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Study on corrosion resistance and roughness of micro-plasma oxidation ceramic coatings on Ti alloy by EIS technique
Electrochimica Acta 52 (2007) 4539–4546
Study on corrosion resistance and roughness of micro-plasma oxidation ceramic coatings on Ti alloy by EIS technique Zhongping Yao ∗ , Zhaohua Jiang, Fuping Wang Department of Applied Chemistry, Harbin Institute of Technology, Harbin 150001, PR China Received 20 October 2006; received in revised form 21 December 2006; accepted 25 December 2006 Available online 21 January 2007
Abstract Micro-plasma oxidation (MPO) technique has been developed quickly in recent years. The produced ceramic coatings are reported to possess fine properties and promising application prospects in many fields. The aim of this work is to study the corrosion resistance and the roughness of the micro-plasma oxidation ceramic coatings on Ti alloy by electrochemical impedance spectroscopy (EIS) technique. Compound ceramic coatings were prepared on Ti–6Al–4V alloy by pulsed bi-polar micro-plasma oxidation in NaAlO2 solution. The phase composition and element distribution in the coating were investigated by X-ray diffractometry and electron probe micro-analyzer. EIS of the coatings was measured through CHI604 electrochemical analyzer in 3.5% NaCl solution. The ceramic coating is composed of a large amount of Al2 TiO5 and a little ␣-Al2 O3 and rutile TiO2 . The coating is of double-layer structure with the loose outer layer and the dense inner layer. The thickness of the coatings is reduced when the working frequency or the cathode pulse current density is increased, while the thickness is increased when the frequency or the anode current density is increased. The established “equivalent circuit” of the coatings is consistent with the double-layer structure. The electric charge transfer resistance (Rt ) in the equivalent circuit can be used to assess the corrosion resistance of the coatings, which is consistent with the result of the polarizing curves test. And the empirical exponent (n1 ) of the constant phase element (Q1 ) in the equivalent circuit can be used to assess the surface roughness of the coatings, which is consistent with the surface SEM analysis of the coatings. © 2007 Elsevier Ltd. All rights reserved. Keywords: Corrosion resistance; Roughness; Micro-plasma oxidation; Ceramic coatings; EIS
1. Introduction Since G¨unterschulze and Betz first studied the spark discharge at the anode in early 1930s, micro-plasma oxidation (MPO) technique has been developed quickly in recent years. The produced ceramic coatings are of the fine properties like high hardness, corrosion resistance, anti-abrasion property or decorative property, etc. and have the promising application prospect in many fields [1–3]. At present, much work has been focused on the composition, the structure and the properties of the prepared coatings by many researchers through X-ray diffractometry (XRD), scanning electron microscopy (SEM) and other regular studying methods of material science [4–7]. However, it is still far away to get insight into the coatinggrowing mechanism and the relations of the coating structure to the properties. Therefore, it is important and necessary to ful-
∗
Corresponding author. Tel.: +86 451 86413710; fax: +86 451 86413707. E-mail address: [email protected] (Z. Yao).
0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2006.12.052
fill more research work on the MPO technique to turn it into practice quickly. Electrochemical impedance spectroscopy (EIS) technique is a powerful analyzing technique, which can provide a wealth of information on the mass transport, the corrosion reaction and the electrical charge transfer of the ceramic coatings or organic coatings. It offers a number of advantages over conventional electrochemical polarization techniques through fitting and interpretation of EIS spectra [8–11]. Using this technique, micro-plasma oxidation ceramic coatings on Ti–6Al–4V alloy in NaAlO2 solution were studied in our previous work [12]. We established the “equivalent circuit”, and discussed the relations of the corrosion resistance and the surface roughness of the coatings to the different components in the “equivalent circuit” preliminarily. In this work, we prepared ceramic coatings under different electric parameters and further studied the corrosion resistance and the surface roughness of the ceramic coatings through EIS technique. Meanwhile, the evaluation criterion of the coatings on the corrosion resistance and the surface roughness was discussed.
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2. Experimental details 2.1. Preparing of compound ceramic coatings by MPO technique Plate samples of Ti–6Al–4V with a reaction dimension of 15 mm2 were first polished using abrasive paper, and then washed in HF–HNO3 (1:1 in volume) aqueous solution. A home made electrical source with power of 5 kW was used for micro-plasma oxidation of the samples in a water-cooled electrolyser made of stainless steel, which also serves as the counter electrode. The reaction temperature was controlled to below 30 ◦ C by adjusting the cooling water flow. The MPO process equipment used is similar to the one presented by Matthews’ group [1]. The electrolyte used in the experiment is sodium aluminate (8 g/l) and sodium phosphate (1 g/l). The whole MPO process was carried out under different technique parameters (current density and frequency). After the treatment, the coated samples were flushed with water and dried in air. 2.2. Analysis of phase composition and structure of the coatings Phase composition of the coatings was examined with X-ray diffraction (XRD) (D/max-rB, RICOH, Japan) with a Cu K␣ source. The section image of the coating and the element distribution were studied by electron probe micro-analyzer (EPMA) (JEOL SUPERPROBE 733, Japan). Surface images of the prepared coatings were studied with Scanning electron microscopy (SEM) (S-570, Hitachi, Japan). The sample thickness was measured, using an eddy current-based thickness gauge (CTG-10, Time Company, China) with a minimum resolution of 1 m. The average thickness of each sample was obtained from 10 measurements at different positions. 2.3. EIS measuring and fitting of the ceramic coatings EIS spectra of the coatings were measured in a threeelectrode cell (a Pt plate was used as a counter electrode, Ag/AgCl reference electrode, and the coated sample with the area of 1 cm2 as the working electrode) through CHI604 electrochemical analyzer (Shanghai, China) in 3.5% NaCl solution. A sinusoidal ac perturbation of 5 mV was applied to the electrode at the open circuit potential of the coated samples over the frequency range 0.01 Hz–10 kHz. The measured EIS spectra of the coatings were fitted and interpreted by Autolab Electrochemical EIS fitting software. 2.4. The corrosion resistance of the coatings The corrosion resistance of the coatings was evaluated by the polarizing curves in 3.5% NaCl solution, using the same threeelectrode cell as in 2.3. The studying area of the coated sample (working electrode) was 1 cm2 . The polarization curve’s sweeping rate was 1 mV/s, with a scanning range from −0.5 V of open circuit potential to +0.5 V of open circuit potential. The corro-
Fig. 1. XRD patterns of the ceramic coating (Ia /Ic = 8/8 A/dm2 , frequency is 60 Hz and MPO time is 90 min). (a) Surface; (b) 50 m from the interface.
