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Electrodeposition of nanocrystalline zinc on steel for enhanced resistance to corrosive wear
Surface & Coatings Technology 304 (2016) 567–573
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Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Electrodeposition of nanocrystalline zinc on steel for enhanced resistance to corrosive wear Qingyang Li a,b, Hao Lu b, Juan Cui b, Maozhong An a,⁎, Dongyang Li b,⁎ a b
State Key Laboratory of Urban Water Resource and Environment, School of Chemical Engineering and Technology, Harbin Institute of Technology, No. 92 West-Da Zhi Street, Harbin 150001, China Dept. of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 1H9, Canada
a r t i c l e
i n f o
Article history: Received 12 May 2016 Revised 18 July 2016 Accepted in revised form 19 July 2016 Available online 20 July 2016 Keywords: Nanocrystalline zinc Electrodeposition Corrosive wear Mechanical property Electron work function
a b s t r a c t In order to increase the resistance of electrogalvanized steel to corrosive wear, nanocrystalline zinc coating was electrodeposited onto the steel substrate using a sulfate bath with polyacrylamide as grain refiner. Corrosive wear tests were performed in a simulated seawater solution to evaluate the performance of the nanocrystalline zinc coating with its grain size around 40 nm, in comparison with that of coarse-grained zinc coating (grain size ~ 5 μm). It was demonstrated that material loss of the coarse-grained zinc coating was 39 times as large as that of the nanocrystalline one. The considerably higher corrosive wear resistance of the nanocrystalline zinc coating largely benefited from its increased mechanical strength due to nanocrystallization and higher surface activity, which improved the passivation capability with the formation of a more protective oxide scale. Detailed analyses were conducted to clarify the mechanism responsible for the improvements. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Due to their more negative standard potential (−0.76 V vs. standard hydrogen electrode SHE) than iron (−0.44 V vs. SHE) [1], zinc coatings are often used to provide sacrificial cathodic protection for steel against corrosion even they are scratched or damaged. The lower cost and larger reserves of zinc, compared to other metals, as well as nontoxic and recyclable properties make it to be the most widely employed surface protective coating. According to the International Zinc Association, more than 5 million tons of zinc per year is used to protect steel against corrosion all over the world, and retrieves about 2.2 trillion USD annually from the capital lost for repair and replacement of corroded steel components [2]. The utilization of zinc coatings continuously increases with the increase in global demand for steel structures and others (such as energy-harvesting and storage cell) [3,4]. This also requires development of more protective zinc coatings and advanced coating processes. Nano-electrodeposition is one of such coating processes. Compared to conventional coarse-grained zinc coating, nanocrystalline zinc coating has demonstrated the following advantages: smoother and brighter surface, higher hardness, stronger corrosion resistance and improved tribological properties [5–14]. Besides, nano-electrodeposition technique is promising for industrialization, which can be achieved by adding additives in conventional electrolytes without post-treatment. Thus, electrodeposition of nano-zinc has become one of future directions for surface protection against corrosion. ⁎ Corresponding authors. E-mail addresses: [email protected] (M. An), [email protected] (D. Li).
http://dx.doi.org/10.1016/j.surfcoat.2016.07.056 0257-8972/© 2016 Elsevier B.V. All rights reserved.
Our earlier studies have demonstrated enhanced corrosion resistance of zinc coating as the grain size is reduced from micro-scale to nano-scale in simulated seawater (3.5 wt% NaCl solution), which is attributed to enhanced scale of corrosion products. Nano-Zn costing also shows higher dry sliding wear resistance, benefiting from increased hardness of nanocrystalline zinc coating [13,14]. Since in many applications, wear may occur in corrosive environments, it is importance to have a look at the behavior of the nano-Zn coating during corrosive wear and determine whether or not the improved surface scale of corrosion products may play a beneficial role in resisting synergistic attack involving corrosion and wear. This is the motivation for conducting this study on the resistance of nano-Zn coating to corrosive wear in a simulated seawater environment. 2. Experimental 2.1. Electrodeposition and characterization of zinc coatings Nanocrystalline zinc coating was electrodeposited on carbon steel through galvanostatic current control using a basic sulfate bath (ZnSO4·7H2O 100 g L−1 and H3BO3 20 g L−1) with added grain refiner (polyacrylamide 1 g L−1). The optimization process of polyacrylamide concentrations is illustrated in Fig. S1. For comparison purpose, conventional coarse-grained zinc coating was also electrodeposited using the basic sulfate bath. The pH of the baths was maintained at 1–2, and the current density was 3 A dm− 2 with electrodeposition time of 1 h at room temperature (25 ± 1 °C) for all experiments. A detailed description of the bath configuration procedure and operating conditions can
