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A356 composite from ionic liquid as protective coating
Surface & Coatings Technology 213 (2012) 264–270
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The electrodeposition of aluminum on TiB2/A356 composite from ionic liquid as protective coating Wenmao Huang, Mingliang Wang, Haowei Wang, Naiheng Ma, Xianfeng Li ⁎ State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China
a r t i c l e
i n f o
Article history: Received 20 July 2012 Accepted in revised form 18 October 2012 Available online 2 November 2012 Keywords: TiB2/A356 composite Ionic liquid Electrodeposition Al coating Corrosion resistance
a b s t r a c t The electrochemical deposition of aluminum using ionic liquid of AlCl3-1-ethtyl-3-methylimidazolium chloride (AlCl3-[EMIm]Cl) was studied to improve the corrosion resistance of TiB2/A356 composites. The effects of experimental parameters (e.g., current density, deposition time and electrolyte temperature) on the deposit morphology and cathode current efficiency were investigated. The results indicated that the experimental parameters can greatly affect the morphology of Al coatings. Sustaining at a relatively high percentage (≥97.0%), the cathode current efficiencies showed varied dependence on different parameters. The optimum conditions for producing dense and uniform Al coatings were determined, with the current density of 1.0 A/dm2, deposition time of 60 min and electrolyte temperature of 30 °C. The electrochemical behavior was evaluated by potentiodynamic polarization curves and cyclic voltammogram. It was found that the corrosion resistance of the TiB2/A356 composite was greatly enhanced by Al electrodeposition in ionic liquid. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Aluminum alloys reinforced with ceramic particles (Al-CPRMMCs) have received considerable attention due to their superiorities in high stiffness, high strength, high Young modules, good wear resistance and low coefficient of thermal expansion [1–5]. Therefore, they have been widely applied as structural components in military, aerospace sectors and automobile industries [6–10]. However, the presence of reinforcements is inclined to adversely affect the corrosion behavior of these materials [9–14]. Thus, the Al-CPRMMCs are susceptible to corrosion in hostile environments, because of the galvanic reactions between the reinforcements and the matrix. The localized corrosion or selective corrosion should occur at the interface [9–11,13,14]. Therefore, the enhancement of their corrosion resistance is a pressing issue. It is well known that Al possesses such advantages as low density, excellent electrical and thermal conductivity. Actually, Al naturally forms a protective oxide coating and is highly corrosion resistant. Thus, the application of Al coatings for anti-corrosion in materials has been widely developed [15–18]. In the ceramic particles (e.g. TiB2, Al2O3 and SiC) reinforced Al metal matrix composites, there is hardly any potential difference between Al coatings and Al matrix. The application of Al coatings deposited on the surface of Al-CPRMMCs is considered to be an effective anti-corrosion strategy.
⁎ Corresponding author. Tel./fax: +86 21 54747597. E-mail addresses: [email protected] (W. Huang), [email protected] (X. Li). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2012.10.058
Currently, there are several techniques adopted for Al plating. Thermal spraying process is a commercially available technology with the advantage of flexible operation. However, the thermal sprayed Al coating usually contains pores that can be detrimental to the corrosion resistance [19]. Hot-dip coating process prevails in consuming industries. Although this method can produce Al coating with shining surface, it disfavors a well-controlled coating with a uniform thickness [20]. Vapor deposition methods can fabricate dense and uniform metallic coatings for corrosion protection. Nevertheless, both physical vapor deposition and chemical vapor deposition are very expensive and the processes are often slow [21,22]. Electrochemical deposition of metals is a versatile strategy to produce metallic coatings. It is a cost-effective method allowing a precise control of coating thickness and surface smoothness. Since Al is too active to be electrodeposited on the alien surface in water electrolytes, the electrodeposition of Al from non-water electrolytes (e.g., inorganic molten salts and ionic liquids) is received increasing interest. In comparison with the inorganic molten salt electrolytes working at a high temperature [23], the ionic liquid electrolytes are organic molten salts with a lower working temperature. Besides, the ionic liquids possess unique chemical and physical properties such as high conductivity, high thermal stability, negligible vapor pressures, and wide electrochemical window [24,25]. These excellent properties make them suitable media for the electrochemical deposition. To the best of our knowledge, although there are some literatures on Al-CPRMMCs for anti-corrosion, the reports of electrodeposition of Al from ionic liquids on the Al-CPRMMCs for the corrosion protection are few. In this paper, the electrodeposition of Al coatings on in-situ TiB2/ A356 composites from AlCl3-1-ethtyl-3-methylimidazolium chloride
W. Huang et al. / Surface & Coatings Technology 213 (2012) 264–270
