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A study of direct- and pulse-current chromium electroplating on rotating cylinder electrode (RCE)
Applied Surface Science 253 (2007) 6829–6834 www.elsevier.com/locate/apsusc
A study of direct- and pulse-current chromium electroplating on rotating cylinder electrode (RCE) J.H. Chang a, F.Y. Hsu b, M.J. Liao a, C.A. Huang a,* b
a Department of Mechanical Engineering, Chang Gung University, Taoyuan 333, Taiwan Department of Materials Engineering, Ming Chi University of Technology, Taipei 243, Taiwan
Received 12 August 2005; received in revised form 26 January 2007; accepted 27 January 2007 Available online 8 February 2007
Abstract Direct- and pulse-current (DC and PC) chromium electroplating on Cr–Mo steel were performed in a sulfate-catalyzed chromic acid solution at 50 8C using a rotating cylinder electrode (RCE). The electroplating cathodic current densities were at 30, 40, 50 and 60 A dm2, respectively. The relationship between electroplating current efficiency and the rotating speed of the RCE was studied. The cross-sectional microstructure of Crdeposit was examined by transmission electron microscope (TEM). Results showed that DC-plating exhibited higher current efficiency than the PC-plating under the same conditions of electroplating current density and the rotating speed. We found the critical rotating speed of RCE used in the chromium electroplating, above this rotating speed the chromium deposition is prohibited. At the same plating current density, the critical rotating speed for DC-plating was higher than that for PC-plating. The higher plating current density is, the larger difference in critical rotating speeds appears between DC- and PC-electroplating. Equiaxed grains, in a nanoscale size with lower dislocation density, nucleate on the cathodic surface in both DC- and PC-electroplating. Adjacent to the equiaxed grains, textured grains were found in other portion of chromium deposit. Fine columnar grains were observed in the DC-electroplated deposit. On the other hand, very long slender grains with high degree of preferred orientation were detected in PC-electroplated deposit. # 2007 Elsevier B.V. All rights reserved. Keywords: Chromium electroplating; Rotating speed; RCE; TEM microstructure
1. Introduction The technology of the chromium plating with direct-current (DC) has been developed and utilized for decades [1]. In recent years, the Cr-plating with pulse-current (PC) was widely employed due to its better relief of residual stress, higher coulombic efficiency, and higher throwing power for electroplating [2–6]. Despite the above-mentioned advantages, the selection of optimal operating parameters, such as the applied frequency, the pulse duration, additives, and the controlled duty cycle, is a rather complex procedure in DC-plating. Different crystal structures (hcp, bcc and amorphous) and properties of the Cr-deposit can also be developed when different operating parameters are selected [2,7,8]. Leisner et al. [5] show that the current efficiency of the PC-chromium plating was strongly
* Corresponding author. Tel.: +886 3 2118800x5346; fax: +886-3-2118050. E-mail address: [email protected] (C.A. Huang). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.01.124
affected by the anodic and cathodic charge ratio (Qa/Qc). The optimal Qa/Qc value lies between 0.0020 and 0.0085. The current efficiency of DC- and PC-chromium plating is usually lower than 25% in the sulfate-catalyzed acid bath [1]. Hoare [10] used the rotating disc electrode (RDE) to study chromium plating and showed that a rapid flow of the electrolyte over the cathode surface was essential for highspeed chromium plating due to the reduction of the diffusion path in a diffusion-controlled electrode reaction. It is known that a constant flow velocity on the plating surface can be achieved using the rotating cylinder electrode (RCE). A study with RCE is thus expected to contribute a better understanding for the effect of the flow velocity on chromium plating. The cross-sectional microstructure of the chromium deposit has been studied with optical microscope and scanning electron microscope (SEM) [11,12]. The results show that the grain size of as-plated chromium deposit was so fine that the grain size is not available except that the deposit was subsequently annealed at elevated temperatures. The TEM micrograph of the chromium
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Table 1 Chemical composition of Cr–Mo steel Element
wt.%
Fe Cr Mo C Mn V Si S P
Bal. 1.2 1.1 0.22 0.4 0.3 0.4 <0.035 <0.035
deposit was obtained from a thin foil prepared by jet polishing [2]. Lee [13] prepared the cross-sectional specimens of annealed Cr–C deposits with TEM and found the strengthening mechanism of Cr–C deposits owing to precipitation of chromium carbides. However, few literatures have been reported regarding the interfacial microstructure between the Cr-deposit and the steel substrate. Furthermore, little work has been done on the current efficiency of chromium deposit plated at different rotating speeds with RCE, by which a uniform mass transport over the electrode surface can be obtained. In this paper the aforementioned aspects are addressed systematically. 2. Experimental procedure The electroplating substrate was cold-swaged Cr–Mo steel in this study. Table 1 shows the chemical composition of the steel. The substrate was machined in a cylinder form with a diameter of 16 mm and a length of 5 mm. The cylinder was adopted to be the rotating cylinder electrode (RCE). The RCE specimen with an exposing area of 2.51 cm2 was used as a cathode. A platinized titanium mesh with a dimension of 25 mm 35 mm was used as an anode. All chromium plating was performed at 50 8C in a sulfate-catalyzed chromic acid bath containing 250 g L1 of CrO3 and 2.5 g L1 of H2SO4. The cathodic current densities chosen for DC-plating were 30, 40,
Fig. 1. Schematic representation of the parameters for PC-electroplating with rectangular current waveform.
