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Polyoxadiazole-based coating for corrosion protection of magnesium alloy
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Surface & Coatings Technology 202 (2008) 4598 – 4601 www.elsevier.com/locate/surfcoat
Polyoxadiazole-based coating for corrosion protection of magnesium alloy M. Bobby Kannan a,⁎, D. Gomes b , W. Dietzel a , V. Abetz b a b
Institute of Materials Research, GKSS-Forschungszentrum Geesthacht GmbH, D-21502 Geesthacht, Germany Institute of Polymer Research, GKSS-Forschungszentrum Geesthacht GmbH, D-21502 Geesthacht, Germany Received 3 December 2007; accepted in revised form 18 March 2008 Available online 8 April 2008
Abstract In this study, polyoxadiazole-based coatings were molecularly designed by attaching two different functional groups, i.e., diphenyl-ether and diphenyl-hexafluoropropane, in the main polymer chain for the purpose of low water permeability and eventually for high corrosion protection of AM50 magnesium alloy. Potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) were used to evaluate the coating performance of the two polymers. Electrochemical experiments showed that POD-6FP (poly(4,4′-diphenyl-hexafluoropropane-1,3,4-oxadiazole)) coated alloy exhibited 3–4 orders of magnitude higher corrosion resistance as compared to the POD-DPE (poly (4,4′-diphenyl-ether-1,3,4oxadiazole)) coated alloy. The high coating performance of the POD-6FP polymer can be attributed to the hydrophobic group attached to the polyoxadiazole chain. © 2008 Elsevier B.V. All rights reserved. Keywords: Polyoxadiazole; Coating; Corrosion; Magnesium alloys
1. Introduction Magnesium alloys are susceptible to various forms of corrosion such as general and localized corrosion which hinders their widespread applications [1–5]. Rare-earth addition was shown to improve the corrosion resistance of magnesium alloys to some extent [6,7]. However, further enhancement in the corrosion resistance of magnesium alloys is required and this only seems to be viable through suitable surface coating. Polymer coatings are one of the surface coating technologies which are used to protect metallic materials from corrosion as well as to enhance the abrasion and wear properties [8]. In recent years, studies have shown that conducting polymers such as polyaniline and polypyrrole offer corrosion protection for iron, steel, aluminium and magnesium alloys [8–11]. It is noted that polymers with electron-withdrawing units like aromatic oxadiazole rings have a strong electron affinity and, as a result, are capable of enhancing their electron transport properties. Hence, this aspect in polyoxadiazole seems to be promising in the application of conducting protective coatings. In fact,
⁎ Corresponding author. Tel.: +61 3 99055173; fax: +61 3 99055686. E-mail address: [email protected] (M.B. Kannan). 0257-8972/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.03.027
substituted oxadiazoles were shown to act as corrosion inhibitor for mild steel [12]. The oxadiazole ring is electronically equivalent to the phenylene ring, being able to delocalize πelectrons. The redox property of polyoxadiazole has been reported by Janietz et al. [13]. Another advantage of oxadiazole containing polymers is their high thermal, chemical and mechanical stability [14], which makes them good candidates for applications that require flame-resistance, fire-resistance or self-extinguishing properties. The degradation temperature of such polymers can be as high as 500 °C [15]. In addition, due to the high molecular weight and mechanical stability of oxadiazole polymers, low thickness films can be coated on the metallic surface. Hence, it is worthwhile to evaluate the coating performance of oxadiazole-based polymers for their potential application in magnesium corrosion protection. In general, water permeation through coatings is one of the important factors to be considered in coating formulation. Mostly, water becomes the major cause of swelling and loss of adhesion of the coating and eventually allows the electrolyte to contact the bare metal and accelerates the corrosion process. Hence, it is necessary to have a control on the permeation of water for better performance of the coating. Water is essentially transported through the coating by two mechanisms: (i) convection through pores and imperfections, and (ii) diffusion
M. Bobby Kannan et al. / Surface & Coatings Technology 202 (2008) 4598–4601
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Table 1 Polarization experiment data of AM50 magnesium alloy with and without polymer coatings tested in 0.1 M NaCl solution Coating Bare metal POD-DPE POD-6FP
Ecorr (mV) − 1378 − 1485 − 1364
icorr (mA/cm2) −3
5.6 × 10 1.2 × 10− 3 1.4 × 10− 6
Ebd (mV)
Eprot (mV)
− 1378 − 1285 –
– 200 N900
Note: Ecorr (corrosion potential), icorr (corrosion current), Ebd (breakdown potential), Eprot (protective potential) =Ebd −Ecorr.
