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Electrodeposition of Al-Mn-Zr ternary alloy films from the Lewis acidic aluminum chloride-1-ethyl-3-methylimidazolium chloride ionic liquid and their corrosion properties
Surface & Coatings Technology 321 (2017) 45–51
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Electrodeposition of Al-Mn-Zr ternary alloy films from the Lewis acidic aluminum chloride-1-ethyl-3-methylimidazolium chloride ionic liquid and their corrosion properties Jing Yang a, Ling Chang a, Li Jiang b, Kai Wang a, Liangai Huang a, Zhishun He a, Haibo Shao a, Jianming Wang a,⁎, Chu-nan Cao a a b
Department of Chemistry, Zhejiang University, Hangzhou 310027, PR China College of Materials Science and Engineering, China Jiliang University, Hangzhou 310018, PR China
a r t i c l e
i n f o
Article history: Received 31 January 2017 Revised 23 April 2017 Accepted in revised form 24 April 2017 Available online 25 April 2017 Keywords: Electrodeposition Ionic liquid Aluminum alloy Corrosion Zirconium Manganese
a b s t r a c t The electrodeposition of Al-Mn and Al-Mn-Zr alloy films on low carbon steel substrate from the Lewis acidic aluminum chloride-1-ethyl-3-methylimidazolium chloride (AlCl3/[Emim]Cl) ionic liquids with different metal salts is investigated. The addition of ZrCl4 in the bath with 0.04M MnCl2 induces the higher overpotential for the electrodeposition, which decreases the size of the electrodeposited primary crystallites. Rounded nodular particles composed of many primary nanocrystallites are obtained by the electrodeposition in the bath containing 0.04 M MnCl2 and 5 mM ZrCl4. The close packing of these nodular particles results in the formation of the AlMn-Zr ternary alloy film with a dense structure. Compared with the bare carbon steel electrode, the as-electrodeposited Al-Mn and Al-Mn-Zr alloy film electrodes shows much more negative corrosion potentials, indicating their cathodic protection effect on the substrate. As the Zr content in the alloy films becomes larger, the corrosion current density first decreases, and then increases. The Al-Mn-Zr ternary alloy film with 1.871 at.% Zr, which is prepared in the bath with 0.04 M MnCl2 and 5 mM ZrCl4, manifests the lowest corrosion current density. This can be attributed to its dense film structure and good combination with the substrate. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Aluminum has many attractive properties such as light weight, low cost, high strength and environmentally benign nature [1–5]. Especially, in atmospheric environments, aluminum exhibits excellent corrosion resistance, resulting from the spontaneous formation of surface oxide layer [1]. However, the thin surface oxide layer is unstable in chloridecontaining liquid media, thus pitting corrosion occurs on the surface of aluminum [1,4]. In order to improve the properties of aluminum, various aluminum alloys have been prepared [6–9]. As a typical aluminum alloy, Al-Mn alloy has received considerable attention due to its attractive luster, good mechanical performance and high corrosion resistance [10–17]. In general, compared with other common approaches such as hot dipping [18], spraying [19], physical vapor deposition (PVD) [20] and chemical vapor deposition (CVD) [21], electrodeposition is the preferred method for the preparation of the Al-Mn alloy coatings due to its convenient operation and relatively facile control on the microstructure of deposited films. The electrodepositions of Al-Mn alloy films in nonaqueous media have been conducted [10–17]. The manganese content ⁎ Corresponding author. E-mail address: [email protected] (J. Wang).
http://dx.doi.org/10.1016/j.surfcoat.2017.04.061 0257-8972/© 2017 Elsevier B.V. All rights reserved.
of the electrodeposit was reported to increase with increasing MnCl2 concentration in the deposition electrolyte, independent of current density [10]. As the manganese content increased, the deposit film first underwent a transformation from a microcrystalline solid solution with rough angular surface morphology to a nanocrystalline solid-solution phase with a nodular surface morphology, and an amorphous phase is finally achieved [14]. The improvement on the corrosion resistance of Al-Mn alloy electrodeposited films in chloride-containing solutions remains a challenge [12,16,17]. One commonly used approach for improving the microstructure and performance of aluminum is to add appropriate alloy elements [8,17, 22–25]. Zirconium (Zr) is one of the alloy elements that can improve the corrosion performance of Aluminum. Tsuda et al. reported that the pitting corrosion potential of the electrodeposited Al84.3Zr15.7 alloy in 0.1 M NaCl solution was about +0.3 V more positive than that of pure aluminum [23]. Shiomi et al. prepared a bright Al-Zr alloy coating from dimethylsulfone-based melt containing AlCl3, and the as-prepared Al-Zr alloy coating showed improved anticorrosion performance compared with pure aluminum in 0.1M NaCl solution [24]. The electrodeposition of Al-Zr alloy films with various compositions from AlCl3-NaClKCl molten salt was carried out by Ueda et al., and it was reported that the pitting corrosion potentials of Al-Zr alloys with 523 at.% Zr in
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J. Yang et al. / Surface & Coatings Technology 321 (2017) 45–51
Table 1 Chemical composition (wt%) of low carbon steel. Element
C
Si
Mn
P
S
Cr
Mo
Ni
Cu
Fe
Content
0.19
0.23
0.50
0.016
0.019
0.017
0.001
0.010
0.013
balance
0.1 M NaCl solution shifted from +0.1 to +0.3 V compared with that of pure Al [25]. The above investigations imply that the corrosion performance of Al-Mn alloy electrodeposited films in chloride-containing solutions may be improved by introduction of Zr alloy element. In this work, we investigate the electrodeposition of Al-Mn and AlMn-Zr alloy films from the Lewis acidic aluminum chloride-1-ethyl-3methylimidazolium chloride (AlCl3/[Emim]Cl) ionic liquids with different metal salts, using low carbon steel as substrate. The addition of ZrCl4 in the bath solutions increases the deposition overpotential of alloys, effectively decreasing the size of the electrodeposited primary crystallites. The Al-Mn-Zr ternary alloy films show higher corrosion resistance than the corresponding Al-Mn alloy film in 3.5 wt% NaCl solution. The effects of Zr content on the microstructures and corrosion properties of Al-MnZr ternary alloy films are discussed in detail.
