- Email: [email protected]
Characterization of anodized titanium for hydrometallurgical applications—Evidence for the reduction of cupric on titanium dioxide
G Model APSUSC-25978; No. of Pages 10
ARTICLE IN PRESS Applied Surface Science xxx (2013) xxx–xxx
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Characterization of anodized titanium for hydrometallurgical applications—Evidence for the reduction of cupric on titanium dioxide Jing Liu ∗ , Akram Alfantazi, Edouard Asselin Department of Materials Engineering, The University of British Columbia, Vancouver, BC, Canada V6 T 1Z4
a r t i c l e
i n f o
Article history: Received 23 March 2013 Received in revised form 22 June 2013 Accepted 23 June 2013 Available online xxx Keywords: Titanium Copper leaching AOFs EIS Corrosion resistance XPS Oxidants
a b s t r a c t Anodic oxide films (AOFs) were potentiostatically formed on commercially pure titanium in 0.5 M sulfuric acid solutions at various anodizing voltages (up to 80 V) at room temperature. The subject of this study was the corrosion resistance of the AOFs in synthetic copper sulfide leaching solutions containing 30 g L−1 sulfuric acid as well as 12 g L−1 Cl− , 15 g L−1 Cu2+ and 1 g L−1 Fe3+ . Open circuit potential (OCP) measurement, linear polarization resistance (LPR) and electrochemical impedance spectroscopy (EIS) were used to study the corrosion response of the AOFs in copper sulfide leaching solutions up to 85 ◦ C. Scanning electron microscopy (SEM) was used to investigate the morphology of the AOFs before and after 12 h of immersion at 85 ◦ C. X-ray photoelectron spectroscopy (XPS) was used to examine the surface chemistry of the AOFs after immersion. OCP measurements showed that the final failure of the AOFs occurred in 2 h in de-aerated 30 g L−1 H2 SO4 and 12 g L−1 Cl− solutions at 85 ◦ C. Both LPR and EIS results showed a significant increase in the corrosion resistance of the anodized titanium versus that of freshly polished titanium. Electrochemical results were confirmed by SEM analysis, where the AOF formed at 80 V lead to the best improvement in corrosion resistance. XPS measurements revealed that Cu2+ was reduced to Cu or Cu+ within the titanium oxide film. It was further confirmed that the presence of leaching oxidants would inhibit the reduction of Cu2+ on titanium dioxide in chloride containing copper sulfide leaching solutions. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Hydrometallurgical production of zinc, nickel and copper from their sulfide ores typically involves using concentrated sulfuric acid or a mixed sulfate-chloride system to extract the desired metal at elevated temperatures. This has prompted the use of titanium and its alloys as an autoclave liner material, where it is subjected to severe conditions of acidity, temperature and pressure [1,2]. Titanium is known to provide excellent corrosion resistance due to the chemically unreactive and mechanically strong oxide films formed on its surface. Titanium oxide forms instantly when a freshly polished titanium surface is exposed to air. The protective titanium oxide (TiO2 ) can be enhanced by two methods: thermal oxidation and anodic oxidation [3,4]. Thermal oxidation is simple, but its reproducibility is poor and should be preceded by acid pickling to remove embedded surface contaminants, such as iron [5]. However, anodic oxidation is more consistent and also can be effective as a single step if it is applied for a sufficiently long time to dissolve surface contamination [6].
∗ Corresponding author. Tel.: +1 778 8475031. E-mail address: [email protected] (J. Liu).
A large number of papers and patents have dealt with titanium anodizing [3,4,6–22]. It has been found that the anodic oxide films (AOFs) formed on titanium depend on the substrate surface character, electrolyte type, applied potential and temperature [7,8,13,14,23]. It is difficult to distinguish between purity and surface preparation effects, therefore an effort should be made to determine surface effects by anodizing the same type of titanium substrate with varying surface preparations [14]. A large variety of acidic, basic and neutral electrolytes have been used to produce titanium based anodic films, and sulfuric acid and phosphoric acid were used most frequently and produced by far the best AOFs [7,8,13,14,23]. Sul et al. have found that an increase of electrolyte concentration and electrolyte temperature decreased the anodic forming rate and the current efficiency [13]. As a general trend, anodic film growth on titanium was carried out at room temperature. Meanwhile the color of AOFs has been explained by multiple-beam interference theory and the thickness of the oxide formed dictates the colors [3,15–19]. Since the film thickness depends on the anodizing voltage, it is possible to relate the anodizing voltage to the color and to the oxide thickness. The correlation of color and thickness to anodizing voltage from published results [3,16,17,19] is listed in Table 1. It has been found that AOFs can be formed with the incorporation of species from the electrolyte, affecting the structure
0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.06.168
Please cite this article in press as: J. Liu, et al., Characterization of anodized titanium for hydrometallurgical applications—Evidence for the reduction of cupric on titanium dioxide, Appl. Surf. Sci. (2013), http://dx.doi.org/10.1016/j.apsusc.2013.06.168
ARTICLE IN PRESS
G Model APSUSC-25978; No. of Pages 10
J. Liu et al. / Applied Surface Science xxx (2013) xxx–xxx
2
Table 1 Correlation of color and thickness to titanium anodization voltage. Anodizing voltage (V)
Colors obtained in this work
Colors reported in the literature
Thickness (m) reported in the literature
8
Brown
20
Dark blue
40
Light olive
60
Pink
80
Turquoise
6 V-light brown [17] 10 V-golden brown [17] Violet blue [3] Dark blue [17] Dark blue [19] Pale aqua [3] Light olive [17] Light olive [19] Purple [3] Pink [17] Pink [19] Bronze [3]
6 V-0.036 [17] 10 V-0.049 [17] 0.046 [3] 0.058 [17] 0.063 [16] 0.066 [3] 0.103 [17] 0.09 [16] 0.11 [3] 0.141 [17] 0.141 [16] 0.15 [3] 0.19 [16]
and properties of the final oxide [8,24–26]. Since the purpose of this study was to assess the performance of anodized titanium in sulfate-based copper sulfide leaching environments, sulfuric acid is employed as the anodizing electrolyte in this investigation. AOFs of titanium show a wide range of structural and electrochemical properties, depending on the electrolyte and the applied potential [27]. Diamanti et al. suggested that the optimum concentration of sulfuric acid solution for titanium anodization was 0.5 M, which promoted the oxidation conversion to anatase instead of an amorphous oxide [27]. In the present study, AOFs were produced on commercially pure titanium (CP-Ti) in 0.5 M sulfuric acid solutions at room temperature and in the potential range 20–80 V. The corrosion behavior of the AOFs was investigated in de-aerated sulfuric acid solutions in the presence of 12 g L−1 Cl− , 15 g L−1 Cu2+ and 1 g L−1 Fe3+ , which are their typical tenors in copper sulfide leaching solutions [28]. Open circuit potential (OCP), linear polarization resistance (LPR) and electrochemical impedance spectroscopy (EIS) measurements were carried out on the AOFs up to 85 ◦ C. The surface morphology and chemical composition of the AOFs were studied by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). 2. Experimental
2.3. Electrochemical experiments Electrochemical experiments were carried out using a standard three-electrode cell with thermostated water jacket, a graphite rod as the counter electrode, the anodized titanium sample as the working electrode and an Ag/AgCl ([KCl] = 4 M) reference electrode. The potentiostat used was a Princeton Applied Research Versastat 4 potentiostat/galvanostat. The solutions were introduced into the cell, heated to the desired temperature and de-aerated using high-purity N2 for at least 30 min prior to introducing the working electrode. N2 sparging was maintained during the entire experimental process. OCP, LPR, potentiostatic polarization and EIS tests were conducted to study the electrochemical behavior of the AOFs formed on CP-Ti in the desired solutions at 25 and 85 ◦ C. All potential values used in this work are quoted with respect to the Ag/AgCl reference electrode (0.197 V vs. SHE). Each sample was immersed in the solution for 1 h before starting an EIS or LPR test to stabilize the OCP. LPR tests were performed from −0.020 V vs. OCP up to +0.020 V vs. OCP with a scan rate of 0.167 mV/s. The corrosion mechanism at the oxide film/electrolyte interface was investigated by EIS. The EIS perturbation voltage amplitude was 10 mV (peak to peak). The frequency range for all experiments was 10 kHz to 100 mHz with sampling rate of 10 points per decade. All EIS measurements were taken at OCP.
2.1. Material and electrolyte 2.4. Surface examination The material investigated was commercially pure (CP) titanium, ASTM Grade 2, and its chemical composition is listed in Table 2. The titanium sample was 3 mm thick with a circular surface area of 2.0 cm2 . Deionized (DI) water, sulfuric acid (H2 SO4 , 95.0–98.0%, Fisher Scientific Canada), sodium chloride (NaCl, Cryst./Certified ACS), cupric sulfate pentahydrate (CuSO4 ·5H2 O, Cryst./Certified ACS), iron(III) sulfate pentahydrate (Fe2 (SO4 )3 ·5H2 O, 97%, Acros) were used to prepare the electrolytes. 2.2. Anodizing and sealing Anodization was carried out in a 1 L jacketed cell with a graphite rod as a cathode and a 0.5 M sulfuric acid electrolyte. The temperature of the anodizing electrolyte was kept at 25 ◦ C by a water circulator with an accuracy of ±1 ◦ C. A magnetic stirrer was used to eliminate the bubbles generated during the anodic oxidation. Prior to anodization, the electrode was abraded using successive grades of SiC papers down to 1200 grit, washed with DI water and then washed with 0.5 M sulfuric acid solution. Constant voltages of 20, 40, 60 and 80 V were applied to the titanium sample using a DC power supply. The anodizing time at each specific voltage was 1 h. After anodizing, samples were washed in DI water, sealed in boiling DI water for 30 min, and then dried in air.