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be found in reference [14,15]. Variations in potential with the electrodeposition time of zinc coatings in different baths were analyzed by chronopotentiometry test. All the electrochemical experiments were performed at room temperature in a conventional three electrode cell by Gamry electrochemical workstation with platinum plate, saturated calomel electrode (SCE) and glassy carbon electrode as the auxiliary, reference and working electrode, respectively. Prior to each experiment, the glassy carbon electrode was polished with an aqueous slurry of 0.05 μm alumina, rinsed with deionized water, and then dried with N2. After electrodeposition, zinc coatings were rinsed immediately in deionized water and dried, followed by structure characterization and property evaluation. Grain size and cross-sectional morphology of zinc coatings were characterized by transmission electron microscopy (TEM, JEM-2100), scanning electron microscopy (SEM, Helios Nanolab 600i) and energy dispersive X-ray spectroscopy (EDS). The surface morphology, roughness, electron work function, elastic modulus, deformation, adhesive force of zinc coatings were characterized using a multimode atomic force microscope with a nano-Kelvin Probe (MAFM, Brucker Multimode 8). Through the collection of variations in contact potential between the probe and the sample surfaces by the nano Kelvin Probe in AFM, the in situ AFM analysis reflects changes in properties of the coatings before and after corrosive wear. Both of electron work function and mechanical properties (modulus, deformation and adhesion magnitude) of the samples are proportional to their contact potentials. Detailed discussion on the relationships between the contact potential and the coating properties is given in Supplementary information. 2.2. Corrosive wear testing Corrosive wear tests were performed for the Zn coatings using a pinon-disc tribometer (Neuchatel, CSEM Instruments CH-2007) with a
container, in which simulated seawater was used as a corrosive medium. The disc was the coating and the pin was a SiN ball with a diameter of 6 mm. All tests were performed at a sliding speed of 0.05 cm s− 1 along a circle path of 2 mm in diameter under a load of 1 N for 1 h. The tests were carried at the room temperature with a relative humidity of approximately 50%. During the wear tests, friction coefficient curves were also recorded. Wear scars were analyzed using a ZeGage 3D optical profiler, from which the volume loss was determined by measuring the cross-sectional area of wear scar and then integrating it over the entire wear track. This calculation can be conducted automatically with the ZeGage's software. The surface morphology, element composition, electron work function and mechanical property of the wear scars were analyzed using SEM, EDS and MAFM.
3. Results and discussion 3.1. Electrodeposition of zinc coatings Fig. 1 shows the AFM morphologies of coarse-grained and nanocrystalline zinc coatings electrodeposited using the basic sulfate bath without or with the grain refiner under the same deposition conditions. As shown, the coarse-grained zinc coating has an irregular crystallite size distribution (around 5 μm, Fig. 1a) and its surface is rough (Ra = 306 nm, Fig. 1b), while the nanocrystalline zinc coating shows a uniform, fine and dense grain distribution (less than 100 nm, Fig. 1c) and its surface is much smooth (Ra = 1.3 nm, Fig. 1d). Fig. 2 illustrates a TEM image of the nanocrystalline Zn coating (the dark domains in the TEM image are nanocrystalline zinc.), showing that the average grain size is approximately 40 nm. The thicknesses of coarse-grained and nanocrystalline zinc coatings are about 43 and 38 μm, respectively
Fig. 1. AFM micrographs of coarse-grained (a, b) and nanocrystalline (c, d) zinc coatings.
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Fig. 4. Variations in potential during electrodeposition of coarse-grained and nanocrystalline zinc coatings at constant current.
Fig. 2. TEM image of nanocrystalline zinc scratched off from the coating.
(Fig. 3). Both coatings are adherent to the substrate and appear to be defect-free. Fig. 4 illustrates changes in the applied potential during growth of coarse-grained and nanocrystalline zinc coating at constant current, when deposited using the basic sulfate bath in absence and presence of the grain refiner, respectively. The cathodic potential became more negative as polyacrylamide was added, indicating the cathode polarization occurred. Thus, the formation of nanocrystalline zinc is attributed to the fact that the grain refiner increased the overpotential of the cathode caused by cathode surface absorption of the grain refiner during the electrodeposition process [16]. 3.2. Electron work function and mechanical property of zinc coatings In order to determine how the reduction of grain size affected mechanical properties of zinc coating and corresponding surface electron behavior, several properties were evaluated and mapped using the MAFM microscope, including electron work functions, elastic moduli, deformation magnitudes and adhesive forces of the coarse-grained and nanocrystalline coatings. Maps of these properties and line profiles, along with surface morphologies of scanned areas, are illustrated in Fig. 5. These properties were determined from corresponding contact
potentials. A summary of average contact potentials, to which electron work function, elastic modulus, deformation and adhesive force are proportional, calculated from the maps is given in Table S1. As illustrated, the coarse-grained zinc coating has higher electron work function than nanocrystalline zinc coating, indicating that the latter has a higher surface activity. This is ascribed to the fact that the electron work function is the minimum energy required to remove an electron from inside the material to its surface [17,18]. Thus, the lower electron work function of nanocrystalline zinc coating renders electrons on surface of the nanocrystalline zinc coating more prone to participate in corrosion reactions than those on surface of the coarse-grained coating. This consequently accelerates passivation and increases the stability of formed surface scale of corrosion products [8,13]. Previous studies have demonstrated that the nanocrystalline zinc coating possesses excellent corrosion resistance, compared to the coarse-grained counterpart, suggesting that the higher corrosion resistance of nanocrystalline zinc coatings mainly results from the enhanced protection of the corrosion product scale [7–10]. From the maps and line property profiles of elastic modulus, deformation and adhesive force, one may see that the nanocrystalline zinc coating is stronger with high modulus (Fig. 5c1), lower deformation (Fig. 5c2), larger adhesive force (Fig. 5c3) than the coarse-grained zinc coating. Apparently, the reduction of grain size improved the mechanical strength of the zinc coating. 3.3. Corrosive wear behavior of zinc coatings Resistances of the coarse-grained and nanocrystalline zinc coatings to corrosive wear in stimulated seawater were evaluated using a ball-
Fig. 3. Cross-sectional morphologies of the coarse-grained (a) and nanocrystalline (b) zinc coatings.
