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Investigation of corrosion behavior of high nickel ductile iron by laser surface alloying with copper
Scripta mater. 44 (2001) 2747–2752 www.elsevier.com/locate/scriptamat
INVESTIGATION OF CORROSION BEHAVIOR OF HIGH NICKEL DUCTILE IRON BY LASER SURFACE ALLOYING WITH COPPER Dawen Zeng1, K.C. Yung2 and Changsheng Xie1 1
The State Key Laboratory of Plastic Forming Simulation and Mould Technology, Huazhong University of Science and Technology, Wuhan (430074), Hubei, China 2 Department of Manufacturing Engineering, The Hong Kong Polytechnic University, Hunghom, Kowloon, Hong Kong (Received March 10, 2000) (Accepted September 12, 2000)
Keywords: Laser surface alloying; High nickel ductile iron; Corrosion; X-ray photoelectron spectroscopy (XPS)
Introduction High nickel ductile iron, with nickel contents varying from 13 to 37%, is one of the most widely used materials in corrosive environments due to its excellent resistance to corrosion by alkaline, dilute acids and chloride containing media [1–2]. Despite its successful application in various corrosive environments, this material is still being investigated and tested in order to obtain a better performance in alkali or saline solutions and to widen the scope of its application [3]. Surface addition of copper has been used to improve the corrosion resistance of high nickel grey cast iron [4]. For example, addition of up to 10% copper leads to increase of its corrosion resistance [2]. However, the solubility of copper in ferrite or austenite cast iron is restricted [5]. Any copper present in excess of the solubility of copper limit in the molten cast iron will appear as globules of insoluble primary copper dispersed in the austenitic matrix, which is harmful to high nickel ductile iron due to galvanic corrosion. To overcome the solid solubility constraint, an attempt has been made recently to develop Fe-Cu or Fe-Ni-Cu alloys by laser surface alloying (LSA) to enhance their corrosion resistance or to achieve a higher thermal conductivity [6 –7]. Recent investigation on Cu-Ni (⬍40wt%) alloys had shown that their good corrosion inhibition has to be correlated with a stable passivity in chloride solutions established by the formation of a compact Cu2O oxide film due to its low electronic conductivity [8 –11]. Also, similar to that of Cu-Ni alloys [7], the improvement of the corrosion resistance by LSA of high nickel ductile iron with pure copper is attributed to formation of a compact Cu2O oxide film, but was not substantiated with proof. The aim of this research was to investigate on corrosion behavior of LSA of high nickel ductile iron with pure copper using X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) with an energy dispersion X-ray (EDX) spectrometer.
1359-6462/01/$–see front matter. © 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S1359-6462(01)00958-7
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Figure 1. A cross-section micrograph of alloyed high nickel ductile iron.
Experimental The substrate material used in this study was high nickel ductile iron with chemical composition Fe-37Ni-2.11Cr-1.63Mo-0.38Mn-3.6Si-2.6C (wt. %). The alloying powders were pure coppers (99.99%) with particle size ranging from mesh 150 to 200. LSA was performed in two steps. Initially, the powders were sprayed on the surface of the high nickel ductile iron substrate (surface area 70 ⫻ 35 mm2, thickness 10 mm) by means of a flame spray technique. The coating thickness achieved was 0.2mm. Subsequently, the thermal-sprayed coating was remelted using a 2 kW continuous wave CO2 laser. Experiments were performed to quantify effect of the laser surface alloying parameters on distribution of copper and microstructure of alloyed layer. Within the range of laser parameters employed, the best result (20% content and homogeneous distribution of copper in alloyed layer) was observed at a 2 mm/s scanning speed, 5mm laser beam diameter and 2kW laser power [7]. The microstructure of the laser-surface-alloyed samples were observed by scanning electron microscopy (SEM) with an energy dispersion X-ray (EDX) spectrometer. The latter was used to detect the chemical compositions of the surface alloyed layer and the eroded surface. X-ray photoelectron spectroscopy (XPS) spectra were obtained by means of a Quantum 2000 spectrometer using a non-monochromatized Al K␣excitation radiation. During the measurement, the pressure in the working chamber was kept at below 5.0 ⫻ 10⫺8Pa. The eroded surface was etched, as required, by argon ion bombardment. The corrosion behavior of the alloyed or unalloyed surface of the samples was evaluated by using an EG & G Potentiostat/Galvanostat Model 273A corrosion measurement system. Anodic polarization tests were carried out in a 0.1wt% HCl solution. The samples were driven from an Ecorr of ⫺600 mV to 1.6 V at a scanning rate of 0.5mVs⫺1 to produce potentiodynamic polarization plots. The initial delay was set to 1 hour. All potentials were measured with reference to a standard calomel electrode (SCE). Results and Discussion Corrosion Behavior A cross-sectional view of alloyed high nickel ductile iron is shown in Fig 1. Few features are worth noticed. A smooth, porous-free and crack-free alloyed zone is formed across the depth of the substrate,
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Figure 2. Potentiodynamic anodic polarization curves of the untreated and laser-surface-alloyed samples.
