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The effect of metallic bonding layer on the corrosion behavior of plasma sprayed Al2O3 ceramic coatings in simulated seawater
Vacuum 101 (2014) 6e9
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Vacuum journal homepage: www.elsevier.com/locate/vacuum
Rapid communication
The effect of metallic bonding layer on the corrosion behavior of plasma sprayed Al2O3 ceramic coatings in simulated seawater Zhe Liu, Zhenhua Chu, Yanchun Dong, Yong Yang, Xueguang Chen, Xiangjiao Kong, Dianran Yan* School of Materials Science and Engineering, Hebei University of Technology, No. 29, Guangrong Road, Hongqiao District, 300132 Tianjin, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 June 2013 Received in revised form 1 July 2013 Accepted 23 July 2013
Al2O3 ceramic coatings with different metallic bonding layers, i.e. Ni60, NiAl and FeAl, were prepared by the plasma spraying technique. To study the effect of bonding layer on corrosion behavior of plasma sprayed Al2O3 coatings, metallic coatings were also prepared for direct use as working layer. Phase analyses of the coatings were determined by X-ray diffraction (XRD). The corrosion behavior of the coated samples was evaluated by potentiodynamic polarization tests in simulated seawater. The results showed that Ni60 could make Al2O3 coating attain the best corrosion resistance amongst the three metallic bonding layers. The corrosion resistance of Al2O3 coatings first increased and then decreased with the increased Ni60 bonding layer thickness. Conclusively, it is found that chemical composition and thickness of bonding layers played an important role in the corrosion behavior of the plasma sprayed Al2O3 coatings. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Ceramic coating Plasma spray Corrosion Alumina Bonding layer
It is well known that carbon steels are often exposed to environments, which are both corrosive and erosive. One of the most common routes to prevent steels from corrosion and wear is to deposit a protective coating onto the steel surface [1,2]. Ceramic coating for example, has shown its ability to be used where corrosion and wear co-exist [3e5]. Alumina coatings are widely used for a range of industrial applications to provide wear resistance, corrosion protection and thermal insulation [6e8]. Alumina coatings could be prepared by techniques such as chemical vapor deposition (CVD), physical vapor deposition (PVD), solegel, plasma electrolytic oxidation (PEO) and plasma spray [9]. Among these techniques, plasma spraying is one of the most popular methods because it does not cause deterioration of the substrate, and comparatively thick coatings can be formed at a low cost and high deposition rate [10e12]. As a result of the spraying process, ceramic coatings usually have open porosity and incomplete bonding between lamellae, which are deleterious when the coatings have to perform in an aggressive environment [13e15]. The porosity allows a path for electrolytes from the outer surface to the substrate [16e 18]. Therefore, the internal microstructures of coatings play a big role in preventing or reducing corrosion attacks. One of the main concerns when applying the ceramic coating is the adhesion between the substrate and the coating. Mismatch * Corresponding author. Tel.: þ86 22 60204810; fax: þ86 22 26564810. E-mail addresses: [email protected], [email protected] (D. Yan). 0042-207X/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.vacuum.2013.07.039
between the thermal expansion coefficients of ceramic and metals leads to the development of excessive stresses at the interface [19e 21]. Thus, a metallic bonding layer should be used to provide a good thermal expansion match between the ceramic coating and the metal substrate and consequently, give a better effect to the mechanical properties [22,23]. However, there is few reports about the role of the metallic bonding layer on the electrochemical properties of ceramic coatings. In the present work, Al2O3 ceramic coatings with different metallic bonding layers, i.e. Ni60, NiAl and FeAl, were prepared by the plasma spraying technique, and their corrosion behaviors in simulated seawater were evaluated using the potentiodynamic polarization. In addition, the effect of the bonding layer thickness on the corrosion behavior of Al2O3 coatings was also investigated. Q235 carbon steel (0.14e0.22 wt.%C) was used as substrate which was machined into samples of 10 mm 10 mm 15 mm. Before spraying process the substrate was sand-blasted. Three kinds of metallic powders were applied to the treated substrate surface act as bonding layers by plasma spraying, and their characteristics are listed in Table 1. For comparison, three bonding layers were prepared with the same thickness of around 100 mm. The size of Al2O3 powders was 50e74 mm. The coatings were prepared by plasma spraying equipment, which includes a GP-80 electric power of 80 KW and a BT-G3 gun manufactured by Taixing Yeyuan Device Company in China. All powders were sprayed at a current of 500 A, a voltage of 60e70 V and AreH2 gases. The gun-