Two samples were prepared for each anodizing condition using the procedure previously described at 20 V, 40 V, 60 V and 80 V for a total of eight samples. One of each of these pairs was set aside with no further treatment, while the second was further immersed in 1 L de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ + 1 g L−1 Fe3+ solution for 12 h at 85 ± 1 ◦ C. After immersion, samples were ultrasonically washed in DI water for 5 min and dried naturally. The surface morphology of all eight samples was observed using a Zeiss Sigma field emission SEM. In order to investigate the surface chemistry of 80 V-AOFs in mixed Cu-Cl solutions, XPS analysis was carried-out on four 80 V-anodized titanium samples, each immersed in one of the following four solutions at 85 ◦ C for 12 h, (1) N2 -de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ solution; (2) N2 -deaerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ + 1 g L−1 Fe3+ solution; (3) N2 -de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ + 5.0 g L−1 Fe3+ solution; (4) O2 -saturated 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ solution. After the immersion, all samples were ultrasonically washed in DI water for 5 min and dried in an N2 stream. The surface chemical composition of these samples was analyzed by a Leybold MAX2000 XPS spectrometer, using a monochromatic Mg K␣ X-ray source and
Please cite this article in press as: J. Liu, et al., Characterization of anodized titanium for hydrometallurgical applications—Evidence for the reduction of cupric on titanium dioxide, Appl. Surf. Sci. (2013), http://dx.doi.org/10.1016/j.apsusc.2013.06.168
ARTICLE IN PRESS
G Model APSUSC-25978; No. of Pages 10
J. Liu et al. / Applied Surface Science xxx (2013) xxx–xxx
3
Table 2 Chemical composition of the CP-Ti used in this study, ASTM Grade 2 (wt.%). Al
Zr
Fe
Mg
Mn
Mo
Ni
Si
Cu
C
Ti
0.005
0.001
0.14
0.008
0.001
0.001
0.004
0.003
0.001
0.005
Bal.
pass energies of 192 and 48 eV for wide and high resolution scans, respectively. XPS tests were carried out at the take-off angle of 90◦ with no sputtering and no charge neutralization. 3. Results and discussion 3.1. Potentiostatic formation of AOFs Shibata and Zhu have reported that the growth of AOFs under potentiostatic conditions follows the logarithmic rate law, i.e., a linear relationship of log i vs. log t [20]. Shibata and Zhu also point out that two processes are involved during anodization: a growth stage (linear section) and the balanced stage (non-linear part) [20]. In this work, the time scale for the growth stage decreased from 7200 s to 1000 s when the applied potential increased from 2 V to 8 V. This indicates that the AOF grows to a steady thickness within 1 h when the anodizing voltage is higher than 8 V. In the voltage range used in this work (20–80 V) it is therefore assumed that the 1 h of applied anodization was easily sufficient to grow a film of steady thickness in 0.5 M sulfuric acid at 25 ◦ C. A DC power supply was used to provide constant voltages of 20, 40, 60 and 80 V since the maximum applied voltage of most potentiostats is low (the Versastat 4 potentiostat used here has a maximum voltage of 10 V). Because the DC power supply used here did not have the capability to record current, no i vs. t plots for the anodizing processes can be shown. As mentioned above, the AOF will grow to a specific thickness (related to color) at a given applied voltage. The colors of the anodized CP-Ti samples at different potentials as determined by visual inspection are listed in Table 1. The correlation of color and thickness to anodization voltage as reported by other authors [3,16,17,19] are also shown in Table 1. Our results are consistent with previous reports. From Table 1 we may conclude that the thickness of the AOFs obtained here at 80 V exceeds 100 nm.
at 85 ◦ C [28], Fig. 1(b) reveals that at 85 ◦ C the AOFs retain virtually no increased passivation effect over freshly polished Ti within 2 h of immersion (as the OCP shifts to −0.60 V after 2 h). This indicates that all AOFs were evidently completely reduced in the last phase of OCP breakdown at 85 ◦ C. The above discussion suggests that the mixed chloride-sulfuric acid solution used in copper sulfide leaching is extremely corrosive, and that the protective oxide films formed on titanium at 20–80 V are not stable in this mixed chloride-sulfuric acid solution in the absence of an oxidant. The change of OCP for anodized titanium in de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− solutions with 15 g L−1 Cu2+ or 1 g L−1 Fe3+ additions at 85 ◦ C is presented in Fig. 2. The change of OCP over the last 10 min of each OCP measurement in Fig. 2 is less than 5 mV. Therefore, AOFs formed on titanium at 20–80 V reach a steady state
3.2. Electrochemical behavior of anodized titanium The objective of this work is to investigate the corrosion response of AOFs in typical copper sulfide leaching solutions: 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ + 1 g L−1 Fe3+ [28]. To gauge the effect of cupric and ferric it is also important to study the stability of the AOFs in de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− solutions. Electrochemical characterizations of AOFs formed on titanium have previously been reported [6,16,17,27,29–31]. However, there is a lack of publicly available information as to the corrosion of anodized titanium in strong, chloride containing, reducing acid such as that of interest here i.e. de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− solutions. The performance of AOFs in these solutions is first reported below. 3.2.1. Open circuit potential measurements The OCP decay curves for anodized titanium in de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− solutions at 25 and 85 ◦ C are presented in Fig. 1. Fig. 1(a) shows the OCP values of AOFs recorded over 12 h at 25 ◦ C. Both the 40 V-AOF and 80 V-AOF reached stable OCP values at 25 ◦ C, which were maintained for the final 6 h of immersion. The OCP values of the 20 V and 60 V-AOF kept decaying and did not reach a stable value over the 12 h of immersion. Since the OCP of titanium with a freshly polished surface is around −0.60 V in de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− solution
Fig. 1. Open circuit potential decay curves for anodized titanium in de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− solutions: (a) T = 25 ◦ C and (b) T = 85 ◦ C.
Please cite this article in press as: J. Liu, et al., Characterization of anodized titanium for hydrometallurgical applications—Evidence for the reduction of cupric on titanium dioxide, Appl. Surf. Sci. (2013), http://dx.doi.org/10.1016/j.apsusc.2013.06.168
G Model APSUSC-25978; No. of Pages 10 4
ARTICLE IN PRESS J. Liu et al. / Applied Surface Science xxx (2013) xxx–xxx
Fig. 2. Open circuit potential recorded on (a) 20 V-, (b) 40 V-, (c) 60 V-, and (d) 80 V-anodized titanium in de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− solutions with different additions, T = 85 ◦ C.
thickness [32] within 1 h in de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− solutions with 15 g L−1 Cu2+ or 1 g L−1 Fe3+ additions at 85 ◦ C. The passive region of titanium with a freshly polished surface starts at approximately −0.25 V in de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− solution at 85 ◦ C [33]. Fig. 2 shows that the presence of Cu2+ and Fe3+ ions kept the OCP values of anodized titanium in the range 0.3–0.5 V, which is well within the passive region of titanium at 85 ◦ C. 3.2.2. Linear polarization resistance Fig. 3(a) and (b) present the LPR results of the AOFs after 1 h immersion in de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ + 1 g L−1 Fe3+ solutions at both 25 and 85 ◦ C. The dashed lines in Fig. 3 show the previously published Rp-LPR results for titanium with a freshly polished surface under the same conditions [28]. Fig. 4 presents the Bode plots of freshly polished titanium after 1 h immersion in de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ + 1 g L−1 Fe3+ solution at both 25 and 85 ◦ C. It is shown that the impedance magnitude of the freshly polished Ti electrode in
this solution was around 1000 cm2 at 25 ◦ C and 10,000 cm2 at 85 ◦ C, which agrees with the previously published Rp-LPR results under the same conditions. It is evident that Rp-LPR values for anodized titanium are much higher than those for freshly polished titanium at both 25 and 85 ◦ C. Fig. 3 also shows that the Rp-LPR value of anodized titanium increased as the anodized voltage increased from 20 to 80 V. Thus, an 80 V anodization treatment lead to the best improvement of the corrosion resistance of Ti in this work. 3.2.3. Electrochemical impedance spectroscopy It has been proposed by many authors that equivalent electrical circuits (EECs) are a good method to characterize the structure of AOFs [29,30,34–37]. Among them, the equivalent circuit Rs (Qpr (Rpr (Rb Qb ))) is the one used most frequently for anodized titanium [29,30,34]. In this EEC it is assumed that the AOF is a two-layer film, including an inner barrier layer and an outer porous layer. The parameters of this EEC are electrolyte resistance (Rs ), outer porous film phase constant (Qpr ), outer porous film resistance (Rpr ), inner
Please cite this article in press as: J. Liu, et al., Characterization of anodized titanium for hydrometallurgical applications—Evidence for the reduction of cupric on titanium dioxide, Appl. Surf. Sci. (2013), http://dx.doi.org/10.1016/j.apsusc.2013.06.168
G Model APSUSC-25978; No. of Pages 10
ARTICLE IN PRESS J. Liu et al. / Applied Surface Science xxx (2013) xxx–xxx
5
Fig. 4. Bode plots for freshly polished titanium after 1 h immersion in N2 -de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ + 1 g L−1 Fe3+ solution at 25 ◦ C and 85 ◦ C.
Fig. 3. Rp-LPR of anodized titanium after 1 h of OCP measurement in N2 -de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ + 1 g L−1 Fe3+ solutions: (a) T = 25 ◦ C and (b) T = 85 ◦ C. The dashed lines show the previously published Rp-LPR results for titanium with freshly polished surface under otherwise identical conditions [28].
barrier film resistance (Rb ) and inner barrier film phase constant (Qpr ). Fig. 5 presents electrochemical impedance data for AOFs in 30 g L−1 H2 SO4 + 12 g L−1 Cl− solutions with different additives at 85 ◦ C in the form of Nyquist plots, Bode |Z| and Bode Phase diagrams. The fitting curves obtained by using the EEC of Rs (Qpr (Rpr (Rb Qb ))) are also present in Fig. 5. The corresponding analyzed data is presented in Table 3 and the Chi square (2 ) values were all around 1 × 10−4 . As shown, very good agreement between experimental and simulated data is obtained. It could be concluded that the twolayer EEC model satisfactorily describes the AOFs in this work. It can be observed in Fig. 5 that the impedance magnitude of anodized titanium is increased with increasing applied anodization voltage at 85 ◦ C. The influence of anodization voltage on the film resistance, Rpr and Rb , is shown in Table 3. The Rb values for the AOFs in all three solutions followed the same trend as the Rp-LPR values obtained in the LPR tests. That is, the Rb value of anodized titanium increased as the applied potential increased from 20 V to 80 V. Regarding the values of Rpr , it is also observed that the 40 VAOFs have the lowest value compared with the other AOFs in the same solution (Table 3). Based on these observations, it can be predicted that the thickness of the barrier layer of the AOFs increases as the anodization voltage increases, and that the 40 V-anodization produces a more porous outer layer than the others.
Table 3 EIS parameters of equivalent circuit Rs (Qpr (Rpr (Rb Qb ))) for anodized titanium in de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− solutions with different additives, T = 85 ◦ C. Additives
Anodizing voltage (V)
Rs ( cm2 )
Qpr − Y0 (×10−5 , S sn cm−2 )
Qpr − n
Rpr ( cm2 )
Rb ( cm2 )
Qb − Y0 (×10−5 , S sn cm−2 )
Qb − n
2 (×10−4 )
15 g L−1 Cu2+
20 40 60 80
2.08 2.04 2.34 2.46
3.96 3.48 3.58 4.09
0.87 0.89 0.91 0.92
269 170 240 482
512 545 966 1177
7.34 13.90 8.97 7.13
0.76 0.68 0.70 0.76
0.65 0.59 0.69 1.41
15 g L−1 Cu2+ + 1 g L−1 Fe3+
20 40 60 80
2.42 2.81 1.62 1.51
3.13 3.49 3.56 3.95
0.85 0.89 0.90 0.92
367 193 282 457
412 1313 1375 2042
18.70 9.22 9.18 7.91
0.60 0.67 0.67 0.70
1.42 0.70 0.55 1.53
1 g L−1 Fe3+
20 40 60 80
2.03 1.53 1.61 1.37
2.71 3.52 3.20 4.06
0.90 0.88 0.86 0.84
339 188 308 352
1086 1612 1636 2585
9.69 10.20 10.50 11.50
0.70 0.66 0.78 0.76
2.66 0.73 2.77 2.10
Note: 2 is the Chi square.
