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Application of magnesium phosphate coating on low carbon steel via electrochemical cathodic method and investigation of its corrosion resistance
Accepted Manuscript Application of magnesium phosphate coating on low carbon steel via electrochemical cathodic method and investigation of its corrosion resistance M.R. Dayyari, A. Amadeh, S. Sadreddini PII:
S0925-8388(15)30181-X
DOI:
10.1016/j.jallcom.2015.06.063
Reference:
JALCOM 34405
To appear in:
Journal of Alloys and Compounds
Received Date: 7 March 2015 Revised Date:
5 May 2015
Accepted Date: 7 June 2015
Please cite this article as: M.R. Dayyari, A. Amadeh, S. Sadreddini, Application of magnesium phosphate coating on low carbon steel via electrochemical cathodic method and investigation of its corrosion resistance, Journal of Alloys and Compounds (2015), doi: 10.1016/j.jallcom.2015.06.063. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
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Application of magnesium phosphate coating on low carbon steel via electrochemical cathodic method and investigation of its corrosion resistance M.R. Dayyari1,*, A. Amadeh2, S. Sadreddini1
SC
1- Department of Materials Science and Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran.
M AN U
2- Department of Materials Science and Engineering, university of Tehran, Tehran, Iran.
(٭Corresponding Author, Tel: +989302059595, [email protected])
TE D
Abstract
EP
In this study, magnesium phosphate coating was applied to low carbon steel via electrochemical method, its morphology was examined by scanning electron microscopy (SEM) and its crystalline structure was investigated by X-ray diffraction (XRD). Corrosion behavior of coated samples was evaluated by electrochemical impedance spectroscopy (EIS) and polarization techniques. The results showed that the application of magnesium phosphate coating significantly improved corrosion resistance behavior by forming Newberyite phase.
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Keywords: Magnesium phosphate coating; Thin films; Corrosion; Microstructure; X-ray techniques; electrochemical impedance spectroscopy.
1.
Introduction
Phosphate coating is typical conversion coating widely used in various industries [1, 2]. For applying this kind of coatings we usually use a solution of phosphoric acid and magnesium salts to apply this kind of coating by immersing the sample in a solution with or without electric current or by spraying the solution on the sample[3]. The main properties of this coating are increased corrosion resistance, improved adhesion between 1
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Experimental
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2.
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the topcoat and base metal and enhanced absorption of metal lubricant [4-8]. These coatings are applied on different metals and alloys such as steel, cast iron, aluminum, magnesium, etc [3, 9]. Phosphate coatings generally used in industry are calcium phosphate, manganese phosphate, etc [10-13]. Although the phosphate coating has good quality, we need to develop the current technology in all fields of industry. Moreover, the common phosphate coating is characterized with problems such as low thickness and inappropriate microstructure that causes some problems [14]. Applying low temperature in phosphating operation causes slowing down the progress, therefore using an accelerator is needed. Adding nitrites as an accelerator is the most common way in phosphating bath. However increasing nitrites concentrate causes increasing in phosphate deposit, but the environmental protection agency (EPA) determined nitride as a toxic, hence using nitrites as an accelerator could cause disposal problems [7], hence electrochemical method can be replaced with chemical methods. Thus, developing a new coating with higher quality can also help increase the performance. Because of low density of magnesium metal and its high specific strength, it's widely used in various industries. A side from these properties, given the low equilibrium standard potential of the magnesium, it is employed as sacrificial anode so as to improve the corrosion resistance of adjacent metal [1518]. Therefore, if these properties of magnesium can be exploited in phosphate coating, significant progress will appear in the coating industry, because magnesium has more negative potentials and lower density than that of Zn and Mn, which are widely used in phosphate coating. On the other hand, in determining the quality of a phosphate coating, many parameters such a pH, solvent temperature and immersion time, etc. should be considered. This study seeks to apply magnesium phosphate coating on steel substrates using the electrochemical cathodic method and to study the corrosion resistance of this coating.