Fig. 1. Chemical structure of POD polymer and the functional groups attached to the main polymer chain.
coatings were prepared by casting the polymer solutions mixed with 4 wt.% N-methyl-2-pyrrolidone, on the sample surface and left to dry at 60 °C for 24 h. For residual solvent removal, the
through the polymer matrix [16]. The present study is concerned with the water diffusion through the polymer matrix. A novel approach was adopted by tailoring the polymer with functional groups which are anticipated to reduce the water uptake and consequently reduce the water permeation and enhance the corrosion resistance of the coated material. Two polymers with different functional groups were studied in this work: (i) poly (4,4′-diphenyl-ether-1,3,4oxadiazole) (POD-DPE) having low free volume in the polymer matrix i.e., high efficient packing density which is attributed to the presence of flexible ether linkage [17], and (ii) poly(4,4′-diphenyl-hexafluoropropane-1,3,4-oxadiazole) (POD-6FP) possessing a hydrophobic group [18]. Electrochemical techniques were used to evaluate the coating performance of these polymers. 2. Experimental procedure The polymers POD-DPE and POD-6FP were synthesized according to the previous studies reported elsewhere [19–22], where the optimization of the polyoxadiazole synthesis was performed. The chemical structures of the polymers are shown in Fig. 1. AM50 magnesium alloy (composition: 4.4–5.5% Al, 0.26–0.6% Mn, max 0.22% Zn, max 0.1.% Si, Balance Mg— all in wt.%) was taken as the test sample. Homogeneous
Fig. 2. Potentiodynamic polarization curves of bare and coated AM50 magnesium alloy.
Fig. 3. Impedance spectra of (a) bare metal; (b) POD-DPE; and (c) POD-6FP coated AM50 magnesium alloy.
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M. Bobby Kannan et al. / Surface & Coatings Technology 202 (2008) 4598–4601
coated samples were placed in a vacuum oven at 80 °C for 24 h. The final thicknesses of the polymer coatings were about 1–5 μm. Prior to the coating, the samples were polished with SiC paper up to 2500 grit, washed in distilled water, and ultrasonically cleaned in acetone. Electrochemical studies were carried out using a computer controlled potentiostat/ frequency response analyser (Gill AC, ACM Instruments, UK) to evaluate the performance of the polymer coatings on AM50 magnesium alloy. A typical three electrode system consisting of a platinum mesh as counter electrode, a saturated calomel electrode (SCE) as reference electrode and AM50 bare and coated samples (1 cm2 exposed area) as working electrode, was used. The experiments were conducted in 0.1 M NaCl solution. Potentiodynamic polarisation experiments were done at a scan rate of 0.5 mV/s. Prior to the experiments the samples were exposed for 2 h to establish a relatively stable open circuit potential. Electrochemical impedance spectroscopy (EIS) experiments were performed at open circuit potential with AC amplitude of 10 mV over the frequency range 105 Hz to 10− 2 Hz. The EIS tests were conducted on the coated samples after exposure periods of 2 h, 36 h and 72 h in 0.1 M NaCl solution. Water absorption of the polymers was measured by immersing the polymer film in 0.1 M NaCl test solution for 72 h at 25 °C. Polymer film was prepared by casting the polymer solution on Teflon-coated sample surface and dried under vacuum at 80°C and later the polymer film was pealed off
for water absorption measurements. The water absorption of the polymers was calculated by the following Eq. (1): Water absorption ðwt:%Þ ¼ wwet wdry =wdry 100 ð1Þ where wdry and wwet are the weights of dried polymer film and hydrated polymer film, respectively. 3. Results and discussion The polarisation curves of bare metal and polymer coated samples are shown in Fig. 2. The electrochemical parameters are given in Table 1. The POD-DPE coated sample showed a marginal reduction in the cathodic current compared to the bare metal. Interestingly, the POD-6FP coated sample showed four orders of magnitude lower cathodic current than the POD-DPE coated sample. The Ecorr value of the coated samples was shifted towards the active direction. The Ecorr shift is more prominent in the POD-DPE than in POD-6FP coated samples. The icorr value of the POD-DPE coated sample (1.2 × 10− 3 mA/cm2) is slightly lower than that of the bare metal (5.6 × 10− 3 mA/cm2). However, the POD-6FP coated sample (1.4 × 10− 6 mA/cm2) exhibits three orders of magnitude lower icorr than the POD-DPE coated sample. The anodic polarization curves of the bare metal showed a sharp increase in the current immediately above Ecorr. This indicates that the Ecorr is connected with the pitting potential. Such a
Fig. 4. Photographs of (a) bare metal; (b) POD-DPE; and (c) POD-6FP coated AM50 magnesium alloy, exposed to 0.1 M NaCl for 72 h.