Please cite this article in press as: J. Liu, et al., Characterization of anodized titanium for hydrometallurgical applications—Evidence for the reduction of cupric on titanium dioxide, Appl. Surf. Sci. (2013), http://dx.doi.org/10.1016/j.apsusc.2013.06.168
G Model APSUSC-25978; No. of Pages 10
ARTICLE IN PRESS J. Liu et al. / Applied Surface Science xxx (2013) xxx–xxx
6
Fig. 5. Nyquist (a), (c) and (e) and Bode (b), (d), and (f) plots for anodized titanium in de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− solutions with different cupric and ferric additions after 1 h OCP measurements, DC potential = OCP (as shown in Fig. 2), T = 85 ◦ C.
3.3. The morphology of surfaces Fig. 6a shows the surface morphology of AOFs produced on CP-Ti at different anodizing voltages in 0.5 M sulfuric acid for 1 h. Fig. 6a20 V shows the fish scale-like anodized surface obtained at 20 V. When the anodization voltage was 40 V, the outer layer of the AOFs started peeling and generated oxide buds on the surface, as
shown in Fig. 6a-40 V. The detailed morphology of the 40 V-AOF is presented in Fig. 7. Similar but much scarcer oxide buds were obtained also on the AOF obtained at 60 V, as shown in Fig. 6a-60 V. These oxide buds have been reported by other authors under similar conditions [29,38]. The 80 V condition gave a much smoother surface morphology, as shown in Fig. 6a-80 V. The above discussion confirms the EIS results, where the 40 V-AOFs have the lowest Rpr
Please cite this article in press as: J. Liu, et al., Characterization of anodized titanium for hydrometallurgical applications—Evidence for the reduction of cupric on titanium dioxide, Appl. Surf. Sci. (2013), http://dx.doi.org/10.1016/j.apsusc.2013.06.168
G Model APSUSC-25978; No. of Pages 10
ARTICLE IN PRESS J. Liu et al. / Applied Surface Science xxx (2013) xxx–xxx
7
Fig. 6. Surface morphology of anodized titanium: (a) samples anodized at different voltages and prior to immersion; (b) anodized samples after 12 h of immersion in N2 -de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ + 1 g L−1 Fe3+ solution, T = 85 ◦ C.
value compared with the other AOFs in the same solution at 85 ◦ C (Table 3). Fig. 6b shows the surface morphology of the AOFs obtained at different anodizing voltages after 12 h immersion in the de-aerated leaching solution, i.e., 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ + 1 g L−1 Fe3+ at 85 ◦ C. Due to the coarse morphology of the 20 and 40 V-AOFs, the active surface area was large and would lead to severe corrosion. As expected, the most severe corrosion (as judged by the smoothness of the surface) occurred on the AOFs obtained at 20 and 40 V as localized corrosion (black pits) was visible on these two samples. Fig. 6b-60 V shows a much better surface with scarcer pitting compared with Fig. 6b-20 V and Fig. 6b-40 V. Uniform corrosion with little to no pitting is observed on the AOF obtained at 80 V due to its dense and smooth structure, as shown in Fig. 6b-80 V. These SEM results corroborate the electrochemical results, that is,
the 80 V anodization of titanium leads to the best improvement of its corrosion resistance in the de-aerated leaching solution. As shown in Fig. 6, after 12 h of immersion in N2 -de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ + 1 g L−1 Fe3+ solution at 85 ◦ C, only the 80 V-AOFs gave a uniform surface morphology. In order to generate the most reproducible oxide film, only 80 V-AOFs were studied in the following section. 3.4. Effects of leaching oxidants on the 80 V-AOFs and the presence of reduced copper in the oxide film It has been confirmed that AOFs on titanium obtained at 80 V (80 V-AOFs) show the best improvement in corrosion resistance in sulfuric acid solutions relevant to copper sulfide leaching. It has been proposed in our previous study that the presence of cupric
Please cite this article in press as: J. Liu, et al., Characterization of anodized titanium for hydrometallurgical applications—Evidence for the reduction of cupric on titanium dioxide, Appl. Surf. Sci. (2013), http://dx.doi.org/10.1016/j.apsusc.2013.06.168
G Model APSUSC-25978; No. of Pages 10 8
ARTICLE IN PRESS J. Liu et al. / Applied Surface Science xxx (2013) xxx–xxx
Fig. 8. Open-circuit potential recorded on anodized titanium in 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ solutions with different oxidants, T = 85 ◦ C.
30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ + 5.0 g LL−1 Fe3+ solution at 85 ◦ C. Based on our thermodynamic calculation presented above, the OCP results from Fig. 8 indicate that there would be no Cu0 formed in the AOFs in the latter two solutions. To confirm these results, XPS analyses were undertaken. Fig. 7. Surface morphology of the AOF formed in 0.5 M sulfuric acid at 40 V for 1 h on CP-Ti.
ions may affect the titanium oxide films in de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ solution at 85 ◦ C, and the mechanism behind this effect can be described by Eqs. (1) and (2) [33]. CuCl2 + 2H+ + 2e → Cu + 2HCl, E = 0.440 Vvs.Ag/AgCl,T = 85 ◦ C
(1)
TiO2 + 4H+ + 4e → Ti + 2H2 O, E = −0.800 Vvs.Ag/AgCl,T = 85 ◦ C
(2)
The OCP of the 80 V-AOF was around 0.37 V in de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ solution at 85 ◦ C and was thus between the equilibrium potentials of reaction (1) and reaction (2). As discussed above, AOFs formed in this work have a porous outer layer. Therefore, Cu0 can be formed as described in reaction (1) and be trapped in these pores when the Ti working electrode is immersed in de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ solution at 85 ◦ C, provided that OCP is lower than 0.440 V vs. Ag/AgCl. Also, if a higher potential (>0.440 V vs. Ag/AgCl) is applied on the Ti working electrode, the Cu0 associated with the oxide film will be oxidized to cupric ions, or may not be formed in the first place. In order to examine this idea, OCP measurements and XPS analyses were carried out as detailed below. 3.4.1. Open circuit potential measurements It is well known that both oxygen and ferric are extensively used as oxidants in copper leaching processes [39–41]. To raise the corrosion potential of the Ti working electrode, ferric and oxygen were added respectively in the 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ solution at 85 ◦ C. The OCP was recorded on anodized titanium for 1 h in 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ solutions with different oxidants at 85 ◦ C, as shown in Fig. 8. The OCP value of the 80 V-AOFs was below 0.440 V in de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ solutions with or without 1 g L−1 Fe3+ solution at 85 ◦ C. On the other hand, the OCP value of 80 V-AOFs was 0.445 V in O2 -saturated 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ , and 0.510 V in de-aerated
3.4.2. XPS characterization In order to further investigate the surface chemistry of the 80 V-AOFs in mixed Cu-Cl solutions, four titanium samples were immersed for 12 h as described above. The wide-scan XPS spectra presented in Fig. 9a show that the two samples from solutions (1) and (2) had Cu 2p signals in the same locations, and there was no Cu signal on the samples from solutions (3) and (4). Ti 2p spectra of these four samples are shown in Fig. 9b. All four samples had two peaks in the same locations, the peak at 458.7 eV is attributed to Ti4+ 2p3/2 and that at 464.2 eV is attributed to Ti4+ 2p1/2 , both for TiO2 [42,43]. The Cu 2p peaks of samples from solutions (1) and (2) are shown in Fig. 9c. The peak at 932 eV is attributed to Cu 2p3/2 and that at 952 eV is attributed to Cu 2p1/2 , both refer to Cu0 [44,45]. The XPS signals of Cu0 and Cu+ are basically the same, but they are different from Cu2+ signals [44–46]. Since the OCP value of 80 V-AOFs in solution (3) is 0.510 (>0.440 V), there should be no detectable Cu signal from the corresponding sample, as shown in Fig. 9c line (3). A very weak Cu signal at 932 eV from the sample exposed to solution (4) is observed in Fig. 9c line (4), which is probably due to the fact that the OCP value of the 80 V-AOFs in solution (4) (0.445 V) was close to 0.440 V. In aggregate, these results provided evidence that cupric ions were reduced to Cu0 (or perhaps Cu+ ) on the AOFs in solutions (1) and (2). However, in keeping with thermodynamic predictions, under oxidative conditions this reduction was inhibited. The reduction of cupric ions may adversely affect the performance of both AOFs and naturally formed oxides on titanium in mixed Cu-Cl solutions [33]. However, the presence of oxidants will inhibit this reduction, and 5 g L−1 Fe3+ is more effective than O2 saturation to this end. It is important to note that during operation at elevated temperatures it is difficult to maintain high ferric concentrations as this ion quickly hydrolyzes and precipitates as hematite or jarosite. On the other hand, high oxygen partial pressures are achievable during leaching at elevated temperatures and pressures. Nonetheless, the findings presented herein result in a practical suggestion for the protection of titanium liners used in pressure leaching autoclaves i.e. that oxidative conditions should be
Please cite this article in press as: J. Liu, et al., Characterization of anodized titanium for hydrometallurgical applications—Evidence for the reduction of cupric on titanium dioxide, Appl. Surf. Sci. (2013), http://dx.doi.org/10.1016/j.apsusc.2013.06.168
G Model APSUSC-25978; No. of Pages 10
ARTICLE IN PRESS J. Liu et al. / Applied Surface Science xxx (2013) xxx–xxx
9
Fig. 9. (a) Wide-scan XPS spectra, (b) Ti 2p spectra and (c) Cu 2p spectra for 80 V-anodized titanium samples after 12 h immersion in (1) N2 -de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ solution; (2) N2 -de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ + 1 g L−1 Fe3+ solution; (3) N2 -de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ + 5.0 g L−1 Fe3+ solution; (4) O2 -saturated 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ solution, T = 85 ◦ C. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
