liquids formed were used without further purification, in order to investigate the electrodeposition of aluminium in a condition, which could be transferred to industry. The cyclic voltammogram measurements and electrodeposition experiments were carried out in a conventional three electrode cell using the potentiostat model “Wenking LB 94 L (Auto Range) Laboratory Potentiostat”, controlled by a PC via the software program CPCDAU 41. Al wire and Al sheet (both 99.99%) were used as reference electrode and counter electrode, respectively. Working electrode used was either platinum or low carbon steel.
Low carbon steel strips (grade A 516 with nominal wt.% composition: 0.21 C, 0.13–0.45 Si, 0.55–0.98 Mn, 0.035 P and 0.040 S) with dimensions of 100 × 20 × 2 mm were used as substrate specimens. Each specimen was ground down to 2400 grit SiC paper and polished with diamond paste to have a mirror-like surface. The specimen was then subjected to acidic pickling for 5 min in 10% HCl, followed by tap water washing, rinsing in acetone and drying. The specimen was finally coated by an appropriate lacquer exposing free surface of 10 × 10 mm for CV measurements or 15 × 15 mm for electrodeposition experiments. Pt sheet, used also as a working electrode for CV measurements, was prepared through treating it with dilute HCl, ultrasonic cleaning in acetone bath, rinsing in distilled water, and then drying. Thereafter, it was coated with lacquer exposing a free surface of 10 × 10 mm to be the effective electrode area. All cyclic voltammetry measurements and electrodeposition experiments were performed at room temperature in about 80 ml static ionic liquid, using a conventional three-electrode cell which consisted of the ionic liquid-contained beaker, in which Al sheet (80 × 15 × 1 mm) is immersed and connected as anode. Low carbon steel sheet was immersed and connected as cathode. The cell was covered with a PVC sheet with openings for holding the electrodes, with keeping a constant distance of 3 cm between the working and counter electrodes, as will as keeping the Al wire, connected as reference electrode, about 2 cm far from both other electrodes. After electrodeposition experiments, Al-plated steel specimens were rinsed in ethanol, dried and stored in desiccator for next microstructure investigations and corrosion testing. Potentiodynamic polarisation experiments for characterisation of corrosion behaviour of Al deposits were carried out in 0.1 M NaCl aqueous solution at 25 ◦ C using the same potentiostat aforementioned above with an Avesta-cell [38]. As-deposited surface of specimen, with area of 1 cm2 , was exposed to 500 ml solution for 15 min prior to polarisation, by which time a stable potential (open circuit potential “OCP”) was monitored. With reference to saturated calomel electrode (SCE), the polarisation was obtained by scanning from −1100vs. SCE at a rate of 20 mV/min. Scanning continued in the noble direction until a sharp rise in the current, indicating the onset of pitting, was obtained. The scan was then reversed at the current density of 0.2 mA/cm2 to obtain cyclic polarisation measurements. The reverse scan continued until the curve intersected the forward scan curve, thereby forming a loop. The intersection determines the protection potential (Eprot ). The area enclosed by the cyclic polarisation loop is commonly known as the hysteresis area. Each test was repeated two times and the average value of corrosion potential (Ecorr ), pitting potential (Ep ), corrosion current density (Icorr ), protection potential (Eprot ) and the hysteresis area were determined by using the software program CPCDAU 41. The Icorr was determined by extrapolation of the Tafel lines of each polarisation curve. Microstructural investigations of Al layers, as deposited and as corroded surfaces, were conducted using an optical microscope and a scanning electron microscope (model CamScan Series 4) coupled with an energy dispersive X-ray analyser (EDX). X-ray diffraction patterns of Al deposits were recorded by D5000 powder diffractometer using Cu K␣ radiation (wavelength = 0.15406 nm) with a nickel filter at 40 kV and 30 mA. Diffraction signal intensity throughout the scan was monitored and processed with DIFFRACplus software. 3. Results and discussion 3.1. Cyclic voltammetry The cyclic voltammograms of platinum and low carbon steel in AlCl3 /EMIC ionic liquid (IL) shown in this section revealed the shape of peaks well known for aluminium elctrodeposition reported for
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213
(a) 1200
40 -1
Epa
-1
30
50 mV.S
800
-1
20 mV.S 20
-1
10 mV.S
Ep vs. Al / mV
Current density / mA cm-2
100 mV.S
-1
5 mV.S
10 0
400 0 1
2
3
4
5
6
7
8
9
10
11
12
10
11
12
-400
-10
Epc
-800
-20 -1200
-30
Scanning rate, v
1/2
-1500
-1000
-500
0
500
1000
1500
2000
Potential vs. Al / mV
/ mV
(b) 40
-1/2
s
Ipa
30
Fig. 1. Cyclic voltammograms recorded on Pt sheet electrode in AlCl3 /EMIC (60/40 mol%) at room temperature with different scan rates.