100, FEI Co. Ltd.) coupled with energy dispersive X-ray spectrometry (EDS). The X-ray diffraction (XRD) patterns of the deposits were recorded by employing a Rigaku D/Max 2550 X-ray diffractometer with Cu Kα radiation (λ = 0.15405 nm) at 40 kV and 300 mA. 2.4. Corrosion tests
2. Experimental
A three-electrode cell was employed to investigate the corrosion behaviors of various electrodeposited films in 3.5 wt% NaCl solution at ambient temperature, using a platinum foil as counter electrode and a Ag/ AgCl electrode (SSE) in saturated KCl solution as the reference electrode. Potentiodynamic polarization curves were measured at a scan rate of 1 mV s−1 by a potentiostat (CHI 630D). Electrochemical impedance spectroscopy (EIS) measurements were recorded by an electrochemical analyzer (Parstat 2273), with the frequency range of 100 kHz to 0.01 Hz and a.c. signal amplitude of 10 mV.
2.1. Preparation of plating baths
3. Results and discussion
The bath preparation and electrochemical experiments in the baths were conducted in an argon-filled glove box (MIKROUNA Co., China). [Emim]Cl (Shanghai Chengjie Co.) was dried under vacuum condition at 60 °C for 12 h to remove residual moisture. Anhydrous AlCl3 (Sinopharm Chemical Reagent Co., AR), MnCl2 (Sigma, 99.99%) and ZrCl4 (Aldrich, 99.5%) were used as received. AlCl3 was mixed with [Emim]Cl in a 2:1 M ratio to prepare the electrolyte solution. MnCl2 (0.04 M) was added into the electrolyte solution and agitated for 24 h. Different amounts of ZrCl4 were added in the MnCl2-containing electrolyte solution to prepare the plating baths with various contents of ZrCl4 (1, 5 and 10 mM).
3.1. Electrodeposition and microstructures of alloy films
2.2. Electrodeposition of Al-Mn-Zr ternary alloy films Electrodeposition was carried out in a two-electrode cell in the argon-filled glove box, using an electrochemical workstation (CHI660D). Al wires (Alfa, 99.999%) were used as the anode, and the low carbon steel with the composition in Table 1 was used as the cathode. The distance between the anode and the cathode was about 20mm. Prior to each experiment, the carbon steel electrode was polished with 2000# grit waterproof abrasive paper, ultrasonically cleaned in acetone, deionized water and dichloromethane for 10 min, respectively. The samples were then transferred into the glove box. The electrodeposition was conducted at a constant current of 6 mA cm−2 for 3 h. After the electrodeposition, the samples were immediately taken out from the glove box, and washed in acetone, deionized water and ethanol, respectively, then dried in nitrogen stream. The Al-Mn-Zr ternary alloy films prepared by the electrodeposition in the baths containing 1, 5 and 10 mM ZrCl4 were designated as AlMnZr-1, AlMnZr-5 and AlMnZr-10, respectively. The electrochemical investigations on the electrodeposition behaviors of various films were conducted in a three-electrode cell in the glove box at ambient temperature. The cathodic polarization curves were measured at a scan rate of 1 mV s−1 by a potentiostat (CHI 630D), using Al wires (99.999%) as reference electrode and Pt foil (99.99%) as counter electrode. 2.3. Physical characterizations The surface morphologies and compositions of various alloy films were determined by a scanning electron microscopy (SEM, SIRION-
Fig. 1 depicts the cathodic polarization curves of mild steel substrate in the electrolyte solutions without and with metal salts. The electrode potential is negatively scanned from the open circuit potential at a scan rate of 1 mV s−1. The deposition of Al starts at −80 mV in the neat ionic liquid bath [26]. The addition of 0.04 M MnCl2 in the bath decreases the initial deposition potential of metals. As the concentration of ZrCl4 in the MnCl2-containing bath increases, the initial deposition potential of metals becomes more negative, implying that the deposition overpotential of metals gradually increases. For the identical cathodic deposition potential in the range of −200 to −800 mV, the deposition current is effectively decreased by the addition of MnCl2 and/or ZrCl4, which results from the co-electrodeposition of different metals [8]. It is noted that the increase in the concentration of ZrCl4 in the baths decreases the cathodic deposition current at the identical deposition
Fig. 1. Cathodic polarization curves of mild steel substrate in the electrolyte solutions without and with metal salts at a scan rate of 1 mV s−1. (a) Neat melt, (b) 0.04 M MnCl2, (c) 0.04 M MnCl2 + 1 mM ZrCl4, (d) 0.04 M MnCl2 + 5 mM ZrCl4, (e) 0.04 M MnCl2 + 10 mM ZrCl4.
J. Yang et al. / Surface & Coatings Technology 321 (2017) 45–51
Fig. 2. XRD patterns of the alloy films obtained by the electrodeposition at a current density of 6 mA cm−2 in the electrolyte solutions with MnCl2 and/or ZrCl4. (a) 0.04 M MnCl2, (b) 0.04 M MnCl2 + 1 mM ZrCl4, (c) 0.04 M MnCl2 + 5 mM ZrCl4, (d) 0.04 M MnCl2 + 10 mM ZrCl4.