Z. Liu et al. / Vacuum 101 (2014) 6e9 Table 1 The characteristic of the bonding layer materials. Designation
Composition (wt.%)
Powder size (mm)
Ni60 NiAl FeAl
60Ni-17Cr-5Fe-4.5Si-3Mo-2.7B 95Ni-5Al 50Fe-50Al
50e76 30e40 30e90
to-substrate distance of 80e100 mm was kept constant for deposition of the powders. The microstructures of coatings and bonding layer were examined by means of scanning electron microscopes (SEM, S-4800, Hitachi, Japan). Phase analyses of the coatings were determined by X-ray diffraction (XRD, Philips X0-Pert MPD) with Cu Ka radiation. The copper conducting wires were soldered on the corrosion tested samples surface without coatings. Then, the samples were inset with epoxy resin, leaving only an exposed area of coating of about 1 cm2. Before corrosion test, the samples were washed with distilled water, degreased with acetone and dried with a jet of air. The electrochemical corrosion tests of coating samples were performed in a three-electrode cell with graphite as the counter electrode, a saturated calomel electrode (SCE) as the reference electrode and coating samples as the working electrode. The electrolyte was simulated seawater, and the composition of it is 96.5 wt.% H2O, 2.73 wt.% NaCl, 0.24 wt.% MgCl2, 0.34 wt.% MgSO4, 0.11 wt.% CaCl2 and 0.08 wt.% KCl. Gamry PCI4/750 integrated electrochemical test equipment was used for the corrosion tests. The polarization curves were generated by a scan rate of 0.2 mV/s and a scan range of 0.2 Ve0.4 V vs. EOCP after 1 h immersion. Three repeated tests were performed for each set of measurements. Fig. 1 shows SEM images of as-sprayed surfaces of Al2O3 coatings and three kinds of bonding layers. Evidently, all the coatings exhibited lamellar splat structures. The structure was formed during plasma spraying when the molten droplets strike the substrate at a high velocity, resulting in splat morphology [5]. The Al2O3 ceramic coatings showed more porous structure compared to the metallic coatings. This result agreed with that obtained by Celik et al. [24]. On one hand, this can be attributed to the high melting
7
point of Al2O3. On the other hand, the thermal conductivity of metal is higher than that of alumina, and therefore, the metal powders are more efficiently molten than alumina in the plasma spraying. For the three bonding layers, FeAl coating exhibited large micro cracks, however, the defects in NiAl coating and Ni60 coating were mainly in the form of micro pores but not micro cracks. Ni60 coating was relatively compact, and the pores in NiAl coating were more and bigger. The occurrence of cracks could be attributed to the accumulation of residual stresses in the layer during spraying and coating solidification. In general the thinner the coating, the corresponding residual stress was smaller. X-ray diffraction (XRD) analyses were used to investigate the chemical composition of the three bonding layers prepared by plasma spraying. The XRD pattern, as shown in Fig. 2a, reveals that the Ni60 coating was mainly composed of Ni and CrB. For the NiAl coating, it can be seen from Fig. 2b that the characteristic peaks of Ni and Ni3Al phases were pronounced. During plasma spraying of NiAl powder, particles were injected into the plasma and heated, and then, the NieAl reactions took place and formed intermetallic compounds. Fig. 2c shows that the FeAl coating contained Fe3Al, Fe4Al13 and FeAl2, which remained the same as the phase composition of powders. Fig. 3a shows the potentiodynamic polarization curves of Al2O3 ceramic coatings with different metallic bonding layers in simulated seawater. The corrosion potential of Al2O3/Ni60 coating was much more positive than the other two coatings. This meant that the thermodynamic tendency of corrosion for the Al2O3/Ni60 coating was lowest. The Al2O3/NiAl coating had lower corrosion current density than Al2O3/FeAl coating. The corrosion current density of Al2O3/Ni60 coating was only about one tenth of that of Al2O3/NiAl coating. According to the potentiodynamic polarization curves, the Al2O3 coatings with Ni60 bonding layer possessed much higher corrosion resistance than the other two coatings. The corrosion resistance of the Al2O3 coatings with NiAl bonding layer was higher than that of Al2O3 coatings with FeAl bonding layer. In order to explain the effect of the bonding layers on the corrosion behavior, an additional test was carried out: the potentiodynamic polarization curves test of the three metallic bonding
Fig. 1. As-sprayed surfaces of (a) Al2O3 coating, (b) Ni60 coating, (c) NiAl coating and (d) FeAl coating.
