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Chrome-free neodymium-based protective coatings for magnesium alloys
Materials Letters 65 (2011) 1145–1147
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Chrome-free neodymium-based protective coatings for magnesium alloys Guo Jin a,⁎, Yuyun Yang a, Xiufang Cui b, Erbao Liu a, Zhongyu Wang a, Qingfen Li a a b
Center for Biomedical Materials and Engineering, School of Materials Science and Chemical Engineering, Harbin Engineering University, 145 Nantong St., Harbin 150001, China School of Materials Science and Chemical Engineering, Harbin Engineering University, 145 Nantong St., Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 3 November 2010 Accepted 8 December 2010 Available online 15 December 2010 Keywords: Microstructure Corrosion Neodymium conversion coating Morphology
a b s t r a c t The microstructure of chrome-free neodymium-based conversion coating on magnesium alloy was investigated and the corrosion resistance was evaluated as well. The micro-morphology, transverse section, crystal structure and composition of the coating were observed by scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS), respectively. The corrosion resistance was evaluated by potentiodynamic polarization curve and electrochemical impedance spectroscopy (EIS). The results revealed that the morphology of neodymium conversion coating is of crack-mud structure. Tiny cracks distribute in the compact coating deposited by neodymium oxides. EDS results characterize that the coating is made of neodymium oxides. The potentiodynamic polarization curve, EIS and OCP indicate that the neodymium conversion coating can improve the corrosion resistance of magnesium alloys. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The lanthanides ions of rare earth elements, as Ce3+, La3+, Y3+, Nd3+, Pr3+, have a lower toxicity and are considered as the best candidate for replacing the hexavalent chromium compounds of high toxicity. In previous work, lanthanum and cerium conversion coating are well known to inhibit the corrosion processes on some alloys such as aluminium alloys [1–3], magnesium alloys [4–9] and zinc [10,11]. But there is no paper reported on neodymium conversion coating. Commonly, neodymium is used as the isotopic reference material [12] in geology. In this article, newly developed environmentally surface treatments were proposed based on Nd3+as alternatives to toxic chromate-based systems. Micro-morphology, microstructure, element area profile of the transverse section and corrosion performance of the neodymium conversion coating were studied according to priority. Surface and transverse section examinations were performed by SEM, EDS and XRD. Corrosion behavior of the neodymium coating on AZ91 surface in 3.5% NaCl solution were carried out by potentiodynamic polarization curve and EIS.
2. Experimental methods 2.1. Preparation of samples The neodymium conversion coating was obtained on the matrix of die-cast AZ91. Test samples (15 mm × 10 mm × 5 mm) were polished using waterproof abrasive paper from 360 grits to 2500 grits, then fine ⁎ Corresponding author. Tel.: +86 451 82518173. E-mail address: [email protected] (G. Jin). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.12.019
polished using diamond paste of 3.5 μm, and then they were degreased with absolute ethanol in ultrasonic bath for 15 min and subsequently dried by cold air in room temperature. 2.2. Conversion coating treatment The optimal technical parameters determined by orthogonal experiment were the neodymium nitrate, Nd(NO3)3, with concentrations of 5 g/L, and 20 ml/L of hydrogen dioxide solution, H2O2 (Fisher Scientific, 30%) used as the accelerant. The obtained solution had been mixed for 5 min with a magnetic stirrer prior to deposition. The samples were immersed in the conversion solution at 50 °C for 20 min. After all these treatments, the samples were thoroughly rinsed with deionized water and then dried in cold air. 2.3. Characterization and corrosion test The micro-morphology was observed by a FEI Quant200 SEM equipped with EDS. The crystal structure of the conversion coating was studied by XRD. The anticorrosion performance was evaluated by recording the corrosion potential and corrosion current density of the samples in 3.5% sodium chloride (NaCl) solution upon immersion, which was conducted on a CHI660B electrochemical workstation. The electrochemical cell used a classic three-electrode system which consists of a reference electrode (a saturated calomel electrode), a counter electrode (a platinum foil) and a working electrode. The potentiodynamic polarization curves were performed at a scanning rate of 1 mV/s. OCP and EIS measurements were carried out at corrosion potential in a frequency range between 0.01 Hz and 100,000 Hz using a 10 mV amplitude perturbation.