50 and 60 A dm2, respectively. In PC-electroplating, a rectangular current waveform with cathodic current densities the same as those for the DC-plating was employed and the anodic current density was fixed at 8 A dm2. Based on the Leisner’s research on the optimal current efficiency of PCelectroplating in the industrial application, the cathodic and anodic periods of the pulse cycle were set as 250 and 1 ms, respectively [5,9]. Fig. 1 shows schematic representation of the parameters for PC-electroplating with rectangular current waveform. The rotating speeds of RCE were controlled in the range of 5–3000 rpm in the EG&G RDE system (EG&G Models 636 and 616). Before electroplating, the RCE specimens were anodically polarized at 40 A dm2, 200 rpm for 30 s. The current efficiency of DC-electroplating was evaluated with three RCE specimens at a constant cathodic charge of 1.33 Ah. The weight increment of the chromium deposit was measured and compared with the theoretical weight increment according to Faraday’s law. The same calculation method was also employed
Fig. 2. The preparation process of a cross-sectional TEM specimen by using ion-milling method.
J.H. Chang et al. / Applied Surface Science 253 (2007) 6829–6834
for that of PC-eletroplating, where the total amount of electroplating charge was the difference between total cathodic charge and total anodic charge. It means that the total plating charge was 1.33 Ah for PC-electroplating. After electroplating, the RCE specimen was rinsed in acetone, dried with hot air, and then prepared for current efficiency evaluation as well as microstructure examination. The microstructure study was made with TEM (JEOL 2010). As shown in Fig. 2, crosssectional TEM specimens were prepared as follows: two pieces of Cr-deposit were sectioned with diamond saw in a dimension of 1 mm thick, 1.5 mm long and 1 mm wide, glued face to face with M-Bond 610, and mounted vertically in a copper ring 3 mm in diameter with G 1 epoxy (Gatan company) mixed with BaTiO3. The specimen was then mechanically ground into a disc form with a thickness of ca. 100 mm. The middle of disc specimen was further mechanically dimpled to a thickness of ca. 10 mm by using Dimpler (VCR, D 500i). Finally, a low angle (88) Ar+-ion beam (VCR, XLA 2000) at 5 kV was used to mill the specimen until a tiny hole was produced in the adhered chromium deposit, around which the deposit was so thin that made TEM image observation and electron diffraction feasible. During ion milling, liquid nitrogen was used to cool the specimen stand to minimize the heating effect on the microstructure. Details of the preparation procedure were reported in literature [14]. 3. Results and discussion 3.1. Current efficiency and critical rotating speed The relationship between the current efficiency and the rotating speed of RCE for varying densities of the applied cathodic current is shown in Fig. 3. The results clearly demonstrate that the current efficiency increases with increasing the current density for both DC- and the PC-electroplating. Under the same applied cathodic current density and the rotating speed, the current efficiency of the DC-electroplating is obviously higher than that of the PC-electroplating. In the DCelectroplating the maximum current efficiency is 27% at 60 A dm2, 1700 rpm; whereas in the PC-electroplating the
Fig. 3. The relationship between current efficiency and the rotating speed of RCE.