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Fig. 5. Maps of electron work function (a, b), elastic modulus (a1, b1), deformation (a2, b2) and adhesive force (a3, b3) of coarse-grained (a1-a3) and nanocrystalline zinc coatings (b1-b3), and their line property profiles for comparison: (c) the contact potential of electron work function VElectron work function, (c1) the contact potential of elastic modulus VElastic modulus, (c2) the contact potential of deformation VDeformation and (c3) the contact potential of adhesive force VAdhesive force. The insets of (a) and (b) are surface morphologies of scan areas on coarse-grained and nanocrystalline zinc coatings, respectively.
on-disc wear tester. During the tests, variations in the coefficient of friction with sliding time were recorded. As shown in Fig. 6, the friction coefficients of both the coarse-grained and nanocrystalline zinc coatings are stable after the initial running-in period. The friction coefficient of the nanocrystalline zinc coating is lower than that of coarse-grained coating, which should be attributed to it higher surface strength that reduced the contact area with the sliding counter-face. Fig. 7 illustrates morphologies and profiles of wear tracks of the worn coatings. Corresponding local electron work function, elastic modulus, deformation and adhesive force maps on the wear scars are also presented. From the wear track morphologies (Fig. 7a and b) and corresponding profiles (Fig. 7a1 and b1), the coarse-grained zinc coating
suffered more damage from the corrosive wear, showing a wider and deeper wear track than the nanocrystalline zinc coating. The volume losses of the coarse-grained and nanocrystalline zinc coatings are 1.165 × 107 and 2.979 × 106 μm3, respectively. Such a large difference in volume loss indicates that the corrosive wear resistance of the zinc coating can be increased by almost 39 times as the grain size is reduced the nanoscale. The obtained information on the electron work function, elastic modulus, deformation and adhesive force of the wear scar (presented in Fig. 7) helps better understand the improvement in the resistance to corrosive wear by the reduction of grain size from micro- to nanoscale. Table S2 lists average contact potentials related to electron work
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grained coating, compositions of the scales were analyzed with EDS. Obtained EDS spectra of wear scars on the coarse-grained (Fig. 9A and a) and nanocrystalline (Fig. 9B and b) zinc coatings show that the scales mainly contain Zn and O, i.e. they are primarily composed of ZnO, consistent with results of reported studies on zinc coatings exposed to stimulated seawater [19–21]. The scale on wear scar of nanocrystalline zinc coating shows higher intensity of oxygen (in the maps shown in Fig. 9, the brighter zones correspond to higher elemental content) with denser distribution than that in the scale on the wear scar of coarse-grained zinc coating. This may imply that the ZnO scale formed on the wear scar of nanocrystalline zinc coating is relatively intact than that on wear scar of the coarse-grained zinc coating. Or in other words, the ZnO scale on the wear scar of nanocrystalline zinc coating was more durable and protective during the corrosive wear test, benefiting from the improved properties as shown in Fig. 8. Fig. 6. Fiction coefficient curves of the coarse-grained and nanocrystalline zinc coatings in 3.5 wt% NaCl solution at 25 ± 1 °C.