M. Bobby Kannan et al. / Surface & Coatings Technology 202 (2008) 4598–4601
behavior is reported even in aluminium alloys [23]. The POD-DPE coated sample showed no sharp increase in the anodic current immediately above Ecorr; instead, a protective region of ∼ 200 mV was observed above Ecorr. However, the coating failed at − 1285 mV showing a breakdown potential, i.e., a sharp increase in the current density with a small rise in potential. This shows that the electrolyte permeated through the coating and consequently initiated pitting corrosion. Interestingly, the POD-6FP coated sample showed no breakdown potential even up to − 450 mV, i.e.∼ 900 mV above Ecorr, after which the experiment was stopped. This clearly shows the protective properties of the POD-6FP coating. The Bode plots obtained for the bare metal and the polymer coated samples are shown in Fig. 3a–c. The bare metal showed a polarization resistance of 2.6 × 103 ohm cm2 after an exposure period of 2 h to the test solution. The polarisation resistance decreased with increasing exposure time, i.e., after 36 h immersion the polarisation resistance was reduced to 1.3 × 103 ohm cm2 and further decreased to 9.9 × 102 ohm. cm2 after 72 h immersion. The POD-DPE coated sample showed a impedance of 3.2 × 104 ohm cm2 after 2 h exposure period. The impedance increased marginally with exposure period (i.e., 3.9 × 104 ohm cm2 and 4.0 × 104 ohm cm2 for 36 h and 72 h exposure time, respectively), which could be attributed to initial corrosion of magnesium alloy, which in turn increases the pH of the electrolyte at the substrate-coating interface and consequently passivate the alloy. However, the impedance value of POD-DPE coated sample reveals that the protection is only marginal. Whereas, the POD-6FP coated sample showed a very high impedance of 6.6 × 107 ohm cm2 after 2 h exposure period. The impedance further slightly increased with exposure time, i.e., 8.0 × 107 ohm cm2 and 7.5 × 107 ohm cm2 for 36 h and 72 h exposure, respectively. Comparing the two polymer coatings, POD-6FP showed four orders of magnitude higher impedance than POD-DPE. The photographs of the bare metal and of the coated samples (1 cm2 area) after exposure for 72 h in 0.1 M NaCl solution are shown in Fig. 4a–c. A large amount of corrosion products is visible on the bare metal (Fig. 4a). In the POD-DPE coated sample, a hump is evident on the exposed surface showing electrolyte collection inside the coating (Fig. 4b). In addition, corrosion products are also seen. However, in the POD-6FP coated sample there is no evidence of any corrosion or electrolyte uptake (Fig. 4c). The coating was intact on the sample surface. Water absorption measurements showed that the water absorption capacity is higher in POD-DPE than in POD-6FP polymers. Thus, the POD-DPE film showed water absorption of 20% whereas the POD-6FP film showed no water absorption (0%) even after 72 h of immersion in the electrolyte. It is interesting to note that the POD-6FP possess a higher free volume in the polymer matrix than the POD-DPE [17], which can pave way for water permeation, exhibited higher impedance. This shows that the hydrophobicity of the polymer is more critical than the free volume present in the polymer matrix to reduce water permeation. Thus, the high coating