1/2
(i) Epc becomes more cathodic as the scan rate (v) increases, Fig. 2a. (ii) Ep (|Epc −Epa |) increases as v increases, Fig. 2a. (iii) The cathodic peak current Ipc , as well as the anodic peak current Ipa , increases in a linear relationship with the square root of scan rate (v½ ), Fig. 2b. (iv) The ratio Ipa /Ipc is greater than unity and decreases towards unity as v increases, Table 1. (v) The current efficiency increases as v increases, Fig. 2c. The first four relationships are in agreement with the criteria for quasi-reversible charge transfer mechanism according to Brown and Large [39]. In general, kinetics of an electrochemical reaction, expressed quantitatively as the current, is determined by two processes; charge transfer process (O + ne− = R), and mass transfer (diffusion) of electroactive species. If the rate of charge transfer process is very fast and the mass transfer is the only process that determines the flux of electroactive species at the electrode
Ip / mA cm
10 0 1
2
3
4
5
6
7
8
9
-10
Ipc
-20 -30 -40
Scanning rate, v / mV 1/2
1/2
-1/2
s
(c) 100 95 50 mV.s-1
90
100 mV.s-1
20 mV.s-1 85
Efficiency (%)
this IL system [10,17,20]. The onset potential of Al deposition was about −200 mVvs Al , indicating that relatively large overpotential is necessary for nucleation. Fig. 1 shows a typical set of cyclic voltammograms, obtained at various scan rates, on Pt electrode. The data obtained were characterised using the following parameters normally employed [14,22,30,39–41]: the cathodic peak current (Ipc ), the anodic peak current (Ipa ), the cathodic peak potential (Epc ), the anodic peak potential (Epa ), and the peak potential difference (Ep ) between the cathodic and anodic peaks, as well as the coulombic current efficiency () of Al deposition. is the ratio of anodic charge consumed by Al dissolution (determined as the area enclosed by anodic peak) against the cathodic charge during the cathodic polarisation (determined as the area enclosed by the cathodic peak). All of these parameters were analysed as a function of scan rate (v), or square root of scan rate (v½ ), and shown in Table 1 and Fig. 2. The precise assessments of these voltammograms and the parameters characterised can lead to the following observations:
-2
20
10 mV.s-1
80 75
5 mV.s-1
70 65 60 55 -10
-15
-20
-25
-30
I pc / mA cm
-2
Fig. 2. Relationships between the electrochemical parameters deduced from cyclic voltammograms depicted in Fig. 1, at different scan rates; (a) variations of cathodic (Epc ) and anodic (Epa ) peak potentials with the square root of scan rate (v), (b) variations of cathodic (Ipc ) and anodic (Ipa ) peak currents with the square root of scan rate (v), (c) variations of coulombic current efficiency of Al deposition on Pt, as a function of cathodic current density peak (Ipc ), with the scan rates.