function, elastic modulus, deformation and adhesive force of the wear scars on coarse-grained and nanocrystalline zinc coatings calculated from the maps. It was observed that the wear scar of nanocrystalline zinc coating showed higher electron work function than that of the coarse-grained zinc coating. As showed earlier, the work function of nanocrystalline coating was lower than that of the coarse-grained zinc coating before the corrosive wear testing. Thus, one may conclude that the corrosion product scale formed on the wear scar surface of the nanocrystalline coating is more protective and hinder electrons to participate in corrosion during the corrosive wear test, thus reducing the role of corrosion in assisting wear attack or corrosion-wear synergy. The scale of corrosion products also contribute to the resistance to surface attacks, playing a role in protection the coating against corrosive wear. As illustrated from Fig. 7 (a4–a6 and b4–b6) and Fig. 8, the scale of corrosion products on wear scar of nanocrystalline zinc coating shows higher work function, increased elastic modulus and smaller deformation than the scale on wear scar of the coarse-grained zinc coating. The enhanced surface stability (i.e. high EWF) and mechanical strength indicate that the scale of corrosion products on wear scar of the nanocrystalline zinc coating is more protective against corrosive wear than that on wear scar of the coarse-grained counterpart. In order to explain why the scale on the worn surface of the nanocrystalline coating showed superior properties than that of the coarse-
4. Conclusions Effect of nanocrystallization of electrodeposited Zn coating on its resistance to corrosive wear in a dilute NaCl solution was investigated. It was shown that the corrosive wear resistance of the zinc coating was increased by almost 39 times as the grain size was reduced from 5 μm to 40 nm. Based on surface composition analysis and local property mapping, including modulus, deformation magnitude and electron work function, using a multi-mode atomic force microscope, the dramatic improvement in corrosive wear resistance of the nanocrystalline coating is ascribed to its enhanced mechanical strength, faster passivation and the formation of a more protective surface scale of corrosion products. The elevated mechanical strength and higher surface stability rendered the nanocrystalline Zn coating to possess a markedly enhanced resistance to synergistic attack of wear and corrosion. Acknowledgement The authors are grateful for financial support from the Natural Sciences and Engineering Research Council of Canada, Suncor Energy Inc., Shell Canada Ltd., Magna International Inc. and Volant Products Inc. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.surfcoat.2016.07.056.
Fig. 7. AFM morphologies of coarse-grained (a) and nanocrystalline (b) coatings after the corrosive wear testing. Corresponding local wear scar profiles (a1, b1), surface morphologies (a2, b2), electron work functions (a3, b3), elastic modulus (a4, b4), deformations (a5, b5) and adhesive forces (a6, b6) are also presented.
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Fig. 8. Line property profiles for comparison: the contact potential of electron work function VElectron work function (a), elastic modulus VElastic modulus (b), deformation VDeformation (c) and adhesive force VAdhesive force (d) for coarse-grained and nanocrystalline zinc coatings after corrosive wear.
Fig. 9. EDS spectrums and corresponding elemental mappings of the wear scars on coarse-grained (A–A2, a–a2) and nanocrystalline (B–B2, b–b2) zinc coatings.
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References [1] R. Winand, Modern Electroplating, fifth ed. John Wiley & Sons, Inc., New Jersey, 2010. [2] International Zinc Association, Coatings, www.zinc.org/coatings (Accessed 27 March 2016). [3] L.F. Kong, Y.N. Bao, W.W. Guo, L. Cheng, J. Du, R.L. Liu, Y.D. Wang, X. Fan, C.Y. Tao, Low-cost zinc-plated photoanode for fabric-type dye-sensitized solar cells, Appl. Surf. Sci. 363 (2016) 323–327. [4] N.N. Zhang, J. Chen, Y. Huang, W.W. Guo, J. Yang, J. Du, X. Fan, C.Y. Tao, A wearable all-solid photovoltaic textile, Adv. Mater. 28 (2016) 263–269. [5] K.O. Nayana, T.V. Venkatesha, Synergistic effects of additives on morphology, texture and discharge mechanism of zinc during electrodeposition, J. Electroanal. Chem. 663 (2011) 98–107. [6] K. Saber, C.C. Koch, P.S. Fedkiw, Pulse current electrodeposition of nanocrystalline zinc, Mater. Sci. Eng. A 341 (2003) 174–181. [7] H.B. Muralidhara, Y.A. Naik, Studies on nanocrystalline zinc coating, Bull. Mater. Sci. 31 (2008) 585–591. [8] K.M. Youssef, C.C. Koch, P.S. Fedkiw, Improved corrosion behavior of nanocrystalline zinc produced by pulse-current electrodeposition, Corros. Sci. 46 (2004) 51–64. [9] R. Ramanauskas, L. Gudavičiūtė, R. Juškėnas, O. Ščit, Structural and corrosion characterization of pulse plated nanocrystalline zinc coatings, Electrochim. Acta 53 (2007) 1801–1810. [10] M.C. Li, L.L. Jiang, W.Q. Zhang, Y.H. Qian, S.Z. Luo, J.N. Shen, Electrochemical corrosion behavior of nanocrystalline zinc coatings in 3.5% NaCl solutions, J. Solid State Electrochem. 11 (2007) 1319–1325. [11] M.S. Chandrasekar, P. Malathy, Synergetic effects of pulse constraints and additives in electrodeposition of nanocrystalline zinc: corrosion, structural and textural characterization, Mater. Chem. Phys. 124 (2010) 516–528.