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performance of the POD-6FP polymer can be attributed to the hydrophobic group attached to the polyoxadiazole chain. 4. Conclusion Potentiodynamic polarisation and electrochemical impedance spectroscopy experiments showed that polyoxadiazolebased coatings improve the corrosion resistance of AM50 magnesium alloy. The poly(4,4′-diphenyl-hexafluoropropane1,3,4-oxadiazole) (POD-6FP) coated AM50 magnesium alloy exhibited a 3–4 orders of magnitude higher corrosion resistance as compared to the poly (4,4′-diphenyl-ether-1,3,4-oxadiazole) (POD-DPE) coated alloy. The high coating performance of the POD-6FP polymer can be attributed to the hydrophobic group attached to the polyoxadiazole chain. Acknowledgement One of the authors (MBK) expresses his sincere thanks for the financial support through the German Helmholtz-DAAD Postdoctoral programme. References [1] G.L. Song, A. Atrens, Adv. Eng. Mater. 1 (1999) 11. [2] E. Ghali, W. Dietzel, K.U. Kainer, J. Mater. Eng. Perform. 13 (2004) 7. [3] M. Bobby Kannan, W. Dietzel, R.K. Singh Raman, P. Lyon, Scripta Mater. 57 (2007) 579. [4] M. Bobby Kannan, W. Dietzel, C. Blawert, S. Riekehr, M. Kocak, Mater. Sci. Eng.A 444 (2007) 220. [5] M. Bobby Kannan, W. Dietzel, R. Zeng, R. Zettler, J.F. dos Santos, Mater. Sci. Eng. A 460–461 (2007) 243. [6] M. Bobby Kannan, W. Dietzel, C. Blawert, A. Atrens, P. Lyon, Mater. Sci. Eng. A 480 (2008) 529. [7] J.H. Nordlien, K. Nisancioglu, S. Ono, N. Masuko, J. Electrochem. Soc. 144 (1997) 461. [8] J.E. Gray, B. Luan, J. Alloys Compd. 336 (2002) 88. [9] J. Yano, K. Nakatani, Y. Harima, A. Kitani, Mater. Lett. 61 (2007) 1500. [10] V.T. Truong, P.K. Lai, B.T. Moore, R.F. Muscat, M.S. Russo, Synth. Met. 110 (2000) 7. [11] A. Yfantis, I. Paloumpa, D. Schmeiser, D. Yfantis, Surf. Coat. Technol. 151–152 (2002) 400. [12] M. Lagrenee, B. Mernari, N. Chaibi, M. Traisnel, H. Vezin, F. Bentiss, Corros. Sci. 43 (2001) 951. [13] S. Janietz, S. Anlauf, A. Wedel, Macromol. Chem. Phys. 203 (2002) 433. [14] H.H. Yang, Aromatic High-strength Fibers, John Wiley & Sons, New York, 1989. [15] D. Gomes, J.C. Pinto, C.P. Borges, Polymer 44 (2003) 6223. [16] E.C. Bucharsky, E.B. Castro, S.G. Real, J. Corros. Sci. Eng. 2 (19) (2000) 1–13. [17] M.E. Sena, C.T. Andrade, Polym. Bull. 34 (1995) 643. [18] E. David, A. Lazar, A. Armeanu, J. Mater. Processing Technol. 157–158 (2004) 284. [19] D. Gomes, S.P. Nunes, J. Membr. Sci. (2008), doi:10.1016/j.memsci. 2007.11.041. [20] D. Gomes, C.P. Borges, J.C. Pinto, Polymer 42 (2001) 851. [21] D. Gomes, C.P. Borges, J.C. Pinto, Polymer 45 (2004) 4997. [22] D. Gomes, J. Roeder, M.L. Ponce, S.P. Nunes, J. Membr. Sci. 295 (2007) 121. [23] M. Bobby Kannan, V.S. Raja, Metall. Mater. Trans. A 38A (2007) 2843.