sion potential and the corrosion current density were obtained through the linear analysis of Tafel approximation. 3. Results and discussion 3.1. Phase composition of the coating The phase composition of the coating at different depth is shown in Fig. 1. Clearly, the coating consists of Al2 TiO5 , ␣Al2 O3 and rutile TiO2 , of which Al2 TiO5 is the main crystal phase through the whole coating. The content of ␣-Al2 O3 on the coating surface is more than that inside the coating. On the contrary, the content of rutile TiO2 on the coating surface is less than that inside the coating. 3.2. Section image and elemental analysis of the coating The section image and the elemental distribution of the coating obtained in the early study [12] are presented in Fig. 2 due to the relevance to the present work. Firstly, panel (a) shows that the coating is composed of double layers, which are the loose outer layer and the dense inner layer. Secondly, it can be seen clearly that Al in the coating is much more than that in the substrate, which means that Al in the electrolyte that took part in the MPO process may be more than that in the substrate. Because the diffusion of Al from the electrolyte became more and more difficult with extending MPO time, the content of Al was increased gradually from inner layer to outer layer. But the maximum of Al emerged in the middle part of the coating, which was due to more and more micro-holes and cracks formed in the outer layer. Thirdly, Ti in the coating is much less than that in the substrate. Interestingly, the distribution of Ti is relatively uniform in comparison with Al, and its distribution in the coating can be divided into two parts approximately: the outer layer with low content of Ti and the inner layer with high content of Ti, which is just consistent with the double-layer structure of the coatings. 3.3. Surface morphology of the coatings Electric parameters play an important role in the coating growth, which affect the coatings’ thickness and structure. Fig. 3
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Fig. 3. Thickness of the coatings produced under different electric parameters.
of the coatings produced under different current densities of both pulses. The coating surface is very coarse and there are not apparent differences in the roughness among the three samples. Fig. 5 is surface images of the coatings produced under different working frequencies. It is apparent that the number of discharge channel is increased with increasing working frequency while the discharge channel diameters are clearly decreased. Besides, the sintered particles on the coating surface become smaller with the increasing frequency while their numbers are increased greatly. This means that the surface roughness of the coatings is decreased with the increasing frequency. 3.4. EIS spectra of the ceramic coatings under different technique parameters
Fig. 2. EPMA of the ceramic coating (Ia /Ic = 8/8 A/dm2 , frequency is 60 Hz and MPO time is 120 min). (a) The backward pattern; (b) line scanning pattern of Al and Ti.
is the thickness of the coatings prepared under different conditions. Ia is the current density of the anode pulse while Ic is the current density of the cathode pulse. The increase of the anode current density leads to the increase of the coating thickness, while the increase of the cathode current density leads to the decrease of the coating thickness. Besides, the thickness of the coating is decreased gradually with increasing working frequency. Surface morphology of the coatings prepared under different electric parameters is shown in Figs. 4 and 5. Clearly, the micrographs clearly indicate the presence of discharge channels appearing as dark circular spots distributed all over the coating surface where there also exists the sintered particles of different sizes. Fig. 4 is surface images (with the same magnification)
The established “equivalent circuit” in our previous work [12] is shown in Fig. 6. All the capacitance elements in the “equivalent circuit” are replaced with the constant phase elements (CPE) in the fitting of the EIS. It is a modified Randles circuit. Rs is the solution resistance; Q1 is related to the surface state and the outer layer of the coating, which is defined as the CPE of the outer layer; Q2 is corresponding to Warburg impedance, which reflects the characteristics of the inner layer; and Rt is the electric charge transfer resistance. Meantime, two relations were found as follows: the corrosion resistance of the coated sample was improved with the increase of Rt ; and the coating roughness of the coating was increased with the decrease of the value of Q1 − n1 . In order to verify the above relations further, the EIS spectra of the ceramic coatings produced under different current densities and frequencies were investigated, and the above two relations were further discussed. 3.4.1. EIS spectra of the ceramic coatings under different current densities The coatings were prepared under different current densities of both pulses and the whole MPO process was divided into two stages: (1) a constant current density period when the constant current density was applied to the sample for 60 min, then (2) a current-descending period when current density was allowed
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Fig. 4. Surface images of the coatings produced under different current densities. (a) Ia /Ic = 8/10 A/dm2 ; (b) Ia /Ic = 8/8 A/dm2 ; (c) Ia /Ic = 10/8 A/dm2 .