1146
G. Jin et al. / Materials Letters 65 (2011) 1145–1147
12000
-Nd2 O3 -Mg
Intensity(a.u.)
10000 the treated sample 8000 6000 4000 the bare sample
2000 0 20
25
30
35
40
45
50
55
60
65
2θ ( ° ) Fig. 3. XRD pattern of Nd conversion coating and the bare magnesium alloy.
Fig. 1. SEM morphology of conversion coating on magnesium alloy.
3. Results and discussions 3.1. Morphology and composition of conversion coating SEM was used to reveal the morphology of the neodymium conversion coating. As shown in Fig. 1, the tiny and thick cracks distributed in the compact coating were obtained at the optimal preparation process. The magnified map, placed at the lower right corner of Fig. 1, characterized the crack-mud structure of the neodymium conversion coating. The coating has being forming by depositing neodymium oxides which were determined via XRD and EDS measurements. The atomic percentage (at.%) histogram of Mg, O, and Nd in neodymium conversion coating is shown as following: O (70.58 at.%) dominated in the coating, Mg was 4.84 at.%, Al was 0.97 at.% and Nd was 23.6 at.% in the neodymium coating. The column bar was consistent with the phenomenon revealed by SEM. All these results could be tested and verified by the transverse section of conversion specimen and the element area profile.
The morphology of the transverse section of conversion specimen and Element area profile were shown in Fig. 2. The conversion coating was about 4–5 μm approximately consisted of two layers. The first layer formed on the surface of magnesium alloys was loose with cracks and the second layer covered above the first layer was compact with less and tiny cracks. As shown in element area profile, Mg concentrated in the matrix and simultaneously, there was a handful of Mg near the coating. Al also concentrated in matrix, particularly abundant in second phase [13]. Nd was found rich in coating, which indicated the neodymium conversion coating formed on the surface of magnesium alloy. O spread in matrix and coating, especially in coating. It revealed that coating was mainly made of the oxides of Nd. 3.2. XRD analysis of coating XRD patterns of sample are shown in Fig. 3. The peaks of the neodymium element and its chemical compound appeared at low angle. The appearance of the bread-shaped peak from 21° to 29° in the XRD pattern indicated that the formed lanthanum conversion coating on the matrix was of amorphous structure. High intensity La2O3 peaks was present in the figure. The results of the XRD patterns in Fig. 4 corresponded with the results from SEM and EDS.
Fig. 2. SEM morphology of the cross section of conversion specimen and element area profile.
G. Jin et al. / Materials Letters 65 (2011) 1145–1147
2400
the treated sample the untreated sample
2100
0.0 the treated sample
Ecorr(V vs. SEC)
1800
-Z''/ohm
1147
1500 1200 900 600
the untreated sample
-0.5 -1.0 -1.5
300 -2.0
0 0
1000
2000
3000
4000
5000
10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1
Z'/ohm
100
icorr(A/cm2)
Fig. 4. EIS spectra for the treated and the untreated samples. Fig. 5. Potentiodynamic polarization curves of the treated and untreated samples.
3.3. Corrosion resistance of conversion coating
Acknowledgements
In Fig. 4 the impedance spectra of the untreated magnesium electrode and the neodymium-treated electrode recorded in 3.5% NaCl solution after about 10 min immersion was shown. EIS of the treated sample showed only a capacitive loop and the untreated one had capacitive loop in the high frequency range and an inductive loop in the intermediate frequency range. The total impedance value of the treated sample was 4.5e+ 3 Ω cm2 and that of the untreated one was 1.3e+ 3 Ω cm2. Potentiodynamic polarization curve was employed to investigate corrosion resistance of the sample. In a typical polarization curve, lower corrosion densities correspond to lower corrosion rates and better corrosion resistance [14]. The sample immersed in 3.5 wt.% NaCl aqueous at room temperature after 30 min and the potentiodynamic polarization curves for the treated and untreated one are plotted in Fig. 5. Neodymium conversion coating on magnesium alloys surface decreased the corrosion current density (Icorr) about two orders of magnitude compared with the substrate, partially blocking the cathodic reaction and shifting the polarization curves toward lower current density values. It can also be obviously seen that the corrosion potential (Ecorr) of the coating was about 250 mV higher than that of the substrate. These results demonstrated that the anticorrosion capability of the magnesium alloys had been increased through the neodymium conversion treatment.