and spheroidal graphite and coarse eutectic ledeburite are not seen. The microstructure of the alloyed zone is essentially fine austenite dendrite. Our previous results have showed that the profile of Ni and Cu elements across the melted pool was fairly uniform due to intensive mass transfer by convection [7]. The solubility of copper in high nickel ductile iron is ⬃20% in the alloyed zone at this processing condition, and significantly exceeds the maximum solubility due to non-equilibrium rapid solidification. These results are good for enhancing the corrosion resistance of laser-surface-alloyed sample. Some typical potentiodynamic anodic polarization plots of the untreated sample and the lasersurface-alloyed sample are shown in Fig. 2. The experimental results show that the curve of the laser-surface-alloyed sample was shifted downward and to the right of the curve of the untreated sample, which indicates that laser-surface-alloyed sample was nobler than the untreated sample and the corrosion current was much reduced. In general, the corrosion current (icorr) was reduced by 50% after LSA. Comparing the corrosion potential of the laser-surface-alloyed sample and the untreated sample, an increase of 200 mV was obtained for the laser-surface-alloyed sample. Fig. 3 compares the corroded surface morphology of the untreated and laser-surface-alloyed sample. The untreated surface was found to experience localized attack at the boundaries of the spheroidal graphite particles and at the austenitic grain boundary, resulting in the formation of the network of fine cracks at the austenitic grain boundary that was covered with a non-homogeneous and loose scale-like layer. Some spheroidal graphite peeled off and floated on the corroded surface and the untreated surface
Figure 3. Corroded surface morphology: (a) untreated, and (b) laser-surface–alloyed sample.
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Figure 4. EDX spectra of corroded surface morphology: (a) untreated, and (b) laser-surface–alloyed sample.
was badly eroded. On the contrary, a much cleaner surface was obtained for the laser-surface-alloyed sample, apart from absorbing a few undissolved graphite spheroids in it. The improvement of the corrosion resistance of the high nickel ductile iron by LSA with pure copper is therefore confirmed in Fig. 3. Fig. 4 presents the EDX spectra from the eroded surface of the untreated and laser-surface-alloyed sample. As seen from Fig. 4, O signals are found for both samples. This indicates that oxide film formed on the eroded surface. For laser-surface-alloyed sample, Cu signals are detected and its intensity is higher than that of nickel. Chemical compositions of the eroded surface of the untreated and alloyed sample are listed in Table 1. It is found that the Cu content of the eroded surface (42.85%) is two times more than that before corrosion (⬃20%) for the laser-surface-alloyed sample. This indicates that the Cu could play a significant role for the improvement of the corrosion resistance of LSA of high nickel ductile iron. X-Ray Photoelectron Spectroscopy (XPS) The difference in the corrosion behavior of the untreated and laser-surface-alloyed sample was investigated by XPS. The XPS survey spectra of both samples and Cu 2p spectra after corrosion in 0.1% HCl solution are presented in Fig. 5. For the untreated sample, the XPS survey spectrum obtained from the eroded surface did not show the pronounced peak of Ni and Fe, and only peaks of oxygen (O1s 532eV) and carbon (C 1s at 284.8eV) were detected (Fig. 5a). This is due to existence of a loose scale-like layer, which is confirmed by the SEM picture (Fig. 3a). After 4 minutes of surface etching, which is equivalent to 20nm etched thickness, the characteristic peaks of nickel (Ni 2p at 871.2eV and 853.6eV, Ni LMM at 642.4eV), iron (Fe LMM at 784eV, Fe 2p at 721.6eV and 708eV) and molybdenum (Mo 2p at 412.8eV and 394.4eV, Mo 3d at 231.2eV) were detected besides peaks of TABLE 1 EDX Results of Corroded Surface of the Untreated and Alloyed Samples (wt. %) Type
O
Si
Cr
Cl
Mo
Fe
Ni
Cu
Untreated Alloyed
6.34 3.43
3.99 0.79
3.92 2.94
— 1.79
4.81 3.12
46.93 33.66
33.99 11.42
— 42.85
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Figure 5. XPS spectra (a) survey spectrum at depth of 20 nm from the corroded surface of the untreated sample, (b) survey spectrum obtained from the corroded surface of the alloyed sample and (c) Cu 2p spectra of the alloyed sample.