(AlCl3-[EMIm]Cl) ionic liquid was investigated. The effects of experimental parameters (including current density, deposition time and electrolyte temperature) on the deposit morphology and cathode current efficiency were studied. The optimum experimental conditions for producing dense and uniform Al coatings were determined. The results of electrochemical behavior measurement indicated that the corrosion resistance of the TiB2/A356 composite was greatly enhanced by Al electrodeposition in AlCl3-[EMIm]Cl ionic liquid. 2. Experimental procedures The Al-CPRMMC used in this investigation was in-situ TiB2/A356 composite (Al-6.5–7.5 wt.% Si-0.25–0.45 wt.% Mg-0.08–0.20 wt.% Ti-13.5 wt.% TiB2). The specimens with the size of 40 mm ×10 mm× 2 mm were ground using SiC paper down to 2000 grit under Ar atmosphere, in order to achieve clear and reproducible surfaces. The preparation of ionic liquid electrolyte was also carried out under Ar atmosphere. The ionic liquid (20 ml) was prepared by mixing precise quantities of anhydrous AlCl3 and 1-ethtyl-3-methylimidazolium chloride ([EMIm]Cl) with a molar ratio of 2:1. The mixing process was exothermic that AlCl3 was carefully added in small quantities to a beaker containing [EMIm]Cl at room temperature with magnetic stirring. After the liquid was fully mixed and cooled down naturally, the ionic liquid electrolyte was achieved. The electrochemical deposition of Al was performed in the vacuum glove box filled with Ar gas. In a two-electrode electrochemical cell, the cathode was TiB2/A356 composite, and the anode was Al rod. The distance between anode and cathode was about 2 cm. The pure Al layer was electrodeposited on the surface of TiB2/A356 composite at varied conditions, where the current density ranged from 0.5 to 2.0 A/dm 2, deposition time from 30 to 90 min, and temperature from 30 to 120 °C. Once the deposition is completed, the samples were thoroughly cleaned with deionized water and dried in air. In order to evaluate the current efficiency (η) of Al electrodeposition, the current efficiency was defined as the ratio of the mass of practically deposited Al to that expected theoretically [26,27]: η¼
m2 m1
ð1Þ
where m1 is the theoretical mass of deposited Al on cathode, while m2 is the actual mass. According to Faraday's law, the theoretical mass (m1) gained on the cathode was expressed by the following equation [26,27]: m1 ¼
ItM F n
ð2Þ
where I is the current, t is the deposition time, M/n is the equivalent weight of the metal Al, and F is Faraday's constant (96,487 C). The actual mass of deposited Al (m2) was calculated from the mass change derived from the cathode before and after the electrodeposition. Overall, the current efficiency (η) of Al electrodeposition can be expressed as following: η¼
F n m2 : ItM
composite and pure Al were used as working electrode in turn. The saturated calomel electrode (SCE) and platinum electrode were performed as reference electrode and auxiliary electrode, respectively. In order to stabilize the open-circuit potential, the specimens were kept in the 3.5 wt.% NaCl solution for at least 60 min prior to the electrochemical measurements. The scanning speed of the potentiodynamic polarization curve measurement was 1 mV/s. The cyclic voltammetry measurement was carried out using an electrochemical cell (25 ml glass beaker) with three electrodes. TiB2/A356 composite was used as working electrode. Al sheet and Al wire were used as counter and reference electrodes, respectively. The electrolyte was AlCl3-[EMIm]Cl ionic liquid with 2:1 of molar ratio, and the electrolyte temperature was controlled at 30 °C. The scanning speed was 10 mV/s. The exposed surface of the working electrodes was about 1.0 cm2. In both measurements, three tests were performed for each condition to confirm the validity of these tests. 3. Results and discussion 3.1. SEM micrograph of TiB2/A356 composite Fig. 1 shows the SEM micrograph of the TiB2/A356 composite before electrodeposition. It was clearly observed that the surface of the composite contained countless TiB2 particles, and some Si phase was surrounded by these particle aggregates. 3.2. Effect of current density The effects of current density on the morphology of Al coatings and cathode current efficiency were studied in this section. The electrodeposition performed here was under the following conditions: current density being kept constant in each test in the range of 0.5– 2.0 A/dm 2, the temperature of 30 °C and the deposition time of 60 min. The effect of current density on the morphologies of Al deposits is shown in Fig. 2. Fig. 2(a) shows the Al deposit presents a cloud-shaped morphology and roughly covers the substrate surface at the lower current density of 0.5 A/dm 2. At the higher current density of 1.0 A/dm 2, shown in Fig. 2(b), the Al deposit becomes dense and uniform, and continuously covers the substrate. As the current density increases to 1.5 A/dm 2, the Al deposit layer is observed as nodular and not so compact. The growth of the Al grains occurred preferentially on limited sites, as seen in Fig. 2(c). In Fig. 2(d), it is noted that the higher current density of 2.0 A/dm 2 resulted in cauliflower structure deposits with rough surface, dendritic growth and poor adherence. Such clear observations indicate that the optimum current density can be determined to be 1.0 A/dm 2. The variation of surface morphology with current density was due to the increased deposition rate [28,29]. At the lower current density, the deposition of Al did not occur at all sites for the low cathode overpotential. The
ð3Þ
The surface morphologies of the specimens were characterized by scanning electron microscope (SEM) equipped with energy dispersive spectroscopy (EDS). A CHI660C electrochemical measurement system was used to evaluate the electrochemical behavior of the specimens including the measurements of potentiodynamic polarization curve and cyclic voltammogram. The measurement of potentiodynamic polarization curve was carried out in a 3.5 wt.% NaCl solution in a 250 ml glass beaker at room temperature. A three-electrode cell was adopted, in which the Al-coated TiB2/A356 composite, TiB2/A356
265
Fig. 1. SEM micrograph of TiB2/A356 composite before electrodeposition.