Table 1 Typical cyclic voltammetry data for Pt electrode in AlCl3 /EMIC (0.6/0.4 mol%) at different scan rates. Scan rate (mV s−1 )
Ep (mV)
Ipc (mA cm−2 )
Ipa (mA cm−2 )
Ipa /Ipc
Ipc /v½ (mA cm−2 s½ mV−½ )
Ipa /v½ (mA cm−2 s½ mV−½ )
5 10 20 50 100
1239 1317 1457 1896 2196
10.01 12.90 15.93 21.79 27.40
14.79 16.73 19.38 25.80 34.66
1.48 1.30 1.22 1.18 1.26
4.48 4.08 3.56 3.08 2.74
6.61 5.29 4.33 3.65 3.47
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(a)
20 till -1200 mV till -1000 mV
Current density / mA cm
-2
15 till -1500 mV
10 5 0
till -800 mV till -600 mV
-5
till -400 mV
-10 -15 -1500
-1000
-500
0
500
1000
1500
Potential vs. Al / mV
(b) 100 90 80
Efficiency (%)
surface, as well as there is no coupled chemical reaction involved in the total electrochemical reaction, then the reaction is called a reversible charge transfer reaction. On the other hand, irreversible reaction is characterised by the slower charge transfer process to be the rate determining step. A quasi- reversible reaction is one in which the current, is controlled partly by diffusion of electroactive species and partly by charge transfer kinetics [39,40]. In the deposition process studied here the cathodic charge transfer reaction may be typically represented by the Eq. (1), although the actual process may be more or less complex. A quasi-reversible process is therefore one in which this reaction proceeds at a rate comparable to that of diffusion process. The current function Ip /v½ has to be virtually independent on v for the formal quasi-reversible process [39]. This prediction is not fulfilled by our data in Table 1. However, the clear linear relationship of cathodic current peak with the square root of the scan rate (the plot Ipc vs. v½ ), Fig. 2b, indicates that the deposition rate is determined by the diffusion process [10,14,22,30,40,41]. It is seen that the representative line does not intercept with the origin as expected for a simple diffusion controlled process. This is in agreement with Refs. [14,22,41], in which the ILs were used without purification as the same circumstance of our ILs, and in disagreement with Refs. [10,30,40], in which the ILs were previously purified. This non-interception indicates a contribution of a side reaction with the deposition reaction [14,22,40]. The side reaction has been assumed to be the reduction of some electroactive impurities, probably including water, in the ionic liquid ingredients [40]. In the same manner, the plot Ipa vs. v½ does not pass through the origin, indicating that the dissolution kinetics can also be influenced by a chemical reaction, which is mostly likely to be the chemical corrosion of Al film deposited by the ionic liquid or its impurities. High corrosion impact of AlCl3 /EMIC IL on metallic materials has been reported, so that titanium was severely corroded with absence of its well known passive behaviour in aqueous solutions [42]. Fig. 2c illustrates the dependence of the coulombic efficiency (ratio of the stripping charge to the deposition charge) on the cathodic peak current density Ipc estimated from various scan rates. The higher efficiency at higher current densities Ipc indicates that the contribution of deposition reaction, rather than side reactions, is maximising. Also, relatively higher scan rates minimise the contribution of chemical corrosion reaction of Al during the striping peak [22,30]. Fig. 3a shows a set of cyclic voltammograms monitored by low carbon steel substrate with different reversal potentials. It is clear that the more the negative sweep is, i.e. the higher the reductive charge is, the higher is the oxidative charge and the wider is the stripping peak. This indicates that the bulk deposition of Al on steel continues with increasing the negative charge regardless the presence of side reactions at far negative potentials. However, polarisation to negative potentials behind the Al deposition peak, namely −650 mV, presumes the higher contribution of side reactions, which are likely to be the dissociation of ionic liquid ingredients. Cathodic decomposition of the imidazolium ions to a considerable extent has been reported after exposure of the Lewis acidic IL AlCl3 /EMIC to an electrode potential of −700 mVvs Al [43]. This is clear for extreme negative polarisation to −1500 mV, where the CV plot depicts lower anodic current peak with appearance of a side peak. Note that the successive increase of current, followed its gradual decrease after oxidation peaks for some CV plots, is due to the corrosion of steel alloy substrate. Also, narrower stripping peaks monitored for CV plots with less negative reversal potential (e.g.−400 mV) show a secondary side anodic peak, that can be ascribed to dissolution of iron aluminide deposited at steel/aluminium interface due to presence of Fe++ ions arisen during free immersion of steel substrate [17].