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EP
In this study low carbon steel sheet (30 mm ×30 mm ×2 mm) was used as a substrate metal. The surface of the samples was polished with a series of emery papers up to 1500 grit. After grinding, they were degreased in acetone for 15 min (in ultrasonic bath). To keep the proper surface and activate the sample surface, the sheet was acid pickled using 10 wt% sulfuric acid for 20 seconds, which was then rinsed by deionized water. In this study electrochemical cathodic method were used to create magnesium phosphate coating. The final electrochemical coating of phosphate process was performed in a solution with a pH of 4.5±0.1 at 85 ± 1°Cfor 10 min and current density 5 mA/cm2. The low carbon steel sheet sample is used as a cathode and graphite is used as anode. A constant cathodic current was applied using a potentiostat. The composition of bath includes 16 ml/L of H3PO4 and 2 g/L of MgO. The pH was adjusted by NaOH solution. After phosphating scanning electron microscope (SEM, Seron Technology, AIS-2100) was used to study the surface morphology of coating. Afterwards, the phase analyze of the coating was performed by X-ray diffractometer (XRD, Phillips Xpert pro).The XRD spectra were obtained from Cu Kα radiation within a 2θ range from 10° to 90° and λ= 1.54 °A. The corrosion behavior of the coatings was evaluated the room temperature using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The set up consisted of a saturated calomel reference electrode (SCE), a platinum auxiliary electrode and a working electrode (the sample). The polarization curve of coated carbon steel was obtained by a EG&G 273A equipment with a scanning rate of 0.5 mV/s in the range of -0.6 to 0.6 V with respect to OCP in 3.5 wt% NaCl solution and then, the results were assessed by Corrview software. Moreover, the Nyquist diagram of coated carbon steel was evaluated using 2
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an EG&G 1025A in a frequency range of 10 kHz to 0.01 Hz in 3.5 wt% NaCl solution. The charge transfer resistance (Rct) and double layer capacity (Cdl) were evaluated by Zview software.
Results and discussion
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3.
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Fig1 shows SEM images of magnesium phosphate coating with two different magnifications. The entire surface of the substrate is covered by numerous crystals. In the presence of the coating, a coating thickness of 42 µm was achieved. After applying coating on the samples, as shown in Fig 2, the coating phase analysis was performed by X-ray diffraction (XRD). As can be seen, the two peaks at 17° and44° angles are indicative of the presence of iron generated by the infiltration of X-rays to the substrates of the steel. Other peaks are related to Newberyite (MgHPO4.3H2O).
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Fig 1 - SEM micrographs of surface morphology magnesium phosphate at different magnifications.
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Fig 2- XRD pattern of magnesium phosphate
The reactions leading to the formation of Newberyite are as follows: MeO → Me (OH)2 → Me (H2PO4)2 → MeHPO4 → Me3 (PO4)2
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EP
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First, metal oxide in contact with water and phosphoric acid turns into metal hydroxide, producing the primary (Me(H2PO4)2), secondary (MeHPO4) and tertiary (Me3(PO4)2) metal phosphates. The primary metal phosphates are completely soluble in water. Because of the low solubility of the secondary phosphate magnesium (MgHPO4) in water (0.0026 g/l), at this stage of the reaction, the secondary magnesium phosphate of the solution is deposited, obviating the possibility of tertiary magnesium phosphate precipitate deposition [18]. Fig 3 shows potentiodynamic polarization curves of magnesium phosphate coated samples. The results show that the corrosion potential of magnesium phosphate coating on low carbon steel shifts towards more positive potentials and the corrosion current decreases from 21.3 to 1.22 µA/cm2 in respect with substrate, indicating improved corrosion behavior. Table1 presents the results obtained from the drawing of Tafel curves.
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Fig 3 - Polarization curves for coated samples.