potential, indicating that the deposition of Al and Mn are inhibited by the addition of ZrCl4. The XRD patterns of the alloy films obtained by the electrodeposition in the electrolyte solutions with MnCl2 and/or ZrCl4 are illustrated in Fig. 2, and the results of the texture calculations for various alloy films obtained from the XRD patterns in Fig. 2 are displayed in Table S1, the Supporting Information. The four main XRD peaks of the Al-Mn deposit (Fig. 2a) can be indexed as the characteristic diffraction peaks of (111), (200), (220) and (311) crystal planes for Al with a face-centered cubic (fcc) structure in terms of JCPDS file 01–1180. No other XRD peaks for impurities are detected. This shows that the Al-Mn deposit could be a solid solution of fcc Al [14]. The AlMnZr-1 sample displays a similar fcc
47
structure (Fig. 2b). As the Zr content in the Al-Mn-Zr deposits increases, the XRD peaks slightly shift to lower 2θ values, and the peak intensities gradually decrease, implying the decrease of primary crystallite sizes, as illustrated in Figs. 2c and d. The broad (220) and (311) peaks with low intensities suggest the emergence of an amorphous phase in AlMnZr-5 and AlMnZr-10 samples [14]. The results of texture calculations for various alloy films in Table S1 show that the preferred orientations in AlMn and AlMnZr-1 alloys are (111) and (220) crystal planes, and the bath solutions with higher ZrCl4 concentrations result in the alloy films (AlMnZr-5 and AlMnZr-10) with only a strong preferred (111) reflection [27,28]. The average crystallite sizes of Al-Mn, AlMnZr-1, AlMnZr-5 and AlMnZr-10 samples are estimated to be 75, 69, 50 and 38 nm according to the Scherrer equation (the Supporting Information), respectively [26,27]. This shows that the primary crystallite size is gradually decreased with the increase of ZrCl4 concentration in the bath solutions. The top-view SEM images of the alloy films obtained by the electrodeposition in the electrolyte solutions with MnCl2 and/or ZrCl4 are displayed in Fig. 3 and Fig. S1 (the Supporting Information). The electrodeposition from the electrolyte solution without ZrCl4 produces the AlMn alloy film consisting of large uneven angular or prismoid-like particles with a diameter of 3–13 μm (Fig. 3a). Some voids are observed in the Al-Mn alloy film, which is disadvantageous to improving its anticorrosion performance. As illustrated by the surface topography in Fig. 3b and Fig. S1 [29], a compact alloy film is prepared by the electrodeposition in the electrolyte solution with 1 mM ZrCl4. The electrodeposition in the bath with 5 mM ZrCl4 leads to the formation of a dense alloy film (AlMnZr-5) consisting of rounded nodular particles with a diameter of 3–15 μm (Fig. 3c). When the concentration of ZrCl4 in the bath is increased to 10 mM, a porous electrodeposited film (AlMnZr-10) formed by large agglomerated nodular particles is obtained (Fig. 3d). Combining the SEM results in Fig. 3 with the Scherrer's analysis of the XRD patterns in Fig. 2, it is concluded that the particles in Al-Mn and Al-Mn-Zr films are composed of numerous primary nanocrystallites. The addition of ZrCl4 in the baths increases the deposition overpotential of metals, as indicated by Fig. 1. Larger overpotential can result in higher nucleation rate, thus the size of the electrodeposited primary crystallites
Fig. 3. Top-view SEM images of the alloy films obtained by the electrodeposition at a current density of 6 mA cm−2 in the electrolyte solutions with MnCl2 and/or ZrCl4. (a) 0.04 M MnCl2, (b) 0.04 M MnCl2 + 1 mM ZrCl4, (c) 0.04 M MnCl2 + 5 mM ZrCl4, (d) 0.04 M MnCl2 + 10 mM ZrCl4.
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J. Yang et al. / Surface & Coatings Technology 321 (2017) 45–51
Fig. 4. Cross-sectional SEM images of Al-Mn alloy (a) and AlMnZr-5 (b) films.
thin surface oxide layer and the deposition solutions, respectively. It is noted from Table 2 that the O and Cl contents in the AlMnZr-5 alloy film are significantly higher than the corresponding values in other alloy films. Compared with the Al-Mn and AlMnZr-1 films, the AlMnZr-5 film has smaller primary crystallite size. The smaller primary crystallites are more easily oxidized in atmospheric environments, responsible for the higher O content in the AlMnZr-5 film. Simultaneously, the AlMnZr-5 film composed of smaller primary crystallites has higher specific surface area, which results in the adsorption of more Cl ions on its surface. Although the AlMnZr-10 film has smaller primary crystallite size than the AlMnZr-5 film, the porous nature is responsible for relatively lower apparent O and Cl contents on its top surface layer. 3.2. Corrosion properties of alloy films
Fig. 5. EDS spectra of various electrodeposited alloy films. (a) Al-Mn, (b) AlMnZr-1, (c) AlMnZr-5, (d) AlMnZr-10.
decreases [23,26,30]. This is the reason that smaller electrodeposited primary crystallites are obtained in the baths with larger ZrCl4 concentrations. The cross-sectional SEM images of Al-Mn alloy and AlMnZr-5 films are illustrated in Fig. 4. The two alloy films well adhere onto the substrate, with a uniform thickness. The Al-Mn alloy film with a thickness of 19 μm exhibits porous nature (Fig. 4a), while the Al-Mn-Zr alloy film with a thickness of 17 μm is much dense (Fig. 4b). This is consistent with their top-view SEM images in Fig. 3a and c. The EDS spectra of various electrodeposited alloy films are presented in Fig. 5, and the corresponding elemental compositions of different alloy films are displayed in Table 2. In the EDS spectrum of Al-Mn deposit film (Fig. 5a), stronger Al and Mn peaks as well as weak Cl and O peaks are observed. The addition of ZrCl4 in the baths leads to the occurrence of Zr peak, and its peak intensity increases as the concentration of ZrCl4 becomes larger, as shown in Fig. 5b-d. This indicates that the Al-Mn-Zr ternary alloy films are obtained by the electrodeposition in the ionic liquid baths with MnCl2 and ZrCl4 salts, and the Zr content in alloy films increases with increasing ZrCl4 concentration in the deposition solutions (Table 2). O and Cl elements in various alloy films are originated from
The polarization curves of Al-Mn and Al-Mn-Zr alloy film electrodes as well as the low carbon steel electrode were measured in 3.5wt% NaCl solution at a scan rate of 1 mV s−1 in order to evaluate their corrosion performances, and the corresponding results are depicted in Fig. 6 and Table 3. The low carbon steel electrode shows a typical anodic active dissolution behavior, with a large corrosion current density (Icorr) of 10.96 μA cm-2. The Al-Mn and Al-Mn-Zr alloy film electrodes deliver much more negative corrosion potentials (Ecorr) than the low carbon steel electrode, suggesting their cathodic protective effect on the carbon steel substrate. The Al-Mn and Al-Mn-Zr alloy film electrodes show the passivation behaviors in the anodic potential regions, which may be ascribed to the spontaneously formed surface oxide layer [1,12]. The corrosion current density (2.497 μA cm−2) of the Al-Mn alloy film
Table 2 Elemental compositions of various alloy films obtained from the EDS spectra in Fig. 5. Element
Al-Mn
AlMnZr-1
AlMnZr-5
AlMnZr-10
O (at.%) Al (at.%) Cl (at.%) Mn (at.%) Zr (at.%)
4.024 91.545 0.428 4.002 /
2.252 93.224 0.414 3.555 0.556
6.096 87.759 1.314 2.960 1.871
3.678 88.098 0.900 3.669 3.655
Fig. 6. Polarization curves of different alloy film electrodes and low carbon steel electrode in 3.5 wt% NaCl solution at a scan rate of 1 mV s−1.
J. Yang et al. / Surface & Coatings Technology 321 (2017) 45–51 Table 3 Corrosion parameters obtained from the polarization curves in Fig. 6.