70 60 50 40 30 -200
-400
-600
-800
-1000
-1200
-1400
-1600
Reversing Potential vs. Al / mV Fig. 3. (a) A set of cyclic voltammograms for low carbon steel sheet electrode in AlCl3 /EMIC (60/40 mol%) at room temperature with different reversal potentials. (b) Variation of coulombic current efficiency () of Al electrodeposition with the CV reversal potentials; is ratio of the stripping charge to the deposition charge obtained from CV diagrams in (a).
In a comparison observation between CV on Pt substrate and that on steel substrate, it can be stated that there is no significant difference, and the reduction peak started at about −200 mV on each substrate, indicating that the Al reduction kinetics on Pt and steel substrates are similar in AlCl3 /EMIC ionic liquid. In contrast,
-11 -10
Current density / mA cm-2
214
-9 -8
- 700 mV
-7
- 600 mV
-6 -5
- 500 mV
-4 -3
- 400 mV
-2
- 300 mV
-1 0 1
10
100
1000
Time / s Fig. 4. Potentiostatic current transients for Al deposition on low carbon steel substrate in AlCl3 /EMIC (60/40 mol%) at room temperature.
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Fig. 5. SEM micrograph Al layers electrodeposited at (a) −400 mV for 90 min, (b) −600 mV for 60 min, (c) −700 mV for 50 min, (d) EDX spectrum of the area shown in (a), and (e) EDX spectrum of the area shown in (c).
significant discrepancies in reduction kinetics in other IL systems depending on the substrate materials have been reported [29,44]. Fig. 3b shows the variation of coulombic efficiency with the reversing potential reached by cathodic polarisation. The efficiency initially increases with the increase of reversing potential but decreases at higher reversing potential. Relatively low efficiency at low reversing potential indicates that a side reaction other than deposition occurs. The fall of efficiency at high reversing potential is attributed to more significant contribution of side reactions, which are most likely to comprise partial dissociation of IL ingredients.
3.2. Potentiostatic elctrodeposition Fig. 4 shows the typical current-time transients resulting from potentiostatic electrodeposition experiments on low carbon steel substrate. Studying of the initial stage of electrodeposition, named as chronoamperometry, has been extensively reported with the aim at understanding useful information about nucleation and growth processes [10,14,22,30,45]. The current-time transients presented here are not as informative as that reported in literature. The time step of the potentiostat used was limited to 1 s as the fastest measurement. However, it can be observed from initial
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stage in Fig. 4 that the current increased, to be maximum at the 2nd second, due to sudden deposition of Al to form nuclei. The following decrease in current is suggested to be caused by formation of depletion layer of Al ions. For higher applied potential the current increases significantly at longer times to be 2000 s for −600 mV and 800 s for −700 mV. This subsequent increase in current density is presumed to be due to arising of side reactions accompanying the depletion of Al ion in the electrolyte, where a partial dissociation of the ionic liquid could be expected. 3.3. Microstructure
3.4. X-ray diffraction A further crystal structure assessment was conducted using X-ray diffraction (XRD) pattern analysis. Fig. 6 shows the XRD patterns of Al layers deposited at different potentials. All diffraction peaks detected are attributed to pure Al with FCC structure. The Al deposits at higher potentials are preferentially textured taking the plane orientation (200). A close analysis of the (200) peak showed observable broadening, indicating rather small crystalline size of Al electrodeposits at higher potentials. The full width at half maximum (FWHM) of the (200) peak was determined in radians and made up in the Scherrer equation [47] for estimation of the average crystallite size to be 177 nm and 28 nm for Al layers deposited at −600 mV and at −700 mV, respectively. It is notable that the estimated crystallite size by XRD is in a good consensus with the SEM micrographs in Fig. 5. In a comparison observation of the d-value for the (200) representing peaks, it decreased from 2.0238 for Al layer deposited at −400 mV to be 2.0082 and 2.0078 for deposits at–600 mV and at −700 mV, respectively. This indicates the presence of uniform elastic strain in deposits at high potentials, expressed by maximum d/d = 0.0079. Because the d-value decreased, i.e. obviously, the peak positions for high potential deposits shifted to the right side of that for low potential deposit, the indicated strain is compressive [47]. Stressed Al deposits at high potential were reported to
Fig. 6. XRD patterns of Al layers deposited at different potentials on low carbon steel substrate from AlCl3 /EMIC (60/40 mol%).