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maximum current efficiency is only 23% at 60 A dm2, 700 rpm. According to the results from Fig. 3, there exists a critical rotating speed, above which the reduction from chromium ion to metallic chromium is fully prohibited and hydrogen bubbles can be visually observed on the RCE surface. It can be clearly seen that a higher current density results in a higher critical rotating speed in both DC- and PC-electroplating. However, the critical rotating speed in the DC-electroplating is higher than that in the PC-electroplating, when the same current density is applied. The difference in the critical rotating speed between DC- and PC-electroplating depends strongly on the electroplating current density, the higher current density and the larger difference. For example, at the current density of 30 A dm2, the critical rotating speed for DC- and PC-electroplating is nearly the same (at 200 rpm). Nevertheless, at 60 A dm2 the critical speed is 2000 rpm for DC-electroplating, but only 1000 rpm for PC-electroplating (see Fig. 3). From above-mentioned results, the current efficiency of Crdeposit with DC-electroplating is higher than that with PCelectroplating under the same plating current density. Lower current efficiency of Cr-deposit with PC-electroplating could be attributed to the effect of surface double layer charging. In the plating process, the applied cathodic current charges the interface first between the electrode and the electrolyte, and then the reduction of cations occurs. The charging current, or called nonfaradaic current, acts only as a capacitance for electrical double layer and do not provide the reduction of any cation from the electrolyte on the electrode. Thus the reduction of chromium ion on the cathode does not take place with this charging current. In this study, a rectangular current waveform was utilized in the PC-electroplating. The interfacial charging current was required by each pulse-current cycle, therefore, leading to less current efficiency in the PC-electroplating than that in the DC-electroplating at the same applied plating charge. In this study we confirm that the reduction of chromium ions could be fully excluded in the sulfate-catalyzed chromic electrolyte bath when the rotating speed of RCE is higher than the critical rotating speed. From the viewpoint of metallic chromium electrocrystallization, it has been accepted for a long time that a viscous cathodic film and the presence of SO42 are needed to prevent Cr3+ forming a stable complex with water, Cr(H2O)63+, from which the reduction from chromium ion to metallic chromium was obstructed [15]. The reaction mechanism among the viscous cathodic film, consisting of SO42 and Cr(H2O)63+, was studied and elucidated by many researchers [16–18]. In this study, the reduction of chromium ion to form metallic deposit is fully prohibited when the rotating speed of RCE is higher the critical rotating speed. Hence, it may suggest properly that the viscous film is not possibly present or the function of SO42 totally disappears under this plating condition. Reduction competition between hydrogen ion and chromium ion could be another reason to explain the occurrence of the nil-current efficiency. In the chromium electroplating in the acid bath, both Cr6+ and H+ cations may be reduced simultaneously to metallic chromium and hydrogen (H2) on the electrode surface at the same time. Since the reduction
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Fig. 4. The interfacial microstructure between chromium deposit and steel substrate under DC-electroplating (TEM cross-sectional examination): (a) 30 A dm2, 100 rpm and (b) 50 A dm2, 350 rpm.
Fig. 5. Texture structure of some area of the chromium deposit with DC-electroplating: (a) bright-field image and (b) dark-field image and its SAD pattern.
potentials of Cr6+/Cr3+ and Cr3+/Cr are more negative than that of H+/Hads, therefore, the reduction from hydrogen ions into hydrogen bubble could be always found during the chromium plating. However, the generally accepted rate-controlling step is that H2 bubbles, the final reduction product, being driven away from the cathodic surface. This would become easier at a higher rotating speed during the chromium plating. Thus, as the rotating speed of RCE reaches a certain value, the H+ reduction reaction would fully dominated, and the reduction of chromium ions into metallic chromium preclude and leads to nil-current efficiency. 3.2. Microstructure examination A typical interfacial microstructure between the Cr–Mo steel substrate and the chromium DC-deposit plated with DC is shown in Fig. 4. The electroplating conditions were at 30 A dm2, 100 rpm and at 50 A dm2, 350 rpm, respectively. According to the grain size and dislocation density shown in Fig. 4, two kinds of the deposit grains are detected
Fig. 6. The interfacial microstructure between chromium deposit and steel substrate under PC-electroplating (TEM cross-sectional examination).
