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The influence of overpotential on electrodeposited aluminum coating onto zincated ZM5 magnesium alloy in ionic liquid
Materials Letters 258 (2020) 126814
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
The influence of overpotential on electrodeposited aluminum coating onto zincated ZM5 magnesium alloy in ionic liquid Liman Chen a, Guixiang Wang b, Yandong He b, Yanli Wang b, Meng Zhang a,⇑ a b
Fundamental Science on Nuclear Safety and Simulation Technology Laboratory, China Laboratory of Superlight Materials and Surface Technology, Ministry of Education, Harbin Engineering University, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 5 July 2019 Received in revised form 5 August 2019 Accepted 12 October 2019 Available online 16 October 2019 Keywords: ZM5 magnesium alloy Aluminum Electrodeposition Overpotential Ionic liquid
a b s t r a c t Aluminum electrodeposited in ionic liquid using current pulse electrodeposition was used as a protective layer on the surface of magnesium alloy. In the process of electrodepositing aluminum, the overpotential greatly affects the morphology and size of the aluminum coating. The relationship between overpotential and the morphologies of aluminum grains was consistent with the classical nucleation theory, showing that the aluminum nuclei size was proportional to the inverse of overpotential. Moreover, it can be found that temperature and current density are two main factors affecting overpotential. The results showed that small grain sizes and well-adhered deposits were achieved at 20 °C and 15 mA/cm2. Ó 2019 Elsevier B.V. All rights reserved.
1. Introduction Mg alloys as lightest structural metallic materials have some beneficial properties that make it an excellent material for many fields, including aerospace and automobile products. However, a major cause restricting their use stems from their poor corrosion resistance because of their loose oxide film [1]. Owing to the good corrosion resistance property of Al coatings, they have attracted much attention as a potential alternative to substitute toxic chromium for electrodeposited on Mg alloys [2]. Compared with other electrolytes, ionic liquid is the appropriate alternative for electrodeposition Al at ambient temperatures [3]. In order to remove the oxide film on the surface of Mg and prevent its regeneration, zincated coating on the Mg base is applied. Several studies have reported the electrodepositing of Al on Mg or on zincated Mg alloy in ionic liquid [4,5]. By comparison, pulse electrodeposition has the capability to suppress metal ion depletion near the electrode and further to control the number density as well as the particle size [6]. Obviously, the mechanism governing the nucleation and growth of Al on zincated Mg alloy in ionic liquid has not been investigated so far. Thus, it is necessary to investigate the influence of overpotential at the stage of nucleation and the mechanism of growth Al coating. In particular, an in-depth mathematical analysis ⇑ Corresponding author. E-mail address: [email protected] (M. Zhang). https://doi.org/10.1016/j.matlet.2019.126814 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.
was provided to give a more straightforward explanation for the influence of overpotential. 2. Experimental section The method of pretreating ZM5 Mg alloy was shown in supporting information (Table 1). Al sheet was first cleaned with 50% phosphoric acid. Thereafter, the sheet was rinsed with ultrapure water and dried with cold N2 flow. All electrochemical experiments were carried out using a potentiostat. The Al plate was used as the counter electrode, an Al wire served as reference electrode, and the zinc-coated specimen as working electrode. Al depositions were performed using a pulse power in a glove box (frequency f = 1000 HZ, duty ratio rc = 20%). A zinc-coated sample was acted as cathode substrate and a high-purity Al plate was anode. The adhesion of the aluminum coating to the substrate was tested by a pull-off adhesion tester. The details of characterization devices and material preparation were displayed in supporting information. 3. Results and discussion The morphologies of ZM5 specimens after the pickling treatment and zincate treatment are shown in Fig. 1. Pickling helps to remove the oxide on the surface of Mg alloys. Because the content of Mg in a phase is higher (92.79% wt.), it is more active than that in the Al-rich b phase (70.14% wt.). Therefore, in the acid pickling
2
L. Chen et al. / Materials Letters 258 (2020) 126814
Fig. 1. SEM images and EDS spectra of the ZM5 specimens after (a) pickling and (b) zincate treatment.