This work was financially supported by the National Natural Science Foundation of China (No. 50905038, 50875053), the Foundation of Heilongjiang (No. QC2010108, E201026), the Fundamental Research Funds for the Central Universities (No. HEUCF 101018, 101009).
4. Conclusion The neodymium conversion coating was a compact coating with crack-mud morphology. The coating was mainly made of neodymium oxides. The electrochemical experiments indicated that the corrosion resistance was improved through the neodymium conversion treatment.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
Campestrini P, Terryn H, Hovestad A, Wit J. Surf Coat Technol 2004;176:365–81. Heller D, Fahrenholtz W, Keefe M. Corros Sci 2010;52:360–8. Chen D, Li W, Gong W, Wu G, Wu J. Trans Nonferrous Met Soc China 2009;19:592–600. Rudd A, Breslin C, Mansfeld F. Corros Sci 2000;42:275–88. Yang L, Li J, Yu X, Zhang M, Huang X. Appl Surf Sci 2008;255:2338–41. Wang C, Zhu S, Jiang F, Wang F. Corros Sci 2009;51:2916–23. Yang X, Wang G, Dong G, Gong F, Zhang M. J Alloys Compd 2009;487:64–8. Montemor M, Simoes A, Carmezim M. Appl Surf Sci 2007;253:6992-31. Dabala M, Brunelli K, Napolitani E, Magrini M. Surf Coat Technol 2003;172:227–32. Montemor M, Simoes A, Ferreira M. Prog Org Coat 2002;44:111–20. Aramaki K. Corrors Sci 2001;43:2201–15. Tanaka T, Togashi S, Kamioka H, Amakawa H, Kagami H, Hamamoto T, et al. Chem Geol 2000;168:279–81. [13] Liu M, Uggowitzer P, Nagasekhar A, Schmutz P, Easton M, Song G, et al. Corrors Sci 2009;51:602–19. [14] Bethencourt M, Botana F, Cano M, Marcos M, Sánchez J, González L. Corrors Sci 2008;50:1376–84.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Chrome-free neodymium-based protective coatings for magnesium alloys Guo Jin a,⁎, Yuyun Yang a, Xiufang Cui b, Erbao Liu a, Zhongyu Wang a, Qingfen Li a a b
Center for Biomedical Materials and Engineering, School of Materials Science and Chemical Engineering, Harbin Engineering University, 145 Nantong St., Harbin 150001, China School of Materials Science and Chemical Engineering, Harbin Engineering University, 145 Nantong St., Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 3 November 2010 Accepted 8 December 2010 Available online 15 December 2010 Keywords: Microstructure Corrosion Neodymium conversion coating Morphology
a b s t r a c t The microstructure of chrome-free neodymium-based conversion coating on magnesium alloy was investigated and the corrosion resistance was evaluated as well. The micro-morphology, transverse section, crystal structure and composition of the coating were observed by scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS), respectively. The corrosion resistance was evaluated by potentiodynamic polarization curve and electrochemical impedance spectroscopy (EIS). The results revealed that the morphology of neodymium conversion coating is of crack-mud structure. Tiny cracks distribute in the compact coating deposited by neodymium oxides. EDS results characterize that the coating is made of neodymium oxides. The potentiodynamic polarization curve, EIS and OCP indicate that the neodymium conversion coating can improve the corrosion resistance of magnesium alloys. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The lanthanides ions of rare earth elements, as Ce3+, La3+, Y3+, Nd3+, Pr3+, have a lower toxicity and are considered as the best candidate for replacing the hexavalent chromium compounds of high toxicity. In previous work, lanthanum and cerium conversion coating are well known to inhibit the corrosion processes on some alloys such as aluminium alloys [1–3], magnesium alloys [4–9] and zinc [10,11]. But there is no paper reported on neodymium conversion coating. Commonly, neodymium is used as the isotopic reference material [12] in geology. In this article, newly developed environmentally surface treatments were proposed based on Nd3+as alternatives to toxic chromate-based systems. Micro-morphology, microstructure, element area profile of the transverse section and corrosion performance of the neodymium conversion coating were studied according to priority. Surface and transverse section examinations were performed by SEM, EDS and XRD. Corrosion behavior of the neodymium coating on AZ91 surface in 3.5% NaCl solution were carried out by potentiodynamic polarization curve and EIS.