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[12] G.Z. Meng, L. Zhang, Y.W. Shao, T. Zhang, F.H. Wang, Study of the electrochemical behaviour of nanocrystalline zinc by statistical methods, Corros. Sci. 51 (2009) 1685–1689. [13] Q.Y. Li, Z.B. Feng, L.H. Liu, H. Xu, W. Ge, F.H. Li, M.Z. An, Deciphering the formation mechanism of a protective corrosion product layer from electrochemical and natural corrosion behaviors of a nanocrystalline zinc coating, RSC Adv. 5 (2015) 32479–32490. [14] Q.Y. Li, Z.B. Feng, L.H. Liu, J. Sun, Y.T. Qu, F.H. Li, M.Z. An, Research on the tribological behavior of a nanocrystalline zinc coating prepared by pulse reverse electrodeposition, RSC Adv. 5 (2015) 12025–12033. [15] Q.Y. Li, Z.B. Feng, J.Q. Zhang, P.X. Yang, F.H. Li, M.Z. An, Pulse reverse electrodeposition and characterization of nanocrystalline zinc coatings, RSC Adv. 4 (2014) 52562–52570. [16] Q.Y. Li, W. Ge, P.X. Yang, J.Q. Zhang, M.Z. An, Insight into the role and its mechanism of polyacrylamide as an additive in sulfate electrolytes for nanocrystalline zinc electrodeposition, J. Electrochem. Soc. 163 (2016) D127–D132. [17] M. Nonnenmacher, M.P. o'Boyle, H.K. Wickramasinghe, Kelvin probe force microscopy, Appl. Phys. Lett. 58 (1991) 2921–2923. [18] C. Kittel, Introduction to Solid State Physics, eighth ed. John Wiley & Sons, Inc., Hoboken, 2005. [19] D. De la Fuente, J.G. Castano, M. Morcillo, Long-term atmospheric corrosion of zinc, Corros. Sci. 49 (2007) 1420–1436. [20] J.D. Yoo, P. Volovitch, A.A. Aal, C. Allely, K. Ogle, The effect of an artificially synthesized simonkolleite layer on the corrosion of electrogalvanized steel, Corros. Sci. 70 (2013) 1–10. [21] S. Thomas, N. Birbilis, M.S. Venkatraman, I.S. Cole, Self-repairing oxides to protect zinc: review, discussion and prospects, Corros. Sci. 69 (2013) 11–22.
Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Electrodeposition of nanocrystalline zinc on steel for enhanced resistance to corrosive wear Qingyang Li a,b, Hao Lu b, Juan Cui b, Maozhong An a,⁎, Dongyang Li b,⁎ a b
State Key Laboratory of Urban Water Resource and Environment, School of Chemical Engineering and Technology, Harbin Institute of Technology, No. 92 West-Da Zhi Street, Harbin 150001, China Dept. of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 1H9, Canada
a r t i c l e
i n f o
Article history: Received 12 May 2016 Revised 18 July 2016 Accepted in revised form 19 July 2016 Available online 20 July 2016 Keywords: Nanocrystalline zinc Electrodeposition Corrosive wear Mechanical property Electron work function
a b s t r a c t In order to increase the resistance of electrogalvanized steel to corrosive wear, nanocrystalline zinc coating was electrodeposited onto the steel substrate using a sulfate bath with polyacrylamide as grain refiner. Corrosive wear tests were performed in a simulated seawater solution to evaluate the performance of the nanocrystalline zinc coating with its grain size around 40 nm, in comparison with that of coarse-grained zinc coating (grain size ~ 5 μm). It was demonstrated that material loss of the coarse-grained zinc coating was 39 times as large as that of the nanocrystalline one. The considerably higher corrosive wear resistance of the nanocrystalline zinc coating largely benefited from its increased mechanical strength due to nanocrystallization and higher surface activity, which improved the passivation capability with the formation of a more protective oxide scale. Detailed analyses were conducted to clarify the mechanism responsible for the improvements. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Due to their more negative standard potential (−0.76 V vs. standard hydrogen electrode SHE) than iron (−0.44 V vs. SHE) [1], zinc coatings are often used to provide sacrificial cathodic protection for steel against corrosion even they are scratched or damaged. The lower cost and larger reserves of zinc, compared to other metals, as well as nontoxic and recyclable properties make it to be the most widely employed surface protective coating. According to the International Zinc Association, more than 5 million tons of zinc per year is used to protect steel against corrosion all over the world, and retrieves about 2.2 trillion USD annually from the capital lost for repair and replacement of corroded steel components [2]. The utilization of zinc coatings continuously increases with the increase in global demand for steel structures and others (such as energy-harvesting and storage cell) [3,4]. This also requires development of more protective zinc coatings and advanced coating processes. Nano-electrodeposition is one of such coating processes. Compared to conventional coarse-grained zinc coating, nanocrystalline zinc coating has demonstrated the following advantages: smoother and brighter surface, higher hardness, stronger corrosion resistance and improved tribological properties [5–14]. Besides, nano-electrodeposition technique is promising for industrialization, which can be achieved by adding additives in conventional electrolytes without post-treatment. Thus, electrodeposition of nano-zinc has become one of future directions for surface protection against corrosion. ⁎ Corresponding authors. E-mail addresses: [email protected] (M. An), [email protected] (D. Li).
http://dx.doi.org/10.1016/j.surfcoat.2016.07.056 0257-8972/© 2016 Elsevier B.V. All rights reserved.