maintained at all times. The nature of sulfide pressure leaching is such that oxidative conditions are usually maintained, however, upset conditions and plant shutdowns can result in inadvertent contact with reducing or non-oxidative solutions. Furthermore, there is certainly also the possibility for localized and potentially reducing conditions under scale formed on reactor surfaces. 4. Conclusions Based on the plots of log i vs. log t obtained in potentiostatic polarization tests, the growth of AOFs in this work is complete within 1 h. OCP measurements demonstrated that the mixed chloride-sulfuric acid solution used in copper sulfide pressure leaching is extremely detrimental to AOFs even at relatively
mild temperatures. The AOFs formed at 20–80 V failed within 2 h in de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− solutions without any oxidant at 85 ◦ C. LPR and EIS results showed that 80 V-anodization of titanium lead to the best improvement of corrosion resistance of titanium in the de-aerated copper leaching solution. It was also shown from EIS results that the 40 V-AOFs had the lowest Rpr value, which indicated that the 40 V-anodization produced a more porous layer than the other conditions tested. SEM results confirmed the LPR and EIS findings. The effect of cupric ions on titanium oxide films was investigated by OCP and XPS measurements. Cupric can reduce on TiO2 in the presence of chloride and this has been shown in previous studies to affect the performance of the protective oxide. The maintenance of oxidative conditions would inhibit the reduction of Cu2+ on TiO2 by shifting the OCP to a more positive
Please cite this article in press as: J. Liu, et al., Characterization of anodized titanium for hydrometallurgical applications—Evidence for the reduction of cupric on titanium dioxide, Appl. Surf. Sci. (2013), http://dx.doi.org/10.1016/j.apsusc.2013.06.168
G Model APSUSC-25978; No. of Pages 10 10
ARTICLE IN PRESS J. Liu et al. / Applied Surface Science xxx (2013) xxx–xxx
value; 5 g L−1 Fe3+ is more effective than O2 saturation to obtain this inhibiting effect at 85 ◦ C. Acknowledgements The authors gratefully acknowledge the financial support of the Natural Sciences and Engineering Research Council of Canada. We also wish to thank Dr. Ken Wong from the Department of Chemistry, The University of British Columbia for his assistance in obtaining the XPS data in this work. References [1] J. Vaughan, P. Reid, A. Alfantazi, Corrosion of Ti-2 and Ti-7 relevant to nickel acid leach chemistry, Hydrometallurgy 101 (2010) 156–165. [2] J. Grauman, T. Say, Titanium for hydrometallurgical extraction equipment, Advanced Materials & Processes 157 (2000) 25–29. [3] E. Gaul, Coloring titanium and related metals by electrochemical oxidation, Journal of Chemical Education 70 (1993) 176. [4] R.W. Schutz, L.C. Covington, Effect of oxide films on the corrosion resistance of titanium, Corrosion 37 (1981) 585–591. [5] J.A. McMaster, The case for specified ranges of interstitial elements and iron in titanium welding filler metal, in: Wah Chang Corrosion Solutions Conference, Sunriver, Oreg, 2001, pp. 129–138. [6] J. Cotton, Using titanium in the chemical plant, Chemical Engineering Process 66 (1970) 57–62. [7] C.E.B. Marino, L.H. Mascaro, Electrochemical tests to evaluate the stability of the anodic films on dental implants, International Journal of Electrochemistry 2011 (2011) 1–7, http://dx.doi.org/10.4061/2011/574502, Article ID 574502. [8] C.E.B. Marino, P.A.P. Nascente, S.R. Biaggio, R.C. Rocha-Filho, N. Bocchi, XPS characterization of anodic titanium oxide films grown in phosphate buffer solutions, Thin Solid Films 468 (2004) 109–112. [9] J.L. Delplancke, A. Garnier, Y. Massiani, R. Winand, Influence of the anodizing procedure on the structure and the properties of titanium oxide films and its effect on copper nucleation, Electrochimica Acta 39 (1994) 1281–1289. [10] J.L. Delplancke, R. Winand, Galvanostatic anodization of titanium—I. Structures and compositions of the anodic films, Electrochimica Acta 33 (1988) 1539–1549. [11] J.L. Delplancke, R. Winand, Galvanostatic anodization of titanium—II. Reactions efficiencies and electrochemical behaviour model, Electrochimica Acta 33 (1988) 1551–1559. [12] J.L. Delplancke, M. Degrez, A. Fontana, R. Winand, Self-colour anodizing of titanium, Surface Technology 16 (1982) 153–162. [13] Y.T. Sul, C.B. Johansson, Y. Jeong, T. Albrektsson, The electrochemical oxide growth behaviour on titanium in acid and alkaline electrolytes, Medical Engineering & Physics 23 (2001) 329–346. [14] M.E. Sibert, Electrochemical oxidation of titanium surfaces, Journal of the Electrochemical Society 110 (1963) 65–72. [15] Z. Yan, T. Guo, H. Pan, J. Yu, Influences of electrolyzing voltage on chromatics of anodized titanium dentures, Materials Transactions 43 (2002) 3142–3145. [16] A. Karambakhsh, A. Afshar, S. Ghahramani, P. Malekinejad, Pure commercial titanium color anodizing and corrosion resistance, Journal of Materials Engineering and Performance 20 (2011) 1690–1696. [17] A.K. Sharma, Anodizing titanium for space applications, Thin Solid Films 208 (1992) 48–54. [18] A. Cigada, M. Cabrini, P. Pedeferri, Increasing of the corrosion resistance of the Ti6Al4V alloy by high thickness anodic oxidation, Journal of Materials Science: Materials in Medicine 3 (1992) 408–412. [19] C. Chen, J. Chen, C. Chao, W.C. Say, Electrochemical characteristics of surface of titanium formed by electrolytic polishing and anodizing, Journal of materials science 40 (2005) 4053–4059. [20] T. Shibata, Y.C. Zhu, The effect of temperature on the growth of anodic oxide film on titanium, Corrosion Science 37 (1995) 133–144.
[21] K.O. Beck, Titanium anodizing process, US Patent 2,949,411 (1960). [22] C.J. Bernard, Method of improving the corrosion resistance of titanium metals, US Patent 3,102,086 (1963). [23] G. Critchlow, D. Brewis, Review of surface pretreatments for titanium alloys, International Journal of Adhesion and Adhesives 15 (1995) 161–172. [24] E.M. Oliveira, C.E.B. Marino, S.R. Biaggio, R.C. Rocha-Filho, Reactivation of passive titanium: the enhancement of O2 evolution after potentiodynamic cyclings, Electrochemistry Communications 2 (2000) 254–258. [25] J.S.L. Leach, B.R. Pearson, Crystallization in anodic oxide films, Corrosion Science 28 (1988) 43–56. [26] H. Habazaki, K. Shimizu, S. Nagata, P. Skeldon, G.E. Thompson, G.C. Wood, Ionic transport in amorphous anodic titania stabilised by incorporation of silicon species, Corrosion Science 44 (2002) 1047–1055. [27] M.V. Diamanti, M.P. Pedeferri, Effect of anodic oxidation parameters on the titanium oxides formation, Corrosion Science 49 (2007) 939–948. [28] J. Liu, E. Asselin, A. Alfantazi, Uniform Corrosion of Titanium Under Medium Temperature Chalcopyrite Concentrate Leaching, Niagara, Canada, 2012, pp. 269–279. [29] S.A. Fadl-allah, Q. Mohsen, Characterization of native and anodic oxide films formed on commercial pure titanium using electrochemical properties and morphology techniques, Applied Surface Science 256 (2010) 5849–5855. [30] M.E.P. Souza, M. Ballester, C.M.A. Freire, EIS characterisation of Ti anodic oxide porous films formed using modulated potential, Surface and Coatings Technology 201 (2007) 7775–7780. [31] C.C. Ma, E.M. Peres, Corrosion resistance of anodized and unanodized titanium, Industrial & Engineering Chemistry 43 (1951) 675–679. [32] R.G. Kelly, J.R. Scully, D. Shoesmith, R.G. Buchheit, Electrochemical Techniques in Corrosion Science and Engineering, Marcel Dekker, Inc., New York, 2002. [33] J. Liu, A. Alfantazi, E. Asselin, Influence of cupric, ferric and chloride on the corrosion of titanium in sulfuric acid solutions up to 85 ◦ C, Corrosion (2013) (submitted for publication). [34] J. Pan, D. Thierry, C. Leygraf, Electrochemical impedance spectroscopy study of the passive oxide film on titanium for implant application, Electrochimica Acta 41 (1996) 1143–1153. [35] J. Marsh, D. Gorse, A photoelectrochemical and ac impedance study of anodic titanium oxide films, Electrochimica Acta 43 (1997) 659–670. [36] M. Aziz-Kerrzo, K.G. Conroy, A.M. Fenelon, S.T. Farrell, C.B. Breslin, Electrochemical studies on the stability and corrosion resistance of titanium-based implant materials, Biomaterials 22 (2001) 1531–1539. [37] S. Piazza, G.L. Biundo, M.C. Romano, C. Sunseri, F. Di Quarto, In situ characterization of passive films on Al-Ti alloy by photocurrent and impedance spectroscopy, Corrosion Science 40 (1998) 1087–1108. [38] Q. Mohsen, S. Fadl-Allah, Improvement in corrosion resistance of commercial pure titanium for the enhancement of its biocompatibility, Materials and Corrosion 62 (2010) 310–319. [39] J. Defreyne, W. Grieve, D. Jones, K. Mayhew, The Role of Iron in the CESL Process, CESL company, Richmond, BC, Canada, 2006, pp. 205–220. [40] T. Hirato, H. Majima, Y. Awakura, The leaching of chalcopyrite with ferric sulfate, Metallurgical Transactions B 18 (1987) 489–496. [41] T. Hirato, M. Kinoshita, Y. Awakura, H. Majima, The leaching of chalcopyrite with ferric chloride, Metallurgical and Materials Transactions B 17 (1986) 19–28. [42] G. Hopfengärtner, D. Borgmann, I. Rademacher, G. Wedler, E. Hums, G.W. Spitznagel, XPS studies of oxidic model catalysts: Internal standards and oxidation numbers, Journal of Electron Spectroscopy and Related Phenomena 63 (1993) 91–116. [43] X. He, J.J. Noël, D.W. Shoesmith, Temperature dependence of crevice corrosion initiation on titanium grade-2, Journal of the Electrochemical Society 149 (2002) B440–B449. [44] A. Rochefort, M. Abon, P. Delichère, J.C. Bertolini, Alloying effect on the adsorption properties of Pd50Cu50{111} single crystal surface, Surface Science 294 (1993) 43–52. [45] J.G. Jolley, G.G. Geesey, M.R. Hankins, R.B. Wright, P.L. Wichlacz, Auger electron and X-ray photoelectron spectroscopic study of the biocorrosion of copper by alginic acid polysaccharide, Applied Surface Science 37 (1989) 469–480. [46] D. Chadwick, T. Hashemi, Adsorbed corrosion inhibitors studied by electron spectroscopy: benzotriazole on copper and copper alloys, Corrosion Science 18 (1978) 39–51.