Surface & Coatings Technology 202 (2008) 4598 – 4601 www.elsevier.com/locate/surfcoat
Polyoxadiazole-based coating for corrosion protection of magnesium alloy M. Bobby Kannan a,⁎, D. Gomes b , W. Dietzel a , V. Abetz b a b
Institute of Materials Research, GKSS-Forschungszentrum Geesthacht GmbH, D-21502 Geesthacht, Germany Institute of Polymer Research, GKSS-Forschungszentrum Geesthacht GmbH, D-21502 Geesthacht, Germany Received 3 December 2007; accepted in revised form 18 March 2008 Available online 8 April 2008
Abstract In this study, polyoxadiazole-based coatings were molecularly designed by attaching two different functional groups, i.e., diphenyl-ether and diphenyl-hexafluoropropane, in the main polymer chain for the purpose of low water permeability and eventually for high corrosion protection of AM50 magnesium alloy. Potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) were used to evaluate the coating performance of the two polymers. Electrochemical experiments showed that POD-6FP (poly(4,4′-diphenyl-hexafluoropropane-1,3,4-oxadiazole)) coated alloy exhibited 3–4 orders of magnitude higher corrosion resistance as compared to the POD-DPE (poly (4,4′-diphenyl-ether-1,3,4oxadiazole)) coated alloy. The high coating performance of the POD-6FP polymer can be attributed to the hydrophobic group attached to the polyoxadiazole chain. © 2008 Elsevier B.V. All rights reserved. Keywords: Polyoxadiazole; Coating; Corrosion; Magnesium alloys
1. Introduction Magnesium alloys are susceptible to various forms of corrosion such as general and localized corrosion which hinders their widespread applications [1–5]. Rare-earth addition was shown to improve the corrosion resistance of magnesium alloys to some extent [6,7]. However, further enhancement in the corrosion resistance of magnesium alloys is required and this only seems to be viable through suitable surface coating. Polymer coatings are one of the surface coating technologies which are used to protect metallic materials from corrosion as well as to enhance the abrasion and wear properties [8]. In recent years, studies have shown that conducting polymers such as polyaniline and polypyrrole offer corrosion protection for iron, steel, aluminium and magnesium alloys [8–11]. It is noted that polymers with electron-withdrawing units like aromatic oxadiazole rings have a strong electron affinity and, as a result, are capable of enhancing their electron transport properties. Hence, this aspect in polyoxadiazole seems to be promising in the application of conducting protective coatings. In fact,
⁎ Corresponding author. Tel.: +61 3 99055173; fax: +61 3 99055686. E-mail address: [email protected] (M.B. Kannan). 0257-8972/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.03.027
substituted oxadiazoles were shown to act as corrosion inhibitor for mild steel [12]. The oxadiazole ring is electronically equivalent to the phenylene ring, being able to delocalize πelectrons. The redox property of polyoxadiazole has been reported by Janietz et al. [13]. Another advantage of oxadiazole containing polymers is their high thermal, chemical and mechanical stability [14], which makes them good candidates for applications that require flame-resistance, fire-resistance or self-extinguishing properties. The degradation temperature of such polymers can be as high as 500 °C [15]. In addition, due to the high molecular weight and mechanical stability of oxadiazole polymers, low thickness films can be coated on the metallic surface. Hence, it is worthwhile to evaluate the coating performance of oxadiazole-based polymers for their potential application in magnesium corrosion protection. In general, water permeation through coatings is one of the important factors to be considered in coating formulation. Mostly, water becomes the major cause of swelling and loss of adhesion of the coating and eventually allows the electrolyte to contact the bare metal and accelerates the corrosion process. Hence, it is necessary to have a control on the permeation of water for better performance of the coating. Water is essentially transported through the coating by two mechanisms: (i) convection through pores and imperfections, and (ii) diffusion
M. Bobby Kannan et al. / Surface & Coatings Technology 202 (2008) 4598–4601
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Table 1 Polarization experiment data of AM50 magnesium alloy with and without polymer coatings tested in 0.1 M NaCl solution Coating Bare metal POD-DPE POD-6FP
Ecorr (mV) − 1378 − 1485 − 1364
icorr (mA/cm2) −3
5.6 × 10 1.2 × 10− 3 1.4 × 10− 6
Ebd (mV)
Eprot (mV)
− 1378 − 1285 –
– 200 N900
Note: Ecorr (corrosion potential), icorr (corrosion current), Ebd (breakdown potential), Eprot (protective potential) =Ebd −Ecorr.