8
Z. Liu et al. / Vacuum 101 (2014) 6e9
Fig. 4. Potentiodynamic polarization curves of Al2O3 coatings with Ni60 bonding layer of different thicknesses.
Fig. 2. X-ray diffraction patterns of (a) Ni60 coating, (b) NiAl coating and (c) FeAl coating.
layers after 1 h immersion in simulated seawater. The test results are presented in Fig. 3b. For comparison, the potentiodynamic polarization curve of Q235 carbon steel is also shown in Fig. 3b. The open circuit potential values of four kinds of samples in simulated seawater in descending were Ni60, NiAl, Q235 and FeAl, which could be ascribed to their chemical composition. The corrosion potential of Ni60 was higher than that of the others due to the existence of Cr. The similar result was reported by Cho et al. [25]. The corrosion potential of NiAl was higher than that of FeAl because Ni was more passive than Fe. For three metallic bonding layers, Ni60 coating possessed the highest corrosion resistance, thus, Ni60 as bonding layer could make Al2O3 coating attain the best corrosion resistance in the present study. The impact of the thickness of the Ni60 bonding layer on the corrosion resistance of Al2O3 coatings was further studied. Fig. 4 depicts potentiodynamic polarization curves of Al2O3 coatings with Ni60 bonding layer of different thicknesses, 50 mm, 100 mm and 200 mm. It can be seen from Fig. 4 that the corrosion resistance of Al2O3 coatings first increased and then decreased with the increased bonding layer thickness. Ni60 bonding layer of 100 mm possessed the noblest corrosion potential and the lowest corrosion current density. Thinner bonding layer let the electrolyte go through the coating because it was not thick enough to correctly protect the base steel. Thicker bonding layer permitted the pass of the electrolyte due to the stresses generated during coating deposition and the corresponding crack formation between different layers. When the thickness of Ni60 bonding layer was 100 mm, Al2O3 coatings could provide best corrosion protect for the substrate. All the three metallic coatings (Ni60, NiAl and FeAl) could provide corrosion protect for the substrate. Of the three metallic bonding layers, Ni60 could make Al2O3 coating attain the best corrosion resistance. The corrosion resistance of Al2O3 coatings first increased and then decreased with the increased bonding layer thickness. It can be inferred that chemical composition and thickness of metallic bonding layers were the important factors influencing the corrosion resistance of the plasma sprayed Al2O3 coatings in simulated seawater. Acknowledgments
Fig. 3. Potentiodynamic polarization curves of (a) Al2O3 ceramic coatings with different metallic bonding layers and (b) three metallic bonding layers after 1 h immersion in simulated seawater.
The authors gratefully acknowledge the financial supports of the National Natural Science Foundation of China (51072045, 51102074
Z. Liu et al. / Vacuum 101 (2014) 6e9
and 51272065), Natural Science Foundation of Hebei Province (E2012202042), China Postdoctoral Science Foundation (20110490979) and Project (20101317120005) supported by Doctoral Program Specialized Research Foundation for Universities, China and Outstanding Youth Fund for Science and Technology Research of Universities in Hebei Province, China (Y2012003). References [1] Toma D, Brandl W, Marginean G. Surf Coat Technol 2001;138:149e58. [2] Ruhi G, Modi OP, Sinha ASK, Singh IB. Corros Sci 2008;50:639e49. [3] Wang Y, Jiang S, Wang MD, Wang SH, Xiao TD, Strutt PR. Wear 2000;237: 176e85. [4] Yan DR, He JN, Wu JJ, Qiu WQ, Ma J. Surf Coat Technol 1997;89:191e5. [5] Singh VP, Sil A, Jayaganthan R. Mater Des 2011;32:584e91. [6] Rico A, Poza P, Rodríguez J. Vacuum 2013;88:149e54. [7] Mindivan H, Tekmen C, Dikici B, Tsunekawa Y, Gavgali M. Mater Des 2009;30: 4516e20. [8] Yang Y, Yan DR, Dong YC, Chen XG, Wang L, Chu ZH, et al. Vacuum 2013;96: 39e45.