to decrease freely until the whole reaction time reached 90 min. Fig. 7 is EIS (Bode plot) of MPO ceramic coatings under different current densities. Using the “equivalent circuit” in Fig. 6 to fit the EIS spectra, the Chi-squared is within 10−4 during the fitting process. The impedance fitting values are shown in Table 1. The
value of Rt changes in terms of the changes of current densities of both pulses: Rt of Ia /Ic = 10/8 A/dm2 is 741.6 cm2 , which is the largest among the three samples, then Rt of Ia /Ic = 8/8 A/dm2 and Rt of Ia /Ic = 8/10 A/dm2 in turn. Besides, the polarization curves of the coated samples under different current densities
Table 1 Impedance fitting values of the coatings produced under different current densities Ia /Ic (A/dm2 )
Rs ( cm2 )
Q1 − Y0 (× 10−4 −1 s−n /cm2 )
n1
Rt ( cm2 )
Q2 − Y0 (−1 s−n /cm2 )
n2
10/8 8/8 8/10
58.04 97.7 85.46
2.801 2.697 3.085
0.3826 0.3737 0.3569
741.6 730.6 610.3
0.0008730 0.0007751 0.0009607
0.6272 0.6265 0.6420
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Fig. 5. Surface images of the coatings produced under different working frequencies. (a) 30 Hz; (b) 60 Hz; (c) 120 Hz; (d) 240 Hz.
Fig. 6. Equivalent circuit of MPO coatings on Ti–6Al–4V alloy in 3.5% NaCl solution Rs : solution resistance; Q1 : CPE of the outer layer of the coating; Q2 : CPE of the inner layer of the coating; Rt : electric charge transfer resistance.
in 3.5% NaCl solution is shown in Fig. 8, with the corrosion current density and the corrosion potential presented in Table 2. It is well known that the corrosion resistance is determined by the corrosion current density: the smaller the corrosion current density, the better is the corrosion resistance. Among the three samples, the best one is the coating of Ia /Ic = 10/8 A/dm2 with Table 2 Corrosion current density and corrosion potential of the coated samples produced under different current densities Ia /Ic (A/dm2 )
Corrosion current density (×10−7 A/cm2 )
Corrosion potential (V)
8/8 8/10 10/8
7.234 7.454 5.222
0.270 0.348 0.340
the minimum current density. Also, the corrosion current densities of the coatings of Ia /Ic = 8/8 A/dm2 is a little lower than that of Ia /Ic = 8/10 A/dm2 . Therefore, the changes of Rt are consistent with the changes of the corrosion current densities. Moreover, it is well known that the empirical exponent (n) of CPE depends not only on the surface roughness, but also on the gradients in dielectric properties (composition/phase) of the coating [13,14]. According the Fig. 2, a gradient in dielectric constant is highly probable. However, it is assumed that the dielectric property within each layer of the coating is the same approximately in view of two-layer structure. In this way, n is only related to the surface roughness, and the relation of n to the fracture dimension (DF ) is shown in the following formula [15,16]: n=
1 DF − 1
and
DF =
1 n+1
(1)
The values of Q1 − n1 are almost the same among the three samples; the maximum difference is about 0.02. Consequently, the value of DF is also similar, which means that the surface roughness of the coating is almost the same. This is consistent with the results of the SEM analysis in Fig. 4. There are no obvious changes in roughness and corrosion resistance for the coating prepared under different current
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Fig. 7. EIS (Bode plot) of MPO ceramic coatings produced under different current densities.
Fig. 9. EIS (Bode plot) of MPO ceramic coatings produced under different frequencies in aerated 3.5% NaCl solution.
densities, which may be related to the following factors: (1) the differences of the current density of both pulses were not great enough, which means that the growing speed of the coatings did not change greatly; (2) the MPO time is all the same and comparatively much longer among the three conditions. Therefore, all the coatings were much thick and dense, which reduced the differences of the corrosion resistance and the surface roughness, especially for the latter.
Fig. 8. Polarizing curves of the coated samples produced under different current densities in 3.5% NaCl.
3.4.2. EIS spectra of the ceramic coatings under different frequencies Since the coatings produced under different current densities for 90 min were similar in the corrosion resistance and the surface roughness, the MPO time was reduced to 60 min and the current density of both pulses remained at 8 A/dm2 when the coatings were prepared under different frequencies. Fig. 9 is EIS (Bode plot) of MPO ceramic coatings under different working frequencies. The impedance fitting values are shown in Table 3. The value of Rt of 60 Hz is 741.6 cm2 , which is the maximum; then the value of Rt is decreased from
Z. Yao et al. / Electrochimica Acta 52 (2007) 4539–4546
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Table 3 Impedance fitting values of the coatings produced under different frequencies Frequency (Hz) 30 60 120 240
Rs ( cm2 )
Q1 − Y0 (−1 s−n /cm2 )
n1
Rt ( cm2 )
Q2 − Y0 (−1 s−n /cm2 )
n2
50.6 42.78 33.27 33.97
2.366 × 10−4
0.3546 0.4099 0.4455 0.4986
688.9 741.6 608.4 444.6
0.001384 0.001173 0.008900 0.001178
0.6048 0.6241 0.5700 0.5735
2.069 × 10−4 1.224 × 10−4 9.591 × 10−5
3.5. Evaluation criterion of the coatings
Fig. 10. Polarizing curves of the coated samples produced under different frequencies in 3.5% NaCl.