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Table 1- The influence of magnesium phosphate coating on carbon steel corrosion behavior Type
βa (mv/decade)
βc (mv/decade)
Icorr (µA/cm2)
Ecorr (V vs. SCE)
Cdl (µF.cm-2)
Rct (ohm.cm2)
Coated sample
35
46
1.22
-0.45
10.42
3110
39
41
21.3
-0.70
82.1
736
EP
Sample
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Fig 4 shows the Nyquist plotting of the magnesium phosphate coating on the low carbon steel substrate using the electrodeposition method. All specimens were immersed in 3.5 wt% sodium chloride solution for 20 min. Both curves have been drawn in a singular semicircular in the frequency range of 10 kHz to 0.01 Hz, which represents the charge control reaction. The difference in the area under curve is indicative of the corrosion resistance difference [19]. The values of Rct and Cdl are listed in Table 1. When magnesium phosphate applied, the area under the curve was greater than substrate. The Cdl values are related to the coating porosity [20]. According to Cdl values, it can be understood that the coating porosity decreases.
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4. Conclusion
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Fig 4 - Nyquist plot of magnesium phosphate carbon steel coated sample in 3.5 wt% NaCl solution.
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Magnesium phosphate was successfully applied on steel substrate using electrochemical cathodic method with the test revealing that no additional materials are required to be applied to the coating. With the formation of magnesium phosphate coating, the Newberyite phase is generated, and the corrosion resistance is improved. By applying the coating, Icorr was decreased from 21.3 to 1.22 µA/cm2.
Reference
[1] T.K. Rout, H.K. Pradhan, T. Venugopalan, Surface and Coatings Technology, 201 (2006) 3496-3501. [2] P.E. Tegehall, N.G. Vannerberg, Corrosion Science, 32 (1991) 635-652. [3] J. Weber, T. Biestek, Electrolytic and Chemical Conversion Coatings, A Concise Survey of Their Production, Properties and Testing, Portcullis Press, 1976, pp. 128-224. [4] D. Weng, P. Jokiel, A. Uebleis, H. Boehni, Surface and Coatings Technology, 88 (1996) 147-156. [5] M. Zubielewicz, E.K. Tarnawska, A. Kozlowska, Process in Organic Coatings, 53 (2005) 276-285. 6
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[6] P.K. Sinha, R. Feser, Electrochemical Acta, 51 (2005) 247-256. [7] S. Jegannathan, T.S. Narayanan, K. Ravichandran, S. Rajeswari, Surface and Coatings Technology, 200
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M AN U
SC
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(2006) 4117-4126. [8] S. Jegannathan, T.S. Narayanan, K. Ravichandran, S. Rajeswari, Progress in Organic Coatings, 57 (2006) 392-399. [9] J.R. Davis, Surface Engineering for Corrosion and Wear Resistance, ASM International, 2001, pp. 95124. [10] B. Mayor, J. Arias, S. Chiussi, F. Garcia, J. Pou, B.L. Fong, M.P. Amor, Thin Solid Films, 317 (1998) 363-366 [11] M. Hamdian, A.L. Ektessabi, Surface and Coatings Technology, 201 (2006) 3123-3128. [12] C.Y. Zhang, R.C. Zang, C.L. Liu, J.C. Gao, Surface and Coatings Technology, 204 (2010) 3636–3640. [13] C.M. Wang, H.C. Liau, W.T. Tasai, Surface and Coatings Technology, 201 (2006) 2994-3001. [14] G. Li, L. Niu, J. Lian, Z. Jiang, Surface and Coatings Technology, 176 (2004) 215-221. [15] M.F. Morks, Materials Lettrs, 58 (2004) 3316-3319. [16] T. Ishizaki, I. Shigematsu, N. Saito, Surface and Coatings Technology, 203 (2009) 2288-2291. [17] M. Fouladi, A. Amadeh, Materials Letters, 98 (2013) 1-4. [18] M. Fouladi, A. Amadeh, Electrochmica Acta, 106 (2013) 1-12. [19] S. Sadreddini, A. Afshar, Applied Surface Science, 303 (2014) 125–130. [20] S. Sadreddini, Z. Salehi, H. Rassaie, Applied Surface Science, 324 (2015) 393-398.