9
[9] Guidi F, Moretti G, Carta G, Natali M, Rossetto G, Pierino Z, et al. Electrochim Acta 2005;50:4609e14. [10] Feng WR, Yan DR, He JN. Appl Surf Sci 2005;243:204e13. [11] Ning XJ, Li CX, Li CJ, Yang GJ. Vacuum 2006;80:1261e5. [12] Singh H, Sidhu BS, Puri D, Prakash S. Mater Corros 2007;58:92e102. [13] Ctibor P, Neufuss K, Zahalka F. Wear 2007;262:1274e80. [14] Shanmugavelayutham G, Yano S, Kobayashi A. Vacuum 2006;80:1336e40. [15] Suegama PH, Fugivara CS, Benedetti AV, Fernández J, Delgado J, Guilemany JM. Corros Sci 2005;47:605e20. [16] Li CJ, Li CX, Ning XJ. Vacuum 2004;73:699e703. [17] Liscano S, Gil L, Staia MH. Surf Coat Technol 2004;188e189:135e9. [18] Espallargas N, Berget J, Guilemany JM, Benedetti AV, Suegama PH. Surf Coat Technol 2008;202:1405e17. [19] Yusoff NHN, Ghazali MJ, Isa MC, Muchtar A, Forghani SM. Int J Mech Mater Eng 2011;6:385e90. [20] Yilmaz S, Ipek M, Celebi GF, Bindal C. Vacuum 2005;77:315e21. [21] Goller G. Ceram Int 2004;30:351e5. [22] Gómez-García J, Rico A, Garrido-Maneiro MA, Múnez CJ, Poza P, Utrilla V. Surf Coat Technol 2009;204:812e5. [23] Sarikaya O. Mater Des 2005;26:53e7. [24] Celik E, Ozdemir I, Avci E, Tsunekawa Y. Surf Coat Technol 2005;193:297e302. [25] Cho JE, Kim KY, Hwang SY. Surf Coat Technol 2006;200:2653e62.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Rapid communication
The effect of metallic bonding layer on the corrosion behavior of plasma sprayed Al2O3 ceramic coatings in simulated seawater Zhe Liu, Zhenhua Chu, Yanchun Dong, Yong Yang, Xueguang Chen, Xiangjiao Kong, Dianran Yan* School of Materials Science and Engineering, Hebei University of Technology, No. 29, Guangrong Road, Hongqiao District, 300132 Tianjin, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 June 2013 Received in revised form 1 July 2013 Accepted 23 July 2013
Al2O3 ceramic coatings with different metallic bonding layers, i.e. Ni60, NiAl and FeAl, were prepared by the plasma spraying technique. To study the effect of bonding layer on corrosion behavior of plasma sprayed Al2O3 coatings, metallic coatings were also prepared for direct use as working layer. Phase analyses of the coatings were determined by X-ray diffraction (XRD). The corrosion behavior of the coated samples was evaluated by potentiodynamic polarization tests in simulated seawater. The results showed that Ni60 could make Al2O3 coating attain the best corrosion resistance amongst the three metallic bonding layers. The corrosion resistance of Al2O3 coatings first increased and then decreased with the increased Ni60 bonding layer thickness. Conclusively, it is found that chemical composition and thickness of bonding layers played an important role in the corrosion behavior of the plasma sprayed Al2O3 coatings. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Ceramic coating Plasma spray Corrosion Alumina Bonding layer
It is well known that carbon steels are often exposed to environments, which are both corrosive and erosive. One of the most common routes to prevent steels from corrosion and wear is to deposit a protective coating onto the steel surface [1,2]. Ceramic coating for example, has shown its ability to be used where corrosion and wear co-exist [3e5]. Alumina coatings are widely used for a range of industrial applications to provide wear resistance, corrosion protection and thermal insulation [6e8]. Alumina coatings could be prepared by techniques such as chemical vapor deposition (CVD), physical vapor deposition (PVD), solegel, plasma electrolytic oxidation (PEO) and plasma spray [9]. Among these techniques, plasma spraying is one of the most popular methods because it does not cause deterioration of the substrate, and comparatively thick coatings can be formed at a low cost and high deposition rate [10e12]. As a result of the spraying process, ceramic coatings usually have open porosity and incomplete bonding between lamellae, which are deleterious when the coatings have to perform in an aggressive environment [13e15]. The porosity allows a path for electrolytes from the outer surface to the substrate [16e 18]. Therefore, the internal microstructures of coatings play a big role in preventing or reducing corrosion attacks. One of the main concerns when applying the ceramic coating is the adhesion between the substrate and the coating. Mismatch * Corresponding author. Tel.: þ86 22 60204810; fax: þ86 22 26564810. E-mail addresses: [email protected], [email protected] (D. Yan). 0042-207X/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.vacuum.2013.07.039