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Table 4 Parameter values obtained by the fitting of the Nyquist plots in Fig. 7.
Material
Ecorr (mV)
Icorr (μA cm−2)
Material
Rs (Ω cm2)
Rct (kΩ cm2)
CPE1 (μF cm−2)
n
Low carbon steel Al-Mn alloy AlMnZr-1 AlMnZr-5 AlMnZr-10
−567 −991 −898 825 729
10.96 2.497 1.630 0.621 1.843
Low carbon steel Al-Mn alloy AlMnZr-1 AlMnZr-5 AlMnZr-10
2.34 2.13 3.64 2.15 1.83
1.164 15.00 23.84 45.37 20.04
897 35.6 33.7 17.9 37.1
0.800 0.921 0.903 0.786 0.701
electrode is much lower than that of the carbon steel electrode. As the Zr content in the alloy films becomes larger, the corrosion potential shifts towards the positive direction, and the corrosion current density first decreases and then increases. The AlMnZr-5 alloy film electrode shows the lowest corrosion current density (0.621 μA cm−2), which is about 3 and 16 times lower than the corresponding values of the AlMn alloy film and carbon steel electrodes, respectively. This results from its dense film and good combination with the steel substrate (Figs. 3c and 4). For the AlMnZr-10 alloy film electrode, the aggregation of nodular particles during the electrodeposition leads to the occurrence of voids in the film, which is responsible for its relatively higher corrosion current density (1.843 μA cm−2) than that of the AlMnZr-5 alloy film electrode. The EIS measurements were conducted to further investigate the corrosion properties of the as-electrodeposited alloy film electrodes and the low carbon steel electrode in 3.5 wt% NaCl solution. The Nyquist plots of the electrodes at open circuit state are given in Fig. 7a. A capacitive loop is observed in the Nyquist plots, which is originated from the electrochemical charge transfer resistance (Rct) in parallel with the interface capacitance (CPE1) [30–32]. The high-frequency intercept of the capacitive loop on the real axis represents the ohmic resistance (Rs) of the electrodes. The diameter of the capacitive loop yields the charge transfer resistance (Rct), and larger Rct value implies better anticorrosion performance [30,32]. The equivalent circuit in accordance with the above Nyquist plots is given in Fig. 7b [31], and the corresponding fitted results are displayed in Table 4. Considering the dispersion effect of the Nyquist plots induced by the roughness and other inhomogenities of the alloy film electrodes, it is necessary to use a constant phase element (CPE) instead of double layer capacity (Cdl) in Fig.
7b [33,34]. It can be seen from Fig. 7a and Table 4 that the Rct values of different electrodes have the following sequence: low carbon steel b Al-Mn alloy b AlMnZr-10 b AlMnZr-1 b AlMnZr-5. The AlMnZr-5 alloy film electrode manifests the highest corrosion resistance. This is in agreement with the results of polarization curves in Fig. 6. The lowest CPE value of the AlMnZr-5 electrode (Table 4) means the decreased electrochemical active interface area [31], which confirms its dense film structure. This may be the main reason that the AlMnZr-5 electrode shows much lower corrosion rate. Fig. 8 illustrates the SEM images of the electrodeposited alloy films after the immersion in 3.5 wt% NaCl solution for 7 days at room temperature. The Al-Mn alloy film suffers serious corrosion, presenting many cracks with a width of 1–2 μm in the corroded film surface (Fig. 8a). The voids in the pristine Al-Mn alloy film could be mainly responsible for its serious corrosion. The corroded AlMnZr-1 alloy film is covered by some loose corrosion products, and tiny cracks appear in the surface layer (Fig. 8b). Compared with the Al-Mn alloy film, the AlMnZr-1 alloy film shows improved anticorrosion performance, resulting from its compact surface. After the immersion, the AlMnZr-5 alloy film still retains the initial dense structure formed by the close packing of rounded nodular particles, and no any distinct crack is observed, although some small corrosion product particles appear on the film surface, as illustrated in Fig. 8c. For the AlMnZr-10 alloy film, a large number of loose corrosion products fill in the voids/cracks of the film after the immersion, showing its heavy corrosion (Fig. 8d). The voids/cracks in the film may provide easy access of the surfaces to NaCl solution, responsible for the heavy corrosion of the AlMnZr-10 alloy film. The preceding SEM observation results of the corroded alloy films show that the AlMnZr-5 alloy film has the best anticorrosion performance, which is
Fig. 7. (a) Nyquist plots of different alloy film electrodes and low carbon steel electrode in 3.5 wt% NaCl solution at open circuit state. Inset is the magnified Nyquist plot of low carbon steel electrode. (b) Corresponding equivalent circuit for the Nyquist plots in (a).
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J. Yang et al. / Surface & Coatings Technology 321 (2017) 45–51
Fig. 8. SEM images of different alloy films after immersion in 3.5 wt% NaCl solution for 7 days at room temperature. (a) Al-Mn, (b) AlMnZr-1, (c) AlMnZr-5, (d) AlMnZr-10.
consistent with the results of electrochemical corrosion experiments in Figs. 6 and 7.
4. Conclusions Al-Mn and Al-Mn-Zr alloy films are successfully prepared on low carbon steel substrate by the electrodeposition in the ionic liquid [Emim]Cl/AlCl3 with various metal salts. The addition of MnCl2 and/or ZrCl4 in the neat ionic liquid bath results in more negative initial deposition potentials and larger overpotentials for the electrodeposition of alloy films. As the concentration of ZrCl4 in the bath increases, the size of the electrodeposited primary crystallites is effectively decreased. A dense Al-Mn-Zr ternary alloy film (AlMnZr-5) formed by the close packing of rounded nodular particles is obtained by the electrodeposition in the bath with 5 mM ZrCl4. The nodular particles in the AlMnZr-5 film consist of many primary nanocrystallites. The introduction of Zr in Al\\Mn alloy film significantly enhances the corrosion resistance of the alloy film in 3.5 wt% NaCl solution. The AlMnZr-5 alloy film electrode manifests the best anticorrosion performance, which may be ascribed to its dense film structure and good combination with the low carbon steel substrate. These attractive results demonstrate that the as-constructed dense Al-Mn-Zr ternary alloy films with excellent anticorrosion performance have a promising potential as the coatings of active metal substrates.
Acknowledgments This work was supported by the National Natural Science Foundation of China (Nos. 51271168 and 51401198), the Zhejiang Provincial Natural Science Foundation of China (LY17B030004) and the Science and Technology Plan Project of Zhejiang Province (No. 2017C31078). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.surfcoat.2017.04.061.