7
S0925-8388(15)30181-X
DOI:
10.1016/j.jallcom.2015.06.063
Reference:
JALCOM 34405
To appear in:
Journal of Alloys and Compounds
Received Date: 7 March 2015 Revised Date:
5 May 2015
Accepted Date: 7 June 2015
Please cite this article as: M.R. Dayyari, A. Amadeh, S. Sadreddini, Application of magnesium phosphate coating on low carbon steel via electrochemical cathodic method and investigation of its corrosion resistance, Journal of Alloys and Compounds (2015), doi: 10.1016/j.jallcom.2015.06.063. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
RI PT
Application of magnesium phosphate coating on low carbon steel via electrochemical cathodic method and investigation of its corrosion resistance M.R. Dayyari1,*, A. Amadeh2, S. Sadreddini1
SC
1- Department of Materials Science and Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran.
M AN U
2- Department of Materials Science and Engineering, university of Tehran, Tehran, Iran.
(٭Corresponding Author, Tel: +989302059595, [email protected])
TE D
Abstract
EP
In this study, magnesium phosphate coating was applied to low carbon steel via electrochemical method, its morphology was examined by scanning electron microscopy (SEM) and its crystalline structure was investigated by X-ray diffraction (XRD). Corrosion behavior of coated samples was evaluated by electrochemical impedance spectroscopy (EIS) and polarization techniques. The results showed that the application of magnesium phosphate coating significantly improved corrosion resistance behavior by forming Newberyite phase.
AC C
Keywords: Magnesium phosphate coating; Thin films; Corrosion; Microstructure; X-ray techniques; electrochemical impedance spectroscopy.
1.
Introduction
Phosphate coating is typical conversion coating widely used in various industries [1, 2]. For applying this kind of coatings we usually use a solution of phosphoric acid and magnesium salts to apply this kind of coating by immersing the sample in a solution with or without electric current or by spraying the solution on the sample[3]. The main properties of this coating are increased corrosion resistance, improved adhesion between 1
ACCEPTED MANUSCRIPT
Experimental
TE D
2.
M AN U
SC
RI PT
the topcoat and base metal and enhanced absorption of metal lubricant [4-8]. These coatings are applied on different metals and alloys such as steel, cast iron, aluminum, magnesium, etc [3, 9]. Phosphate coatings generally used in industry are calcium phosphate, manganese phosphate, etc [10-13]. Although the phosphate coating has good quality, we need to develop the current technology in all fields of industry. Moreover, the common phosphate coating is characterized with problems such as low thickness and inappropriate microstructure that causes some problems [14]. Applying low temperature in phosphating operation causes slowing down the progress, therefore using an accelerator is needed. Adding nitrites as an accelerator is the most common way in phosphating bath. However increasing nitrites concentrate causes increasing in phosphate deposit, but the environmental protection agency (EPA) determined nitride as a toxic, hence using nitrites as an accelerator could cause disposal problems [7], hence electrochemical method can be replaced with chemical methods. Thus, developing a new coating with higher quality can also help increase the performance. Because of low density of magnesium metal and its high specific strength, it's widely used in various industries. A side from these properties, given the low equilibrium standard potential of the magnesium, it is employed as sacrificial anode so as to improve the corrosion resistance of adjacent metal [1518]. Therefore, if these properties of magnesium can be exploited in phosphate coating, significant progress will appear in the coating industry, because magnesium has more negative potentials and lower density than that of Zn and Mn, which are widely used in phosphate coating. On the other hand, in determining the quality of a phosphate coating, many parameters such a pH, solvent temperature and immersion time, etc. should be considered. This study seeks to apply magnesium phosphate coating on steel substrates using the electrochemical cathodic method and to study the corrosion resistance of this coating.