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Fig. 2. SEM images of Al deposits at current density: (a) 0.5 A/dm2, (b) 1.0 A/dm2, (c) 1.5 A/dm2, and (d) 2.0 A/dm2 at temperature of 30 °C and deposition time of 60 min.
growth of the deposit was slow for the low deposition rate. At a moderate high current density, the initial deposition of Al occurred at various sites for the larger cathode overpotential. The deposit grew uniformly in all directions with increasing deposition rate. At a higher current density, the surface exhibited obvious nodule or cauliflowers morphology due to the larger deposition rate. Similar results have also been reported previously [29–31]. As the current density increases, the theoretical mass of deposited Al also increases according to Eq. (2). However, the actual mass of deposited Al on the cathode does not follow the same trend as that of current density. It increases relatively slowly, because the current loss (polarization effect) exists during the electrolysis process [32,33]. Thus, it can be derived that the current efficiency decreases with increasing current density. Fig. 3 shows the experimentally measured variation of current efficiency with current density, it is seen that the current efficiency decreases with increase in the current density in the range of 97.0–99.7%. The experimental results are coincident with that expected theoretically.
found in Fig. 4(c). When the deposition time reached 90 min, agglomeration of particles was observed, the deposits showed an uneven surface with poor adherence (Fig. 4(d)), since the residual stress caused diminished adhesion and tended to drive the separation of the thin film from the substrate [28]. This phenomenon was aggravated with increasing deposition time because the residual stress was likely to increase with the increasing film thickness. Experimentally, it can be determined that the optimum deposition time is about 60 min. The variation of current efficiency with deposition time is shown in Fig. 5. Overall, the current efficiencies of ≥99.0% are observed at
3.3. Effect of deposition time The effects of deposition time on the morphology of Al coatings and cathode current efficiency were studied. The experimental conditions included the current density of 1.0 A/dm 2, temperature of 30 °C, and deposition time varied in the range of 30–90 min. Fig. 4(a) shows the morphology of Al deposit at the earlier deposition stage of 30 min. The Al deposit shows the isolated fine and circular-shaped particles, which partly cover the substrate. Since the nucleation of Al grains did not complete in this stage, some sites were left empty. When the deposition time prolonged to 45 min, new deposits were formed at other sites. A coarse thin film gradually covered the substrate and the growth of redeposits on some Al grains can be also observed, the morphology turned out uneven (Fig. 4(b)). When the deposition time reached 60 min, the Al grains grew and finally integrated into a whole. The more uniform and compact deposits were
Fig. 3. The variation of current efficiency with current density at temperature of 30 °C and deposition time of 60 min.
W. Huang et al. / Surface & Coatings Technology 213 (2012) 264–270
267
Fig. 4. SEM images of Al deposits at deposition time: (a) 30 min, (b) 45 min, (c) 60 min, and (d) 90 min at current density of 1.0 A/dm2 and temperature of 30 °C.
all deposition times. The current efficiency varies slightly with increase in the deposition time in the range of 99.1–99.2%. Conclusively, the effect of deposition time on current efficiency is insignificant, which might due to the similar trend of the theoretical and actual mass of deposited Al with the increasing deposition time.
3.4. Effect of temperature In this section, the effects of temperature on morphology of Al coatings and cathode current efficiency were investigated. The experimental
Fig. 5. The variation of current efficiency with deposition time at current density of 1.0 A/dm2 and temperature of 30 °C.
conditions were as follows: a fixed current density of 1.0 A/dm2, deposition time of 60 min and the temperature was kept constant in each test in the range of 30–120 °C. Fig. 6 shows the deposit morphology obtained under different electrolyte temperatures. Fig. 6(a) shows a dense and uniform coating which continuously covers the substrate. However, at a higher temperature, further growth of the pre-deposits Al is likely to occur rather than new deposition at the other empty places. It is seen in Fig. 6(b) that some particles grow bigger, while some sites appear blank. The deposits show a less compact and poorly adherent morphology. This trend becomes more serious when the experiment was performed under a higher temperature, such as 90 °C, as shown in Fig. 6(c), the deposits grow in some fixed direction with lamellar shape. A neither compact nor uniform morphology is displayed. The deposits obtained at 120 °C (Fig. 6(d)) are chained to several dendritic sheets, and new Al grain can be observed on the surface of Al sheet. It has an enhanced surface roughness. It is observed that the roughness of Al surface increases significantly with increasing temperature. This phenomenon probably results from the variation of electrical conductivities and diffusion rate of ions in ionic liquid at different temperatures. High deposition rate and growth rate of metal nuclei in some sites are prone to promote the dendritic growth, which often leads to rough coatings [27,34]. Thus, the optimum temperature is found to be 30 °C. The effect of temperature on the current efficiency is shown in Fig. 7. It is seen that, current efficiencies higher than 99.0% are obtained at all the temperatures. The current efficiencies are found to slightly increase with increasing temperature within a range of 99.1–99.6%, which is consistent with Ref. [29]. As mentioned in Section 3.3, the variation in current efficiency is affected by the actual amount deposited at the cathode and the theoretical one. Since the theoretical amount deposited is related to the current and deposition time, which is fixed if both two parameters are constant [33]. The increase of current efficiency with temperature can be ascribed to the increasing amount of Al deposited at the cathode.
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Fig. 6. SEM images of Al deposits at temperature: (a) 30 °C, (b) 60 °C, (c) 90 °C, and (d) 120 °C at current density of 1.0 A/dm2 and deposition time of 60 min.
3.5. Electrochemical tests
Al2Cl7− is the only Al-contained ion which can be reduced in the electrochemical window range as Eq. (7):
3.5.1. Cyclic voltammogram The components of AlCl3-[EMIm]Cl (molar ratio of 2:1) ionic liquid can be expressed as follows [35,36].
4Al2 Cl7
þ
AlCl3 þ ½EMImCl→½EMIm þ AlCl4 −
−
AlCl3 þ AlCl4 →Al2 Cl7 :
−
ð4Þ ð5Þ
Since Al2Cl4− is easily oxidized, the oxidation reaction can be shown as follows: −
4AlCl4 →2Al2 Cl7
−
−
þ Cl2 ↑ þ 2e :
ð6Þ
Fig. 7. The variation of current efficiency with temperature at current density of 1.0 A/dm2 and deposition time of 60 min.