between the thermal expansion coefficients of ceramic and metals leads to the development of excessive stresses at the interface [19e 21]. Thus, a metallic bonding layer should be used to provide a good thermal expansion match between the ceramic coating and the metal substrate and consequently, give a better effect to the mechanical properties [22,23]. However, there is few reports about the role of the metallic bonding layer on the electrochemical properties of ceramic coatings. In the present work, Al2O3 ceramic coatings with different metallic bonding layers, i.e. Ni60, NiAl and FeAl, were prepared by the plasma spraying technique, and their corrosion behaviors in simulated seawater were evaluated using the potentiodynamic polarization. In addition, the effect of the bonding layer thickness on the corrosion behavior of Al2O3 coatings was also investigated. Q235 carbon steel (0.14e0.22 wt.%C) was used as substrate which was machined into samples of 10 mm 10 mm 15 mm. Before spraying process the substrate was sand-blasted. Three kinds of metallic powders were applied to the treated substrate surface act as bonding layers by plasma spraying, and their characteristics are listed in Table 1. For comparison, three bonding layers were prepared with the same thickness of around 100 mm. The size of Al2O3 powders was 50e74 mm. The coatings were prepared by plasma spraying equipment, which includes a GP-80 electric power of 80 KW and a BT-G3 gun manufactured by Taixing Yeyuan Device Company in China. All powders were sprayed at a current of 500 A, a voltage of 60e70 V and AreH2 gases. The gun-
Z. Liu et al. / Vacuum 101 (2014) 6e9 Table 1 The characteristic of the bonding layer materials. Designation
Composition (wt.%)
Powder size (mm)
Ni60 NiAl FeAl
60Ni-17Cr-5Fe-4.5Si-3Mo-2.7B 95Ni-5Al 50Fe-50Al
50e76 30e40 30e90
to-substrate distance of 80e100 mm was kept constant for deposition of the powders. The microstructures of coatings and bonding layer were examined by means of scanning electron microscopes (SEM, S-4800, Hitachi, Japan). Phase analyses of the coatings were determined by X-ray diffraction (XRD, Philips X0-Pert MPD) with Cu Ka radiation. The copper conducting wires were soldered on the corrosion tested samples surface without coatings. Then, the samples were inset with epoxy resin, leaving only an exposed area of coating of about 1 cm2. Before corrosion test, the samples were washed with distilled water, degreased with acetone and dried with a jet of air. The electrochemical corrosion tests of coating samples were performed in a three-electrode cell with graphite as the counter electrode, a saturated calomel electrode (SCE) as the reference electrode and coating samples as the working electrode. The electrolyte was simulated seawater, and the composition of it is 96.5 wt.% H2O, 2.73 wt.% NaCl, 0.24 wt.% MgCl2, 0.34 wt.% MgSO4, 0.11 wt.% CaCl2 and 0.08 wt.% KCl. Gamry PCI4/750 integrated electrochemical test equipment was used for the corrosion tests. The polarization curves were generated by a scan rate of 0.2 mV/s and a scan range of 0.2 Ve0.4 V vs. EOCP after 1 h immersion. Three repeated tests were performed for each set of measurements. Fig. 1 shows SEM images of as-sprayed surfaces of Al2O3 coatings and three kinds of bonding layers. Evidently, all the coatings exhibited lamellar splat structures. The structure was formed during plasma spraying when the molten droplets strike the substrate at a high velocity, resulting in splat morphology [5]. The Al2O3 ceramic coatings showed more porous structure compared to the metallic coatings. This result agreed with that obtained by Celik et al. [24]. On one hand, this can be attributed to the high melting
7
point of Al2O3. On the other hand, the thermal conductivity of metal is higher than that of alumina, and therefore, the metal powders are more efficiently molten than alumina in the plasma spraying. For the three bonding layers, FeAl coating exhibited large micro cracks, however, the defects in NiAl coating and Ni60 coating were mainly in the form of micro pores but not micro cracks. Ni60 coating was relatively compact, and the pores in NiAl coating were more and bigger. The occurrence of cracks could be attributed to the accumulation of residual stresses in the layer during spraying and coating solidification. In general the thinner the coating, the corresponding residual stress was smaller. X-ray diffraction (XRD) analyses were used to investigate the chemical