be so high that led to peeling off of Al films from gold substrate [20]. However, in the present study, the resultant residual stresses are significantly lower than the adhesion strength between the Al deposits and steel substrate, where the Al coats were still adherent as long with time. Nanocrystalline electrodeposits have been reported to contain considerable levels of residual stresses. When the individual nanocrystals grow, attractive forces tend to bridge the gap between them, but since the interaction with the substrate or the underlying deposit hinders this approach, stress originates. Conversely, stress relief can be provided by grain growth in the coarse-grained deposits [37]. 3.5. Corrosion behaviour The corrosion behaviour of micro- and nanocrystalline Al layers, deposited at different potentials, were obtained by potentiodynamic polarisation in 0.1 M NaCl solution (Fig. 7 and Table 2). The results illustrated that the nanocrystalline Al coats deposited at −600 mV and −700 mV have higher corrosion resistance, monitoring lower corrosion current densities (Icorr ) to be about one order of magnitude lower than that of the microcrystalline Al layer deposited at −400 mV. Also, Nanocrystalline Al layers showed passivation behaviour, and the Al layer with grain size = 28 nm, 1 0.1
Current density / mA cm-2
Fig. 5 shows the microstructure of the outer surface of Al layers deposited at different potentials. It is clear that higher overpotentials (resulting higher current densities) improved deposit compactness and reduced the grain size to reach down to nano scale, which was out of the resolution ability of the SEM employed. However, it can be observed from Al layer deposited at −600 mV (Fig. 5b) that the grain size is roughly <200 nm, which was ascertained by XRD assessments shown below. Visually, the Al deposits were ranged from dull, satiny white, to bright with increase of the applied potential, and the nanocrystalline Al layer deposited at −700 mV showed a goldish shining surface, as the same appearance reported in Refs. [20,43]. Fig. 5d and Fig. 5e depict the typical EDX spectra of the deposited layers revealed in Fig. 5a and Fig. 5c respectively. A notably small Cl peak in layers deposited at lower potential (Fig. 5d) can be attributed to entrapment of electrolyte impurities in the deposit bumps and fissures. The EDX analysis of nanocrystalline Al layers deposited at high potentials (e.g. Fig. 5e), on the other hand, show pure aluminium with no evidence of Cl or other impurities. The small oxygen peak is due to atmospheric oxidation of Al surface during exposure to air. In conclusion, nanocrystalline Al can be deposited at high over potentials. A large overpotential brings about a large current density and thus a large rate of nucleation at the expense of rate of growth [46]. Also, depletion of electroactive Al species as a result of high discharge rate may allow some organic species to be adsorbed on the Al nuclei and suppress their growth [43,46]. Application of higher potential have been reported to lead to reduction of imidazolium cation ([EMIm]+ ) in AlCl3 /EMIC IL and result in nanocrystalline Al deposits [43].
0.01 1E-3 1E-4 1E-5 1E-6
Al deposit at -400 mV Al deposit at -600 mV Al deposit at -700 mV -1100 -1000
-900
-800
-700
-600
-500
-400
Potentialvs. SCE / mV Fig. 7. Potentiodynamic polarisation diagrams in 0.1 M NaCl aqueous solution for microcrystalline, and 177 nm- and 28 nm-crystalline Al layers deposited, at −400 mV, −600 mV, and −700 mV, respectively, from AlCl3 /EMIC (60/40 mol%).
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Table 2 Corrosion characteristics, obtained by polarisation in 0.1 M NaCl solution, of microcrstalline and nanocrystalline Al electrodeposits, compared with that of bulk hp Al in Ref. [48]. Material
Icorr (A/cm2 )
Ecorr (mVvs.