J.H. Chang et al. / Applied Surface Science 253 (2007) 6829–6834
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Fig. 7. Texture structure of the chromium deposit with PC-electroplating: (a) bright-field image and (b) dark-field image and its SAD pattern.
adjacent to the interfaces shown in dashed line. Equiaxed grains, about 200 nm in size, with low dislocation density were observed on the surface of the substrate. These grains are much larger than the neighboring fine columnar grains. It implies that the equiaxed grains nucleated on the steel substrate in the initial electrodeposition stage. As shown in Fig. 5, the fine columnar grains with a dimension about 20 nm 50 nm were found in the deposit elsewhere except on the cathodic surface. The result of the selected area diffraction (SAD) on the fine columnar grains is shown in Fig. 5(b), in which gh1 1 0i a streaking pattern rather than a ring is observed. This implies that the grains have a preferred orientation, i.e. a textured structure. The same results of the grain distribution can be also found in the Cr-deposits plated at 40, 50 and 60 A dm2. Interfacial micrographs between the substrate and Crdeposit with the PC-electroplating are shown in Fig. 6. Contrary to the DC-electroplating, very small equiaxed grains with a size about several nanometers can be found directly from the chromium deposit/steel interface. These fine equiaxed grains grow only one grain thick adjacent to the cathodic steel substrate. Next to the fine equiaxed grains, grains with a long slender shape were developed (see Fig. 7). From the dark field image shown in Fig. 7(b), the dimension of slender grains is about 10 nm wide and 500 nm long. Similar to the DC-deposit, it exhibited a textured structure as shown by SAD pattern (Fig. 7(b)). Despite that little difference exists in SAD patterns shown in Figs. 6 and 7, the textured grains in the PC-deposit were in long slender shape, which is apparently different from the fine columnar grains in the DC-deposit. It implies that the PC-electroplated grains have higher degree of preference than that of DC-electroplated grains. Therefore, in the PCelectrocrystallization of the Cr-deposit, the growth of equiaxed grains is suppressed and the equiaxed grains have a size only a few nanometers. In summary, although the ratio between anodic and cathodic periods was only 1/250 in PC-electroplating cycle and anodic plating current density was performed about one-fourth smaller than that of the cathodic plating current density, there is an
obvious difference in the current efficiency and the critical rotating speed between DC- and PC-electroplating. Moreover, the grain size and grain morphology in the chromium deposit are significantly affected by this short anodic electroplating period. 4. Conclusions The current efficiency of hexavalent chromium electrodeposition with DC and PC depends strongly on the electroplating current density and the rotating speed of the RCE. A critical rotating speed was found, above which the reduction of the chromium ion is fully prohibited, i.e. nil-current efficiency of chromium deposition. The critical rotating speed increases with increasing of the applied cathodic current density. However, the critical rotating speed in the DCelectroplating is higher than that in the PC-electroplating. Owing to the double layer charging effect, the current efficiency of PC-electroplating is obviously lower than that of DC-electroplating under the same electroplating current density and the rotating speed. Microstructures of the DC- and PC-electroplated chromium deposits were investigated with TEM by using a cross-sectional specimen. The results showed that larger equiaxed grains about 250 nm nucleate on the cathodic surface of Cr–Mo steel substrate during initial electrodeposition stage with DCelectroplating, but only very fine equiaxed grains of nanometer scale with PC-electroplating. Adjacent to the equiaxed grains, a texture structure was developed in both DC- and PC-chromium deposits. Unlike the fine columnar grains in DC-electroplated deposit, very long slender-shaped grains with high degree of preferred orientation could be obtained in PC-electroplated deposit. Acknowledgement The authors wish to thank the National Science Council (NSC) of Republic of China (ROC) for the support of this work under contract No. NSC 88-2623-D-182-001.
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G.A. Lausmann, Surf. Coat. Technol. 86/87 (1996) 814. D.-T. Chin, H. Zhang, Electrochim. Acta 31 (1986) 299. J.C. Saiddington, Plating Surf. Finish. (1978) 45. S. Venkatesh, D.T. Chin, J. Electrochem. Soc. 128 (1981) 2588. P. Leisner, G. Bech-Nielsen, P. Møller, J. Appl. Electrochem. 23 (1993) 1232. [6] R.M. Krishnan, S. Sriveeraraghavan, S.R. Natarajan, Metal Finish. (1993) 65. [7] T. Pearson, K. Dennis, Plating Surf. Finish. (1989) 64. [8] R.-Y. Tsai, S.-T. Wu, J. Electrochem. Soc. 138 (1991) 2622.