−
−
þ 3e →7AlCl4
−
þ Al:
ð7Þ
Fig. 8 shows the cyclic voltammogram recorded on the composite electrode in AlCl3-[EMIm]Cl ionic liquid electrolyte at 30 °C. It is seen that, an anodic peak was identified at a potential of about 1.53 V. This peak corresponds to the dissolution of Al (deposited Al on the composite) as follows: −
−
−
Al−3e þ 7AlCl4 →4Al2 Cl7 :
ð8Þ
The cathodic peak located at a potential of about − 1.57 V is the reduction peak of Al2Cl7−. On the cathode, Al2Cl7− ions get discharged to form Al atoms. The cathode reaction is shown in Eq. (7). The electrochemical process of electrodeposition involves dissolution of Al from anode and deposition Al at cathode. When the potential is applied between the Al anode and composite cathode, the AlCl4− ions that existed in electrolyte can react with Al from anode to produce Al2Cl7− ions (Eq. (8)). Al2Cl7− ions formed by dissolving Al traverse to cathode and get discharged to produce Al deposit (Eq. (7)). 3.5.2. Corrosion performance In order to evaluate the corrosion resistance of TiB2/A356 composite before and after electrodeposition, the potentiodynamic polarization experiments were performed. The samples tested are Al-coated TiB2/A356 composite (current density of 1.0 A/dm 2, deposition time of 60 min and temperature of 30 °C), TiB2/A356 composite and pure Al. The chemical compositions of Al-coated TiB2/A356 composite measured by EDS are shown in Fig. 9. It is confirmed Al is the major composition of the coating with the mass fraction 96.66%. A small quantity of oxygen is presented for the oxidation of coating in the air. Conclusively, Al coatings cover essentially the entire surface of the composite. In other words, Al coatings cover Si and TiB2 particles presented on the surface. Fig. 10 shows the cross-section micrograph
W. Huang et al. / Surface & Coatings Technology 213 (2012) 264–270
269
Fig. 10. Cross-section morphology of Al coating (1.0 A/dm2, 30 °C, 60 min). Fig. 8. Cyclic voltammogram recorded on the TiB2/A356 composite in AlCl3-[EMIm]Cl with 2:1 of molar ratio at 30 °C with scan rate of 10 mV/s.
of deposited Al coating/composite interface. It is observed that no detachment is detectable at the interface. The deposited Al layer is continuous and uniform, with the thickness of ~ 17 μm. Fig. 11 shows the potentiodynamic polarization curves obtained for the specimens. It is found that, compared with bare composite, the anodic and cathodic arms of the curve of the Al-coated TiB2/A356 composite move to lower current densities. The result implies the cathodic and anodic reaction processes are restrained by the Al coating. In Fig. 11, the TiB2/A356 composite exhibited the highest corrosion current density. The Al-coated TiB2/A356 composite shows a lower current density, which is close to that of pure Al. Table 1 summarizes the corrosion potential (Ecorr) and corrosion current density (Icorr) of the specimens. The bare composite shows the lowest Ecorr with a value of −0.797 V and the highest Icorr with a value of 4.077× 10−5 A/cm2. While Al-coated composite exhibits a higher Ecorr with a value of −0.737 V and a lower Icorr with a value of 1.947× 10−6 A/cm2, and these two values are so close to the corresponding ones of pure Al (−0.735 V, 1.509 × 10−6 A/cm2). It is concluded that TiB2/A356 composite after ionic-liquid electrodeposition exhibits enhanced corrosion resistance, it has the similar corrosion resistance to pure Al. For the TiB2/A356 composite, many agglomerates of TiB2 and Si presented on the composites surface (Fig. 1) lead to the discontinuities in the nature film and adversely affect the corrosion protection of this material. Thus, the surface of the composite undergoes active dissolution in chloride ion solution during polarization, leading to the increase in polarization current. After electrodeposition, the Al coating can prevent the composite matrix from contacting with the
Fig. 9. Chemical compositions of Al coating (1.0 A/dm2, 30 °C, 60 min).
solution, and hold back the corrosion process to a certain degree. So the decrease of corrosion current is considerable. The polarization results reveal that integrated Al coatings provide effective protections for the composite, and the treatment of electrodeposited Al coating for in-situ TiB2/A356 composite is feasible. 4. Conclusions This research studied the electrodeposition of Al coatings on in-situ TiB2/A356 composites from AlCl3-[EMIm]Cl ionic liquid. The conclusions of this study can be drawn specifically as follows: (1) Ionic-liquid electrodeposition was performed to enhance the corrosion resistance of TiB2/A356 composite, which can eliminate the adverse affection of TiB2 particle. (2) The experimental parameters, including current density, deposition time and electrolyte temperature, can greatly affect the morphology of the coatings. (3) The cathode current efficiencies showed varied dependence on different experimental parameters. The current efficiency decreased with the increase of the current density. Increasing the temperature, the current efficiency increased slightly. The effect of deposition time on current efficiency is insignificant.
Fig. 11. Potentiodynamic polarization curves of pure Al, TiB2/A356 composite and Al-coated TiB2/A356 composite in a 3.5 wt.% NaCl solution.