Fig. 1. Chemical structure of POD polymer and the functional groups attached to the main polymer chain.
coatings were prepared by casting the polymer solutions mixed with 4 wt.% N-methyl-2-pyrrolidone, on the sample surface and left to dry at 60 °C for 24 h. For residual solvent removal, the
through the polymer matrix [16]. The present study is concerned with the water diffusion through the polymer matrix. A novel approach was adopted by tailoring the polymer with functional groups which are anticipated to reduce the water uptake and consequently reduce the water permeation and enhance the corrosion resistance of the coated material. Two polymers with different functional groups were studied in this work: (i) poly (4,4′-diphenyl-ether-1,3,4oxadiazole) (POD-DPE) having low free volume in the polymer matrix i.e., high efficient packing density which is attributed to the presence of flexible ether linkage [17], and (ii) poly(4,4′-diphenyl-hexafluoropropane-1,3,4-oxadiazole) (POD-6FP) possessing a hydrophobic group [18]. Electrochemical techniques were used to evaluate the coating performance of these polymers. 2. Experimental procedure The polymers POD-DPE and POD-6FP were synthesized according to the previous studies reported elsewhere [19–22], where the optimization of the polyoxadiazole synthesis was performed. The chemical structures of the polymers are shown in Fig. 1. AM50 magnesium alloy (composition: 4.4–5.5% Al, 0.26–0.6% Mn, max 0.22% Zn, max 0.1.% Si, Balance Mg— all in wt.%) was taken as the test sample. Homogeneous
Fig. 2. Potentiodynamic polarization curves of bare and coated AM50 magnesium alloy.
Fig. 3. Impedance spectra of (a) bare metal; (b) POD-DPE; and (c) POD-6FP coated AM50 magnesium alloy.
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M. Bobby Kannan et al. / Surface & Coatings Technology 202 (2008) 4598–4601
coated samples were placed in a vacuum oven at 80 °C for 24 h. The final thicknesses of the polymer coatings were about 1–5 μm. Prior to the coating, the samples were polished with SiC paper up to 2500 grit, washed in distilled water, and ultrasonically cleaned in acetone. Electrochemical studies were carried out using a computer controlled potentiostat/ frequency response analyser (Gill AC, ACM Instruments, UK) to evaluate the performance of the polymer coatings on AM50 magnesium alloy. A typical three electrode system consisting of a platinum mesh as counter electrode, a saturated calomel electrode (SCE) as reference electrode and AM50 bare and coated samples (1 cm2 exposed area) as working electrode, was used. The experiments were conducted in 0.1 M NaCl solution. Potentiodynamic polarisation experiments were done at a scan rate of 0.5 mV/s. Prior to the experiments the samples were exposed for 2 h to establish a relatively stable open circuit potential. Electrochemical impedance spectroscopy (EIS) experiments were performed at open circuit potential with AC amplitude of 10 mV over the frequency range 105 Hz to 10− 2 Hz. The EIS tests were conducted on the coated samples after exposure periods of 2 h, 36 h and 72 h in 0.1 M NaCl solution. Water absorption of the polymers was measured by immersing the polymer film in 0.1 M NaCl test solution for 72 h at 25 °C. Polymer film was prepared by casting the polymer solution on Teflon-coated sample surface and dried under vacuum at 80°C and later the polymer film was pealed off
for water absorption measurements. The water absorption of the polymers was calculated by the following Eq. (1): Water absorption ðwt:%Þ ¼ wwet wdry =wdry 100 ð1Þ where wdry and wwet are the weights of dried polymer film and hydrated polymer film, respectively. 3. Results and discussion The polarisation curves of bare metal and polymer coated samples are shown in Fig. 2. The electrochemical parameters are given in Table 1. The POD-DPE coated sample showed a marginal reduction in the cathodic current compared to the bare metal. Interestingly, the POD-6FP coated sample showed four orders of magnitude lower cathodic current than the POD-DPE coated sample. The Ecorr value of the coated samples was shifted towards the active direction. The Ecorr shift is more prominent in the POD-DPE than in POD-6FP coated samples. The icorr value of the POD-DPE coated sample (1.2 × 10− 3 mA/cm2) is slightly lower than that of the bare metal (5.6 × 10− 3 mA/cm2). However, the POD-6FP coated sample (1.4 × 10− 6 mA/cm2) exhibits three orders of magnitude lower icorr than the POD-DPE coated sample. The anodic polarization curves of the bare metal showed a sharp increase in the current immediately above Ecorr. This indicates that the Ecorr is connected with the pitting potential. Such a
Fig. 4. Photographs of (a) bare metal; (b) POD-DPE; and (c) POD-6FP coated AM50 magnesium alloy, exposed to 0.1 M NaCl for 72 h.