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Electrodeposition of Al-Mn-Zr ternary alloy films from the Lewis acidic aluminum chloride-1-ethyl-3-methylimidazolium chloride ionic liquid and their corrosion properties Jing Yang a, Ling Chang a, Li Jiang b, Kai Wang a, Liangai Huang a, Zhishun He a, Haibo Shao a, Jianming Wang a,⁎, Chu-nan Cao a a b
Department of Chemistry, Zhejiang University, Hangzhou 310027, PR China College of Materials Science and Engineering, China Jiliang University, Hangzhou 310018, PR China
a r t i c l e
i n f o
Article history: Received 31 January 2017 Revised 23 April 2017 Accepted in revised form 24 April 2017 Available online 25 April 2017 Keywords: Electrodeposition Ionic liquid Aluminum alloy Corrosion Zirconium Manganese
a b s t r a c t The electrodeposition of Al-Mn and Al-Mn-Zr alloy films on low carbon steel substrate from the Lewis acidic aluminum chloride-1-ethyl-3-methylimidazolium chloride (AlCl3/[Emim]Cl) ionic liquids with different metal salts is investigated. The addition of ZrCl4 in the bath with 0.04M MnCl2 induces the higher overpotential for the electrodeposition, which decreases the size of the electrodeposited primary crystallites. Rounded nodular particles composed of many primary nanocrystallites are obtained by the electrodeposition in the bath containing 0.04 M MnCl2 and 5 mM ZrCl4. The close packing of these nodular particles results in the formation of the AlMn-Zr ternary alloy film with a dense structure. Compared with the bare carbon steel electrode, the as-electrodeposited Al-Mn and Al-Mn-Zr alloy film electrodes shows much more negative corrosion potentials, indicating their cathodic protection effect on the substrate. As the Zr content in the alloy films becomes larger, the corrosion current density first decreases, and then increases. The Al-Mn-Zr ternary alloy film with 1.871 at.% Zr, which is prepared in the bath with 0.04 M MnCl2 and 5 mM ZrCl4, manifests the lowest corrosion current density. This can be attributed to its dense film structure and good combination with the substrate. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Aluminum has many attractive properties such as light weight, low cost, high strength and environmentally benign nature [1–5]. Especially, in atmospheric environments, aluminum exhibits excellent corrosion resistance, resulting from the spontaneous formation of surface oxide layer [1]. However, the thin surface oxide layer is unstable in chloridecontaining liquid media, thus pitting corrosion occurs on the surface of aluminum [1,4]. In order to improve the properties of aluminum, various aluminum alloys have been prepared [6–9]. As a typical aluminum alloy, Al-Mn alloy has received considerable attention due to its attractive luster, good mechanical performance and high corrosion resistance [10–17]. In general, compared with other common approaches such as hot dipping [18], spraying [19], physical vapor deposition (PVD) [20] and chemical vapor deposition (CVD) [21], electrodeposition is the preferred method for the preparation of the Al-Mn alloy coatings due to its convenient operation and relatively facile control on the microstructure of deposited films. The electrodepositions of Al-Mn alloy films in nonaqueous media have been conducted [10–17]. The manganese content ⁎ Corresponding author. E-mail address: [email protected] (J. Wang).
http://dx.doi.org/10.1016/j.surfcoat.2017.04.061 0257-8972/© 2017 Elsevier B.V. All rights reserved.
of the electrodeposit was reported to increase with increasing MnCl2 concentration in the deposition electrolyte, independent of current density [10]. As the manganese content increased, the deposit film first underwent a transformation from a microcrystalline solid solution with rough angular surface morphology to a nanocrystalline solid-solution phase with a nodular surface morphology, and an amorphous phase is finally achieved [14]. The improvement on the corrosion resistance of Al-Mn alloy electrodeposited films in chloride-containing solutions remains a challenge [12,16,17]. One commonly used approach for improving the microstructure and performance of aluminum is to add appropriate alloy elements [8,17, 22–25]. Zirconium (Zr) is one of the alloy elements that can improve the corrosion performance of Aluminum. Tsuda et al. reported that the pitting corrosion potential of the electrodeposited Al84.3Zr15.7 alloy in 0.1 M NaCl solution was about +0.3 V more positive than that of pure aluminum [23]. Shiomi et al. prepared a bright Al-Zr alloy coating from dimethylsulfone-based melt containing AlCl3, and the as-prepared Al-Zr alloy coating showed improved anticorrosion performance compared with pure aluminum in 0.1M NaCl solution [24]. The electrodeposition of Al-Zr alloy films with various compositions from AlCl3-NaClKCl molten salt was carried out by Ueda et al., and it was reported that the pitting corrosion potentials of Al-Zr alloys with 523 at.% Zr in
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J. Yang et al. / Surface & Coatings Technology 321 (2017) 45–51
Table 1 Chemical composition (wt%) of low carbon steel. Element
C
Si
Mn
P
S
Cr
Mo
Ni
Cu
Fe
Content
0.19
0.23
0.50
0.016
0.019
0.017
0.001
0.010
0.013
balance
0.1 M NaCl solution shifted from +0.1 to +0.3 V compared with that of pure Al [25]. The above investigations imply that the corrosion performance of Al-Mn alloy electrodeposited films in chloride-containing solutions may be improved by introduction of Zr alloy element. In this work, we investigate the electrodeposition of Al-Mn and AlMn-Zr alloy films from the Lewis acidic aluminum chloride-1-ethyl-3methylimidazolium chloride (AlCl3/[Emim]Cl) ionic liquids with different metal salts, using low carbon steel as substrate. The addition of ZrCl4 in the bath solutions increases the deposition overpotential of alloys, effectively decreasing the size of the electrodeposited primary crystallites. The Al-Mn-Zr ternary alloy films show higher corrosion resistance than the corresponding Al-Mn alloy film in 3.5 wt% NaCl solution. The effects of Zr content on the microstructures and corrosion properties of Al-MnZr ternary alloy films are discussed in detail.