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Table 1 The results of the polarization test in 3.5 wt.% NaCl solution. Specimens
Pure Al
TiB2/A356 composite
Al-coated TiB2/A356 composite
Ecorr (V) Icorr (A/cm2)
−0.735 1.509 × 10−6
−0.797 4.077 × 10−5
−0.737 1.947 × 10−6
(4) The optimum experimental conditions for producing dense and uniform Al were current density of 1.0 A/dm 2, deposition time of 60 min and temperature of 30 °C. (5) The result of electrochemical behavior measurements indicated that the corrosion resistance of the TiB2/A356 composite was greatly enhanced by electrodeposition of Al in ionic liquid. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
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Contents lists available at SciVerse ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
The electrodeposition of aluminum on TiB2/A356 composite from ionic liquid as protective coating Wenmao Huang, Mingliang Wang, Haowei Wang, Naiheng Ma, Xianfeng Li ⁎ State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China
a r t i c l e
i n f o
Article history: Received 20 July 2012 Accepted in revised form 18 October 2012 Available online 2 November 2012 Keywords: TiB2/A356 composite Ionic liquid Electrodeposition Al coating Corrosion resistance
a b s t r a c t The electrochemical deposition of aluminum using ionic liquid of AlCl3-1-ethtyl-3-methylimidazolium chloride (AlCl3-[EMIm]Cl) was studied to improve the corrosion resistance of TiB2/A356 composites. The effects of experimental parameters (e.g., current density, deposition time and electrolyte temperature) on the deposit morphology and cathode current efficiency were investigated. The results indicated that the experimental parameters can greatly affect the morphology of Al coatings. Sustaining at a relatively high percentage (≥97.0%), the cathode current efficiencies showed varied dependence on different parameters. The optimum conditions for producing dense and uniform Al coatings were determined, with the current density of 1.0 A/dm2, deposition time of 60 min and electrolyte temperature of 30 °C. The electrochemical behavior was evaluated by potentiodynamic polarization curves and cyclic voltammogram. It was found that the corrosion resistance of the TiB2/A356 composite was greatly enhanced by Al electrodeposition in ionic liquid. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Aluminum alloys reinforced with ceramic particles (Al-CPRMMCs) have received considerable attention due to their superiorities in high stiffness, high strength, high Young modules, good wear resistance and low coefficient of thermal expansion [1–5]. Therefore, they have been widely applied as structural components in military, aerospace sectors and automobile industries [6–10]. However, the presence of reinforcements is inclined to adversely affect the corrosion behavior of these materials [9–14]. Thus, the Al-CPRMMCs are susceptible to corrosion in hostile environments, because of the galvanic reactions between the reinforcements and the matrix. The localized corrosion or selective corrosion should occur at the interface [9–11,13,14]. Therefore, the enhancement of their corrosion resistance is a pressing issue. It is well known that Al possesses such advantages as low density, excellent electrical and thermal conductivity. Actually, Al naturally forms a protective oxide coating and is highly corrosion resistant. Thus, the application of Al coatings for anti-corrosion in materials has been widely developed [15–18]. In the ceramic particles (e.g. TiB2, Al2O3 and SiC) reinforced Al metal matrix composites, there is hardly any potential difference between Al coatings and Al matrix. The application of Al coatings deposited on the surface of Al-CPRMMCs is considered to be an effective anti-corrosion strategy.
⁎ Corresponding author. Tel./fax: +86 21 54747597. E-mail addresses: [email protected] (W. Huang), [email protected] (X. Li). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2012.10.058
Currently, there are several techniques adopted for Al plating. Thermal spraying process is a commercially available technology with the advantage of flexible operation. However, the thermal sprayed Al coating usually contains pores that can be detrimental to the corrosion resistance [19]. Hot-dip coating process prevails in consuming industries. Although this method can produce Al coating with shining surface, it disfavors a well-controlled coating with a uniform thickness [20]. Vapor deposition methods can fabricate dense and uniform metallic coatings for corrosion protection. Nevertheless, both physical vapor deposition and chemical vapor deposition are very expensive and the processes are often slow [21,22]. Electrochemical deposition of metals is a versatile strategy to produce metallic coatings. It is a cost-effective method allowing a precise control of coating thickness and surface smoothness. Since Al is too active to be electrodeposited on the alien surface in water electrolytes, the electrodeposition of Al from non-water electrolytes (e.g., inorganic molten salts and ionic liquids) is received increasing interest. In comparison with the inorganic molten salt electrolytes working at a high temperature [23], the ionic liquid electrolytes are organic molten salts with a lower working temperature. Besides, the ionic liquids possess unique chemical and physical properties such as high conductivity, high thermal stability, negligible vapor pressures, and wide electrochemical window [24,25]. These excellent properties make them suitable media for the electrochemical deposition. To the best of our knowledge, although there are some literatures on Al-CPRMMCs for anti-corrosion, the reports of electrodeposition of Al from ionic liquids on the Al-CPRMMCs for the corrosion protection are few. In this paper, the electrodeposition of Al coatings on in-situ TiB2/ A356 composites from AlCl3-1-ethtyl-3-methylimidazolium chloride