AC C
EP
In this study low carbon steel sheet (30 mm ×30 mm ×2 mm) was used as a substrate metal. The surface of the samples was polished with a series of emery papers up to 1500 grit. After grinding, they were degreased in acetone for 15 min (in ultrasonic bath). To keep the proper surface and activate the sample surface, the sheet was acid pickled using 10 wt% sulfuric acid for 20 seconds, which was then rinsed by deionized water. In this study electrochemical cathodic method were used to create magnesium phosphate coating. The final electrochemical coating of phosphate process was performed in a solution with a pH of 4.5±0.1 at 85 ± 1°Cfor 10 min and current density 5 mA/cm2. The low carbon steel sheet sample is used as a cathode and graphite is used as anode. A constant cathodic current was applied using a potentiostat. The composition of bath includes 16 ml/L of H3PO4 and 2 g/L of MgO. The pH was adjusted by NaOH solution. After phosphating scanning electron microscope (SEM, Seron Technology, AIS-2100) was used to study the surface morphology of coating. Afterwards, the phase analyze of the coating was performed by X-ray diffractometer (XRD, Phillips Xpert pro).The XRD spectra were obtained from Cu Kα radiation within a 2θ range from 10° to 90° and λ= 1.54 °A. The corrosion behavior of the coatings was evaluated the room temperature using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The set up consisted of a saturated calomel reference electrode (SCE), a platinum auxiliary electrode and a working electrode (the sample). The polarization curve of coated carbon steel was obtained by a EG&G 273A equipment with a scanning rate of 0.5 mV/s in the range of -0.6 to 0.6 V with respect to OCP in 3.5 wt% NaCl solution and then, the results were assessed by Corrview software. Moreover, the Nyquist diagram of coated carbon steel was evaluated using 2
ACCEPTED MANUSCRIPT
an EG&G 1025A in a frequency range of 10 kHz to 0.01 Hz in 3.5 wt% NaCl solution. The charge transfer resistance (Rct) and double layer capacity (Cdl) were evaluated by Zview software.
Results and discussion
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3.
EP
TE D
M AN U
SC
Fig1 shows SEM images of magnesium phosphate coating with two different magnifications. The entire surface of the substrate is covered by numerous crystals. In the presence of the coating, a coating thickness of 42 µm was achieved. After applying coating on the samples, as shown in Fig 2, the coating phase analysis was performed by X-ray diffraction (XRD). As can be seen, the two peaks at 17° and44° angles are indicative of the presence of iron generated by the infiltration of X-rays to the substrates of the steel. Other peaks are related to Newberyite (MgHPO4.3H2O).
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Fig 1 - SEM micrographs of surface morphology magnesium phosphate at different magnifications.
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Fig 2- XRD pattern of magnesium phosphate
The reactions leading to the formation of Newberyite are as follows: MeO → Me (OH)2 → Me (H2PO4)2 → MeHPO4 → Me3 (PO4)2
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EP
TE D
First, metal oxide in contact with water and phosphoric acid turns into metal hydroxide, producing the primary (Me(H2PO4)2), secondary (MeHPO4) and tertiary (Me3(PO4)2) metal phosphates. The primary metal phosphates are completely soluble in water. Because of the low solubility of the secondary phosphate magnesium (MgHPO4) in water (0.0026 g/l), at this stage of the reaction, the secondary magnesium phosphate of the solution is deposited, obviating the possibility of tertiary magnesium phosphate precipitate deposition [18]. Fig 3 shows potentiodynamic polarization curves of magnesium phosphate coated samples. The results show that the corrosion potential of magnesium phosphate coating on low carbon steel shifts towards more positive potentials and the corrosion current decreases from 21.3 to 1.22 µA/cm2 in respect with substrate, indicating improved corrosion behavior. Table1 presents the results obtained from the drawing of Tafel curves.
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Fig 3 - Polarization curves for coated samples.