2. Experimental methods 2.1. Preparation of samples The neodymium conversion coating was obtained on the matrix of die-cast AZ91. Test samples (15 mm × 10 mm × 5 mm) were polished using waterproof abrasive paper from 360 grits to 2500 grits, then fine ⁎ Corresponding author. Tel.: +86 451 82518173. E-mail address: [email protected] (G. Jin). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.12.019
polished using diamond paste of 3.5 μm, and then they were degreased with absolute ethanol in ultrasonic bath for 15 min and subsequently dried by cold air in room temperature. 2.2. Conversion coating treatment The optimal technical parameters determined by orthogonal experiment were the neodymium nitrate, Nd(NO3)3, with concentrations of 5 g/L, and 20 ml/L of hydrogen dioxide solution, H2O2 (Fisher Scientific, 30%) used as the accelerant. The obtained solution had been mixed for 5 min with a magnetic stirrer prior to deposition. The samples were immersed in the conversion solution at 50 °C for 20 min. After all these treatments, the samples were thoroughly rinsed with deionized water and then dried in cold air. 2.3. Characterization and corrosion test The micro-morphology was observed by a FEI Quant200 SEM equipped with EDS. The crystal structure of the conversion coating was studied by XRD. The anticorrosion performance was evaluated by recording the corrosion potential and corrosion current density of the samples in 3.5% sodium chloride (NaCl) solution upon immersion, which was conducted on a CHI660B electrochemical workstation. The electrochemical cell used a classic three-electrode system which consists of a reference electrode (a saturated calomel electrode), a counter electrode (a platinum foil) and a working electrode. The potentiodynamic polarization curves were performed at a scanning rate of 1 mV/s. OCP and EIS measurements were carried out at corrosion potential in a frequency range between 0.01 Hz and 100,000 Hz using a 10 mV amplitude perturbation.
1146
G. Jin et al. / Materials Letters 65 (2011) 1145–1147
12000
-Nd2 O3 -Mg
Intensity(a.u.)
10000 the treated sample 8000 6000 4000 the bare sample
2000 0 20
25
30
35
40
45
50
55
60
65
2θ ( ° ) Fig. 3. XRD pattern of Nd conversion coating and the bare magnesium alloy.
Fig. 1. SEM morphology of conversion coating on magnesium alloy.
3. Results and discussions 3.1. Morphology and composition of conversion coating SEM was used to reveal the morphology of the neodymium conversion coating. As shown in Fig. 1, the tiny and thick cracks distributed in the compact coating were obtained at the optimal preparation process. The magnified map, placed at the lower right corner of Fig. 1, characterized the crack-mud structure of the neodymium conversion coating. The coating has being forming by depositing neodymium oxides which were determined via XRD and EDS measurements. The atomic percentage (at.%) histogram of Mg, O, and Nd in neodymium conversion coating is shown as following: O (70.58 at.%) dominated in the coating, Mg was 4.84 at.%, Al was 0.97 at.% and Nd was 23.6 at.% in the neodymium coating. The column bar was consistent with the phenomenon revealed by SEM. All these results could be tested and verified by the transverse section of conversion specimen and the element area profile.
The morphology of the transverse section of conversion specimen and Element area profile were shown in Fig. 2. The conversion coating was about 4–5 μm approximately consisted of two layers. The first layer formed on the surface of magnesium alloys was loose with cracks and the second layer covered above the first layer was compact with less and tiny cracks. As shown in element area profile, Mg concentrated in the matrix and simultaneously, there was a handful of Mg near the coating. Al also concentrated in matrix, particularly abundant in second phase [13]. Nd was found rich in coating, which indicated the neodymium conversion coating formed on the surface of magnesium alloy. O spread in matrix and coating, especially in coating. It revealed that coating was mainly made of the oxides of Nd. 3.2. XRD analysis of coating XRD patterns of sample are shown in Fig. 3. The peaks of the neodymium element and its chemical compound appeared at low angle. The appearance of the bread-shaped peak from 21° to 29° in the XRD pattern indicated that the formed lanthanum conversion coating on the matrix was of amorphous structure. High intensity La2O3 peaks was present in the figure. The results of the XRD patterns in Fig. 4 corresponded with the results from SEM and EDS.