Please cite this article in press as: J. Liu, et al., Characterization of anodized titanium for hydrometallurgical applications—Evidence for the reduction of cupric on titanium dioxide, Appl. Surf. Sci. (2013), http://dx.doi.org/10.1016/j.apsusc.2013.06.168
ARTICLE IN PRESS Applied Surface Science xxx (2013) xxx–xxx
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Characterization of anodized titanium for hydrometallurgical applications—Evidence for the reduction of cupric on titanium dioxide Jing Liu ∗ , Akram Alfantazi, Edouard Asselin Department of Materials Engineering, The University of British Columbia, Vancouver, BC, Canada V6 T 1Z4
a r t i c l e
i n f o
Article history: Received 23 March 2013 Received in revised form 22 June 2013 Accepted 23 June 2013 Available online xxx Keywords: Titanium Copper leaching AOFs EIS Corrosion resistance XPS Oxidants
a b s t r a c t Anodic oxide films (AOFs) were potentiostatically formed on commercially pure titanium in 0.5 M sulfuric acid solutions at various anodizing voltages (up to 80 V) at room temperature. The subject of this study was the corrosion resistance of the AOFs in synthetic copper sulfide leaching solutions containing 30 g L−1 sulfuric acid as well as 12 g L−1 Cl− , 15 g L−1 Cu2+ and 1 g L−1 Fe3+ . Open circuit potential (OCP) measurement, linear polarization resistance (LPR) and electrochemical impedance spectroscopy (EIS) were used to study the corrosion response of the AOFs in copper sulfide leaching solutions up to 85 ◦ C. Scanning electron microscopy (SEM) was used to investigate the morphology of the AOFs before and after 12 h of immersion at 85 ◦ C. X-ray photoelectron spectroscopy (XPS) was used to examine the surface chemistry of the AOFs after immersion. OCP measurements showed that the final failure of the AOFs occurred in 2 h in de-aerated 30 g L−1 H2 SO4 and 12 g L−1 Cl− solutions at 85 ◦ C. Both LPR and EIS results showed a significant increase in the corrosion resistance of the anodized titanium versus that of freshly polished titanium. Electrochemical results were confirmed by SEM analysis, where the AOF formed at 80 V lead to the best improvement in corrosion resistance. XPS measurements revealed that Cu2+ was reduced to Cu or Cu+ within the titanium oxide film. It was further confirmed that the presence of leaching oxidants would inhibit the reduction of Cu2+ on titanium dioxide in chloride containing copper sulfide leaching solutions. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Hydrometallurgical production of zinc, nickel and copper from their sulfide ores typically involves using concentrated sulfuric acid or a mixed sulfate-chloride system to extract the desired metal at elevated temperatures. This has prompted the use of titanium and its alloys as an autoclave liner material, where it is subjected to severe conditions of acidity, temperature and pressure [1,2]. Titanium is known to provide excellent corrosion resistance due to the chemically unreactive and mechanically strong oxide films formed on its surface. Titanium oxide forms instantly when a freshly polished titanium surface is exposed to air. The protective titanium oxide (TiO2 ) can be enhanced by two methods: thermal oxidation and anodic oxidation [3,4]. Thermal oxidation is simple, but its reproducibility is poor and should be preceded by acid pickling to remove embedded surface contaminants, such as iron [5]. However, anodic oxidation is more consistent and also can be effective as a single step if it is applied for a sufficiently long time to dissolve surface contamination [6].
∗ Corresponding author. Tel.: +1 778 8475031. E-mail address: [email protected] (J. Liu).
A large number of papers and patents have dealt with titanium anodizing [3,4,6–22]. It has been found that the anodic oxide films (AOFs) formed on titanium depend on the substrate surface character, electrolyte type, applied potential and temperature [7,8,13,14,23]. It is difficult to distinguish between purity and surface preparation effects, therefore an effort should be made to determine surface effects by anodizing the same type of titanium substrate with varying surface preparations [14]. A large variety of acidic, basic and neutral electrolytes have been used to produce titanium based anodic films, and sulfuric acid and phosphoric acid were used most frequently and produced by far the best AOFs [7,8,13,14,23]. Sul et al. have found that an increase of electrolyte concentration and electrolyte temperature decreased the anodic forming rate and the current efficiency [13]. As a general trend, anodic film growth on titanium was carried out at room temperature. Meanwhile the color of AOFs has been explained by multiple-beam interference theory and the thickness of the oxide formed dictates the colors [3,15–19]. Since the film thickness depends on the anodizing voltage, it is possible to relate the anodizing voltage to the color and to the oxide thickness. The correlation of color and thickness to anodizing voltage from published results [3,16,17,19] is listed in Table 1. It has been found that AOFs can be formed with the incorporation of species from the electrolyte, affecting the structure
0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.06.168
Please cite this article in press as: J. Liu, et al., Characterization of anodized titanium for hydrometallurgical applications—Evidence for the reduction of cupric on titanium dioxide, Appl. Surf. Sci. (2013), http://dx.doi.org/10.1016/j.apsusc.2013.06.168
ARTICLE IN PRESS
G Model APSUSC-25978; No. of Pages 10
J. Liu et al. / Applied Surface Science xxx (2013) xxx–xxx
2
Table 1 Correlation of color and thickness to titanium anodization voltage. Anodizing voltage (V)
Colors obtained in this work
Colors reported in the literature
Thickness (m) reported in the literature
8
Brown
20
Dark blue
40
Light olive
60
Pink
80
Turquoise
6 V-light brown [17] 10 V-golden brown [17] Violet blue [3] Dark blue [17] Dark blue [19] Pale aqua [3] Light olive [17] Light olive [19] Purple [3] Pink [17] Pink [19] Bronze [3]
6 V-0.036 [17] 10 V-0.049 [17] 0.046 [3] 0.058 [17] 0.063 [16] 0.066 [3] 0.103 [17] 0.09 [16] 0.11 [3] 0.141 [17] 0.141 [16] 0.15 [3] 0.19 [16]
and properties of the final oxide [8,24–26]. Since the purpose of this study was to assess the performance of anodized titanium in sulfate-based copper sulfide leaching environments, sulfuric acid is employed as the anodizing electrolyte in this investigation. AOFs of titanium show a wide range of structural and electrochemical properties, depending on the electrolyte and the applied potential [27]. Diamanti et al. suggested that the optimum concentration of sulfuric acid solution for titanium anodization was 0.5 M, which promoted the oxidation conversion to anatase instead of an amorphous oxide [27]. In the present study, AOFs were produced on commercially pure titanium (CP-Ti) in 0.5 M sulfuric acid solutions at room temperature and in the potential range 20–80 V. The corrosion behavior of the AOFs was investigated in de-aerated sulfuric acid solutions in the presence of 12 g L−1 Cl− , 15 g L−1 Cu2+ and 1 g L−1 Fe3+ , which are their typical tenors in copper sulfide leaching solutions [28]. Open circuit potential (OCP), linear polarization resistance (LPR) and electrochemical impedance spectroscopy (EIS) measurements were carried out on the AOFs up to 85 ◦ C. The surface morphology and chemical composition of the AOFs were studied by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). 2. Experimental
2.3. Electrochemical experiments Electrochemical experiments were carried out using a standard three-electrode cell with thermostated water jacket, a graphite rod as the counter electrode, the anodized titanium sample as the working electrode and an Ag/AgCl ([KCl] = 4 M) reference electrode. The potentiostat used was a Princeton Applied Research Versastat 4 potentiostat/galvanostat. The solutions were introduced into the cell, heated to the desired temperature and de-aerated using high-purity N2 for at least 30 min prior to introducing the working electrode. N2 sparging was maintained during the entire experimental process. OCP, LPR, potentiostatic polarization and EIS tests were conducted to study the electrochemical behavior of the AOFs formed on CP-Ti in the desired solutions at 25 and 85 ◦ C. All potential values used in this work are quoted with respect to the Ag/AgCl reference electrode (0.197 V vs. SHE). Each sample was immersed in the solution for 1 h before starting an EIS or LPR test to stabilize the OCP. LPR tests were performed from −0.020 V vs. OCP up to +0.020 V vs. OCP with a scan rate of 0.167 mV/s. The corrosion mechanism at the oxide film/electrolyte interface was investigated by EIS. The EIS perturbation voltage amplitude was 10 mV (peak to peak). The frequency range for all experiments was 10 kHz to 100 mHz with sampling rate of 10 points per decade. All EIS measurements were taken at OCP.
2.1. Material and electrolyte 2.4. Surface examination The material investigated was commercially pure (CP) titanium, ASTM Grade 2, and its chemical composition is listed in Table 2. The titanium sample was 3 mm thick with a circular surface area of 2.0 cm2 . Deionized (DI) water, sulfuric acid (H2 SO4 , 95.0–98.0%, Fisher Scientific Canada), sodium chloride (NaCl, Cryst./Certified ACS), cupric sulfate pentahydrate (CuSO4 ·5H2 O, Cryst./Certified ACS), iron(III) sulfate pentahydrate (Fe2 (SO4 )3 ·5H2 O, 97%, Acros) were used to prepare the electrolytes. 2.2. Anodizing and sealing Anodization was carried out in a 1 L jacketed cell with a graphite rod as a cathode and a 0.5 M sulfuric acid electrolyte. The temperature of the anodizing electrolyte was kept at 25 ◦ C by a water circulator with an accuracy of ±1 ◦ C. A magnetic stirrer was used to eliminate the bubbles generated during the anodic oxidation. Prior to anodization, the electrode was abraded using successive grades of SiC papers down to 1200 grit, washed with DI water and then washed with 0.5 M sulfuric acid solution. Constant voltages of 20, 40, 60 and 80 V were applied to the titanium sample using a DC power supply. The anodizing time at each specific voltage was 1 h. After anodizing, samples were washed in DI water, sealed in boiling DI water for 30 min, and then dried in air.