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(AlCl3-[EMIm]Cl) ionic liquid was investigated. The effects of experimental parameters (including current density, deposition time and electrolyte temperature) on the deposit morphology and cathode current efficiency were studied. The optimum experimental conditions for producing dense and uniform Al coatings were determined. The results of electrochemical behavior measurement indicated that the corrosion resistance of the TiB2/A356 composite was greatly enhanced by Al electrodeposition in AlCl3-[EMIm]Cl ionic liquid. 2. Experimental procedures The Al-CPRMMC used in this investigation was in-situ TiB2/A356 composite (Al-6.5–7.5 wt.% Si-0.25–0.45 wt.% Mg-0.08–0.20 wt.% Ti-13.5 wt.% TiB2). The specimens with the size of 40 mm ×10 mm× 2 mm were ground using SiC paper down to 2000 grit under Ar atmosphere, in order to achieve clear and reproducible surfaces. The preparation of ionic liquid electrolyte was also carried out under Ar atmosphere. The ionic liquid (20 ml) was prepared by mixing precise quantities of anhydrous AlCl3 and 1-ethtyl-3-methylimidazolium chloride ([EMIm]Cl) with a molar ratio of 2:1. The mixing process was exothermic that AlCl3 was carefully added in small quantities to a beaker containing [EMIm]Cl at room temperature with magnetic stirring. After the liquid was fully mixed and cooled down naturally, the ionic liquid electrolyte was achieved. The electrochemical deposition of Al was performed in the vacuum glove box filled with Ar gas. In a two-electrode electrochemical cell, the cathode was TiB2/A356 composite, and the anode was Al rod. The distance between anode and cathode was about 2 cm. The pure Al layer was electrodeposited on the surface of TiB2/A356 composite at varied conditions, where the current density ranged from 0.5 to 2.0 A/dm 2, deposition time from 30 to 90 min, and temperature from 30 to 120 °C. Once the deposition is completed, the samples were thoroughly cleaned with deionized water and dried in air. In order to evaluate the current efficiency (η) of Al electrodeposition, the current efficiency was defined as the ratio of the mass of practically deposited Al to that expected theoretically [26,27]: η¼
m2 m1
ð1Þ
where m1 is the theoretical mass of deposited Al on cathode, while m2 is the actual mass. According to Faraday's law, the theoretical mass (m1) gained on the cathode was expressed by the following equation [26,27]: m1 ¼
ItM F n
ð2Þ
where I is the current, t is the deposition time, M/n is the equivalent weight of the metal Al, and F is Faraday's constant (96,487 C). The actual mass of deposited Al (m2) was calculated from the mass change derived from the cathode before and after the electrodeposition. Overall, the current efficiency (η) of Al electrodeposition can be expressed as following: η¼
F n m2 : ItM
composite and pure Al were used as working electrode in turn. The saturated calomel electrode (SCE) and platinum electrode were performed as reference electrode and auxiliary electrode, respectively. In order to stabilize the open-circuit potential, the specimens were kept in the 3.5 wt.% NaCl solution for at least 60 min prior to the electrochemical measurements. The scanning speed of the potentiodynamic polarization curve measurement was 1 mV/s. The cyclic voltammetry measurement was carried out using an electrochemical cell (25 ml glass beaker) with three electrodes. TiB2/A356 composite was used as working electrode. Al sheet and Al wire were used as counter and reference electrodes, respectively. The electrolyte was AlCl3-[EMIm]Cl ionic liquid with 2:1 of molar ratio, and the electrolyte temperature was controlled at 30 °C. The scanning speed was 10 mV/s. The exposed surface of the working electrodes was about 1.0 cm2. In both measurements, three tests were performed for each condition to confirm the validity of these tests. 3. Results and discussion 3.1. SEM micrograph of TiB2/A356 composite Fig. 1 shows the SEM micrograph of the TiB2/A356 composite before electrodeposition. It was clearly observed that the surface of the composite contained countless TiB2 particles, and some Si phase was surrounded by these particle aggregates. 3.2. Effect of current density The effects of current density on the morphology of Al coatings and cathode current efficiency were studied in this section. The electrodeposition performed here was under the following conditions: current density being kept constant in each test in the range of 0.5– 2.0 A/dm 2, the temperature of 30 °C and the deposition time of 60 min. The effect of current density on the morphologies of Al deposits is shown in Fig. 2. Fig. 2(a) shows the Al deposit presents a cloud-shaped morphology and roughly covers the substrate surface at the lower current density of 0.5 A/dm 2. At the higher current density of 1.0 A/dm 2, shown in Fig. 2(b), the Al deposit becomes dense and uniform, and continuously covers the substrate. As the current density increases to 1.5 A/dm 2, the Al deposit layer is observed as nodular and not so compact. The growth of the Al grains occurred preferentially on limited sites, as seen in Fig. 2(c). In Fig. 2(d), it is noted that the higher current density of 2.0 A/dm 2 resulted in cauliflower structure deposits with rough surface, dendritic growth and poor adherence. Such clear observations indicate that the optimum current density can be determined to be 1.0 A/dm 2. The variation of surface morphology with current density was due to the increased deposition rate [28,29]. At the lower current density, the deposition of Al did not occur at all sites for the low cathode overpotential. The
ð3Þ
The surface morphologies of the specimens were characterized by scanning electron microscope (SEM) equipped with energy dispersive spectroscopy (EDS). A CHI660C electrochemical measurement system was used to evaluate the electrochemical behavior of the specimens including the measurements of potentiodynamic polarization curve and cyclic voltammogram. The measurement of potentiodynamic polarization curve was carried out in a 3.5 wt.% NaCl solution in a 250 ml glass beaker at room temperature. A three-electrode cell was adopted, in which the Al-coated TiB2/A356 composite, TiB2/A356
265
Fig. 1. SEM micrograph of TiB2/A356 composite before electrodeposition.