30, 120 to 240 Hz in sequence. The value of Rt of 240 Hz is only 444.6 cm2 . Besides, the polarizing curves of the coated samples under different frequencies in 3.5% NaCl is shown in Fig. 10, with the corrosion current density and the corrosion potential of the coated samples presented in Table 4. According to the corrosion current density of the coated samples, the corrosion resistance of the coated sample is decreased from 60, 30, 120 to 240 Hz in sequence. The changes of Rt completely comply with the changes of the corrosion resistance of the coated samples; and therefore, Rt reflects the corrosion resistance of the coated samples. Besides, the value of Q1 − n1 of the coatings is increased with the increasing working frequency, as shown in Table 3. According to the formula (1), the fracture dimension (DF ) of the coating surface is increased consequently, which means that the surface roughness of the coatings is decreased. This is also consistent with the changes of the surface SEM of the coatings under different frequencies in Fig. 5. Therefore, the surface roughness of the coatings can be reflected by the changes of the value of Q1 − n1 .
3.5.1. Criterion of the corrosion resistance Generally, the corrosion resistance is related to the thickness and the density of coatings, and the composition as well. Because all the coatings prepared in the experiments are similar in the composition, the key factors would then be attributed to the thickness and the density. If the coating is thicker and denser, the corrosion resistance of the coated sample will become better. Besides, based on the foregoing analyses, we can find the changes of the electric charge transfer resistance (Rt ) in the “equivalent circuit” are consistent with the changes of the corrosion current density. Therefore, the corrosion resistance could be evaluated not only by the corrosion current density but also by the electric charge transfer resistance (Rt ). Besides, corrosion current density is comparatively less accurate to evaluate the coating performance since it is being measured in the dynamic polarization experiment that imposes many changes on the coating, compared to the EIS technique. Instead, Rt is usually considered as more appropriate factor for evaluating the coating resistance once it is modeled in an accurate equivalent circuit model. Therefore, Rt can be considered as a criterion of the corrosion resistance for the coated samples of the same kind. 3.5.2. Criterion of the surface roughness Surface roughness is an important property for the MPO ceramic coatings in its present and potential applications. The fracture dimension (DF ) is a physics variable corresponding to the surface roughness, however, it is difficult to calculate it because it needs a lot of knowledge about mathematics models and theoretical calculation. Similarly, it can be noted from the above analysis that Q1 − n1 is related to the fracture dimension, i.e. DF can be calculated through the value of Q1 − n1 approximately. In other words; the surface roughness of the coated samples can be evaluated by the change of the value of n1 consequently. Therefore, Q1 − n1 can be selected as a criterion of the surface roughness of coated samples of the same kind. 4. Conclusions
Table 4 Corrosion current density and corrosion potential of the coated samples produced under different frequencies Frequency (Hz)
Corrosion current density (×10−6 A/cm2 )
Corrosion potential (V)
30 60 120 240
6.224 1.879 7.242 10.30
0.087 0.047 −0.025 −0.059
Ceramic coatings on Ti alloy were prepared under different electric parameters in NaAlO2 solution by bi-polar pulsed micro-plasma oxidation. The structure, composition and EIS spectra of the coatings were investigated and the following conclusions can be drawn: (1) The ceramic coating is composed of a large amount of Al2 TiO5 and a little ␣-Al2 O3 and rutile TiO2 . The coating is
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of double layer structure with the loose outer layer and the dense inner layer. The coating thickness is reduced when the working frequency or the cathode pulse current density is increased, while the thickness is increased when the working frequency or the anode current density is increased. Furthermore, the surface roughness of the coating is decreased with the increasing frequency. (2) Through the analysis of EIS spectra of the MPO coatings, the corrosion resistance of the coated samples is related to Rt in the “equivalent circuit”: the bigger the value of Rt , the better is the corrosion resistance. Therefore, the electric charge transfer resistance (Rt ) can be considered as a criterion of the corrosion resistance. (3) The surface roughness of the coating is related to the empirical exponent (n1 ) of the constant phase elements (Q1 ) in the “equivalent circuit”: the surface roughness of the coatings is increased with the decrease of n1 . Therefore, n1 can be as a criterion of the surface roughness of coated samples of the same kind. Acknowledgements This work was financially supported by National natural science foundation of China (Grant No. 50171026) and Harbin Special Foundation of Fellow Creation for Science and Technology of China (Grant No. 2006RFQXG032).