oxygen and carbon (see Fig. 5a). These results show that Ni, Fe and Mo are in oxide states. Therefore, the corrosion behavior of the untreated sample can be related to formation of a layer of the metallic oxides, which are responsible for the good corrosion resistance of high nickel ductile iron. In Fig. 5(b), the XPS survey spectrum obtained from the eroded surface of the laser-surface-alloyed sample shows clear Cu peaks (Cu 2p3/2 at 932.8eV and Cu 2p1/2 at 952.8eV, Cu LMM at 569.6eV, the small Cu 3p at 75eV), oxygen peak (O 1s at 530.4eV) and carbon peak, and no signal of nickel and iron were evident. The results of XPS analysis indicate that besides O and C, the eroded surface contains Cu only, which plays an important effect on the corrosion resistance improvement of high nickel ductile iron by LSA. Further, the Cu and O peaks obtained are in considerable agreement with the XPS data of Cu2O that have been reported by Vasquez (Cu 2p3/2 at 932.5eV, Cu LMM at 569.7eV and O 1s at 530.3eV) [12]. This means that Cu2O oxide film was formed on the alloyed surface in the chloride solution. Cu 2p spectra of not-eroded and eroded surface of laser-surface-alloyed sample are presented in Fig. 5(c). For both samples, peaks clearly appeared at the same binding energies of 932.8eV and 952.8eV, which corresponded to Cu 2p3/2 and Cu 2p1/2 respectively. This substantiates the conclusion that the corrosion product of laser-surface-alloyed sample is Cu2O, because the binding energies of Cu 2p3/2 and Cu 2p1/2 of Cu2O are very close to the values of metallic Cu [13]. On the other hand, small satellite peaks were clearly observed at the higher binding energy side of the main peak. This is another proof of presence of Cu2O oxide film on the eroded surface of laser-surface-alloyed sample. Therefore, a compact Cu2O passive oxide film on the alloyed surface in the chloride solution is mainly responsible
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for the enhancement of corrosion resistance for the high nickel ductile iron by LSA with pure copper due to its low conductivity, as already reported by several authors [8 –11]. Further, Barbucci and co-workers [11] reported that Ni could enter the defective structure of Cu2O, reducing its conductivity and hence, enhancing corrosion inhibition. In this study, however, no nickel existed in Cu2O. The reason is not clear and needs to be studied further. Conclusions The corrosion resistance of the high nickel ductile iron was improved after laser surface alloying with pure copper. XPS analysis results for the corrosion products of the laser-surface-alloyed sample show that a compact Cu2O oxide passive film on the alloyed surface in the chloride solution is formed. Due to its low electric conductivity, the compact Cu2O oxide passive film on the alloyed surface is mainly responsible for the enhancement of corrosion resistance of the high nickel ductile iron by LSA with pure copper. Acknowledgments Financial supports by Foundation of Huazhong University of Science and Technology (HUST), Talent Foundation of HUST and Foundation of the State Key Laboratory of Plastic Forming Simulation and Mould Technology are gratefully acknowledged. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
C. Labrecque and M. Gagne´, Can. Metall. Q. 37, 343 (1998). J. R. Davis, ASM Specialty Handbook: Cast Irons, p. 437, ASM International, Materials Park, OH (1996). H. M. Shalaby, S. Attari, W. T. Riad, and V. K. Gouda, Corrosion. 48, 206 (1992). A. Belfrouh, C. Masson, D. Vouagner, A. M. De Becdelievre, N. S. Prakash, and J. P. Audouard, Corros. Sci. 38, 1639 (1996). J. G. Pearce and K. Bromage, Copper in Cast Iron, Hutchinson, London (1964). K. K. Prashanth, K. Chattopadhyay, and J. Mazumder, J. Mater. Sci. 34, 3437 (1999). D. Zeng, C. Xie, Q. Ho, and K. C. Yung, Scripta Mater. submitted for publication. R. G. Blundy and M. J. Pryor, Corros. Sci. 12, 65 (1972). R. F. North and M. J. Pryor, Corros. Sci. 10, 297 (1970). I. Milosev and M. Metikos-Hukovic, Electrochim. Acta, 42, 1537 (1997). A. Barbucci, G. Farne`, P. Matteazzi, R. Riccieri, and G. Cerisola, Corros. Sci. 41, 463 (1999). R. P. Vasquez, Surf. Sci. Spectra, 5, 257 (1998).