Fig. 2. SEM morphology of the cross section of conversion specimen and element area profile.
G. Jin et al. / Materials Letters 65 (2011) 1145–1147
2400
the treated sample the untreated sample
2100
0.0 the treated sample
Ecorr(V vs. SEC)
1800
-Z''/ohm
1147
1500 1200 900 600
the untreated sample
-0.5 -1.0 -1.5
300 -2.0
0 0
1000
2000
3000
4000
5000
10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1
Z'/ohm
100
icorr(A/cm2)
Fig. 4. EIS spectra for the treated and the untreated samples. Fig. 5. Potentiodynamic polarization curves of the treated and untreated samples.
3.3. Corrosion resistance of conversion coating
Acknowledgements
In Fig. 4 the impedance spectra of the untreated magnesium electrode and the neodymium-treated electrode recorded in 3.5% NaCl solution after about 10 min immersion was shown. EIS of the treated sample showed only a capacitive loop and the untreated one had capacitive loop in the high frequency range and an inductive loop in the intermediate frequency range. The total impedance value of the treated sample was 4.5e+ 3 Ω cm2 and that of the untreated one was 1.3e+ 3 Ω cm2. Potentiodynamic polarization curve was employed to investigate corrosion resistance of the sample. In a typical polarization curve, lower corrosion densities correspond to lower corrosion rates and better corrosion resistance [14]. The sample immersed in 3.5 wt.% NaCl aqueous at room temperature after 30 min and the potentiodynamic polarization curves for the treated and untreated one are plotted in Fig. 5. Neodymium conversion coating on magnesium alloys surface decreased the corrosion current density (Icorr) about two orders of magnitude compared with the substrate, partially blocking the cathodic reaction and shifting the polarization curves toward lower current density values. It can also be obviously seen that the corrosion potential (Ecorr) of the coating was about 250 mV higher than that of the substrate. These results demonstrated that the anticorrosion capability of the magnesium alloys had been increased through the neodymium conversion treatment.
This work was financially supported by the National Natural Science Foundation of China (No. 50905038, 50875053), the Foundation of Heilongjiang (No. QC2010108, E201026), the Fundamental Research Funds for the Central Universities (No. HEUCF 101018, 101009).
4. Conclusion The neodymium conversion coating was a compact coating with crack-mud morphology. The coating was mainly made of neodymium oxides. The electrochemical experiments indicated that the corrosion resistance was improved through the neodymium conversion treatment.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
Campestrini P, Terryn H, Hovestad A, Wit J. Surf Coat Technol 2004;176:365–81. Heller D, Fahrenholtz W, Keefe M. Corros Sci 2010;52:360–8. Chen D, Li W, Gong W, Wu G, Wu J. Trans Nonferrous Met Soc China 2009;19:592–600. Rudd A, Breslin C, Mansfeld F. Corros Sci 2000;42:275–88. Yang L, Li J, Yu X, Zhang M, Huang X. Appl Surf Sci 2008;255:2338–41. Wang C, Zhu S, Jiang F, Wang F. Corros Sci 2009;51:2916–23. Yang X, Wang G, Dong G, Gong F, Zhang M. J Alloys Compd 2009;487:64–8. Montemor M, Simoes A, Carmezim M. Appl Surf Sci 2007;253:6992-31. Dabala M, Brunelli K, Napolitani E, Magrini M. Surf Coat Technol 2003;172:227–32. Montemor M, Simoes A, Ferreira M. Prog Org Coat 2002;44:111–20. Aramaki K. Corrors Sci 2001;43:2201–15. Tanaka T, Togashi S, Kamioka H, Amakawa H, Kagami H, Hamamoto T, et al. Chem Geol 2000;168:279–81. [13] Liu M, Uggowitzer P, Nagasekhar A, Schmutz P, Easton M, Song G, et al. Corrors Sci 2009;51:602–19. [14] Bethencourt M, Botana F, Cano M, Marcos M, Sánchez J, González L. Corrors Sci 2008;50:1376–84.