References [1] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, S.J. Dowey, Surf. Coat. Technol. 122 (1999) 73. [2] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, Surf. Coat. Technol. 130 (2000) 195. [3] P.I. Butygin, Y.V. Khokhryakov, A.L. Mamaev, Mater. Lett. 57 (2003) 1784. [4] G.L. Yang, X.Y. Lv, Y.Z. Bai, H.F. Cui, Z.S. Jin, J. Alloy. Compd. 345 (2002) 196. [5] G. Sundrarajan, L. Rama Krishna, Surf. Coat. Technol. 167 (2003) 269. [6] L.O. Snizhko, A.L. Yerokhin, A. Pilkington, N.L. Gurevina, D.O. Misnyankin, A. Leyland, A. Matthews, Electrochim. Acta 49 (2004) 2085. [7] W.B. Xue, C. Wang, Z.W. Deng, R.Y. Chen, T.H. Zhang, Mater. Sci. Technol. 18 (2002) 37. [8] G. Bierwagen, D. Tallman, J.P. Li, et al., Prog. Org. Coatings 46 (2003) 148. [9] F. Fracassi, R. d’Agostino, F. Palumbo, et al., Surf. Coat. Technol. 174–175 (2003) 107. [10] A. Miszczyka, T. Schauer, Prog. Org. Coatings 52 (2005) 298. [11] C. Liu, Q. Bi, A. Mattews, Corros. Sci. 43 (2001) 1953. [12] Z.P. Yao, Z.H. Jiang, S.G. Xin, X.T. Sun, X.H. Wu, Electrochim. Acta 16–17 (2005) 3273. [13] T. Kaplan, L.T. Gray, Phy. Rev. B 32 (1985) 7360. [14] M.L. Shi, T.H. Li, G.D. Zhou, J. Tongji Univ. 23 (1995) 83 (in Chinese). [15] C.M.A. Breet, A.M.O. Breet, Electrochemistry, Principles Methods and Applications [M], Oxford University Press, London, 1993. [16] J.M. Zhang, C.C. Yang, Q.Z. Shi, J.S. Guo, J. Chin. Ceram. Soc. 31 (2003) 127 (in Chinese).
Study on corrosion resistance and roughness of micro-plasma oxidation ceramic coatings on Ti alloy by EIS technique Zhongping Yao ∗ , Zhaohua Jiang, Fuping Wang Department of Applied Chemistry, Harbin Institute of Technology, Harbin 150001, PR China Received 20 October 2006; received in revised form 21 December 2006; accepted 25 December 2006 Available online 21 January 2007
Abstract Micro-plasma oxidation (MPO) technique has been developed quickly in recent years. The produced ceramic coatings are reported to possess fine properties and promising application prospects in many fields. The aim of this work is to study the corrosion resistance and the roughness of the micro-plasma oxidation ceramic coatings on Ti alloy by electrochemical impedance spectroscopy (EIS) technique. Compound ceramic coatings were prepared on Ti–6Al–4V alloy by pulsed bi-polar micro-plasma oxidation in NaAlO2 solution. The phase composition and element distribution in the coating were investigated by X-ray diffractometry and electron probe micro-analyzer. EIS of the coatings was measured through CHI604 electrochemical analyzer in 3.5% NaCl solution. The ceramic coating is composed of a large amount of Al2 TiO5 and a little ␣-Al2 O3 and rutile TiO2 . The coating is of double-layer structure with the loose outer layer and the dense inner layer. The thickness of the coatings is reduced when the working frequency or the cathode pulse current density is increased, while the thickness is increased when the frequency or the anode current density is increased. The established “equivalent circuit” of the coatings is consistent with the double-layer structure. The electric charge transfer resistance (Rt ) in the equivalent circuit can be used to assess the corrosion resistance of the coatings, which is consistent with the result of the polarizing curves test. And the empirical exponent (n1 ) of the constant phase element (Q1 ) in the equivalent circuit can be used to assess the surface roughness of the coatings, which is consistent with the surface SEM analysis of the coatings. © 2007 Elsevier Ltd. All rights reserved. Keywords: Corrosion resistance; Roughness; Micro-plasma oxidation; Ceramic coatings; EIS
1. Introduction Since G¨unterschulze and Betz first studied the spark discharge at the anode in early 1930s, micro-plasma oxidation (MPO) technique has been developed quickly in recent years. The produced ceramic coatings are of the fine properties like high hardness, corrosion resistance, anti-abrasion property or decorative property, etc. and have the promising application prospect in many fields [1–3]. At present, much work has been focused on the composition, the structure and the properties of the prepared coatings by many researchers through X-ray diffractometry (XRD), scanning electron microscopy (SEM) and other regular studying methods of material science [4–7]. However, it is still far away to get insight into the coatinggrowing mechanism and the relations of the coating structure to the properties. Therefore, it is important and necessary to ful-
∗
Corresponding author. Tel.: +86 451 86413710; fax: +86 451 86413707. E-mail address: [email protected] (Z. Yao).
0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2006.12.052
fill more research work on the MPO technique to turn it into practice quickly. Electrochemical impedance spectroscopy (EIS) technique is a powerful analyzing technique, which can provide a wealth of information on the mass transport, the corrosion reaction and the electrical charge transfer of the ceramic coatings or organic coatings. It offers a number of advantages over conventional electrochemical polarization techniques through fitting and interpretation of EIS spectra [8–11]. Using this technique, micro-plasma oxidation ceramic coatings on Ti–6Al–4V alloy in NaAlO2 solution were studied in our previous work [12]. We established the “equivalent circuit”, and discussed the relations of the corrosion resistance and the surface roughness of the coatings to the different components in the “equivalent circuit” preliminarily. In this work, we prepared ceramic coatings under different electric parameters and further studied the corrosion resistance and the surface roughness of the ceramic coatings through EIS technique. Meanwhile, the evaluation criterion of the coatings on the corrosion resistance and the surface roughness was discussed.