[9] Engineering Division, Nan-Ya Plastics Corporation, Internal Technology Document, Republic of China, 1999. [10] J.P. Hoare, Plating Surf. Finish. (1985) 56. [11] C.A. Snavely, C.L. Faust, J. Electrochem. Soc. 97 (1950) 99. [12] J. Pina, A. Dias, M. Francois, J. Lebrun, Surf. Coat. Technol. 96 (1997) 148. [13] J.-W. Lee, Appl. Surf. Sci. 244 (2005) 248–251. [14] C.A. Huang, H.-J. Klaar, Y.L. Kao, DGM Metallographie-Tagung, Rostock, Germany, 1999. [15] G.J. Sargent, Trans. Am. Electrochem. Soc. 37 (1920) 479. [16] H.E. Haring, Metal Ind. (London) 27 (1925) 310. [17] E.A. Ollard, J. Electrodeposit. Tech. Soc. 3 (1929) 5. [18] C.A. Knorr, G. Munster, H. Feigl, Z. Elektrochem. 63 (1959) 59.
A study of direct- and pulse-current chromium electroplating on rotating cylinder electrode (RCE) J.H. Chang a, F.Y. Hsu b, M.J. Liao a, C.A. Huang a,* b
a Department of Mechanical Engineering, Chang Gung University, Taoyuan 333, Taiwan Department of Materials Engineering, Ming Chi University of Technology, Taipei 243, Taiwan
Received 12 August 2005; received in revised form 26 January 2007; accepted 27 January 2007 Available online 8 February 2007
Abstract Direct- and pulse-current (DC and PC) chromium electroplating on Cr–Mo steel were performed in a sulfate-catalyzed chromic acid solution at 50 8C using a rotating cylinder electrode (RCE). The electroplating cathodic current densities were at 30, 40, 50 and 60 A dm2, respectively. The relationship between electroplating current efficiency and the rotating speed of the RCE was studied. The cross-sectional microstructure of Crdeposit was examined by transmission electron microscope (TEM). Results showed that DC-plating exhibited higher current efficiency than the PC-plating under the same conditions of electroplating current density and the rotating speed. We found the critical rotating speed of RCE used in the chromium electroplating, above this rotating speed the chromium deposition is prohibited. At the same plating current density, the critical rotating speed for DC-plating was higher than that for PC-plating. The higher plating current density is, the larger difference in critical rotating speeds appears between DC- and PC-electroplating. Equiaxed grains, in a nanoscale size with lower dislocation density, nucleate on the cathodic surface in both DC- and PC-electroplating. Adjacent to the equiaxed grains, textured grains were found in other portion of chromium deposit. Fine columnar grains were observed in the DC-electroplated deposit. On the other hand, very long slender grains with high degree of preferred orientation were detected in PC-electroplated deposit. # 2007 Elsevier B.V. All rights reserved. Keywords: Chromium electroplating; Rotating speed; RCE; TEM microstructure
1. Introduction The technology of the chromium plating with direct-current (DC) has been developed and utilized for decades [1]. In recent years, the Cr-plating with pulse-current (PC) was widely employed due to its better relief of residual stress, higher coulombic efficiency, and higher throwing power for electroplating [2–6]. Despite the above-mentioned advantages, the selection of optimal operating parameters, such as the applied frequency, the pulse duration, additives, and the controlled duty cycle, is a rather complex procedure in DC-plating. Different crystal structures (hcp, bcc and amorphous) and properties of the Cr-deposit can also be developed when different operating parameters are selected [2,7,8]. Leisner et al. [5] show that the current efficiency of the PC-chromium plating was strongly
* Corresponding author. Tel.: +886 3 2118800x5346; fax: +886-3-2118050. E-mail address: [email protected] (C.A. Huang). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.01.124
affected by the anodic and cathodic charge ratio (Qa/Qc). The optimal Qa/Qc value lies between 0.0020 and 0.0085. The current efficiency of DC- and PC-chromium plating is usually lower than 25% in the sulfate-catalyzed acid bath [1]. Hoare [10] used the rotating disc electrode (RDE) to study chromium plating and showed that a rapid flow of the electrolyte over the cathode surface was essential for highspeed chromium plating due to the reduction of the diffusion path in a diffusion-controlled electrode reaction. It is known that a constant flow velocity on the plating surface can be achieved using the rotating cylinder electrode (RCE). A study with RCE is thus expected to contribute a better understanding for the effect of the flow velocity on chromium plating. The cross-sectional microstructure of the chromium deposit has been studied with optical microscope and scanning electron microscope (SEM) [11,12]. The results show that the grain size of as-plated chromium deposit was so fine that the grain size is not available except that the deposit was subsequently annealed at elevated temperatures. The TEM micrograph of the chromium
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Table 1 Chemical composition of Cr–Mo steel Element