Our earlier studies have demonstrated enhanced corrosion resistance of zinc coating as the grain size is reduced from micro-scale to nano-scale in simulated seawater (3.5 wt% NaCl solution), which is attributed to enhanced scale of corrosion products. Nano-Zn costing also shows higher dry sliding wear resistance, benefiting from increased hardness of nanocrystalline zinc coating [13,14]. Since in many applications, wear may occur in corrosive environments, it is importance to have a look at the behavior of the nano-Zn coating during corrosive wear and determine whether or not the improved surface scale of corrosion products may play a beneficial role in resisting synergistic attack involving corrosion and wear. This is the motivation for conducting this study on the resistance of nano-Zn coating to corrosive wear in a simulated seawater environment. 2. Experimental 2.1. Electrodeposition and characterization of zinc coatings Nanocrystalline zinc coating was electrodeposited on carbon steel through galvanostatic current control using a basic sulfate bath (ZnSO4·7H2O 100 g L−1 and H3BO3 20 g L−1) with added grain refiner (polyacrylamide 1 g L−1). The optimization process of polyacrylamide concentrations is illustrated in Fig. S1. For comparison purpose, conventional coarse-grained zinc coating was also electrodeposited using the basic sulfate bath. The pH of the baths was maintained at 1–2, and the current density was 3 A dm− 2 with electrodeposition time of 1 h at room temperature (25 ± 1 °C) for all experiments. A detailed description of the bath configuration procedure and operating conditions can
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be found in reference [14,15]. Variations in potential with the electrodeposition time of zinc coatings in different baths were analyzed by chronopotentiometry test. All the electrochemical experiments were performed at room temperature in a conventional three electrode cell by Gamry electrochemical workstation with platinum plate, saturated calomel electrode (SCE) and glassy carbon electrode as the auxiliary, reference and working electrode, respectively. Prior to each experiment, the glassy carbon electrode was polished with an aqueous slurry of 0.05 μm alumina, rinsed with deionized water, and then dried with N2. After electrodeposition, zinc coatings were rinsed immediately in deionized water and dried, followed by structure characterization and property evaluation. Grain size and cross-sectional morphology of zinc coatings were characterized by transmission electron microscopy (TEM, JEM-2100), scanning electron microscopy (SEM, Helios Nanolab 600i) and energy dispersive X-ray spectroscopy (EDS). The surface morphology, roughness, electron work function, elastic modulus, deformation, adhesive force of zinc coatings were characterized using a multimode atomic force microscope with a nano-Kelvin Probe (MAFM, Brucker Multimode 8). Through the collection of variations in contact potential between the probe and the sample surfaces by the nano Kelvin Probe in AFM, the in situ AFM analysis reflects changes in properties of the coatings before and after corrosive wear. Both of electron work function and mechanical properties (modulus, deformation and adhesion magnitude) of the samples are proportional to their contact potentials. Detailed discussion on the relationships between the contact potential and the coating properties is given in Supplementary information. 2.2. Corrosive wear testing Corrosive wear tests were performed for the Zn coatings using a pinon-disc tribometer (Neuchatel, CSEM Instruments CH-2007) with a
container, in which simulated seawater was used as a corrosive medium. The disc was the coating and the pin was a SiN ball with a diameter of 6 mm. All tests were performed at a sliding speed of 0.05 cm s− 1 along a circle path of 2 mm in diameter under a load of 1 N for 1 h. The tests were carried at the room temperature with a relative humidity of approximately 50%. During the wear tests, friction coefficient curves were also recorded. Wear scars were analyzed using a ZeGage 3D optical profiler, from which the volume loss was determined by measuring the cross-sectional area of wear scar and then integrating it over the entire wear track. This calculation can be conducted automatically with the ZeGage's software. The surface morphology, element composition, electron work function and mechanical property of the wear scars were analyzed using SEM, EDS and MAFM.