100, FEI Co. Ltd.) coupled with energy dispersive X-ray spectrometry (EDS). The X-ray diffraction (XRD) patterns of the deposits were recorded by employing a Rigaku D/Max 2550 X-ray diffractometer with Cu Kα radiation (λ = 0.15405 nm) at 40 kV and 300 mA. 2.4. Corrosion tests
2. Experimental
A three-electrode cell was employed to investigate the corrosion behaviors of various electrodeposited films in 3.5 wt% NaCl solution at ambient temperature, using a platinum foil as counter electrode and a Ag/ AgCl electrode (SSE) in saturated KCl solution as the reference electrode. Potentiodynamic polarization curves were measured at a scan rate of 1 mV s−1 by a potentiostat (CHI 630D). Electrochemical impedance spectroscopy (EIS) measurements were recorded by an electrochemical analyzer (Parstat 2273), with the frequency range of 100 kHz to 0.01 Hz and a.c. signal amplitude of 10 mV.
2.1. Preparation of plating baths
3. Results and discussion
The bath preparation and electrochemical experiments in the baths were conducted in an argon-filled glove box (MIKROUNA Co., China). [Emim]Cl (Shanghai Chengjie Co.) was dried under vacuum condition at 60 °C for 12 h to remove residual moisture. Anhydrous AlCl3 (Sinopharm Chemical Reagent Co., AR), MnCl2 (Sigma, 99.99%) and ZrCl4 (Aldrich, 99.5%) were used as received. AlCl3 was mixed with [Emim]Cl in a 2:1 M ratio to prepare the electrolyte solution. MnCl2 (0.04 M) was added into the electrolyte solution and agitated for 24 h. Different amounts of ZrCl4 were added in the MnCl2-containing electrolyte solution to prepare the plating baths with various contents of ZrCl4 (1, 5 and 10 mM).
3.1. Electrodeposition and microstructures of alloy films
2.2. Electrodeposition of Al-Mn-Zr ternary alloy films Electrodeposition was carried out in a two-electrode cell in the argon-filled glove box, using an electrochemical workstation (CHI660D). Al wires (Alfa, 99.999%) were used as the anode, and the low carbon steel with the composition in Table 1 was used as the cathode. The distance between the anode and the cathode was about 20mm. Prior to each experiment, the carbon steel electrode was polished with 2000# grit waterproof abrasive paper, ultrasonically cleaned in acetone, deionized water and dichloromethane for 10 min, respectively. The samples were then transferred into the glove box. The electrodeposition was conducted at a constant current of 6 mA cm−2 for 3 h. After the electrodeposition, the samples were immediately taken out from the glove box, and washed in acetone, deionized water and ethanol, respectively, then dried in nitrogen stream. The Al-Mn-Zr ternary alloy films prepared by the electrodeposition in the baths containing 1, 5 and 10 mM ZrCl4 were designated as AlMnZr-1, AlMnZr-5 and AlMnZr-10, respectively. The electrochemical investigations on the electrodeposition behaviors of various films were conducted in a three-electrode cell in the glove box at ambient temperature. The cathodic polarization curves were measured at a scan rate of 1 mV s−1 by a potentiostat (CHI 630D), using Al wires (99.999%) as reference electrode and Pt foil (99.99%) as counter electrode. 2.3. Physical characterizations The surface morphologies and compositions of various alloy films were determined by a scanning electron microscopy (SEM, SIRION-
Fig. 1 depicts the cathodic polarization curves of mild steel substrate in the electrolyte solutions without and with metal salts. The electrode potential is negatively scanned from the open circuit potential at a scan rate of 1 mV s−1. The deposition of Al starts at −80 mV in the neat ionic liquid bath [26]. The addition of 0.04 M MnCl2 in the bath decreases the initial deposition potential of metals. As the concentration of ZrCl4 in the MnCl2-containing bath increases, the initial deposition potential of metals becomes more negative, implying that the deposition overpotential of metals gradually increases. For the identical cathodic deposition potential in the range of −200 to −800 mV, the deposition current is effectively decreased by the addition of MnCl2 and/or ZrCl4, which results from the co-electrodeposition of different metals [8]. It is noted that the increase in the concentration of ZrCl4 in the baths decreases the cathodic deposition current at the identical deposition
Fig. 1. Cathodic polarization curves of mild steel substrate in the electrolyte solutions without and with metal salts at a scan rate of 1 mV s−1. (a) Neat melt, (b) 0.04 M MnCl2, (c) 0.04 M MnCl2 + 1 mM ZrCl4, (d) 0.04 M MnCl2 + 5 mM ZrCl4, (e) 0.04 M MnCl2 + 10 mM ZrCl4.
J. Yang et al. / Surface & Coatings Technology 321 (2017) 45–51
Fig. 2. XRD patterns of the alloy films obtained by the electrodeposition at a current density of 6 mA cm−2 in the electrolyte solutions with MnCl2 and/or ZrCl4. (a) 0.04 M MnCl2, (b) 0.04 M MnCl2 + 1 mM ZrCl4, (c) 0.04 M MnCl2 + 5 mM ZrCl4, (d) 0.04 M MnCl2 + 10 mM ZrCl4.