wt.%
Fe Cr Mo C Mn V Si S P
Bal. 1.2 1.1 0.22 0.4 0.3 0.4 <0.035 <0.035
deposit was obtained from a thin foil prepared by jet polishing [2]. Lee [13] prepared the cross-sectional specimens of annealed Cr–C deposits with TEM and found the strengthening mechanism of Cr–C deposits owing to precipitation of chromium carbides. However, few literatures have been reported regarding the interfacial microstructure between the Cr-deposit and the steel substrate. Furthermore, little work has been done on the current efficiency of chromium deposit plated at different rotating speeds with RCE, by which a uniform mass transport over the electrode surface can be obtained. In this paper the aforementioned aspects are addressed systematically. 2. Experimental procedure The electroplating substrate was cold-swaged Cr–Mo steel in this study. Table 1 shows the chemical composition of the steel. The substrate was machined in a cylinder form with a diameter of 16 mm and a length of 5 mm. The cylinder was adopted to be the rotating cylinder electrode (RCE). The RCE specimen with an exposing area of 2.51 cm2 was used as a cathode. A platinized titanium mesh with a dimension of 25 mm 35 mm was used as an anode. All chromium plating was performed at 50 8C in a sulfate-catalyzed chromic acid bath containing 250 g L1 of CrO3 and 2.5 g L1 of H2SO4. The cathodic current densities chosen for DC-plating were 30, 40,
Fig. 1. Schematic representation of the parameters for PC-electroplating with rectangular current waveform.
50 and 60 A dm2, respectively. In PC-electroplating, a rectangular current waveform with cathodic current densities the same as those for the DC-plating was employed and the anodic current density was fixed at 8 A dm2. Based on the Leisner’s research on the optimal current efficiency of PCelectroplating in the industrial application, the cathodic and anodic periods of the pulse cycle were set as 250 and 1 ms, respectively [5,9]. Fig. 1 shows schematic representation of the parameters for PC-electroplating with rectangular current waveform. The rotating speeds of RCE were controlled in the range of 5–3000 rpm in the EG&G RDE system (EG&G Models 636 and 616). Before electroplating, the RCE specimens were anodically polarized at 40 A dm2, 200 rpm for 30 s. The current efficiency of DC-electroplating was evaluated with three RCE specimens at a constant cathodic charge of 1.33 Ah. The weight increment of the chromium deposit was measured and compared with the theoretical weight increment according to Faraday’s law. The same calculation method was also employed
Fig. 2. The preparation process of a cross-sectional TEM specimen by using ion-milling method.
J.H. Chang et al. / Applied Surface Science 253 (2007) 6829–6834
for that of PC-eletroplating, where the total amount of electroplating charge was the difference between total cathodic charge and total anodic charge. It means that the total plating charge was 1.33 Ah for PC-electroplating. After electroplating, the RCE specimen was rinsed in acetone, dried with hot air, and then prepared for current efficiency evaluation as well as microstructure examination. The microstructure study was made with TEM (JEOL 2010). As shown in Fig. 2, crosssectional TEM specimens were prepared as follows: two pieces of Cr-deposit were sectioned with diamond saw in a dimension of 1 mm thick, 1.5 mm long and 1 mm wide, glued face to face with M-Bond 610, and mounted vertically in a copper ring 3 mm in diameter with G 1 epoxy (Gatan company) mixed with BaTiO3. The specimen was then mechanically ground into a disc form with a thickness of ca. 100 mm. The middle of disc specimen was further mechanically dimpled to a thickness of ca. 10 mm by using Dimpler (VCR, D 500i). Finally, a low angle (88) Ar+-ion beam (VCR, XLA 2000) at 5 kV was used to mill the specimen until a tiny hole was produced in the adhered chromium deposit, around which the deposit was so thin that made TEM image observation and electron diffraction feasible. During ion milling, liquid nitrogen was used to cool the specimen stand to minimize the heating effect on the microstructure. Details of the preparation procedure were reported in literature [14]. 3. Results and discussion 3.1. Current efficiency and critical rotating speed The relationship between the current efficiency and the rotating speed of RCE for varying densities of the applied cathodic current is shown in Fig. 3. The results clearly demonstrate that the current efficiency increases with increasing the current density for both DC- and the PC-electroplating. Under the same applied cathodic current density and the rotating speed, the current efficiency of the DC-electroplating is obviously higher than that of the PC-electroplating. In the DCelectroplating the maximum current efficiency is 27% at 60 A dm2, 1700 rpm; whereas in the PC-electroplating the
Fig. 3. The relationship between current efficiency and the rotating speed of RCE.