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2. Experimental details 2.1. Preparing of compound ceramic coatings by MPO technique Plate samples of Ti–6Al–4V with a reaction dimension of 15 mm2 were first polished using abrasive paper, and then washed in HF–HNO3 (1:1 in volume) aqueous solution. A home made electrical source with power of 5 kW was used for micro-plasma oxidation of the samples in a water-cooled electrolyser made of stainless steel, which also serves as the counter electrode. The reaction temperature was controlled to below 30 ◦ C by adjusting the cooling water flow. The MPO process equipment used is similar to the one presented by Matthews’ group [1]. The electrolyte used in the experiment is sodium aluminate (8 g/l) and sodium phosphate (1 g/l). The whole MPO process was carried out under different technique parameters (current density and frequency). After the treatment, the coated samples were flushed with water and dried in air. 2.2. Analysis of phase composition and structure of the coatings Phase composition of the coatings was examined with X-ray diffraction (XRD) (D/max-rB, RICOH, Japan) with a Cu K␣ source. The section image of the coating and the element distribution were studied by electron probe micro-analyzer (EPMA) (JEOL SUPERPROBE 733, Japan). Surface images of the prepared coatings were studied with Scanning electron microscopy (SEM) (S-570, Hitachi, Japan). The sample thickness was measured, using an eddy current-based thickness gauge (CTG-10, Time Company, China) with a minimum resolution of 1 m. The average thickness of each sample was obtained from 10 measurements at different positions. 2.3. EIS measuring and fitting of the ceramic coatings EIS spectra of the coatings were measured in a threeelectrode cell (a Pt plate was used as a counter electrode, Ag/AgCl reference electrode, and the coated sample with the area of 1 cm2 as the working electrode) through CHI604 electrochemical analyzer (Shanghai, China) in 3.5% NaCl solution. A sinusoidal ac perturbation of 5 mV was applied to the electrode at the open circuit potential of the coated samples over the frequency range 0.01 Hz–10 kHz. The measured EIS spectra of the coatings were fitted and interpreted by Autolab Electrochemical EIS fitting software. 2.4. The corrosion resistance of the coatings The corrosion resistance of the coatings was evaluated by the polarizing curves in 3.5% NaCl solution, using the same threeelectrode cell as in 2.3. The studying area of the coated sample (working electrode) was 1 cm2 . The polarization curve’s sweeping rate was 1 mV/s, with a scanning range from −0.5 V of open circuit potential to +0.5 V of open circuit potential. The corro-
Fig. 1. XRD patterns of the ceramic coating (Ia /Ic = 8/8 A/dm2 , frequency is 60 Hz and MPO time is 90 min). (a) Surface; (b) 50 m from the interface.
sion potential and the corrosion current density were obtained through the linear analysis of Tafel approximation. 3. Results and discussion 3.1. Phase composition of the coating The phase composition of the coating at different depth is shown in Fig. 1. Clearly, the coating consists of Al2 TiO5 , ␣Al2 O3 and rutile TiO2 , of which Al2 TiO5 is the main crystal phase through the whole coating. The content of ␣-Al2 O3 on the coating surface is more than that inside the coating. On the contrary, the content of rutile TiO2 on the coating surface is less than that inside the coating. 3.2. Section image and elemental analysis of the coating The section image and the elemental distribution of the coating obtained in the early study [12] are presented in Fig. 2 due to the relevance to the present work. Firstly, panel (a) shows that the coating is composed of double layers, which are the loose outer layer and the dense inner layer. Secondly, it can be seen clearly that Al in the coating is much more than that in the substrate, which means that Al in the electrolyte that took part in the MPO process may be more than that in the substrate. Because the diffusion of Al from the electrolyte became more and more difficult with extending MPO time, the content of Al was increased gradually from inner layer to outer layer. But the maximum of Al emerged in the middle part of the coating, which was due to more and more micro-holes and cracks formed in the outer layer. Thirdly, Ti in the coating is much less than that in the substrate. Interestingly, the distribution of Ti is relatively uniform in comparison with Al, and its distribution in the coating can be divided into two parts approximately: the outer layer with low content of Ti and the inner layer with high content of Ti, which is just consistent with the double-layer structure of the coatings. 3.3. Surface morphology of the coatings Electric parameters play an important role in the coating growth, which affect the coatings’ thickness and structure. Fig. 3
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Fig. 3. Thickness of the coatings produced under different electric parameters.
of the coatings produced under different current densities of both pulses. The coating surface is very coarse and there are not apparent differences in the roughness among the three samples. Fig. 5 is surface images of the coatings produced under different working frequencies. It is apparent that the number of discharge channel is increased with increasing working frequency while the discharge channel diameters are clearly decreased. Besides, the sintered particles on the coating surface become smaller with the increasing frequency while their numbers are increased greatly. This means that the surface roughness of the coatings is decreased with the increasing frequency. 3.4. EIS spectra of the ceramic coatings under different technique parameters
Fig. 2. EPMA of the ceramic coating (Ia /Ic = 8/8 A/dm2 , frequency is 60 Hz and MPO time is 120 min). (a) The backward pattern; (b) line scanning pattern of Al and Ti.