composition of the three bonding layers prepared by plasma spraying. The XRD pattern, as shown in Fig. 2a, reveals that the Ni60 coating was mainly composed of Ni and CrB. For the NiAl coating, it can be seen from Fig. 2b that the characteristic peaks of Ni and Ni3Al phases were pronounced. During plasma spraying of NiAl powder, particles were injected into the plasma and heated, and then, the NieAl reactions took place and formed intermetallic compounds. Fig. 2c shows that the FeAl coating contained Fe3Al, Fe4Al13 and FeAl2, which remained the same as the phase composition of powders. Fig. 3a shows the potentiodynamic polarization curves of Al2O3 ceramic coatings with different metallic bonding layers in simulated seawater. The corrosion potential of Al2O3/Ni60 coating was much more positive than the other two coatings. This meant that the thermodynamic tendency of corrosion for the Al2O3/Ni60 coating was lowest. The Al2O3/NiAl coating had lower corrosion current density than Al2O3/FeAl coating. The corrosion current density of Al2O3/Ni60 coating was only about one tenth of that of Al2O3/NiAl coating. According to the potentiodynamic polarization curves, the Al2O3 coatings with Ni60 bonding layer possessed much higher corrosion resistance than the other two coatings. The corrosion resistance of the Al2O3 coatings with NiAl bonding layer was higher than that of Al2O3 coatings with FeAl bonding layer. In order to explain the effect of the bonding layers on the corrosion behavior, an additional test was carried out: the potentiodynamic polarization curves test of the three metallic bonding
Fig. 1. As-sprayed surfaces of (a) Al2O3 coating, (b) Ni60 coating, (c) NiAl coating and (d) FeAl coating.
8
Z. Liu et al. / Vacuum 101 (2014) 6e9
Fig. 4. Potentiodynamic polarization curves of Al2O3 coatings with Ni60 bonding layer of different thicknesses.
Fig. 2. X-ray diffraction patterns of (a) Ni60 coating, (b) NiAl coating and (c) FeAl coating.
layers after 1 h immersion in simulated seawater. The test results are presented in Fig. 3b. For comparison, the potentiodynamic polarization curve of Q235 carbon steel is also shown in Fig. 3b. The open circuit potential values of four kinds of samples in simulated seawater in descending were Ni60, NiAl, Q235 and FeAl, which could be ascribed to their chemical composition. The corrosion potential of Ni60 was higher than that of the others due to the existence of Cr. The similar result was reported by Cho et al. [25]. The corrosion potential of NiAl was higher than that of FeAl because Ni was more passive than Fe. For three metallic bonding layers, Ni60 coating possessed the highest corrosion resistance, thus, Ni60 as bonding layer could make Al2O3 coating attain the best corrosion resistance in the present study. The impact of the thickness of the Ni60 bonding layer on the corrosion resistance of Al2O3 coatings was further studied. Fig. 4 depicts potentiodynamic polarization curves of Al2O3 coatings with Ni60 bonding layer of different thicknesses, 50 mm, 100 mm and 200 mm. It can be seen from Fig. 4 that the corrosion resistance of Al2O3 coatings first increased and then decreased with the increased bonding layer thickness. Ni60 bonding layer of 100 mm possessed the noblest corrosion potential and the lowest corrosion current density. Thinner bonding layer let the electrolyte go through the coating because it was not thick enough to correctly protect the base steel. Thicker bonding layer permitted the pass of the electrolyte due to the stresses generated during coating deposition and the corresponding crack formation between different layers. When the thickness of Ni60 bonding layer was 100 mm, Al2O3 coatings could provide best corrosion protect for the substrate. All the three metallic coatings (Ni60, NiAl and FeAl) could provide corrosion protect for the substrate. Of the three metallic bonding layers, Ni60 could make Al2O3 coating attain the best corrosion resistance. The corrosion resistance of Al2O3 coatings first increased and then decreased with the increased bonding layer thickness. It can be inferred that chemical composition and thickness of metallic bonding layers were the important factors influencing the corrosion resistance of the plasma sprayed Al2O3 coatings in simulated seawater. Acknowledgments
Fig. 3. Potentiodynamic polarization curves of (a) Al2O3 ceramic coatings with different metallic bonding layers and (b) three metallic bonding layers after 1 h immersion in simulated seawater.