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Fig. 2. SEM images of Al deposits at current density: (a) 0.5 A/dm2, (b) 1.0 A/dm2, (c) 1.5 A/dm2, and (d) 2.0 A/dm2 at temperature of 30 °C and deposition time of 60 min.
growth of the deposit was slow for the low deposition rate. At a moderate high current density, the initial deposition of Al occurred at various sites for the larger cathode overpotential. The deposit grew uniformly in all directions with increasing deposition rate. At a higher current density, the surface exhibited obvious nodule or cauliflowers morphology due to the larger deposition rate. Similar results have also been reported previously [29–31]. As the current density increases, the theoretical mass of deposited Al also increases according to Eq. (2). However, the actual mass of deposited Al on the cathode does not follow the same trend as that of current density. It increases relatively slowly, because the current loss (polarization effect) exists during the electrolysis process [32,33]. Thus, it can be derived that the current efficiency decreases with increasing current density. Fig. 3 shows the experimentally measured variation of current efficiency with current density, it is seen that the current efficiency decreases with increase in the current density in the range of 97.0–99.7%. The experimental results are coincident with that expected theoretically.
found in Fig. 4(c). When the deposition time reached 90 min, agglomeration of particles was observed, the deposits showed an uneven surface with poor adherence (Fig. 4(d)), since the residual stress caused diminished adhesion and tended to drive the separation of the thin film from the substrate [28]. This phenomenon was aggravated with increasing deposition time because the residual stress was likely to increase with the increasing film thickness. Experimentally, it can be determined that the optimum deposition time is about 60 min. The variation of current efficiency with deposition time is shown in Fig. 5. Overall, the current efficiencies of ≥99.0% are observed at
3.3. Effect of deposition time The effects of deposition time on the morphology of Al coatings and cathode current efficiency were studied. The experimental conditions included the current density of 1.0 A/dm 2, temperature of 30 °C, and deposition time varied in the range of 30–90 min. Fig. 4(a) shows the morphology of Al deposit at the earlier deposition stage of 30 min. The Al deposit shows the isolated fine and circular-shaped particles, which partly cover the substrate. Since the nucleation of Al grains did not complete in this stage, some sites were left empty. When the deposition time prolonged to 45 min, new deposits were formed at other sites. A coarse thin film gradually covered the substrate and the growth of redeposits on some Al grains can be also observed, the morphology turned out uneven (Fig. 4(b)). When the deposition time reached 60 min, the Al grains grew and finally integrated into a whole. The more uniform and compact deposits were
Fig. 3. The variation of current efficiency with current density at temperature of 30 °C and deposition time of 60 min.
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Fig. 4. SEM images of Al deposits at deposition time: (a) 30 min, (b) 45 min, (c) 60 min, and (d) 90 min at current density of 1.0 A/dm2 and temperature of 30 °C.
all deposition times. The current efficiency varies slightly with increase in the deposition time in the range of 99.1–99.2%. Conclusively, the effect of deposition time on current efficiency is insignificant, which might due to the similar trend of the theoretical and actual mass of deposited Al with the increasing deposition time.
3.4. Effect of temperature In this section, the effects of temperature on morphology of Al coatings and cathode current efficiency were investigated. The experimental
Fig. 5. The variation of current efficiency with deposition time at current density of 1.0 A/dm2 and temperature of 30 °C.
conditions were as follows: a fixed current density of 1.0 A/dm2, deposition time of 60 min and the temperature was kept constant in each test in the range of 30–120 °C. Fig. 6 shows the deposit morphology obtained under different electrolyte temperatures. Fig. 6(a) shows a dense and uniform coating which continuously covers the substrate. However, at a higher temperature, further growth of the pre-deposits Al is likely to occur rather than new deposition at the other empty places. It is seen in Fig. 6(b) that some particles grow bigger, while some sites appear blank. The deposits show a less compact and poorly adherent morphology. This trend becomes more serious when the experiment was performed under a higher temperature, such as 90 °C, as shown in Fig. 6(c), the deposits grow in some fixed direction with lamellar shape. A neither compact nor uniform morphology is displayed. The deposits obtained at 120 °C (Fig. 6(d)) are chained to several dendritic sheets, and new Al grain can be observed on the surface of Al sheet. It has an enhanced surface roughness. It is observed that the roughness of Al surface increases significantly with increasing temperature. This phenomenon probably results from the variation of electrical conductivities and diffusion rate of ions in ionic liquid at different temperatures. High deposition rate and growth rate of metal nuclei in some sites are prone to promote the dendritic growth, which often leads to rough coatings [27,34]. Thus, the optimum temperature is found to be 30 °C. The effect of temperature on the current efficiency is shown in Fig. 7. It is seen that, current efficiencies higher than 99.0% are obtained at all the temperatures. The current efficiencies are found to slightly increase with increasing temperature within a range of 99.1–99.6%, which is consistent with Ref. [29]. As mentioned in Section 3.3, the variation in current efficiency is affected by the actual amount deposited at the cathode and the theoretical one. Since the theoretical amount deposited is related to the current and deposition time, which is fixed if both two parameters are constant [33]. The increase of current efficiency with temperature can be ascribed to the increasing amount of Al deposited at the cathode.
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Fig. 6. SEM images of Al deposits at temperature: (a) 30 °C, (b) 60 °C, (c) 90 °C, and (d) 120 °C at current density of 1.0 A/dm2 and deposition time of 60 min.
3.5. Electrochemical tests
Al2Cl7− is the only Al-contained ion which can be reduced in the electrochemical window range as Eq. (7):
3.5.1. Cyclic voltammogram The components of AlCl3-[EMIm]Cl (molar ratio of 2:1) ionic liquid can be expressed as follows [35,36].
4Al2 Cl7
þ
AlCl3 þ ½EMImCl→½EMIm þ AlCl4 −
−
AlCl3 þ AlCl4 →Al2 Cl7 :
−
ð4Þ ð5Þ
Since Al2Cl4− is easily oxidized, the oxidation reaction can be shown as follows: −
4AlCl4 →2Al2 Cl7
−
−
þ Cl2 ↑ þ 2e :
ð6Þ
Fig. 7. The variation of current efficiency with temperature at current density of 1.0 A/dm2 and deposition time of 60 min.