3. Results and discussion 3.1. Electrodeposition of zinc coatings Fig. 1 shows the AFM morphologies of coarse-grained and nanocrystalline zinc coatings electrodeposited using the basic sulfate bath without or with the grain refiner under the same deposition conditions. As shown, the coarse-grained zinc coating has an irregular crystallite size distribution (around 5 μm, Fig. 1a) and its surface is rough (Ra = 306 nm, Fig. 1b), while the nanocrystalline zinc coating shows a uniform, fine and dense grain distribution (less than 100 nm, Fig. 1c) and its surface is much smooth (Ra = 1.3 nm, Fig. 1d). Fig. 2 illustrates a TEM image of the nanocrystalline Zn coating (the dark domains in the TEM image are nanocrystalline zinc.), showing that the average grain size is approximately 40 nm. The thicknesses of coarse-grained and nanocrystalline zinc coatings are about 43 and 38 μm, respectively
Fig. 1. AFM micrographs of coarse-grained (a, b) and nanocrystalline (c, d) zinc coatings.
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Fig. 4. Variations in potential during electrodeposition of coarse-grained and nanocrystalline zinc coatings at constant current.
Fig. 2. TEM image of nanocrystalline zinc scratched off from the coating.
(Fig. 3). Both coatings are adherent to the substrate and appear to be defect-free. Fig. 4 illustrates changes in the applied potential during growth of coarse-grained and nanocrystalline zinc coating at constant current, when deposited using the basic sulfate bath in absence and presence of the grain refiner, respectively. The cathodic potential became more negative as polyacrylamide was added, indicating the cathode polarization occurred. Thus, the formation of nanocrystalline zinc is attributed to the fact that the grain refiner increased the overpotential of the cathode caused by cathode surface absorption of the grain refiner during the electrodeposition process [16]. 3.2. Electron work function and mechanical property of zinc coatings In order to determine how the reduction of grain size affected mechanical properties of zinc coating and corresponding surface electron behavior, several properties were evaluated and mapped using the MAFM microscope, including electron work functions, elastic moduli, deformation magnitudes and adhesive forces of the coarse-grained and nanocrystalline coatings. Maps of these properties and line profiles, along with surface morphologies of scanned areas, are illustrated in Fig. 5. These properties were determined from corresponding contact
potentials. A summary of average contact potentials, to which electron work function, elastic modulus, deformation and adhesive force are proportional, calculated from the maps is given in Table S1. As illustrated, the coarse-grained zinc coating has higher electron work function than nanocrystalline zinc coating, indicating that the latter has a higher surface activity. This is ascribed to the fact that the electron work function is the minimum energy required to remove an electron from inside the material to its surface [17,18]. Thus, the lower electron work function of nanocrystalline zinc coating renders electrons on surface of the nanocrystalline zinc coating more prone to participate in corrosion reactions than those on surface of the coarse-grained coating. This consequently accelerates passivation and increases the stability of formed surface scale of corrosion products [8,13]. Previous studies have demonstrated that the nanocrystalline zinc coating possesses excellent corrosion resistance, compared to the coarse-grained counterpart, suggesting that the higher corrosion resistance of nanocrystalline zinc coatings mainly results from the enhanced protection of the corrosion product scale [7–10]. From the maps and line property profiles of elastic modulus, deformation and adhesive force, one may see that the nanocrystalline zinc coating is stronger with high modulus (Fig. 5c1), lower deformation (Fig. 5c2), larger adhesive force (Fig. 5c3) than the coarse-grained zinc coating. Apparently, the reduction of grain size improved the mechanical strength of the zinc coating. 3.3. Corrosive wear behavior of zinc coatings Resistances of the coarse-grained and nanocrystalline zinc coatings to corrosive wear in stimulated seawater were evaluated using a ball-
Fig. 3. Cross-sectional morphologies of the coarse-grained (a) and nanocrystalline (b) zinc coatings.
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Fig. 5. Maps of electron work function (a, b), elastic modulus (a1, b1), deformation (a2, b2) and adhesive force (a3, b3) of coarse-grained (a1-a3) and nanocrystalline zinc coatings (b1-b3), and their line property profiles for comparison: (c) the contact potential of electron work function VElectron work function, (c1) the contact potential of elastic modulus VElastic modulus, (c2) the contact potential of deformation VDeformation and (c3) the contact potential of adhesive force VAdhesive force. The insets of (a) and (b) are surface morphologies of scan areas on coarse-grained and nanocrystalline zinc coatings, respectively.