Two samples were prepared for each anodizing condition using the procedure previously described at 20 V, 40 V, 60 V and 80 V for a total of eight samples. One of each of these pairs was set aside with no further treatment, while the second was further immersed in 1 L de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ + 1 g L−1 Fe3+ solution for 12 h at 85 ± 1 ◦ C. After immersion, samples were ultrasonically washed in DI water for 5 min and dried naturally. The surface morphology of all eight samples was observed using a Zeiss Sigma field emission SEM. In order to investigate the surface chemistry of 80 V-AOFs in mixed Cu-Cl solutions, XPS analysis was carried-out on four 80 V-anodized titanium samples, each immersed in one of the following four solutions at 85 ◦ C for 12 h, (1) N2 -de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ solution; (2) N2 -deaerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ + 1 g L−1 Fe3+ solution; (3) N2 -de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ + 5.0 g L−1 Fe3+ solution; (4) O2 -saturated 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ solution. After the immersion, all samples were ultrasonically washed in DI water for 5 min and dried in an N2 stream. The surface chemical composition of these samples was analyzed by a Leybold MAX2000 XPS spectrometer, using a monochromatic Mg K␣ X-ray source and
Please cite this article in press as: J. Liu, et al., Characterization of anodized titanium for hydrometallurgical applications—Evidence for the reduction of cupric on titanium dioxide, Appl. Surf. Sci. (2013), http://dx.doi.org/10.1016/j.apsusc.2013.06.168
ARTICLE IN PRESS
G Model APSUSC-25978; No. of Pages 10
J. Liu et al. / Applied Surface Science xxx (2013) xxx–xxx
3
Table 2 Chemical composition of the CP-Ti used in this study, ASTM Grade 2 (wt.%). Al
Zr
Fe
Mg
Mn
Mo
Ni
Si
Cu
C
Ti
0.005
0.001
0.14
0.008
0.001
0.001
0.004
0.003
0.001
0.005
Bal.
pass energies of 192 and 48 eV for wide and high resolution scans, respectively. XPS tests were carried out at the take-off angle of 90◦ with no sputtering and no charge neutralization. 3. Results and discussion 3.1. Potentiostatic formation of AOFs Shibata and Zhu have reported that the growth of AOFs under potentiostatic conditions follows the logarithmic rate law, i.e., a linear relationship of log i vs. log t [20]. Shibata and Zhu also point out that two processes are involved during anodization: a growth stage (linear section) and the balanced stage (non-linear part) [20]. In this work, the time scale for the growth stage decreased from 7200 s to 1000 s when the applied potential increased from 2 V to 8 V. This indicates that the AOF grows to a steady thickness within 1 h when the anodizing voltage is higher than 8 V. In the voltage range used in this work (20–80 V) it is therefore assumed that the 1 h of applied anodization was easily sufficient to grow a film of steady thickness in 0.5 M sulfuric acid at 25 ◦ C. A DC power supply was used to provide constant voltages of 20, 40, 60 and 80 V since the maximum applied voltage of most potentiostats is low (the Versastat 4 potentiostat used here has a maximum voltage of 10 V). Because the DC power supply used here did not have the capability to record current, no i vs. t plots for the anodizing processes can be shown. As mentioned above, the AOF will grow to a specific thickness (related to color) at a given applied voltage. The colors of the anodized CP-Ti samples at different potentials as determined by visual inspection are listed in Table 1. The correlation of color and thickness to anodization voltage as reported by other authors [3,16,17,19] are also shown in Table 1. Our results are consistent with previous reports. From Table 1 we may conclude that the thickness of the AOFs obtained here at 80 V exceeds 100 nm.
at 85 ◦ C [28], Fig. 1(b) reveals that at 85 ◦ C the AOFs retain virtually no increased passivation effect over freshly polished Ti within 2 h of immersion (as the OCP shifts to −0.60 V after 2 h). This indicates that all AOFs were evidently completely reduced in the last phase of OCP breakdown at 85 ◦ C. The above discussion suggests that the mixed chloride-sulfuric acid solution used in copper sulfide leaching is extremely corrosive, and that the protective oxide films formed on titanium at 20–80 V are not stable in this mixed chloride-sulfuric acid solution in the absence of an oxidant. The change of OCP for anodized titanium in de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− solutions with 15 g L−1 Cu2+ or 1 g L−1 Fe3+ additions at 85 ◦ C is presented in Fig. 2. The change of OCP over the last 10 min of each OCP measurement in Fig. 2 is less than 5 mV. Therefore, AOFs formed on titanium at 20–80 V reach a steady state
3.2. Electrochemical behavior of anodized titanium The objective of this work is to investigate the corrosion response of AOFs in typical copper sulfide leaching solutions: 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ + 1 g L−1 Fe3+ [28]. To gauge the effect of cupric and ferric it is also important to study the stability of the AOFs in de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− solutions. Electrochemical characterizations of AOFs formed on titanium have previously been reported [6,16,17,27,29–31]. However, there is a lack of publicly available information as to the corrosion of anodized titanium in strong, chloride containing, reducing acid such as that of interest here i.e. de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− solutions. The performance of AOFs in these solutions is first reported below. 3.2.1. Open circuit potential measurements The OCP decay curves for anodized titanium in de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− solutions at 25 and 85 ◦ C are presented in Fig. 1. Fig. 1(a) shows the OCP values of AOFs recorded over 12 h at 25 ◦ C. Both the 40 V-AOF and 80 V-AOF reached stable OCP values at 25 ◦ C, which were maintained for the final 6 h of immersion. The OCP values of the 20 V and 60 V-AOF kept decaying and did not reach a stable value over the 12 h of immersion. Since the OCP of titanium with a freshly polished surface is around −0.60 V in de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− solution
Fig. 1. Open circuit potential decay curves for anodized titanium in de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− solutions: (a) T = 25 ◦ C and (b) T = 85 ◦ C.
Please cite this article in press as: J. Liu, et al., Characterization of anodized titanium for hydrometallurgical applications—Evidence for the reduction of cupric on titanium dioxide, Appl. Surf. Sci. (2013), http://dx.doi.org/10.1016/j.apsusc.2013.06.168
G Model APSUSC-25978; No. of Pages 10 4
ARTICLE IN PRESS J. Liu et al. / Applied Surface Science xxx (2013) xxx–xxx
Fig. 2. Open circuit potential recorded on (a) 20 V-, (b) 40 V-, (c) 60 V-, and (d) 80 V-anodized titanium in de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− solutions with different additions, T = 85 ◦ C.
thickness [32] within 1 h in de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− solutions with 15 g L−1 Cu2+ or 1 g L−1 Fe3+ additions at 85 ◦ C. The passive region of titanium with a freshly polished surface starts at approximately −0.25 V in de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− solution at 85 ◦ C [33]. Fig. 2 shows that the presence of Cu2+ and Fe3+ ions kept the OCP values of anodized titanium in the range 0.3–0.5 V, which is well within the passive region of titanium at 85 ◦ C. 3.2.2. Linear polarization resistance Fig. 3(a) and (b) present the LPR results of the AOFs after 1 h immersion in de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ + 1 g L−1 Fe3+ solutions at both 25 and 85 ◦ C. The dashed lines in Fig. 3 show the previously published Rp-LPR results for titanium with a freshly polished surface under the same conditions [28]. Fig. 4 presents the Bode plots of freshly polished titanium after 1 h immersion in de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ + 1 g L−1 Fe3+ solution at both 25 and 85 ◦ C. It is shown that the impedance magnitude of the freshly polished Ti electrode in
this solution was around 1000 cm2 at 25 ◦ C and 10,000 cm2 at 85 ◦ C, which agrees with the previously published Rp-LPR results under the same conditions. It is evident that Rp-LPR values for anodized titanium are much higher than those for freshly polished titanium at both 25 and 85 ◦ C. Fig. 3 also shows that the Rp-LPR value of anodized titanium increased as the anodized voltage increased from 20 to 80 V. Thus, an 80 V anodization treatment lead to the best improvement of the corrosion resistance of Ti in this work. 3.2.3. Electrochemical impedance spectroscopy It has been proposed by many authors that equivalent electrical circuits (EECs) are a good method to characterize the structure of AOFs [29,30,34–37]. Among them, the equivalent circuit Rs (Qpr (Rpr (Rb Qb ))) is the one used most frequently for anodized titanium [29,30,34]. In this EEC it is assumed that the AOF is a two-layer film, including an inner barrier layer and an outer porous layer. The parameters of this EEC are electrolyte resistance (Rs ), outer porous film phase constant (Qpr ), outer porous film resistance (Rpr ), inner
Please cite this article in press as: J. Liu, et al., Characterization of anodized titanium for hydrometallurgical applications—Evidence for the reduction of cupric on titanium dioxide, Appl. Surf. Sci. (2013), http://dx.doi.org/10.1016/j.apsusc.2013.06.168
G Model APSUSC-25978; No. of Pages 10
ARTICLE IN PRESS J. Liu et al. / Applied Surface Science xxx (2013) xxx–xxx
5
Fig. 4. Bode plots for freshly polished titanium after 1 h immersion in N2 -de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ + 1 g L−1 Fe3+ solution at 25 ◦ C and 85 ◦ C.
Fig. 3. Rp-LPR of anodized titanium after 1 h of OCP measurement in N2 -de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ + 1 g L−1 Fe3+ solutions: (a) T = 25 ◦ C and (b) T = 85 ◦ C. The dashed lines show the previously published Rp-LPR results for titanium with freshly polished surface under otherwise identical conditions [28].
barrier film resistance (Rb ) and inner barrier film phase constant (Qpr ). Fig. 5 presents electrochemical impedance data for AOFs in 30 g L−1 H2 SO4 + 12 g L−1 Cl− solutions with different additives at 85 ◦ C in the form of Nyquist plots, Bode |Z| and Bode Phase diagrams. The fitting curves obtained by using the EEC of Rs (Qpr (Rpr (Rb Qb ))) are also present in Fig. 5. The corresponding analyzed data is presented in Table 3 and the Chi square (2 ) values were all around 1 × 10−4 . As shown, very good agreement between experimental and simulated data is obtained. It could be concluded that the twolayer EEC model satisfactorily describes the AOFs in this work. It can be observed in Fig. 5 that the impedance magnitude of anodized titanium is increased with increasing applied anodization voltage at 85 ◦ C. The influence of anodization voltage on the film resistance, Rpr and Rb , is shown in Table 3. The Rb values for the AOFs in all three solutions followed the same trend as the Rp-LPR values obtained in the LPR tests. That is, the Rb value of anodized titanium increased as the applied potential increased from 20 V to 80 V. Regarding the values of Rpr , it is also observed that the 40 VAOFs have the lowest value compared with the other AOFs in the same solution (Table 3). Based on these observations, it can be predicted that the thickness of the barrier layer of the AOFs increases as the anodization voltage increases, and that the 40 V-anodization produces a more porous outer layer than the others.
Table 3 EIS parameters of equivalent circuit Rs (Qpr (Rpr (Rb Qb ))) for anodized titanium in de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− solutions with different additives, T = 85 ◦ C. Additives
Anodizing voltage (V)
Rs ( cm2 )
Qpr − Y0 (×10−5 , S sn cm−2 )
Qpr − n
Rpr ( cm2 )
Rb ( cm2 )
Qb − Y0 (×10−5 , S sn cm−2 )
Qb − n
2 (×10−4 )
15 g L−1 Cu2+
20 40 60 80
2.08 2.04 2.34 2.46
3.96 3.48 3.58 4.09
0.87 0.89 0.91 0.92
269 170 240 482
512 545 966 1177
7.34 13.90 8.97 7.13
0.76 0.68 0.70 0.76
0.65 0.59 0.69 1.41
15 g L−1 Cu2+ + 1 g L−1 Fe3+
20 40 60 80
2.42 2.81 1.62 1.51
3.13 3.49 3.56 3.95
0.85 0.89 0.90 0.92
367 193 282 457
412 1313 1375 2042
18.70 9.22 9.18 7.91
0.60 0.67 0.67 0.70
1.42 0.70 0.55 1.53
1 g L−1 Fe3+
20 40 60 80
2.03 1.53 1.61 1.37
2.71 3.52 3.20 4.06
0.90 0.88 0.86 0.84
339 188 308 352
1086 1612 1636 2585
9.69 10.20 10.50 11.50
0.70 0.66 0.78 0.76
2.66 0.73 2.77 2.10
Note: 2 is the Chi square.