is the thickness of the coatings prepared under different conditions. Ia is the current density of the anode pulse while Ic is the current density of the cathode pulse. The increase of the anode current density leads to the increase of the coating thickness, while the increase of the cathode current density leads to the decrease of the coating thickness. Besides, the thickness of the coating is decreased gradually with increasing working frequency. Surface morphology of the coatings prepared under different electric parameters is shown in Figs. 4 and 5. Clearly, the micrographs clearly indicate the presence of discharge channels appearing as dark circular spots distributed all over the coating surface where there also exists the sintered particles of different sizes. Fig. 4 is surface images (with the same magnification)
The established “equivalent circuit” in our previous work [12] is shown in Fig. 6. All the capacitance elements in the “equivalent circuit” are replaced with the constant phase elements (CPE) in the fitting of the EIS. It is a modified Randles circuit. Rs is the solution resistance; Q1 is related to the surface state and the outer layer of the coating, which is defined as the CPE of the outer layer; Q2 is corresponding to Warburg impedance, which reflects the characteristics of the inner layer; and Rt is the electric charge transfer resistance. Meantime, two relations were found as follows: the corrosion resistance of the coated sample was improved with the increase of Rt ; and the coating roughness of the coating was increased with the decrease of the value of Q1 − n1 . In order to verify the above relations further, the EIS spectra of the ceramic coatings produced under different current densities and frequencies were investigated, and the above two relations were further discussed. 3.4.1. EIS spectra of the ceramic coatings under different current densities The coatings were prepared under different current densities of both pulses and the whole MPO process was divided into two stages: (1) a constant current density period when the constant current density was applied to the sample for 60 min, then (2) a current-descending period when current density was allowed
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Fig. 4. Surface images of the coatings produced under different current densities. (a) Ia /Ic = 8/10 A/dm2 ; (b) Ia /Ic = 8/8 A/dm2 ; (c) Ia /Ic = 10/8 A/dm2 .
to decrease freely until the whole reaction time reached 90 min. Fig. 7 is EIS (Bode plot) of MPO ceramic coatings under different current densities. Using the “equivalent circuit” in Fig. 6 to fit the EIS spectra, the Chi-squared is within 10−4 during the fitting process. The impedance fitting values are shown in Table 1. The
value of Rt changes in terms of the changes of current densities of both pulses: Rt of Ia /Ic = 10/8 A/dm2 is 741.6 cm2 , which is the largest among the three samples, then Rt of Ia /Ic = 8/8 A/dm2 and Rt of Ia /Ic = 8/10 A/dm2 in turn. Besides, the polarization curves of the coated samples under different current densities
Table 1 Impedance fitting values of the coatings produced under different current densities Ia /Ic (A/dm2 )
Rs ( cm2 )
Q1 − Y0 (× 10−4 −1 s−n /cm2 )
n1
Rt ( cm2 )
Q2 − Y0 (−1 s−n /cm2 )
n2
10/8 8/8 8/10
58.04 97.7 85.46
2.801 2.697 3.085
0.3826 0.3737 0.3569
741.6 730.6 610.3
0.0008730 0.0007751 0.0009607
0.6272 0.6265 0.6420
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Fig. 5. Surface images of the coatings produced under different working frequencies. (a) 30 Hz; (b) 60 Hz; (c) 120 Hz; (d) 240 Hz.
Fig. 6. Equivalent circuit of MPO coatings on Ti–6Al–4V alloy in 3.5% NaCl solution Rs : solution resistance; Q1 : CPE of the outer layer of the coating; Q2 : CPE of the inner layer of the coating; Rt : electric charge transfer resistance.
in 3.5% NaCl solution is shown in Fig. 8, with the corrosion current density and the corrosion potential presented in Table 2. It is well known that the corrosion resistance is determined by the corrosion current density: the smaller the corrosion current density, the better is the corrosion resistance. Among the three samples, the best one is the coating of Ia /Ic = 10/8 A/dm2 with Table 2 Corrosion current density and corrosion potential of the coated samples produced under different current densities Ia /Ic (A/dm2 )
Corrosion current density (×10−7 A/cm2 )
Corrosion potential (V)
8/8 8/10 10/8
7.234 7.454 5.222
0.270 0.348 0.340
the minimum current density. Also, the corrosion current densities of the coatings of Ia /Ic = 8/8 A/dm2 is a little lower than that of Ia /Ic = 8/10 A/dm2 . Therefore, the changes of Rt are consistent with the changes of the corrosion current densities. Moreover, it is well known that the empirical exponent (n) of CPE depends not only on the surface roughness, but also on the gradients in dielectric properties (composition/phase) of the coating [13,14]. According the Fig. 2, a gradient in dielectric constant is highly probable. However, it is assumed that the dielectric property within each layer of the coating is the same approximately in view of two-layer structure. In this way, n is only related to the surface roughness, and the relation of n to the fracture dimension (DF ) is shown in the following formula [15,16]: n=
1 DF − 1
and
DF =
1 n+1
(1)
The values of Q1 − n1 are almost the same among the three samples; the maximum difference is about 0.02. Consequently, the value of DF is also similar, which means that the surface roughness of the coating is almost the same. This is consistent with the results of the SEM analysis in Fig. 4. There are no obvious changes in roughness and corrosion resistance for the coating prepared under different current
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Fig. 7. EIS (Bode plot) of MPO ceramic coatings produced under different current densities.
Fig. 9. EIS (Bode plot) of MPO ceramic coatings produced under different frequencies in aerated 3.5% NaCl solution.
densities, which may be related to the following factors: (1) the differences of the current density of both pulses were not great enough, which means that the growing speed of the coatings did not change greatly; (2) the MPO time is all the same and comparatively much longer among the three conditions. Therefore, all the coatings were much thick and dense, which reduced the differences of the corrosion resistance and the surface roughness, especially for the latter.
Fig. 8. Polarizing curves of the coated samples produced under different current densities in 3.5% NaCl.