on-disc wear tester. During the tests, variations in the coefficient of friction with sliding time were recorded. As shown in Fig. 6, the friction coefficients of both the coarse-grained and nanocrystalline zinc coatings are stable after the initial running-in period. The friction coefficient of the nanocrystalline zinc coating is lower than that of coarse-grained coating, which should be attributed to it higher surface strength that reduced the contact area with the sliding counter-face. Fig. 7 illustrates morphologies and profiles of wear tracks of the worn coatings. Corresponding local electron work function, elastic modulus, deformation and adhesive force maps on the wear scars are also presented. From the wear track morphologies (Fig. 7a and b) and corresponding profiles (Fig. 7a1 and b1), the coarse-grained zinc coating
suffered more damage from the corrosive wear, showing a wider and deeper wear track than the nanocrystalline zinc coating. The volume losses of the coarse-grained and nanocrystalline zinc coatings are 1.165 × 107 and 2.979 × 106 μm3, respectively. Such a large difference in volume loss indicates that the corrosive wear resistance of the zinc coating can be increased by almost 39 times as the grain size is reduced the nanoscale. The obtained information on the electron work function, elastic modulus, deformation and adhesive force of the wear scar (presented in Fig. 7) helps better understand the improvement in the resistance to corrosive wear by the reduction of grain size from micro- to nanoscale. Table S2 lists average contact potentials related to electron work
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grained coating, compositions of the scales were analyzed with EDS. Obtained EDS spectra of wear scars on the coarse-grained (Fig. 9A and a) and nanocrystalline (Fig. 9B and b) zinc coatings show that the scales mainly contain Zn and O, i.e. they are primarily composed of ZnO, consistent with results of reported studies on zinc coatings exposed to stimulated seawater [19–21]. The scale on wear scar of nanocrystalline zinc coating shows higher intensity of oxygen (in the maps shown in Fig. 9, the brighter zones correspond to higher elemental content) with denser distribution than that in the scale on the wear scar of coarse-grained zinc coating. This may imply that the ZnO scale formed on the wear scar of nanocrystalline zinc coating is relatively intact than that on wear scar of the coarse-grained zinc coating. Or in other words, the ZnO scale on the wear scar of nanocrystalline zinc coating was more durable and protective during the corrosive wear test, benefiting from the improved properties as shown in Fig. 8. Fig. 6. Fiction coefficient curves of the coarse-grained and nanocrystalline zinc coatings in 3.5 wt% NaCl solution at 25 ± 1 °C.
function, elastic modulus, deformation and adhesive force of the wear scars on coarse-grained and nanocrystalline zinc coatings calculated from the maps. It was observed that the wear scar of nanocrystalline zinc coating showed higher electron work function than that of the coarse-grained zinc coating. As showed earlier, the work function of nanocrystalline coating was lower than that of the coarse-grained zinc coating before the corrosive wear testing. Thus, one may conclude that the corrosion product scale formed on the wear scar surface of the nanocrystalline coating is more protective and hinder electrons to participate in corrosion during the corrosive wear test, thus reducing the role of corrosion in assisting wear attack or corrosion-wear synergy. The scale of corrosion products also contribute to the resistance to surface attacks, playing a role in protection the coating against corrosive wear. As illustrated from Fig. 7 (a4–a6 and b4–b6) and Fig. 8, the scale of corrosion products on wear scar of nanocrystalline zinc coating shows higher work function, increased elastic modulus and smaller deformation than the scale on wear scar of the coarse-grained zinc coating. The enhanced surface stability (i.e. high EWF) and mechanical strength indicate that the scale of corrosion products on wear scar of the nanocrystalline zinc coating is more protective against corrosive wear than that on wear scar of the coarse-grained counterpart. In order to explain why the scale on the worn surface of the nanocrystalline coating showed superior properties than that of the coarse-
4. Conclusions Effect of nanocrystallization of electrodeposited Zn coating on its resistance to corrosive wear in a dilute NaCl solution was investigated. It was shown that the corrosive wear resistance of the zinc coating was increased by almost 39 times as the grain size was reduced from 5 μm to 40 nm. Based on surface composition analysis and local property mapping, including modulus, deformation magnitude and electron work function, using a multi-mode atomic force microscope, the dramatic improvement in corrosive wear resistance of the nanocrystalline coating is ascribed to its enhanced mechanical strength, faster passivation and the formation of a more protective surface scale of corrosion products. The elevated mechanical strength and higher surface stability rendered the nanocrystalline Zn coating to possess a markedly enhanced resistance to synergistic attack of wear and corrosion. Acknowledgement The authors are grateful for financial support from the Natural Sciences and Engineering Research Council of Canada, Suncor Energy Inc., Shell Canada Ltd., Magna International Inc. and Volant Products Inc. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.surfcoat.2016.07.056.
Fig. 7. AFM morphologies of coarse-grained (a) and nanocrystalline (b) coatings after the corrosive wear testing. Corresponding local wear scar profiles (a1, b1), surface morphologies (a2, b2), electron work functions (a3, b3), elastic modulus (a4, b4), deformations (a5, b5) and adhesive forces (a6, b6) are also presented.
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Fig. 8. Line property profiles for comparison: the contact potential of electron work function VElectron work function (a), elastic modulus VElastic modulus (b), deformation VDeformation (c) and adhesive force VAdhesive force (d) for coarse-grained and nanocrystalline zinc coatings after corrosive wear.
Fig. 9. EDS spectrums and corresponding elemental mappings of the wear scars on coarse-grained (A–A2, a–a2) and nanocrystalline (B–B2, b–b2) zinc coatings.
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