process, a phase will dissolve preferentially. In order to improve the uniformity and compactness of the Al layer, Zn pretreatment is applied after pickling. It illuminates that Zn particles are rectangular nanosheets which grow into clusters in three-dimensional growth mode and cover the surface of Mg matrix tightly. The entire surface of the Mg alloy is completely covered by a dense and uniform Zn layer, regardless of a or b phase region (94.65 or 98.14% wt.). SEM images of zincated Mg after the Al plating reactions at different current densities at 45 °C are shown in Fig. 2. At low current densities of 3 and 6 mA/cm2 (Fig. 2a,b), Al particles loosely distributed with cracks among them possibly because the size of Al nuclei on the surface is large and cannot afford to the formation of dense Al deposition layer. With the increase of current density (Fig. 2c-e), the crystallizations of the coating tend to be fine and uniform, which are closely packed together. Therefore, the deposited layer becomes compact and smooth, and the underlying bare Mg substrate is nearly unable to be seen. EDS shows that as the current density increases from 3 to 13 mA/cm2, the content of Al increases from 97.39 to 99.48%wt in supporting information (Table 2). Furthermore, better adhesion strength is always obtained with the condition of high current densities which result in smoother and denser surface (Fig. 2f). In conclusion, the increasing of the current densities brings about both the increasing of the coverage ratio and adhesive strength and the decreasing of the electrodeposited Al size. This finding can be explained by the chronopotentiometry. During the electrodeposition, overpotential is the parameter of major
importance in providing the driving force to the nucleationgrowth process. The nucleation overpotential (gn) and plateau overpotential (gp) are important parameters describing nucleation and growth. It can be observed in the chronopotentiometric curve that at the initial stage, the potential decreases to a peak value, which is the nucleation stage. Later, with the rise of potential, the driving force of nucleation is insufficient, and it turns to growth. Because the energy needed for growth is lower than that needed for nucleation. Fig. 2(g) shows chronopotentiometric curves at different current densities on the zincated ZM5 specimens. Assuming a homogeneous nucleation, classical equations can be used to evaluate the effect of an overpotential on the grain size of the electrodeposited aluminum [7]. In the process of electrochemical nucleation, the Gibbs energy (DGnucleation) for forming a spherical nuclei of radius r is the sum of its bulk free energy and surface free energy as shown in Eq. (1):
DGnucleation ¼ 4=3pr 3 DGv þ 4pr 2 s
ð1Þ
where DGv is the free energy change per volume and r is the surface energy of the Al-electrolyte interface. The nucleation overpotential, gn, is related to DGv by Eq. (2)
DGv ¼ 3F jgn j=V m
ð2Þ
where F is Faraday’s constant and Vm is the molar volume of Al. The critical radius can be derived from the equation dDGnucleation/dr = 0
rcrit ¼ 2sV m =3Fjhn j
ð3Þ
L. Chen et al. / Materials Letters 258 (2020) 126814
3
Fig. 2. SEM images of the zincated ZM5 specimens after Al electrodeposition at (a)-3, (b)-6, (c)-8, (d)-10, (e)-13 mA/cm2, (f) adhesive strength of the Al coatings, and (g) chronopotentiometric curves.
The presented formalism shows that the nucleation rate is directly proportional to the overpotential, and therefore inversely affecting the final Al grain size. This conclusion is in good agreement with the experimental observations of Fig. 2. SEM micrographs showing the Al electrodeposits on the zincated Mg substrate at different temperatures for 4000 s at 15 mA/cm2 are depicted in Fig. 3. The diameters of the products raise from around 6 to 60 lm with the temperature increasing from 20 to 90 °C. These results suggest that lower temperatures significantly reduce grain coalescence and decrease deposit porosity. To further analyze the influence of temperature, chronopotentiometry curve was used to understand the process of Al nucleation and growth shown in Fig. 3e. From the chronopotentiometry curves, it can be seen that the overpotential of cathode decreases
with the increase of temperature. This is in accordance with the literature [8]. Fig. 3(f) shows the relationship between temperature and adhesive strength of Al coatings. Evidently, the adhesive strength decreases with increase of temperature. It may be that the increasing of the temperature gives rise to less negative overpotential, which increases the grain size of Al and reduces the adhesive strength. Fig. 4 illustrates the process of pretreatment and how the higher nucleation overpotential can limit the size of Al nuclei deposited on zinc-dipped ZM5 specimens. Specimens are first pickled in an acidic solution to remove the oxide and then treated by zincate pretreatment. Subsequently, both a and b phase are covered by homogeneous Zn deposits. The next step is electrodeposited Al. In the high nucleation overpotential case, small size of Al nuclei
4
L. Chen et al. / Materials Letters 258 (2020) 126814
Fig. 3. SEM images of the zincated ZM5 specimens after Al electrodeposition at (a) 20 °C, (b) 45 °C, (c) 70 °C, (d) 90 °C, (f) chronopotentiometric curves, and (g) adhesive strength of the Al coatings.