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Table 1- The influence of magnesium phosphate coating on carbon steel corrosion behavior Type
βa (mv/decade)
βc (mv/decade)
Icorr (µA/cm2)
Ecorr (V vs. SCE)
Cdl (µF.cm-2)
Rct (ohm.cm2)
Coated sample
35
46
1.22
-0.45
10.42
3110
39
41
21.3
-0.70
82.1
736
EP
Sample
AC C
Fig 4 shows the Nyquist plotting of the magnesium phosphate coating on the low carbon steel substrate using the electrodeposition method. All specimens were immersed in 3.5 wt% sodium chloride solution for 20 min. Both curves have been drawn in a singular semicircular in the frequency range of 10 kHz to 0.01 Hz, which represents the charge control reaction. The difference in the area under curve is indicative of the corrosion resistance difference [19]. The values of Rct and Cdl are listed in Table 1. When magnesium phosphate applied, the area under the curve was greater than substrate. The Cdl values are related to the coating porosity [20]. According to Cdl values, it can be understood that the coating porosity decreases.
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4. Conclusion
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Fig 4 - Nyquist plot of magnesium phosphate carbon steel coated sample in 3.5 wt% NaCl solution.
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EP
Magnesium phosphate was successfully applied on steel substrate using electrochemical cathodic method with the test revealing that no additional materials are required to be applied to the coating. With the formation of magnesium phosphate coating, the Newberyite phase is generated, and the corrosion resistance is improved. By applying the coating, Icorr was decreased from 21.3 to 1.22 µA/cm2.
Reference
[1] T.K. Rout, H.K. Pradhan, T. Venugopalan, Surface and Coatings Technology, 201 (2006) 3496-3501. [2] P.E. Tegehall, N.G. Vannerberg, Corrosion Science, 32 (1991) 635-652. [3] J. Weber, T. Biestek, Electrolytic and Chemical Conversion Coatings, A Concise Survey of Their Production, Properties and Testing, Portcullis Press, 1976, pp. 128-224. [4] D. Weng, P. Jokiel, A. Uebleis, H. Boehni, Surface and Coatings Technology, 88 (1996) 147-156. [5] M. Zubielewicz, E.K. Tarnawska, A. Kozlowska, Process in Organic Coatings, 53 (2005) 276-285. 6
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[6] P.K. Sinha, R. Feser, Electrochemical Acta, 51 (2005) 247-256. [7] S. Jegannathan, T.S. Narayanan, K. Ravichandran, S. Rajeswari, Surface and Coatings Technology, 200
AC C
EP
TE D
M AN U
SC
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(2006) 4117-4126. [8] S. Jegannathan, T.S. Narayanan, K. Ravichandran, S. Rajeswari, Progress in Organic Coatings, 57 (2006) 392-399. [9] J.R. Davis, Surface Engineering for Corrosion and Wear Resistance, ASM International, 2001, pp. 95124. [10] B. Mayor, J. Arias, S. Chiussi, F. Garcia, J. Pou, B.L. Fong, M.P. Amor, Thin Solid Films, 317 (1998) 363-366 [11] M. Hamdian, A.L. Ektessabi, Surface and Coatings Technology, 201 (2006) 3123-3128. [12] C.Y. Zhang, R.C. Zang, C.L. Liu, J.C. Gao, Surface and Coatings Technology, 204 (2010) 3636–3640. [13] C.M. Wang, H.C. Liau, W.T. Tasai, Surface and Coatings Technology, 201 (2006) 2994-3001. [14] G. Li, L. Niu, J. Lian, Z. Jiang, Surface and Coatings Technology, 176 (2004) 215-221. [15] M.F. Morks, Materials Lettrs, 58 (2004) 3316-3319. [16] T. Ishizaki, I. Shigematsu, N. Saito, Surface and Coatings Technology, 203 (2009) 2288-2291. [17] M. Fouladi, A. Amadeh, Materials Letters, 98 (2013) 1-4. [18] M. Fouladi, A. Amadeh, Electrochmica Acta, 106 (2013) 1-12. [19] S. Sadreddini, A. Afshar, Applied Surface Science, 303 (2014) 125–130. [20] S. Sadreddini, Z. Salehi, H. Rassaie, Applied Surface Science, 324 (2015) 393-398.
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