INVESTIGATION OF CORROSION BEHAVIOR OF HIGH NICKEL DUCTILE IRON BY LASER SURFACE ALLOYING WITH COPPER Dawen Zeng1, K.C. Yung2 and Changsheng Xie1 1
The State Key Laboratory of Plastic Forming Simulation and Mould Technology, Huazhong University of Science and Technology, Wuhan (430074), Hubei, China 2 Department of Manufacturing Engineering, The Hong Kong Polytechnic University, Hunghom, Kowloon, Hong Kong (Received March 10, 2000) (Accepted September 12, 2000)
Keywords: Laser surface alloying; High nickel ductile iron; Corrosion; X-ray photoelectron spectroscopy (XPS)
Introduction High nickel ductile iron, with nickel contents varying from 13 to 37%, is one of the most widely used materials in corrosive environments due to its excellent resistance to corrosion by alkaline, dilute acids and chloride containing media [1–2]. Despite its successful application in various corrosive environments, this material is still being investigated and tested in order to obtain a better performance in alkali or saline solutions and to widen the scope of its application [3]. Surface addition of copper has been used to improve the corrosion resistance of high nickel grey cast iron [4]. For example, addition of up to 10% copper leads to increase of its corrosion resistance [2]. However, the solubility of copper in ferrite or austenite cast iron is restricted [5]. Any copper present in excess of the solubility of copper limit in the molten cast iron will appear as globules of insoluble primary copper dispersed in the austenitic matrix, which is harmful to high nickel ductile iron due to galvanic corrosion. To overcome the solid solubility constraint, an attempt has been made recently to develop Fe-Cu or Fe-Ni-Cu alloys by laser surface alloying (LSA) to enhance their corrosion resistance or to achieve a higher thermal conductivity [6 –7]. Recent investigation on Cu-Ni (⬍40wt%) alloys had shown that their good corrosion inhibition has to be correlated with a stable passivity in chloride solutions established by the formation of a compact Cu2O oxide film due to its low electronic conductivity [8 –11]. Also, similar to that of Cu-Ni alloys [7], the improvement of the corrosion resistance by LSA of high nickel ductile iron with pure copper is attributed to formation of a compact Cu2O oxide film, but was not substantiated with proof. The aim of this research was to investigate on corrosion behavior of LSA of high nickel ductile iron with pure copper using X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) with an energy dispersion X-ray (EDX) spectrometer.
1359-6462/01/$–see front matter. © 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S1359-6462(01)00958-7
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Figure 1. A cross-section micrograph of alloyed high nickel ductile iron.