3.4.2. EIS spectra of the ceramic coatings under different frequencies Since the coatings produced under different current densities for 90 min were similar in the corrosion resistance and the surface roughness, the MPO time was reduced to 60 min and the current density of both pulses remained at 8 A/dm2 when the coatings were prepared under different frequencies. Fig. 9 is EIS (Bode plot) of MPO ceramic coatings under different working frequencies. The impedance fitting values are shown in Table 3. The value of Rt of 60 Hz is 741.6 cm2 , which is the maximum; then the value of Rt is decreased from
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Table 3 Impedance fitting values of the coatings produced under different frequencies Frequency (Hz) 30 60 120 240
Rs ( cm2 )
Q1 − Y0 (−1 s−n /cm2 )
n1
Rt ( cm2 )
Q2 − Y0 (−1 s−n /cm2 )
n2
50.6 42.78 33.27 33.97
2.366 × 10−4
0.3546 0.4099 0.4455 0.4986
688.9 741.6 608.4 444.6
0.001384 0.001173 0.008900 0.001178
0.6048 0.6241 0.5700 0.5735
2.069 × 10−4 1.224 × 10−4 9.591 × 10−5
3.5. Evaluation criterion of the coatings
Fig. 10. Polarizing curves of the coated samples produced under different frequencies in 3.5% NaCl.
30, 120 to 240 Hz in sequence. The value of Rt of 240 Hz is only 444.6 cm2 . Besides, the polarizing curves of the coated samples under different frequencies in 3.5% NaCl is shown in Fig. 10, with the corrosion current density and the corrosion potential of the coated samples presented in Table 4. According to the corrosion current density of the coated samples, the corrosion resistance of the coated sample is decreased from 60, 30, 120 to 240 Hz in sequence. The changes of Rt completely comply with the changes of the corrosion resistance of the coated samples; and therefore, Rt reflects the corrosion resistance of the coated samples. Besides, the value of Q1 − n1 of the coatings is increased with the increasing working frequency, as shown in Table 3. According to the formula (1), the fracture dimension (DF ) of the coating surface is increased consequently, which means that the surface roughness of the coatings is decreased. This is also consistent with the changes of the surface SEM of the coatings under different frequencies in Fig. 5. Therefore, the surface roughness of the coatings can be reflected by the changes of the value of Q1 − n1 .
3.5.1. Criterion of the corrosion resistance Generally, the corrosion resistance is related to the thickness and the density of coatings, and the composition as well. Because all the coatings prepared in the experiments are similar in the composition, the key factors would then be attributed to the thickness and the density. If the coating is thicker and denser, the corrosion resistance of the coated sample will become better. Besides, based on the foregoing analyses, we can find the changes of the electric charge transfer resistance (Rt ) in the “equivalent circuit” are consistent with the changes of the corrosion current density. Therefore, the corrosion resistance could be evaluated not only by the corrosion current density but also by the electric charge transfer resistance (Rt ). Besides, corrosion current density is comparatively less accurate to evaluate the coating performance since it is being measured in the dynamic polarization experiment that imposes many changes on the coating, compared to the EIS technique. Instead, Rt is usually considered as more appropriate factor for evaluating the coating resistance once it is modeled in an accurate equivalent circuit model. Therefore, Rt can be considered as a criterion of the corrosion resistance for the coated samples of the same kind. 3.5.2. Criterion of the surface roughness Surface roughness is an important property for the MPO ceramic coatings in its present and potential applications. The fracture dimension (DF ) is a physics variable corresponding to the surface roughness, however, it is difficult to calculate it because it needs a lot of knowledge about mathematics models and theoretical calculation. Similarly, it can be noted from the above analysis that Q1 − n1 is related to the fracture dimension, i.e. DF can be calculated through the value of Q1 − n1 approximately. In other words; the surface roughness of the coated samples can be evaluated by the change of the value of n1 consequently. Therefore, Q1 − n1 can be selected as a criterion of the surface roughness of coated samples of the same kind. 4. Conclusions
Table 4 Corrosion current density and corrosion potential of the coated samples produced under different frequencies Frequency (Hz)
Corrosion current density (×10−6 A/cm2 )
Corrosion potential (V)
30 60 120 240
6.224 1.879 7.242 10.30
0.087 0.047 −0.025 −0.059
Ceramic coatings on Ti alloy were prepared under different electric parameters in NaAlO2 solution by bi-polar pulsed micro-plasma oxidation. The structure, composition and EIS spectra of the coatings were investigated and the following conclusions can be drawn: (1) The ceramic coating is composed of a large amount of Al2 TiO5 and a little ␣-Al2 O3 and rutile TiO2 . The coating is
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of double layer structure with the loose outer layer and the dense inner layer. The coating thickness is reduced when the working frequency or the cathode pulse current density is increased, while the thickness is increased when the working frequency or the anode current density is increased. Furthermore, the surface roughness of the coating is decreased with the increasing frequency. (2) Through the analysis of EIS spectra of the MPO coatings, the corrosion resistance of the coated samples is related to Rt in the “equivalent circuit”: the bigger the value of Rt , the better is the corrosion resistance. Therefore, the electric charge transfer resistance (Rt ) can be considered as a criterion of the corrosion resistance. (3) The surface roughness of the coating is related to the empirical exponent (n1 ) of the constant phase elements (Q1 ) in the “equivalent circuit”: the surface roughness of the coatings is increased with the decrease of n1 . Therefore, n1 can be as a criterion of the surface roughness of coated samples of the same kind. Acknowledgements This work was financially supported by National natural science foundation of China (Grant No. 50171026) and Harbin Special Foundation of Fellow Creation for Science and Technology of China (Grant No. 2006RFQXG032).
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