Micro-crystalline Al deposited at −400 mVvs. Al 177 nm-Crystalline Al deposited at −600 mVvs. Al 28 nm-Crystalline Al deposited at −700 mVvs. Al Bulk hp Al [48]
0.313 0.075 0.039 0.253
−722 −680 −652 −828
SCE )
Ep (mVvs. −617 −581 −477 −615
SCE )
Eprot (mVvs. −907 −675 −626 −695
SCE )
Hysteresis area (mA s/cm2 ) 36.43 18.71 22.49 27.50
deposited at −700 mV, showed a noticeable ability to re-passivate after the polarisation was swept in the reverse direction when pitting clearly began in the cyclic polarisation procedure. Its Eprot lies in the passivation range and is nobler than its Ecorr . This signifies a good pitting performance owing to the re-passivation possibility. In contrast, microcrystalline deposits showed Eprot more negative than Ecorr . Also, there was a clear shift of Ecorr towards noble direction for the finer grain size deposits. This is in agreement with the results of nanocrystalline nickel electrodeposits [33]. The progressive shift of Ecorr towards noble direction for the finer nanocrystalline deposits was related to the modification in cathodic reaction processes [33]. Fig. 8 shows the microscopic observation of the corroded surfaces after cyclic polarisation to Eprot . It is clear that nanocrystalline deposits have shallow pits overshadowed with corrosion products. In contrast, the micro-crystalline deposits showed deleterious defects. Consequently, it can be stated that the nanocrystalline Al layers electrodeposited at high potentials have higher corrosion resistance than that of the microcrystalline deposits. This higher corrosion resistance appeared in clearly lower Icorr values with showing passivation behaviour and a noticeable tendency for re-passivation. Higher corrosion resistance of nanocrystalline electrodeposits compared with microcrystalline ones has been reported [32,33,35,36]. In a comparison observation of corrosion behaviour of micro- and nanocrystalline Al electrodeposits in the present study with that of polished surfaces of highly pure (hp) bulk Al studied in the same electrolyte with the same procedure and reported in a previous study by the authors [48], Al electrodeposits monitored more noble Ecorr values than that of bulk Al. This indicates lower surface activity of Al deposits compared to polished surface of bulk Al. Micro-crystalline Al electrodeposits recorded higher Icorr values than that of bulk Al. This can be attributed to Cl inclusions in deposits at low potentials, as shown in Fig. 5d. On the other hand, nanocrystalline Al deposits showed lower Icorr values than that of bulk Al, indicating a higher resistance of nanocrystalline Al to anodic dissolution.
4. Conclusions The following conclusions have emerged from a detailed experimental study on the electrodeposition of aluminium from AlCl3 /EMIC (60/40 mol%) ionic liquid:
Fig. 8. Photo micrographs of corrosion surfaces following potentiodynamic cyclic polarisation, depicted in Fig. 7, in 0.1 M NaCl aqueous solution for: (a) microcrystalline Al deposited at −400 mV, (b) nanocrystalline Al deposited at −600 mV, and (c) nanocrystalline Al deposited at −700 mV.
• Results obtained from cyclic voltammetry (CV) measurements indicated that electrodeposition reaction of aluminium is a quasireversible process, which is controlled partly by diffusion of electroactive species and partly by charge transfer kinetics. In addition, the deposition kinetics appear to be influenced by a side chemical reaction preceding charge transfer, probably dissociation of water contaminated in the ionic liquid. • Coulombic current efficiency estimated from CV diagrams increased with increasing the farthest negative potential reached in CV, namely reversal potential, and then decreased significantly, indicating co-deposition of organic species at high negative potentials. These organic species are suggested to inhibit the