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maximum current efficiency is only 23% at 60 A dm2, 700 rpm. According to the results from Fig. 3, there exists a critical rotating speed, above which the reduction from chromium ion to metallic chromium is fully prohibited and hydrogen bubbles can be visually observed on the RCE surface. It can be clearly seen that a higher current density results in a higher critical rotating speed in both DC- and PC-electroplating. However, the critical rotating speed in the DC-electroplating is higher than that in the PC-electroplating, when the same current density is applied. The difference in the critical rotating speed between DC- and PC-electroplating depends strongly on the electroplating current density, the higher current density and the larger difference. For example, at the current density of 30 A dm2, the critical rotating speed for DC- and PC-electroplating is nearly the same (at 200 rpm). Nevertheless, at 60 A dm2 the critical speed is 2000 rpm for DC-electroplating, but only 1000 rpm for PC-electroplating (see Fig. 3). From above-mentioned results, the current efficiency of Crdeposit with DC-electroplating is higher than that with PCelectroplating under the same plating current density. Lower current efficiency of Cr-deposit with PC-electroplating could be attributed to the effect of surface double layer charging. In the plating process, the applied cathodic current charges the interface first between the electrode and the electrolyte, and then the reduction of cations occurs. The charging current, or called nonfaradaic current, acts only as a capacitance for electrical double layer and do not provide the reduction of any cation from the electrolyte on the electrode. Thus the reduction of chromium ion on the cathode does not take place with this charging current. In this study, a rectangular current waveform was utilized in the PC-electroplating. The interfacial charging current was required by each pulse-current cycle, therefore, leading to less current efficiency in the PC-electroplating than that in the DC-electroplating at the same applied plating charge. In this study we confirm that the reduction of chromium ions could be fully excluded in the sulfate-catalyzed chromic electrolyte bath when the rotating speed of RCE is higher than the critical rotating speed. From the viewpoint of metallic chromium electrocrystallization, it has been accepted for a long time that a viscous cathodic film and the presence of SO42 are needed to prevent Cr3+ forming a stable complex with water, Cr(H2O)63+, from which the reduction from chromium ion to metallic chromium was obstructed [15]. The reaction mechanism among the viscous cathodic film, consisting of SO42 and Cr(H2O)63+, was studied and elucidated by many researchers [16–18]. In this study, the reduction of chromium ion to form metallic deposit is fully prohibited when the rotating speed of RCE is higher the critical rotating speed. Hence, it may suggest properly that the viscous film is not possibly present or the function of SO42 totally disappears under this plating condition. Reduction competition between hydrogen ion and chromium ion could be another reason to explain the occurrence of the nil-current efficiency. In the chromium electroplating in the acid bath, both Cr6+ and H+ cations may be reduced simultaneously to metallic chromium and hydrogen (H2) on the electrode surface at the same time. Since the reduction
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Fig. 4. The interfacial microstructure between chromium deposit and steel substrate under DC-electroplating (TEM cross-sectional examination): (a) 30 A dm2, 100 rpm and (b) 50 A dm2, 350 rpm.
Fig. 5. Texture structure of some area of the chromium deposit with DC-electroplating: (a) bright-field image and (b) dark-field image and its SAD pattern.
potentials of Cr6+/Cr3+ and Cr3+/Cr are more negative than that of H+/Hads, therefore, the reduction from hydrogen ions into hydrogen bubble could be always found during the chromium plating. However, the generally accepted rate-controlling step is that H2 bubbles, the final reduction product, being driven away from the cathodic surface. This would become easier at a higher rotating speed during the chromium plating. Thus, as the rotating speed of RCE reaches a certain value, the H+ reduction reaction would fully dominated, and the reduction of chromium ions into metallic chromium preclude and leads to nil-current efficiency. 3.2. Microstructure examination A typical interfacial microstructure between the Cr–Mo steel substrate and the chromium DC-deposit plated with DC is shown in Fig. 4. The electroplating conditions were at 30 A dm2, 100 rpm and at 50 A dm2, 350 rpm, respectively. According to the grain size and dislocation density shown in Fig. 4, two kinds of the deposit grains are detected
Fig. 6. The interfacial microstructure between chromium deposit and steel substrate under PC-electroplating (TEM cross-sectional examination).