potential, indicating that the deposition of Al and Mn are inhibited by the addition of ZrCl4. The XRD patterns of the alloy films obtained by the electrodeposition in the electrolyte solutions with MnCl2 and/or ZrCl4 are illustrated in Fig. 2, and the results of the texture calculations for various alloy films obtained from the XRD patterns in Fig. 2 are displayed in Table S1, the Supporting Information. The four main XRD peaks of the Al-Mn deposit (Fig. 2a) can be indexed as the characteristic diffraction peaks of (111), (200), (220) and (311) crystal planes for Al with a face-centered cubic (fcc) structure in terms of JCPDS file 01–1180. No other XRD peaks for impurities are detected. This shows that the Al-Mn deposit could be a solid solution of fcc Al [14]. The AlMnZr-1 sample displays a similar fcc
47
structure (Fig. 2b). As the Zr content in the Al-Mn-Zr deposits increases, the XRD peaks slightly shift to lower 2θ values, and the peak intensities gradually decrease, implying the decrease of primary crystallite sizes, as illustrated in Figs. 2c and d. The broad (220) and (311) peaks with low intensities suggest the emergence of an amorphous phase in AlMnZr-5 and AlMnZr-10 samples [14]. The results of texture calculations for various alloy films in Table S1 show that the preferred orientations in AlMn and AlMnZr-1 alloys are (111) and (220) crystal planes, and the bath solutions with higher ZrCl4 concentrations result in the alloy films (AlMnZr-5 and AlMnZr-10) with only a strong preferred (111) reflection [27,28]. The average crystallite sizes of Al-Mn, AlMnZr-1, AlMnZr-5 and AlMnZr-10 samples are estimated to be 75, 69, 50 and 38 nm according to the Scherrer equation (the Supporting Information), respectively [26,27]. This shows that the primary crystallite size is gradually decreased with the increase of ZrCl4 concentration in the bath solutions. The top-view SEM images of the alloy films obtained by the electrodeposition in the electrolyte solutions with MnCl2 and/or ZrCl4 are displayed in Fig. 3 and Fig. S1 (the Supporting Information). The electrodeposition from the electrolyte solution without ZrCl4 produces the AlMn alloy film consisting of large uneven angular or prismoid-like particles with a diameter of 3–13 μm (Fig. 3a). Some voids are observed in the Al-Mn alloy film, which is disadvantageous to improving its anticorrosion performance. As illustrated by the surface topography in Fig. 3b and Fig. S1 [29], a compact alloy film is prepared by the electrodeposition in the electrolyte solution with 1 mM ZrCl4. The electrodeposition in the bath with 5 mM ZrCl4 leads to the formation of a dense alloy film (AlMnZr-5) consisting of rounded nodular particles with a diameter of 3–15 μm (Fig. 3c). When the concentration of ZrCl4 in the bath is increased to 10 mM, a porous electrodeposited film (AlMnZr-10) formed by large agglomerated nodular particles is obtained (Fig. 3d). Combining the SEM results in Fig. 3 with the Scherrer's analysis of the XRD patterns in Fig. 2, it is concluded that the particles in Al-Mn and Al-Mn-Zr films are composed of numerous primary nanocrystallites. The addition of ZrCl4 in the baths increases the deposition overpotential of metals, as indicated by Fig. 1. Larger overpotential can result in higher nucleation rate, thus the size of the electrodeposited primary crystallites
Fig. 3. Top-view SEM images of the alloy films obtained by the electrodeposition at a current density of 6 mA cm−2 in the electrolyte solutions with MnCl2 and/or ZrCl4. (a) 0.04 M MnCl2, (b) 0.04 M MnCl2 + 1 mM ZrCl4, (c) 0.04 M MnCl2 + 5 mM ZrCl4, (d) 0.04 M MnCl2 + 10 mM ZrCl4.
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J. Yang et al. / Surface & Coatings Technology 321 (2017) 45–51
Fig. 4. Cross-sectional SEM images of Al-Mn alloy (a) and AlMnZr-5 (b) films.
thin surface oxide layer and the deposition solutions, respectively. It is noted from Table 2 that the O and Cl contents in the AlMnZr-5 alloy film are significantly higher than the corresponding values in other alloy films. Compared with the Al-Mn and AlMnZr-1 films, the AlMnZr-5 film has smaller primary crystallite size. The smaller primary crystallites are more easily oxidized in atmospheric environments, responsible for the higher O content in the AlMnZr-5 film. Simultaneously, the AlMnZr-5 film composed of smaller primary crystallites has higher specific surface area, which results in the adsorption of more Cl ions on its surface. Although the AlMnZr-10 film has smaller primary crystallite size than the AlMnZr-5 film, the porous nature is responsible for relatively lower apparent O and Cl contents on its top surface layer. 3.2. Corrosion properties of alloy films
Fig. 5. EDS spectra of various electrodeposited alloy films. (a) Al-Mn, (b) AlMnZr-1, (c) AlMnZr-5, (d) AlMnZr-10.
decreases [23,26,30]. This is the reason that smaller electrodeposited primary crystallites are obtained in the baths with larger ZrCl4 concentrations. The cross-sectional SEM images of Al-Mn alloy and AlMnZr-5 films are illustrated in Fig. 4. The two alloy films well adhere onto the substrate, with a uniform thickness. The Al-Mn alloy film with a thickness of 19 μm exhibits porous nature (Fig. 4a), while the Al-Mn-Zr alloy film with a thickness of 17 μm is much dense (Fig. 4b). This is consistent with their top-view SEM images in Fig. 3a and c. The EDS spectra of various electrodeposited alloy films are presented in Fig. 5, and the corresponding elemental compositions of different alloy films are displayed in Table 2. In the EDS spectrum of Al-Mn deposit film (Fig. 5a), stronger Al and Mn peaks as well as weak Cl and O peaks are observed. The addition of ZrCl4 in the baths leads to the occurrence of Zr peak, and its peak intensity increases as the concentration of ZrCl4 becomes larger, as shown in Fig. 5b-d. This indicates that the Al-Mn-Zr ternary alloy films are obtained by the electrodeposition in the ionic liquid baths with MnCl2 and ZrCl4 salts, and the Zr content in alloy films increases with increasing ZrCl4 concentration in the deposition solutions (Table 2). O and Cl elements in various alloy films are originated from
The polarization curves of Al-Mn and Al-Mn-Zr alloy film electrodes as well as the low carbon steel electrode were measured in 3.5wt% NaCl solution at a scan rate of 1 mV s−1 in order to evaluate their corrosion performances, and the corresponding results are depicted in Fig. 6 and Table 3. The low carbon steel electrode shows a typical anodic active dissolution behavior, with a large corrosion current density (Icorr) of 10.96 μA cm-2. The Al-Mn and Al-Mn-Zr alloy film electrodes deliver much more negative corrosion potentials (Ecorr) than the low carbon steel electrode, suggesting their cathodic protective effect on the carbon steel substrate. The Al-Mn and Al-Mn-Zr alloy film electrodes show the passivation behaviors in the anodic potential regions, which may be ascribed to the spontaneously formed surface oxide layer [1,12]. The corrosion current density (2.497 μA cm−2) of the Al-Mn alloy film
Table 2 Elemental compositions of various alloy films obtained from the EDS spectra in Fig. 5. Element
Al-Mn
AlMnZr-1
AlMnZr-5
AlMnZr-10
O (at.%) Al (at.%) Cl (at.%) Mn (at.%) Zr (at.%)
4.024 91.545 0.428 4.002 /
2.252 93.224 0.414 3.555 0.556
6.096 87.759 1.314 2.960 1.871
3.678 88.098 0.900 3.669 3.655
Fig. 6. Polarization curves of different alloy film electrodes and low carbon steel electrode in 3.5 wt% NaCl solution at a scan rate of 1 mV s−1.
J. Yang et al. / Surface & Coatings Technology 321 (2017) 45–51 Table 3 Corrosion parameters obtained from the polarization curves in Fig. 6.
49
Table 4 Parameter values obtained by the fitting of the Nyquist plots in Fig. 7.