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growth of Al crystallites deposited with higher nucleation rates at high negative potentials, resulting in nano-structured Al deposits. • Nanocrystalline, bright, dense and adherent Al layers were electrodeposited at high negative potentials. XRD patterns showed that the nanocrystalline Al deposits are compressively strained and preferentially textured taking the plane orientation (200) with average crystallite size of 177 nm and 28 nm at −600 mV and at −700 mV, respectively. Nanocrystalline Al deposits showed superior corrosion resistance than microcrystalline Al deposits, and lower corrosion rate than bulk Al. Acknowledgements The authors would like to express their thanks to Dr. Ahmed Hannora, Suez University, for XRD analysis and related discussions. Also, thanks are due to the Duetscher Akademischer Austausch Dienst (DAAD) for partial funding provided to Dr. Ashraf Bakkar. The revision of English by Prof. Samir Abdel Hakeem, Suez University, is also greatly appreciated. References [1] M. Zhang, V. Kamavaram, R.G. Reddy, New electrolytes for aluminum production: ionic liquids, JOM 55 (2003) 54. [2] U. Grjotheim, H. Kvande, Introduction to Aluminum Electrolysis. Understanding the Hall-Heroult Process, Aluminum Verlag GmbH, Germany, 1993, pp. 260. [3] G.R. Stafford, The Electrodeposition of an aluminum–manganese metallic glass from molten salts, Journal of the Electrochemical Society 136 (1989) 635. [4] M. Jafarian, M.G. Mahjani, F. Gobal, I. Danaee, Electrodeposition of aluminum from AlCl3 –NaCl–KCl mixture, Journal of Applied Electrochemistry 36 (2006) 1169. [5] E.M. Moustafa, Ph.D. Thesis, Electrodeposition of aluminium in different air and water s table ionic liquids, TU, Clausthal, Papierflieger, Clausthal-Zellerfeld, 2007. [6] J. Fransaer, E. Leunis, T. Hirato, J.-P. Celis, Aluminium composite coatings containing micrometre and nanometre-sized particles electroplated from a non-aqueous electrolyte, Journal of Applied Electrochemistry 32 (2002) 123. [7] F.H. Hurley, T.P. Wier, The electrodeposition of aluminum from nonaqueous solutions at room temperature, Journal of the Electrochemical Society 98 (1951) 207. [8] J. Robinson, R.A. Osteryoung, The electrochemical behavior of aluminum in the low temperature molten salt system n butyl pyridinium chloride: aluminum chloride and mixtures of this molten salt with benzene, Journal of the Electrochemical Society 127 (1980) 122. [9] R.T. Carlin, W. Crawford, M. Berch, Nucleation and morphology studies of aluminum deposited from an ambient-temperature chloroaluminate molten salt, Journal of the Electrochemical Society 139 (1992) 2720. [10] Y. Zhao, T.J. VanderNoot, Electrodeposition of aluminium from room temperature AlCl3 –TMPAC molten salts, Electrochimica Acta 42 (1997) 1639. [11] Q. Liao, W.R. Pitner, G. Stewart, C.L. Hussey, G.R. Stafford, Electrodeposition of aluminum from the aluminum chloride-1-methyl-3-ethylimidazolium chloride room temperature molten salt + benzene, Journal of the Electrochemical Society 144 (3) (1997) 936. [12] Q. Zhu, C.L. Hussey, G.R. Stafford, Electrodeposition of silver–aluminum alloys from a room-temperature chloroaluminate molten salt, Journal of the Electrochemical Society 148 (2) (2001) C88. [13] Q. Zhu, C.L. Hussey, Galvanostatic pulse plating of Cu–Al alloy in a roomtemperature chloroaluminate molten salt. Rotating ring-disk electrode studies, Journal of the Electrochemical Society 148 (5) (2001) C395. [14] T. Jiang, M.J. Chollier Brym, G. Dube, A. Lasia, G.M. Brisard, Electrodeposition of aluminium from ionic liquids: Part I—electrodeposition and surface morphology of aluminium from aluminium chloride (AlCl3 )–1-ethyl3-methylimidazolium chloride ([EMIm]Cl) ionic liquids, Surface and Coatings Technology 201 (2006) 1. [15] T. Tsuda, C.L. Hussey, G.R. Stafford, Electrochemistry of titanium and the electrodeposition of Al–Ti alloys in the lewis acidic aluminum chloride–1ethyl-3-methylimidazolium chloride melt, Journal of the Electrochemical Society 150 (2003) C234. [16] T. Tsuda, C.L. Hussey, G.R. Stafford, Electrodeposition of Al–Mo alloys from the lewis acidic aluminum chloride–1-ethyl-3-methylimidazolium chloride molten salt, Journal of the Electrochemical Society 151 (2004) C379. [17] Q.X. Liu, S. Zein El Abdeen, F. Endres, Electroplating of mild steel by aluminium in a first generation ionic liquid: a green alternative to commercial Al-plating in organic solvents, Surface and Coatings Technology 201 (2006) 1352.
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