M. Bobby Kannan et al. / Surface & Coatings Technology 202 (2008) 4598–4601
behavior is reported even in aluminium alloys [23]. The POD-DPE coated sample showed no sharp increase in the anodic current immediately above Ecorr; instead, a protective region of ∼ 200 mV was observed above Ecorr. However, the coating failed at − 1285 mV showing a breakdown potential, i.e., a sharp increase in the current density with a small rise in potential. This shows that the electrolyte permeated through the coating and consequently initiated pitting corrosion. Interestingly, the POD-6FP coated sample showed no breakdown potential even up to − 450 mV, i.e.∼ 900 mV above Ecorr, after which the experiment was stopped. This clearly shows the protective properties of the POD-6FP coating. The Bode plots obtained for the bare metal and the polymer coated samples are shown in Fig. 3a–c. The bare metal showed a polarization resistance of 2.6 × 103 ohm cm2 after an exposure period of 2 h to the test solution. The polarisation resistance decreased with increasing exposure time, i.e., after 36 h immersion the polarisation resistance was reduced to 1.3 × 103 ohm cm2 and further decreased to 9.9 × 102 ohm. cm2 after 72 h immersion. The POD-DPE coated sample showed a impedance of 3.2 × 104 ohm cm2 after 2 h exposure period. The impedance increased marginally with exposure period (i.e., 3.9 × 104 ohm cm2 and 4.0 × 104 ohm cm2 for 36 h and 72 h exposure time, respectively), which could be attributed to initial corrosion of magnesium alloy, which in turn increases the pH of the electrolyte at the substrate-coating interface and consequently passivate the alloy. However, the impedance value of POD-DPE coated sample reveals that the protection is only marginal. Whereas, the POD-6FP coated sample showed a very high impedance of 6.6 × 107 ohm cm2 after 2 h exposure period. The impedance further slightly increased with exposure time, i.e., 8.0 × 107 ohm cm2 and 7.5 × 107 ohm cm2 for 36 h and 72 h exposure, respectively. Comparing the two polymer coatings, POD-6FP showed four orders of magnitude higher impedance than POD-DPE. The photographs of the bare metal and of the coated samples (1 cm2 area) after exposure for 72 h in 0.1 M NaCl solution are shown in Fig. 4a–c. A large amount of corrosion products is visible on the bare metal (Fig. 4a). In the POD-DPE coated sample, a hump is evident on the exposed surface showing electrolyte collection inside the coating (Fig. 4b). In addition, corrosion products are also seen. However, in the POD-6FP coated sample there is no evidence of any corrosion or electrolyte uptake (Fig. 4c). The coating was intact on the sample surface. Water absorption measurements showed that the water absorption capacity is higher in POD-DPE than in POD-6FP polymers. Thus, the POD-DPE film showed water absorption of 20% whereas the POD-6FP film showed no water absorption (0%) even after 72 h of immersion in the electrolyte. It is interesting to note that the POD-6FP possess a higher free volume in the polymer matrix than the POD-DPE [17], which can pave way for water permeation, exhibited higher impedance. This shows that the hydrophobicity of the polymer is more critical than the free volume present in the polymer matrix to reduce water permeation. Thus, the high coating