Please cite this article in press as: J. Liu, et al., Characterization of anodized titanium for hydrometallurgical applications—Evidence for the reduction of cupric on titanium dioxide, Appl. Surf. Sci. (2013), http://dx.doi.org/10.1016/j.apsusc.2013.06.168
G Model APSUSC-25978; No. of Pages 10
ARTICLE IN PRESS J. Liu et al. / Applied Surface Science xxx (2013) xxx–xxx
6
Fig. 5. Nyquist (a), (c) and (e) and Bode (b), (d), and (f) plots for anodized titanium in de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− solutions with different cupric and ferric additions after 1 h OCP measurements, DC potential = OCP (as shown in Fig. 2), T = 85 ◦ C.
3.3. The morphology of surfaces Fig. 6a shows the surface morphology of AOFs produced on CP-Ti at different anodizing voltages in 0.5 M sulfuric acid for 1 h. Fig. 6a20 V shows the fish scale-like anodized surface obtained at 20 V. When the anodization voltage was 40 V, the outer layer of the AOFs started peeling and generated oxide buds on the surface, as
shown in Fig. 6a-40 V. The detailed morphology of the 40 V-AOF is presented in Fig. 7. Similar but much scarcer oxide buds were obtained also on the AOF obtained at 60 V, as shown in Fig. 6a-60 V. These oxide buds have been reported by other authors under similar conditions [29,38]. The 80 V condition gave a much smoother surface morphology, as shown in Fig. 6a-80 V. The above discussion confirms the EIS results, where the 40 V-AOFs have the lowest Rpr
Please cite this article in press as: J. Liu, et al., Characterization of anodized titanium for hydrometallurgical applications—Evidence for the reduction of cupric on titanium dioxide, Appl. Surf. Sci. (2013), http://dx.doi.org/10.1016/j.apsusc.2013.06.168
G Model APSUSC-25978; No. of Pages 10
ARTICLE IN PRESS J. Liu et al. / Applied Surface Science xxx (2013) xxx–xxx
7
Fig. 6. Surface morphology of anodized titanium: (a) samples anodized at different voltages and prior to immersion; (b) anodized samples after 12 h of immersion in N2 -de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ + 1 g L−1 Fe3+ solution, T = 85 ◦ C.
value compared with the other AOFs in the same solution at 85 ◦ C (Table 3). Fig. 6b shows the surface morphology of the AOFs obtained at different anodizing voltages after 12 h immersion in the de-aerated leaching solution, i.e., 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ + 1 g L−1 Fe3+ at 85 ◦ C. Due to the coarse morphology of the 20 and 40 V-AOFs, the active surface area was large and would lead to severe corrosion. As expected, the most severe corrosion (as judged by the smoothness of the surface) occurred on the AOFs obtained at 20 and 40 V as localized corrosion (black pits) was visible on these two samples. Fig. 6b-60 V shows a much better surface with scarcer pitting compared with Fig. 6b-20 V and Fig. 6b-40 V. Uniform corrosion with little to no pitting is observed on the AOF obtained at 80 V due to its dense and smooth structure, as shown in Fig. 6b-80 V. These SEM results corroborate the electrochemical results, that is,
the 80 V anodization of titanium leads to the best improvement of its corrosion resistance in the de-aerated leaching solution. As shown in Fig. 6, after 12 h of immersion in N2 -de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ + 1 g L−1 Fe3+ solution at 85 ◦ C, only the 80 V-AOFs gave a uniform surface morphology. In order to generate the most reproducible oxide film, only 80 V-AOFs were studied in the following section. 3.4. Effects of leaching oxidants on the 80 V-AOFs and the presence of reduced copper in the oxide film It has been confirmed that AOFs on titanium obtained at 80 V (80 V-AOFs) show the best improvement in corrosion resistance in sulfuric acid solutions relevant to copper sulfide leaching. It has been proposed in our previous study that the presence of cupric
Please cite this article in press as: J. Liu, et al., Characterization of anodized titanium for hydrometallurgical applications—Evidence for the reduction of cupric on titanium dioxide, Appl. Surf. Sci. (2013), http://dx.doi.org/10.1016/j.apsusc.2013.06.168
G Model APSUSC-25978; No. of Pages 10 8
ARTICLE IN PRESS J. Liu et al. / Applied Surface Science xxx (2013) xxx–xxx
Fig. 8. Open-circuit potential recorded on anodized titanium in 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ solutions with different oxidants, T = 85 ◦ C.
30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ + 5.0 g LL−1 Fe3+ solution at 85 ◦ C. Based on our thermodynamic calculation presented above, the OCP results from Fig. 8 indicate that there would be no Cu0 formed in the AOFs in the latter two solutions. To confirm these results, XPS analyses were undertaken. Fig. 7. Surface morphology of the AOF formed in 0.5 M sulfuric acid at 40 V for 1 h on CP-Ti.
ions may affect the titanium oxide films in de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ solution at 85 ◦ C, and the mechanism behind this effect can be described by Eqs. (1) and (2) [33]. CuCl2 + 2H+ + 2e → Cu + 2HCl, E = 0.440 Vvs.Ag/AgCl,T = 85 ◦ C
(1)
TiO2 + 4H+ + 4e → Ti + 2H2 O, E = −0.800 Vvs.Ag/AgCl,T = 85 ◦ C
(2)
The OCP of the 80 V-AOF was around 0.37 V in de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ solution at 85 ◦ C and was thus between the equilibrium potentials of reaction (1) and reaction (2). As discussed above, AOFs formed in this work have a porous outer layer. Therefore, Cu0 can be formed as described in reaction (1) and be trapped in these pores when the Ti working electrode is immersed in de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ solution at 85 ◦ C, provided that OCP is lower than 0.440 V vs. Ag/AgCl. Also, if a higher potential (>0.440 V vs. Ag/AgCl) is applied on the Ti working electrode, the Cu0 associated with the oxide film will be oxidized to cupric ions, or may not be formed in the first place. In order to examine this idea, OCP measurements and XPS analyses were carried out as detailed below. 3.4.1. Open circuit potential measurements It is well known that both oxygen and ferric are extensively used as oxidants in copper leaching processes [39–41]. To raise the corrosion potential of the Ti working electrode, ferric and oxygen were added respectively in the 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ solution at 85 ◦ C. The OCP was recorded on anodized titanium for 1 h in 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ solutions with different oxidants at 85 ◦ C, as shown in Fig. 8. The OCP value of the 80 V-AOFs was below 0.440 V in de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ solutions with or without 1 g L−1 Fe3+ solution at 85 ◦ C. On the other hand, the OCP value of 80 V-AOFs was 0.445 V in O2 -saturated 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ , and 0.510 V in de-aerated
3.4.2. XPS characterization In order to further investigate the surface chemistry of the 80 V-AOFs in mixed Cu-Cl solutions, four titanium samples were immersed for 12 h as described above. The wide-scan XPS spectra presented in Fig. 9a show that the two samples from solutions (1) and (2) had Cu 2p signals in the same locations, and there was no Cu signal on the samples from solutions (3) and (4). Ti 2p spectra of these four samples are shown in Fig. 9b. All four samples had two peaks in the same locations, the peak at 458.7 eV is attributed to Ti4+ 2p3/2 and that at 464.2 eV is attributed to Ti4+ 2p1/2 , both for TiO2 [42,43]. The Cu 2p peaks of samples from solutions (1) and (2) are shown in Fig. 9c. The peak at 932 eV is attributed to Cu 2p3/2 and that at 952 eV is attributed to Cu 2p1/2 , both refer to Cu0 [44,45]. The XPS signals of Cu0 and Cu+ are basically the same, but they are different from Cu2+ signals [44–46]. Since the OCP value of 80 V-AOFs in solution (3) is 0.510 (>0.440 V), there should be no detectable Cu signal from the corresponding sample, as shown in Fig. 9c line (3). A very weak Cu signal at 932 eV from the sample exposed to solution (4) is observed in Fig. 9c line (4), which is probably due to the fact that the OCP value of the 80 V-AOFs in solution (4) (0.445 V) was close to 0.440 V. In aggregate, these results provided evidence that cupric ions were reduced to Cu0 (or perhaps Cu+ ) on the AOFs in solutions (1) and (2). However, in keeping with thermodynamic predictions, under oxidative conditions this reduction was inhibited. The reduction of cupric ions may adversely affect the performance of both AOFs and naturally formed oxides on titanium in mixed Cu-Cl solutions [33]. However, the presence of oxidants will inhibit this reduction, and 5 g L−1 Fe3+ is more effective than O2 saturation to this end. It is important to note that during operation at elevated temperatures it is difficult to maintain high ferric concentrations as this ion quickly hydrolyzes and precipitates as hematite or jarosite. On the other hand, high oxygen partial pressures are achievable during leaching at elevated temperatures and pressures. Nonetheless, the findings presented herein result in a practical suggestion for the protection of titanium liners used in pressure leaching autoclaves i.e. that oxidative conditions should be
Please cite this article in press as: J. Liu, et al., Characterization of anodized titanium for hydrometallurgical applications—Evidence for the reduction of cupric on titanium dioxide, Appl. Surf. Sci. (2013), http://dx.doi.org/10.1016/j.apsusc.2013.06.168
G Model APSUSC-25978; No. of Pages 10
ARTICLE IN PRESS J. Liu et al. / Applied Surface Science xxx (2013) xxx–xxx
9
Fig. 9. (a) Wide-scan XPS spectra, (b) Ti 2p spectra and (c) Cu 2p spectra for 80 V-anodized titanium samples after 12 h immersion in (1) N2 -de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ solution; (2) N2 -de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ + 1 g L−1 Fe3+ solution; (3) N2 -de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ + 5.0 g L−1 Fe3+ solution; (4) O2 -saturated 30 g L−1 H2 SO4 + 12 g L−1 Cl− + 15 g L−1 Cu2+ solution, T = 85 ◦ C. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