Material
Ecorr (mV)
Icorr (μA cm−2)
Material
Rs (Ω cm2)
Rct (kΩ cm2)
CPE1 (μF cm−2)
n
Low carbon steel Al-Mn alloy AlMnZr-1 AlMnZr-5 AlMnZr-10
−567 −991 −898 825 729
10.96 2.497 1.630 0.621 1.843
Low carbon steel Al-Mn alloy AlMnZr-1 AlMnZr-5 AlMnZr-10
2.34 2.13 3.64 2.15 1.83
1.164 15.00 23.84 45.37 20.04
897 35.6 33.7 17.9 37.1
0.800 0.921 0.903 0.786 0.701
electrode is much lower than that of the carbon steel electrode. As the Zr content in the alloy films becomes larger, the corrosion potential shifts towards the positive direction, and the corrosion current density first decreases and then increases. The AlMnZr-5 alloy film electrode shows the lowest corrosion current density (0.621 μA cm−2), which is about 3 and 16 times lower than the corresponding values of the AlMn alloy film and carbon steel electrodes, respectively. This results from its dense film and good combination with the steel substrate (Figs. 3c and 4). For the AlMnZr-10 alloy film electrode, the aggregation of nodular particles during the electrodeposition leads to the occurrence of voids in the film, which is responsible for its relatively higher corrosion current density (1.843 μA cm−2) than that of the AlMnZr-5 alloy film electrode. The EIS measurements were conducted to further investigate the corrosion properties of the as-electrodeposited alloy film electrodes and the low carbon steel electrode in 3.5 wt% NaCl solution. The Nyquist plots of the electrodes at open circuit state are given in Fig. 7a. A capacitive loop is observed in the Nyquist plots, which is originated from the electrochemical charge transfer resistance (Rct) in parallel with the interface capacitance (CPE1) [30–32]. The high-frequency intercept of the capacitive loop on the real axis represents the ohmic resistance (Rs) of the electrodes. The diameter of the capacitive loop yields the charge transfer resistance (Rct), and larger Rct value implies better anticorrosion performance [30,32]. The equivalent circuit in accordance with the above Nyquist plots is given in Fig. 7b [31], and the corresponding fitted results are displayed in Table 4. Considering the dispersion effect of the Nyquist plots induced by the roughness and other inhomogenities of the alloy film electrodes, it is necessary to use a constant phase element (CPE) instead of double layer capacity (Cdl) in Fig.
7b [33,34]. It can be seen from Fig. 7a and Table 4 that the Rct values of different electrodes have the following sequence: low carbon steel b Al-Mn alloy b AlMnZr-10 b AlMnZr-1 b AlMnZr-5. The AlMnZr-5 alloy film electrode manifests the highest corrosion resistance. This is in agreement with the results of polarization curves in Fig. 6. The lowest CPE value of the AlMnZr-5 electrode (Table 4) means the decreased electrochemical active interface area [31], which confirms its dense film structure. This may be the main reason that the AlMnZr-5 electrode shows much lower corrosion rate. Fig. 8 illustrates the SEM images of the electrodeposited alloy films after the immersion in 3.5 wt% NaCl solution for 7 days at room temperature. The Al-Mn alloy film suffers serious corrosion, presenting many cracks with a width of 1–2 μm in the corroded film surface (Fig. 8a). The voids in the pristine Al-Mn alloy film could be mainly responsible for its serious corrosion. The corroded AlMnZr-1 alloy film is covered by some loose corrosion products, and tiny cracks appear in the surface layer (Fig. 8b). Compared with the Al-Mn alloy film, the AlMnZr-1 alloy film shows improved anticorrosion performance, resulting from its compact surface. After the immersion, the AlMnZr-5 alloy film still retains the initial dense structure formed by the close packing of rounded nodular particles, and no any distinct crack is observed, although some small corrosion product particles appear on the film surface, as illustrated in Fig. 8c. For the AlMnZr-10 alloy film, a large number of loose corrosion products fill in the voids/cracks of the film after the immersion, showing its heavy corrosion (Fig. 8d). The voids/cracks in the film may provide easy access of the surfaces to NaCl solution, responsible for the heavy corrosion of the AlMnZr-10 alloy film. The preceding SEM observation results of the corroded alloy films show that the AlMnZr-5 alloy film has the best anticorrosion performance, which is
Fig. 7. (a) Nyquist plots of different alloy film electrodes and low carbon steel electrode in 3.5 wt% NaCl solution at open circuit state. Inset is the magnified Nyquist plot of low carbon steel electrode. (b) Corresponding equivalent circuit for the Nyquist plots in (a).
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J. Yang et al. / Surface & Coatings Technology 321 (2017) 45–51
Fig. 8. SEM images of different alloy films after immersion in 3.5 wt% NaCl solution for 7 days at room temperature. (a) Al-Mn, (b) AlMnZr-1, (c) AlMnZr-5, (d) AlMnZr-10.
consistent with the results of electrochemical corrosion experiments in Figs. 6 and 7.
4. Conclusions Al-Mn and Al-Mn-Zr alloy films are successfully prepared on low carbon steel substrate by the electrodeposition in the ionic liquid [Emim]Cl/AlCl3 with various metal salts. The addition of MnCl2 and/or ZrCl4 in the neat ionic liquid bath results in more negative initial deposition potentials and larger overpotentials for the electrodeposition of alloy films. As the concentration of ZrCl4 in the bath increases, the size of the electrodeposited primary crystallites is effectively decreased. A dense Al-Mn-Zr ternary alloy film (AlMnZr-5) formed by the close packing of rounded nodular particles is obtained by the electrodeposition in the bath with 5 mM ZrCl4. The nodular particles in the AlMnZr-5 film consist of many primary nanocrystallites. The introduction of Zr in Al\\Mn alloy film significantly enhances the corrosion resistance of the alloy film in 3.5 wt% NaCl solution. The AlMnZr-5 alloy film electrode manifests the best anticorrosion performance, which may be ascribed to its dense film structure and good combination with the low carbon steel substrate. These attractive results demonstrate that the as-constructed dense Al-Mn-Zr ternary alloy films with excellent anticorrosion performance have a promising potential as the coatings of active metal substrates.
Acknowledgments This work was supported by the National Natural Science Foundation of China (Nos. 51271168 and 51401198), the Zhejiang Provincial Natural Science Foundation of China (LY17B030004) and the Science and Technology Plan Project of Zhejiang Province (No. 2017C31078). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.surfcoat.2017.04.061.
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