The authors gratefully acknowledge the financial supports of the National Natural Science Foundation of China (51072045, 51102074
Z. Liu et al. / Vacuum 101 (2014) 6e9
and 51272065), Natural Science Foundation of Hebei Province (E2012202042), China Postdoctoral Science Foundation (20110490979) and Project (20101317120005) supported by Doctoral Program Specialized Research Foundation for Universities, China and Outstanding Youth Fund for Science and Technology Research of Universities in Hebei Province, China (Y2012003). References [1] Toma D, Brandl W, Marginean G. Surf Coat Technol 2001;138:149e58. [2] Ruhi G, Modi OP, Sinha ASK, Singh IB. Corros Sci 2008;50:639e49. [3] Wang Y, Jiang S, Wang MD, Wang SH, Xiao TD, Strutt PR. Wear 2000;237: 176e85. [4] Yan DR, He JN, Wu JJ, Qiu WQ, Ma J. Surf Coat Technol 1997;89:191e5. [5] Singh VP, Sil A, Jayaganthan R. Mater Des 2011;32:584e91. [6] Rico A, Poza P, Rodríguez J. Vacuum 2013;88:149e54. [7] Mindivan H, Tekmen C, Dikici B, Tsunekawa Y, Gavgali M. Mater Des 2009;30: 4516e20. [8] Yang Y, Yan DR, Dong YC, Chen XG, Wang L, Chu ZH, et al. Vacuum 2013;96: 39e45.
9
[9] Guidi F, Moretti G, Carta G, Natali M, Rossetto G, Pierino Z, et al. Electrochim Acta 2005;50:4609e14. [10] Feng WR, Yan DR, He JN. Appl Surf Sci 2005;243:204e13. [11] Ning XJ, Li CX, Li CJ, Yang GJ. Vacuum 2006;80:1261e5. [12] Singh H, Sidhu BS, Puri D, Prakash S. Mater Corros 2007;58:92e102. [13] Ctibor P, Neufuss K, Zahalka F. Wear 2007;262:1274e80. [14] Shanmugavelayutham G, Yano S, Kobayashi A. Vacuum 2006;80:1336e40. [15] Suegama PH, Fugivara CS, Benedetti AV, Fernández J, Delgado J, Guilemany JM. Corros Sci 2005;47:605e20. [16] Li CJ, Li CX, Ning XJ. Vacuum 2004;73:699e703. [17] Liscano S, Gil L, Staia MH. Surf Coat Technol 2004;188e189:135e9. [18] Espallargas N, Berget J, Guilemany JM, Benedetti AV, Suegama PH. Surf Coat Technol 2008;202:1405e17. [19] Yusoff NHN, Ghazali MJ, Isa MC, Muchtar A, Forghani SM. Int J Mech Mater Eng 2011;6:385e90. [20] Yilmaz S, Ipek M, Celebi GF, Bindal C. Vacuum 2005;77:315e21. [21] Goller G. Ceram Int 2004;30:351e5. [22] Gómez-García J, Rico A, Garrido-Maneiro MA, Múnez CJ, Poza P, Utrilla V. Surf Coat Technol 2009;204:812e5. [23] Sarikaya O. Mater Des 2005;26:53e7. [24] Celik E, Ozdemir I, Avci E, Tsunekawa Y. Surf Coat Technol 2005;193:297e302. [25] Cho JE, Kim KY, Hwang SY. Surf Coat Technol 2006;200:2653e62.