Fig. 4. Schematic illustrating the size of Al nuclei deposited on zincated ZM5 specimens.
L. Chen et al. / Materials Letters 258 (2020) 126814
reduces the number of cracks and porosities but increases coverage rate and adhesive strength, resulting in more Al nuclei deposited at substrate.
5
Acknowledgement Supported by the National Natural Science Foundation of China (21876035).
4. Conclusions Appendix A. Supplementary data Zn dipping on Mg surface can eliminate the composition difference between a phase and b phase, which is convenient to obtain uniform Al coating. In the process of electrodepositing Al, overpotential is the decisive factor for the change of morphology and size of electrodeposited Al coating in ionic liquid. It was demonstrated that the Al nuclei size was proportional to the inverse of overpotential. Results reveal that high current density and low temperature can be expected to generate a smooth Al coating with excellent adhesion strength. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Supplementary data to this article can be found online at https://doi.org/10.1016/j.matlet.2019.126814. References [1] Z. Gu, Y. Huang, Y. Wang, N. Yuan, J. Ding, Mater. Lett. 252 (2019) 304–307. [2] G.C. Araujo, M.C. Rangel, Stud. Surf. Sci. Catal. 130 (2000) 1601–1606. [3] J. Wang, Z. Meng, X. Xu, F. Jing, Y. Wang, M. Zhang, H. Wei, Y. Chen, G. Tian, Chem. Res. Chinese Univ. 34 (2018) 8–12. [4] J. Tang, K. Azumi, Surf. Coat. Technol. 208 (2012) 1–6. [5] M. Ueda, T. Ohtsuka, J. Solid State Electrochem. 16 (2012) 3423–3427. [6] S. Li, H. Chen, J. Liu, Y. Deng, X. Han, W. Hu, C. Zhong, ACS Appl. Mater. Interfaces 9 (2017) 27765–27772. [7] A. Pei, G. Zheng, F. Shi, Y. Li, Y. Cui, Nano Lett. 17 (2017) 1132. [8] L. Bing, C. Fan, C. Yan, J. Lou, L. Yan, Electrochim. Acta 56 (2011) 5478–5482.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
The influence of overpotential on electrodeposited aluminum coating onto zincated ZM5 magnesium alloy in ionic liquid Liman Chen a, Guixiang Wang b, Yandong He b, Yanli Wang b, Meng Zhang a,⇑ a b
Fundamental Science on Nuclear Safety and Simulation Technology Laboratory, China Laboratory of Superlight Materials and Surface Technology, Ministry of Education, Harbin Engineering University, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 5 July 2019 Received in revised form 5 August 2019 Accepted 12 October 2019 Available online 16 October 2019 Keywords: ZM5 magnesium alloy Aluminum Electrodeposition Overpotential Ionic liquid
a b s t r a c t Aluminum electrodeposited in ionic liquid using current pulse electrodeposition was used as a protective layer on the surface of magnesium alloy. In the process of electrodepositing aluminum, the overpotential greatly affects the morphology and size of the aluminum coating. The relationship between overpotential and the morphologies of aluminum grains was consistent with the classical nucleation theory, showing that the aluminum nuclei size was proportional to the inverse of overpotential. Moreover, it can be found that temperature and current density are two main factors affecting overpotential. The results showed that small grain sizes and well-adhered deposits were achieved at 20 °C and 15 mA/cm2. Ó 2019 Elsevier B.V. All rights reserved.
1. Introduction Mg alloys as lightest structural metallic materials have some beneficial properties that make it an excellent material for many fields, including aerospace and automobile products. However, a major cause restricting their use stems from their poor corrosion resistance because of their loose oxide film [1]. Owing to the good corrosion resistance property of Al coatings, they have attracted much attention as a potential alternative to substitute toxic chromium for electrodeposited on Mg alloys [2]. Compared with other electrolytes, ionic liquid is the appropriate alternative for electrodeposition Al at ambient temperatures [3]. In order to remove the oxide film on the surface of Mg and prevent its regeneration, zincated coating on the Mg base is applied. Several studies have reported the electrodepositing of Al on Mg or on zincated Mg alloy in ionic liquid [4,5]. By comparison, pulse electrodeposition has the capability to suppress metal ion depletion near the electrode and further to control the number density as well as the particle size [6]. Obviously, the mechanism governing the nucleation and growth of Al on zincated Mg alloy in ionic liquid has not been investigated so far. Thus, it is necessary to investigate the influence of overpotential at the stage of nucleation and the mechanism of growth Al coating. In particular, an in-depth mathematical analysis ⇑ Corresponding author. E-mail address: [email protected] (M. Zhang). https://doi.org/10.1016/j.matlet.2019.126814 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.