−
−
þ 3e →7AlCl4
−
þ Al:
ð7Þ
Fig. 8 shows the cyclic voltammogram recorded on the composite electrode in AlCl3-[EMIm]Cl ionic liquid electrolyte at 30 °C. It is seen that, an anodic peak was identified at a potential of about 1.53 V. This peak corresponds to the dissolution of Al (deposited Al on the composite) as follows: −
−
−
Al−3e þ 7AlCl4 →4Al2 Cl7 :
ð8Þ
The cathodic peak located at a potential of about − 1.57 V is the reduction peak of Al2Cl7−. On the cathode, Al2Cl7− ions get discharged to form Al atoms. The cathode reaction is shown in Eq. (7). The electrochemical process of electrodeposition involves dissolution of Al from anode and deposition Al at cathode. When the potential is applied between the Al anode and composite cathode, the AlCl4− ions that existed in electrolyte can react with Al from anode to produce Al2Cl7− ions (Eq. (8)). Al2Cl7− ions formed by dissolving Al traverse to cathode and get discharged to produce Al deposit (Eq. (7)). 3.5.2. Corrosion performance In order to evaluate the corrosion resistance of TiB2/A356 composite before and after electrodeposition, the potentiodynamic polarization experiments were performed. The samples tested are Al-coated TiB2/A356 composite (current density of 1.0 A/dm 2, deposition time of 60 min and temperature of 30 °C), TiB2/A356 composite and pure Al. The chemical compositions of Al-coated TiB2/A356 composite measured by EDS are shown in Fig. 9. It is confirmed Al is the major composition of the coating with the mass fraction 96.66%. A small quantity of oxygen is presented for the oxidation of coating in the air. Conclusively, Al coatings cover essentially the entire surface of the composite. In other words, Al coatings cover Si and TiB2 particles presented on the surface. Fig. 10 shows the cross-section micrograph
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269
Fig. 10. Cross-section morphology of Al coating (1.0 A/dm2, 30 °C, 60 min). Fig. 8. Cyclic voltammogram recorded on the TiB2/A356 composite in AlCl3-[EMIm]Cl with 2:1 of molar ratio at 30 °C with scan rate of 10 mV/s.
of deposited Al coating/composite interface. It is observed that no detachment is detectable at the interface. The deposited Al layer is continuous and uniform, with the thickness of ~ 17 μm. Fig. 11 shows the potentiodynamic polarization curves obtained for the specimens. It is found that, compared with bare composite, the anodic and cathodic arms of the curve of the Al-coated TiB2/A356 composite move to lower current densities. The result implies the cathodic and anodic reaction processes are restrained by the Al coating. In Fig. 11, the TiB2/A356 composite exhibited the highest corrosion current density. The Al-coated TiB2/A356 composite shows a lower current density, which is close to that of pure Al. Table 1 summarizes the corrosion potential (Ecorr) and corrosion current density (Icorr) of the specimens. The bare composite shows the lowest Ecorr with a value of −0.797 V and the highest Icorr with a value of 4.077× 10−5 A/cm2. While Al-coated composite exhibits a higher Ecorr with a value of −0.737 V and a lower Icorr with a value of 1.947× 10−6 A/cm2, and these two values are so close to the corresponding ones of pure Al (−0.735 V, 1.509 × 10−6 A/cm2). It is concluded that TiB2/A356 composite after ionic-liquid electrodeposition exhibits enhanced corrosion resistance, it has the similar corrosion resistance to pure Al. For the TiB2/A356 composite, many agglomerates of TiB2 and Si presented on the composites surface (Fig. 1) lead to the discontinuities in the nature film and adversely affect the corrosion protection of this material. Thus, the surface of the composite undergoes active dissolution in chloride ion solution during polarization, leading to the increase in polarization current. After electrodeposition, the Al coating can prevent the composite matrix from contacting with the
Fig. 9. Chemical compositions of Al coating (1.0 A/dm2, 30 °C, 60 min).
solution, and hold back the corrosion process to a certain degree. So the decrease of corrosion current is considerable. The polarization results reveal that integrated Al coatings provide effective protections for the composite, and the treatment of electrodeposited Al coating for in-situ TiB2/A356 composite is feasible. 4. Conclusions This research studied the electrodeposition of Al coatings on in-situ TiB2/A356 composites from AlCl3-[EMIm]Cl ionic liquid. The conclusions of this study can be drawn specifically as follows: (1) Ionic-liquid electrodeposition was performed to enhance the corrosion resistance of TiB2/A356 composite, which can eliminate the adverse affection of TiB2 particle. (2) The experimental parameters, including current density, deposition time and electrolyte temperature, can greatly affect the morphology of the coatings. (3) The cathode current efficiencies showed varied dependence on different experimental parameters. The current efficiency decreased with the increase of the current density. Increasing the temperature, the current efficiency increased slightly. The effect of deposition time on current efficiency is insignificant.
Fig. 11. Potentiodynamic polarization curves of pure Al, TiB2/A356 composite and Al-coated TiB2/A356 composite in a 3.5 wt.% NaCl solution.
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Table 1 The results of the polarization test in 3.5 wt.% NaCl solution. Specimens
Pure Al
TiB2/A356 composite
Al-coated TiB2/A356 composite
Ecorr (V) Icorr (A/cm2)
−0.735 1.509 × 10−6
−0.797 4.077 × 10−5
−0.737 1.947 × 10−6
(4) The optimum experimental conditions for producing dense and uniform Al were current density of 1.0 A/dm 2, deposition time of 60 min and temperature of 30 °C. (5) The result of electrochemical behavior measurements indicated that the corrosion resistance of the TiB2/A356 composite was greatly enhanced by electrodeposition of Al in ionic liquid. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
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