maintained at all times. The nature of sulfide pressure leaching is such that oxidative conditions are usually maintained, however, upset conditions and plant shutdowns can result in inadvertent contact with reducing or non-oxidative solutions. Furthermore, there is certainly also the possibility for localized and potentially reducing conditions under scale formed on reactor surfaces. 4. Conclusions Based on the plots of log i vs. log t obtained in potentiostatic polarization tests, the growth of AOFs in this work is complete within 1 h. OCP measurements demonstrated that the mixed chloride-sulfuric acid solution used in copper sulfide pressure leaching is extremely detrimental to AOFs even at relatively
mild temperatures. The AOFs formed at 20–80 V failed within 2 h in de-aerated 30 g L−1 H2 SO4 + 12 g L−1 Cl− solutions without any oxidant at 85 ◦ C. LPR and EIS results showed that 80 V-anodization of titanium lead to the best improvement of corrosion resistance of titanium in the de-aerated copper leaching solution. It was also shown from EIS results that the 40 V-AOFs had the lowest Rpr value, which indicated that the 40 V-anodization produced a more porous layer than the other conditions tested. SEM results confirmed the LPR and EIS findings. The effect of cupric ions on titanium oxide films was investigated by OCP and XPS measurements. Cupric can reduce on TiO2 in the presence of chloride and this has been shown in previous studies to affect the performance of the protective oxide. The maintenance of oxidative conditions would inhibit the reduction of Cu2+ on TiO2 by shifting the OCP to a more positive
Please cite this article in press as: J. Liu, et al., Characterization of anodized titanium for hydrometallurgical applications—Evidence for the reduction of cupric on titanium dioxide, Appl. Surf. Sci. (2013), http://dx.doi.org/10.1016/j.apsusc.2013.06.168
G Model APSUSC-25978; No. of Pages 10 10
ARTICLE IN PRESS J. Liu et al. / Applied Surface Science xxx (2013) xxx–xxx
value; 5 g L−1 Fe3+ is more effective than O2 saturation to obtain this inhibiting effect at 85 ◦ C. Acknowledgements The authors gratefully acknowledge the financial support of the Natural Sciences and Engineering Research Council of Canada. We also wish to thank Dr. Ken Wong from the Department of Chemistry, The University of British Columbia for his assistance in obtaining the XPS data in this work. References [1] J. Vaughan, P. Reid, A. Alfantazi, Corrosion of Ti-2 and Ti-7 relevant to nickel acid leach chemistry, Hydrometallurgy 101 (2010) 156–165. [2] J. Grauman, T. Say, Titanium for hydrometallurgical extraction equipment, Advanced Materials & Processes 157 (2000) 25–29. [3] E. Gaul, Coloring titanium and related metals by electrochemical oxidation, Journal of Chemical Education 70 (1993) 176. [4] R.W. Schutz, L.C. Covington, Effect of oxide films on the corrosion resistance of titanium, Corrosion 37 (1981) 585–591. [5] J.A. McMaster, The case for specified ranges of interstitial elements and iron in titanium welding filler metal, in: Wah Chang Corrosion Solutions Conference, Sunriver, Oreg, 2001, pp. 129–138. [6] J. Cotton, Using titanium in the chemical plant, Chemical Engineering Process 66 (1970) 57–62. [7] C.E.B. Marino, L.H. Mascaro, Electrochemical tests to evaluate the stability of the anodic films on dental implants, International Journal of Electrochemistry 2011 (2011) 1–7, http://dx.doi.org/10.4061/2011/574502, Article ID 574502. [8] C.E.B. Marino, P.A.P. Nascente, S.R. Biaggio, R.C. Rocha-Filho, N. Bocchi, XPS characterization of anodic titanium oxide films grown in phosphate buffer solutions, Thin Solid Films 468 (2004) 109–112. [9] J.L. Delplancke, A. Garnier, Y. Massiani, R. Winand, Influence of the anodizing procedure on the structure and the properties of titanium oxide films and its effect on copper nucleation, Electrochimica Acta 39 (1994) 1281–1289. [10] J.L. Delplancke, R. Winand, Galvanostatic anodization of titanium—I. Structures and compositions of the anodic films, Electrochimica Acta 33 (1988) 1539–1549. [11] J.L. Delplancke, R. Winand, Galvanostatic anodization of titanium—II. Reactions efficiencies and electrochemical behaviour model, Electrochimica Acta 33 (1988) 1551–1559. [12] J.L. Delplancke, M. Degrez, A. Fontana, R. Winand, Self-colour anodizing of titanium, Surface Technology 16 (1982) 153–162. [13] Y.T. Sul, C.B. Johansson, Y. Jeong, T. Albrektsson, The electrochemical oxide growth behaviour on titanium in acid and alkaline electrolytes, Medical Engineering & Physics 23 (2001) 329–346. [14] M.E. Sibert, Electrochemical oxidation of titanium surfaces, Journal of the Electrochemical Society 110 (1963) 65–72. [15] Z. Yan, T. Guo, H. Pan, J. Yu, Influences of electrolyzing voltage on chromatics of anodized titanium dentures, Materials Transactions 43 (2002) 3142–3145. [16] A. Karambakhsh, A. Afshar, S. Ghahramani, P. Malekinejad, Pure commercial titanium color anodizing and corrosion resistance, Journal of Materials Engineering and Performance 20 (2011) 1690–1696. [17] A.K. Sharma, Anodizing titanium for space applications, Thin Solid Films 208 (1992) 48–54. [18] A. Cigada, M. Cabrini, P. Pedeferri, Increasing of the corrosion resistance of the Ti6Al4V alloy by high thickness anodic oxidation, Journal of Materials Science: Materials in Medicine 3 (1992) 408–412. [19] C. Chen, J. Chen, C. Chao, W.C. Say, Electrochemical characteristics of surface of titanium formed by electrolytic polishing and anodizing, Journal of materials science 40 (2005) 4053–4059. [20] T. Shibata, Y.C. Zhu, The effect of temperature on the growth of anodic oxide film on titanium, Corrosion Science 37 (1995) 133–144.
[21] K.O. Beck, Titanium anodizing process, US Patent 2,949,411 (1960). [22] C.J. Bernard, Method of improving the corrosion resistance of titanium metals, US Patent 3,102,086 (1963). [23] G. Critchlow, D. Brewis, Review of surface pretreatments for titanium alloys, International Journal of Adhesion and Adhesives 15 (1995) 161–172. [24] E.M. Oliveira, C.E.B. Marino, S.R. Biaggio, R.C. Rocha-Filho, Reactivation of passive titanium: the enhancement of O2 evolution after potentiodynamic cyclings, Electrochemistry Communications 2 (2000) 254–258. [25] J.S.L. Leach, B.R. Pearson, Crystallization in anodic oxide films, Corrosion Science 28 (1988) 43–56. [26] H. Habazaki, K. Shimizu, S. Nagata, P. Skeldon, G.E. Thompson, G.C. Wood, Ionic transport in amorphous anodic titania stabilised by incorporation of silicon species, Corrosion Science 44 (2002) 1047–1055. [27] M.V. Diamanti, M.P. Pedeferri, Effect of anodic oxidation parameters on the titanium oxides formation, Corrosion Science 49 (2007) 939–948. [28] J. Liu, E. Asselin, A. Alfantazi, Uniform Corrosion of Titanium Under Medium Temperature Chalcopyrite Concentrate Leaching, Niagara, Canada, 2012, pp. 269–279. [29] S.A. Fadl-allah, Q. Mohsen, Characterization of native and anodic oxide films formed on commercial pure titanium using electrochemical properties and morphology techniques, Applied Surface Science 256 (2010) 5849–5855. [30] M.E.P. Souza, M. Ballester, C.M.A. Freire, EIS characterisation of Ti anodic oxide porous films formed using modulated potential, Surface and Coatings Technology 201 (2007) 7775–7780. [31] C.C. Ma, E.M. Peres, Corrosion resistance of anodized and unanodized titanium, Industrial & Engineering Chemistry 43 (1951) 675–679. [32] R.G. Kelly, J.R. Scully, D. Shoesmith, R.G. Buchheit, Electrochemical Techniques in Corrosion Science and Engineering, Marcel Dekker, Inc., New York, 2002. [33] J. Liu, A. Alfantazi, E. Asselin, Influence of cupric, ferric and chloride on the corrosion of titanium in sulfuric acid solutions up to 85 ◦ C, Corrosion (2013) (submitted for publication). [34] J. Pan, D. Thierry, C. Leygraf, Electrochemical impedance spectroscopy study of the passive oxide film on titanium for implant application, Electrochimica Acta 41 (1996) 1143–1153. [35] J. Marsh, D. Gorse, A photoelectrochemical and ac impedance study of anodic titanium oxide films, Electrochimica Acta 43 (1997) 659–670. [36] M. Aziz-Kerrzo, K.G. Conroy, A.M. Fenelon, S.T. Farrell, C.B. Breslin, Electrochemical studies on the stability and corrosion resistance of titanium-based implant materials, Biomaterials 22 (2001) 1531–1539. [37] S. Piazza, G.L. Biundo, M.C. Romano, C. Sunseri, F. Di Quarto, In situ characterization of passive films on Al-Ti alloy by photocurrent and impedance spectroscopy, Corrosion Science 40 (1998) 1087–1108. [38] Q. Mohsen, S. Fadl-Allah, Improvement in corrosion resistance of commercial pure titanium for the enhancement of its biocompatibility, Materials and Corrosion 62 (2010) 310–319. [39] J. Defreyne, W. Grieve, D. Jones, K. Mayhew, The Role of Iron in the CESL Process, CESL company, Richmond, BC, Canada, 2006, pp. 205–220. [40] T. Hirato, H. Majima, Y. Awakura, The leaching of chalcopyrite with ferric sulfate, Metallurgical Transactions B 18 (1987) 489–496. [41] T. Hirato, M. Kinoshita, Y. Awakura, H. Majima, The leaching of chalcopyrite with ferric chloride, Metallurgical and Materials Transactions B 17 (1986) 19–28. [42] G. Hopfengärtner, D. Borgmann, I. Rademacher, G. Wedler, E. Hums, G.W. Spitznagel, XPS studies of oxidic model catalysts: Internal standards and oxidation numbers, Journal of Electron Spectroscopy and Related Phenomena 63 (1993) 91–116. [43] X. He, J.J. Noël, D.W. Shoesmith, Temperature dependence of crevice corrosion initiation on titanium grade-2, Journal of the Electrochemical Society 149 (2002) B440–B449. [44] A. Rochefort, M. Abon, P. Delichère, J.C. Bertolini, Alloying effect on the adsorption properties of Pd50Cu50{111} single crystal surface, Surface Science 294 (1993) 43–52. [45] J.G. Jolley, G.G. Geesey, M.R. Hankins, R.B. Wright, P.L. Wichlacz, Auger electron and X-ray photoelectron spectroscopic study of the biocorrosion of copper by alginic acid polysaccharide, Applied Surface Science 37 (1989) 469–480. [46] D. Chadwick, T. Hashemi, Adsorbed corrosion inhibitors studied by electron spectroscopy: benzotriazole on copper and copper alloys, Corrosion Science 18 (1978) 39–51.
Please cite this article in press as: J. Liu, et al., Characterization of anodized titanium for hydrometallurgical applications—Evidence for the reduction of cupric on titanium dioxide, Appl. Surf. Sci. (2013), http://dx.doi.org/10.1016/j.apsusc.2013.06.168