was provided to give a more straightforward explanation for the influence of overpotential. 2. Experimental section The method of pretreating ZM5 Mg alloy was shown in supporting information (Table 1). Al sheet was first cleaned with 50% phosphoric acid. Thereafter, the sheet was rinsed with ultrapure water and dried with cold N2 flow. All electrochemical experiments were carried out using a potentiostat. The Al plate was used as the counter electrode, an Al wire served as reference electrode, and the zinc-coated specimen as working electrode. Al depositions were performed using a pulse power in a glove box (frequency f = 1000 HZ, duty ratio rc = 20%). A zinc-coated sample was acted as cathode substrate and a high-purity Al plate was anode. The adhesion of the aluminum coating to the substrate was tested by a pull-off adhesion tester. The details of characterization devices and material preparation were displayed in supporting information. 3. Results and discussion The morphologies of ZM5 specimens after the pickling treatment and zincate treatment are shown in Fig. 1. Pickling helps to remove the oxide on the surface of Mg alloys. Because the content of Mg in a phase is higher (92.79% wt.), it is more active than that in the Al-rich b phase (70.14% wt.). Therefore, in the acid pickling
2
L. Chen et al. / Materials Letters 258 (2020) 126814
Fig. 1. SEM images and EDS spectra of the ZM5 specimens after (a) pickling and (b) zincate treatment.
process, a phase will dissolve preferentially. In order to improve the uniformity and compactness of the Al layer, Zn pretreatment is applied after pickling. It illuminates that Zn particles are rectangular nanosheets which grow into clusters in three-dimensional growth mode and cover the surface of Mg matrix tightly. The entire surface of the Mg alloy is completely covered by a dense and uniform Zn layer, regardless of a or b phase region (94.65 or 98.14% wt.). SEM images of zincated Mg after the Al plating reactions at different current densities at 45 °C are shown in Fig. 2. At low current densities of 3 and 6 mA/cm2 (Fig. 2a,b), Al particles loosely distributed with cracks among them possibly because the size of Al nuclei on the surface is large and cannot afford to the formation of dense Al deposition layer. With the increase of current density (Fig. 2c-e), the crystallizations of the coating tend to be fine and uniform, which are closely packed together. Therefore, the deposited layer becomes compact and smooth, and the underlying bare Mg substrate is nearly unable to be seen. EDS shows that as the current density increases from 3 to 13 mA/cm2, the content of Al increases from 97.39 to 99.48%wt in supporting information (Table 2). Furthermore, better adhesion strength is always obtained with the condition of high current densities which result in smoother and denser surface (Fig. 2f). In conclusion, the increasing of the current densities brings about both the increasing of the coverage ratio and adhesive strength and the decreasing of the electrodeposited Al size. This finding can be explained by the chronopotentiometry. During the electrodeposition, overpotential is the parameter of major
importance in providing the driving force to the nucleationgrowth process. The nucleation overpotential (gn) and plateau overpotential (gp) are important parameters describing nucleation and growth. It can be observed in the chronopotentiometric curve that at the initial stage, the potential decreases to a peak value, which is the nucleation stage. Later, with the rise of potential, the driving force of nucleation is insufficient, and it turns to growth. Because the energy needed for growth is lower than that needed for nucleation. Fig. 2(g) shows chronopotentiometric curves at different current densities on the zincated ZM5 specimens. Assuming a homogeneous nucleation, classical equations can be used to evaluate the effect of an overpotential on the grain size of the electrodeposited aluminum [7]. In the process of electrochemical nucleation, the Gibbs energy (DGnucleation) for forming a spherical nuclei of radius r is the sum of its bulk free energy and surface free energy as shown in Eq. (1):
DGnucleation ¼ 4=3pr 3 DGv þ 4pr 2 s
ð1Þ
where DGv is the free energy change per volume and r is the surface energy of the Al-electrolyte interface. The nucleation overpotential, gn, is related to DGv by Eq. (2)
DGv ¼ 3F jgn j=V m
ð2Þ
where F is Faraday’s constant and Vm is the molar volume of Al. The critical radius can be derived from the equation dDGnucleation/dr = 0
rcrit ¼ 2sV m =3Fjhn j
ð3Þ
L. Chen et al. / Materials Letters 258 (2020) 126814
3
Fig. 2. SEM images of the zincated ZM5 specimens after Al electrodeposition at (a)-3, (b)-6, (c)-8, (d)-10, (e)-13 mA/cm2, (f) adhesive strength of the Al coatings, and (g) chronopotentiometric curves.