J.H. Chang et al. / Applied Surface Science 253 (2007) 6829–6834
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Fig. 7. Texture structure of the chromium deposit with PC-electroplating: (a) bright-field image and (b) dark-field image and its SAD pattern.
adjacent to the interfaces shown in dashed line. Equiaxed grains, about 200 nm in size, with low dislocation density were observed on the surface of the substrate. These grains are much larger than the neighboring fine columnar grains. It implies that the equiaxed grains nucleated on the steel substrate in the initial electrodeposition stage. As shown in Fig. 5, the fine columnar grains with a dimension about 20 nm 50 nm were found in the deposit elsewhere except on the cathodic surface. The result of the selected area diffraction (SAD) on the fine columnar grains is shown in Fig. 5(b), in which gh1 1 0i a streaking pattern rather than a ring is observed. This implies that the grains have a preferred orientation, i.e. a textured structure. The same results of the grain distribution can be also found in the Cr-deposits plated at 40, 50 and 60 A dm2. Interfacial micrographs between the substrate and Crdeposit with the PC-electroplating are shown in Fig. 6. Contrary to the DC-electroplating, very small equiaxed grains with a size about several nanometers can be found directly from the chromium deposit/steel interface. These fine equiaxed grains grow only one grain thick adjacent to the cathodic steel substrate. Next to the fine equiaxed grains, grains with a long slender shape were developed (see Fig. 7). From the dark field image shown in Fig. 7(b), the dimension of slender grains is about 10 nm wide and 500 nm long. Similar to the DC-deposit, it exhibited a textured structure as shown by SAD pattern (Fig. 7(b)). Despite that little difference exists in SAD patterns shown in Figs. 6 and 7, the textured grains in the PC-deposit were in long slender shape, which is apparently different from the fine columnar grains in the DC-deposit. It implies that the PC-electroplated grains have higher degree of preference than that of DC-electroplated grains. Therefore, in the PCelectrocrystallization of the Cr-deposit, the growth of equiaxed grains is suppressed and the equiaxed grains have a size only a few nanometers. In summary, although the ratio between anodic and cathodic periods was only 1/250 in PC-electroplating cycle and anodic plating current density was performed about one-fourth smaller than that of the cathodic plating current density, there is an
obvious difference in the current efficiency and the critical rotating speed between DC- and PC-electroplating. Moreover, the grain size and grain morphology in the chromium deposit are significantly affected by this short anodic electroplating period. 4. Conclusions The current efficiency of hexavalent chromium electrodeposition with DC and PC depends strongly on the electroplating current density and the rotating speed of the RCE. A critical rotating speed was found, above which the reduction of the chromium ion is fully prohibited, i.e. nil-current efficiency of chromium deposition. The critical rotating speed increases with increasing of the applied cathodic current density. However, the critical rotating speed in the DCelectroplating is higher than that in the PC-electroplating. Owing to the double layer charging effect, the current efficiency of PC-electroplating is obviously lower than that of DC-electroplating under the same electroplating current density and the rotating speed. Microstructures of the DC- and PC-electroplated chromium deposits were investigated with TEM by using a cross-sectional specimen. The results showed that larger equiaxed grains about 250 nm nucleate on the cathodic surface of Cr–Mo steel substrate during initial electrodeposition stage with DCelectroplating, but only very fine equiaxed grains of nanometer scale with PC-electroplating. Adjacent to the equiaxed grains, a texture structure was developed in both DC- and PC-chromium deposits. Unlike the fine columnar grains in DC-electroplated deposit, very long slender-shaped grains with high degree of preferred orientation could be obtained in PC-electroplated deposit. Acknowledgement The authors wish to thank the National Science Council (NSC) of Republic of China (ROC) for the support of this work under contract No. NSC 88-2623-D-182-001.
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