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performance of the POD-6FP polymer can be attributed to the hydrophobic group attached to the polyoxadiazole chain. 4. Conclusion Potentiodynamic polarisation and electrochemical impedance spectroscopy experiments showed that polyoxadiazolebased coatings improve the corrosion resistance of AM50 magnesium alloy. The poly(4,4′-diphenyl-hexafluoropropane1,3,4-oxadiazole) (POD-6FP) coated AM50 magnesium alloy exhibited a 3–4 orders of magnitude higher corrosion resistance as compared to the poly (4,4′-diphenyl-ether-1,3,4-oxadiazole) (POD-DPE) coated alloy. The high coating performance of the POD-6FP polymer can be attributed to the hydrophobic group attached to the polyoxadiazole chain. Acknowledgement One of the authors (MBK) expresses his sincere thanks for the financial support through the German Helmholtz-DAAD Postdoctoral programme. References [1] G.L. Song, A. Atrens, Adv. Eng. Mater. 1 (1999) 11. [2] E. Ghali, W. Dietzel, K.U. Kainer, J. Mater. Eng. Perform. 13 (2004) 7. [3] M. Bobby Kannan, W. Dietzel, R.K. Singh Raman, P. Lyon, Scripta Mater. 57 (2007) 579. [4] M. Bobby Kannan, W. Dietzel, C. Blawert, S. Riekehr, M. Kocak, Mater. Sci. Eng.A 444 (2007) 220. [5] M. Bobby Kannan, W. Dietzel, R. Zeng, R. Zettler, J.F. dos Santos, Mater. Sci. Eng. A 460–461 (2007) 243. [6] M. Bobby Kannan, W. Dietzel, C. Blawert, A. Atrens, P. Lyon, Mater. Sci. Eng. A 480 (2008) 529. [7] J.H. Nordlien, K. Nisancioglu, S. Ono, N. Masuko, J. Electrochem. Soc. 144 (1997) 461. [8] J.E. Gray, B. Luan, J. Alloys Compd. 336 (2002) 88. [9] J. Yano, K. Nakatani, Y. Harima, A. Kitani, Mater. Lett. 61 (2007) 1500. [10] V.T. Truong, P.K. Lai, B.T. Moore, R.F. Muscat, M.S. Russo, Synth. Met. 110 (2000) 7. [11] A. Yfantis, I. Paloumpa, D. Schmeiser, D. Yfantis, Surf. Coat. Technol. 151–152 (2002) 400. [12] M. Lagrenee, B. Mernari, N. Chaibi, M. Traisnel, H. Vezin, F. Bentiss, Corros. Sci. 43 (2001) 951. [13] S. Janietz, S. Anlauf, A. Wedel, Macromol. Chem. Phys. 203 (2002) 433. [14] H.H. Yang, Aromatic High-strength Fibers, John Wiley & Sons, New York, 1989. [15] D. Gomes, J.C. Pinto, C.P. Borges, Polymer 44 (2003) 6223. [16] E.C. Bucharsky, E.B. Castro, S.G. Real, J. Corros. Sci. Eng. 2 (19) (2000) 1–13. [17] M.E. Sena, C.T. Andrade, Polym. Bull. 34 (1995) 643. [18] E. David, A. Lazar, A. Armeanu, J. Mater. Processing Technol. 157–158 (2004) 284. [19] D. Gomes, S.P. Nunes, J. Membr. Sci. (2008), doi:10.1016/j.memsci. 2007.11.041. [20] D. Gomes, C.P. Borges, J.C. Pinto, Polymer 42 (2001) 851. [21] D. Gomes, C.P. Borges, J.C. Pinto, Polymer 45 (2004) 4997. [22] D. Gomes, J. Roeder, M.L. Ponce, S.P. Nunes, J. Membr. Sci. 295 (2007) 121. [23] M. Bobby Kannan, V.S. Raja, Metall. Mater. Trans. A 38A (2007) 2843.