The presented formalism shows that the nucleation rate is directly proportional to the overpotential, and therefore inversely affecting the final Al grain size. This conclusion is in good agreement with the experimental observations of Fig. 2. SEM micrographs showing the Al electrodeposits on the zincated Mg substrate at different temperatures for 4000 s at 15 mA/cm2 are depicted in Fig. 3. The diameters of the products raise from around 6 to 60 lm with the temperature increasing from 20 to 90 °C. These results suggest that lower temperatures significantly reduce grain coalescence and decrease deposit porosity. To further analyze the influence of temperature, chronopotentiometry curve was used to understand the process of Al nucleation and growth shown in Fig. 3e. From the chronopotentiometry curves, it can be seen that the overpotential of cathode decreases
with the increase of temperature. This is in accordance with the literature [8]. Fig. 3(f) shows the relationship between temperature and adhesive strength of Al coatings. Evidently, the adhesive strength decreases with increase of temperature. It may be that the increasing of the temperature gives rise to less negative overpotential, which increases the grain size of Al and reduces the adhesive strength. Fig. 4 illustrates the process of pretreatment and how the higher nucleation overpotential can limit the size of Al nuclei deposited on zinc-dipped ZM5 specimens. Specimens are first pickled in an acidic solution to remove the oxide and then treated by zincate pretreatment. Subsequently, both a and b phase are covered by homogeneous Zn deposits. The next step is electrodeposited Al. In the high nucleation overpotential case, small size of Al nuclei
4
L. Chen et al. / Materials Letters 258 (2020) 126814
Fig. 3. SEM images of the zincated ZM5 specimens after Al electrodeposition at (a) 20 °C, (b) 45 °C, (c) 70 °C, (d) 90 °C, (f) chronopotentiometric curves, and (g) adhesive strength of the Al coatings.
Fig. 4. Schematic illustrating the size of Al nuclei deposited on zincated ZM5 specimens.
L. Chen et al. / Materials Letters 258 (2020) 126814
reduces the number of cracks and porosities but increases coverage rate and adhesive strength, resulting in more Al nuclei deposited at substrate.
5
Acknowledgement Supported by the National Natural Science Foundation of China (21876035).
4. Conclusions Appendix A. Supplementary data Zn dipping on Mg surface can eliminate the composition difference between a phase and b phase, which is convenient to obtain uniform Al coating. In the process of electrodepositing Al, overpotential is the decisive factor for the change of morphology and size of electrodeposited Al coating in ionic liquid. It was demonstrated that the Al nuclei size was proportional to the inverse of overpotential. Results reveal that high current density and low temperature can be expected to generate a smooth Al coating with excellent adhesion strength. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Supplementary data to this article can be found online at https://doi.org/10.1016/j.matlet.2019.126814. References [1] Z. Gu, Y. Huang, Y. Wang, N. Yuan, J. Ding, Mater. Lett. 252 (2019) 304–307. [2] G.C. Araujo, M.C. Rangel, Stud. Surf. Sci. Catal. 130 (2000) 1601–1606. [3] J. Wang, Z. Meng, X. Xu, F. Jing, Y. Wang, M. Zhang, H. Wei, Y. Chen, G. Tian, Chem. Res. Chinese Univ. 34 (2018) 8–12. [4] J. Tang, K. Azumi, Surf. Coat. Technol. 208 (2012) 1–6. [5] M. Ueda, T. Ohtsuka, J. Solid State Electrochem. 16 (2012) 3423–3427. [6] S. Li, H. Chen, J. Liu, Y. Deng, X. Han, W. Hu, C. Zhong, ACS Appl. Mater. Interfaces 9 (2017) 27765–27772. [7] A. Pei, G. Zheng, F. Shi, Y. Li, Y. Cui, Nano Lett. 17 (2017) 1132. [8] L. Bing, C. Fan, C. Yan, J. Lou, L. Yan, Electrochim. Acta 56 (2011) 5478–5482.