Experimental The substrate material used in this study was high nickel ductile iron with chemical composition Fe-37Ni-2.11Cr-1.63Mo-0.38Mn-3.6Si-2.6C (wt. %). The alloying powders were pure coppers (99.99%) with particle size ranging from mesh 150 to 200. LSA was performed in two steps. Initially, the powders were sprayed on the surface of the high nickel ductile iron substrate (surface area 70 ⫻ 35 mm2, thickness 10 mm) by means of a flame spray technique. The coating thickness achieved was 0.2mm. Subsequently, the thermal-sprayed coating was remelted using a 2 kW continuous wave CO2 laser. Experiments were performed to quantify effect of the laser surface alloying parameters on distribution of copper and microstructure of alloyed layer. Within the range of laser parameters employed, the best result (20% content and homogeneous distribution of copper in alloyed layer) was observed at a 2 mm/s scanning speed, 5mm laser beam diameter and 2kW laser power [7]. The microstructure of the laser-surface-alloyed samples were observed by scanning electron microscopy (SEM) with an energy dispersion X-ray (EDX) spectrometer. The latter was used to detect the chemical compositions of the surface alloyed layer and the eroded surface. X-ray photoelectron spectroscopy (XPS) spectra were obtained by means of a Quantum 2000 spectrometer using a non-monochromatized Al K␣excitation radiation. During the measurement, the pressure in the working chamber was kept at below 5.0 ⫻ 10⫺8Pa. The eroded surface was etched, as required, by argon ion bombardment. The corrosion behavior of the alloyed or unalloyed surface of the samples was evaluated by using an EG & G Potentiostat/Galvanostat Model 273A corrosion measurement system. Anodic polarization tests were carried out in a 0.1wt% HCl solution. The samples were driven from an Ecorr of ⫺600 mV to 1.6 V at a scanning rate of 0.5mVs⫺1 to produce potentiodynamic polarization plots. The initial delay was set to 1 hour. All potentials were measured with reference to a standard calomel electrode (SCE). Results and Discussion Corrosion Behavior A cross-sectional view of alloyed high nickel ductile iron is shown in Fig 1. Few features are worth noticed. A smooth, porous-free and crack-free alloyed zone is formed across the depth of the substrate,
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Figure 2. Potentiodynamic anodic polarization curves of the untreated and laser-surface-alloyed samples.
and spheroidal graphite and coarse eutectic ledeburite are not seen. The microstructure of the alloyed zone is essentially fine austenite dendrite. Our previous results have showed that the profile of Ni and Cu elements across the melted pool was fairly uniform due to intensive mass transfer by convection [7]. The solubility of copper in high nickel ductile iron is ⬃20% in the alloyed zone at this processing condition, and significantly exceeds the maximum solubility due to non-equilibrium rapid solidification. These results are good for enhancing the corrosion resistance of laser-surface-alloyed sample. Some typical potentiodynamic anodic polarization plots of the untreated sample and the lasersurface-alloyed sample are shown in Fig. 2. The experimental results show that the curve of the laser-surface-alloyed sample was shifted downward and to the right of the curve of the untreated sample, which indicates that laser-surface-alloyed sample was nobler than the untreated sample and the corrosion current was much reduced. In general, the corrosion current (icorr) was reduced by 50% after LSA. Comparing the corrosion potential of the laser-surface-alloyed sample and the untreated sample, an increase of 200 mV was obtained for the laser-surface-alloyed sample. Fig. 3 compares the corroded surface morphology of the untreated and laser-surface-alloyed sample. The untreated surface was found to experience localized attack at the boundaries of the spheroidal graphite particles and at the austenitic grain boundary, resulting in the formation of the network of fine cracks at the austenitic grain boundary that was covered with a non-homogeneous and loose scale-like layer. Some spheroidal graphite peeled off and floated on the corroded surface and the untreated surface
Figure 3. Corroded surface morphology: (a) untreated, and (b) laser-surface–alloyed sample.
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Figure 4. EDX spectra of corroded surface morphology: (a) untreated, and (b) laser-surface–alloyed sample.
was badly eroded. On the contrary, a much cleaner surface was obtained for the laser-surface-alloyed sample, apart from absorbing a few undissolved graphite spheroids in it. The improvement of the corrosion resistance of the high nickel ductile iron by LSA with pure copper is therefore confirmed in Fig. 3. Fig. 4 presents the EDX spectra from the eroded surface of the untreated and laser-surface-alloyed sample. As seen from Fig. 4, O signals are found for both samples. This indicates that oxide film formed on the eroded surface. For laser-surface-alloyed sample, Cu signals are detected and its intensity is higher than that of nickel. Chemical compositions of the eroded surface of the untreated and alloyed sample are listed in Table 1. It is found that the Cu content of the eroded surface (42.85%) is two times more than that before corrosion (⬃20%) for the laser-surface-alloyed sample. This indicates that the Cu could play a significant role for the improvement of the corrosion resistance of LSA of high nickel ductile iron. X-Ray Photoelectron Spectroscopy (XPS) The difference in the corrosion behavior of the untreated and laser-surface-alloyed sample was investigated by XPS. The XPS survey spectra of both samples and Cu 2p spectra after corrosion in 0.1% HCl solution are presented in Fig. 5. For the untreated sample, the XPS survey spectrum obtained from the eroded surface did not show the pronounced peak of Ni and Fe, and only peaks of oxygen (O1s 532eV) and carbon (C 1s at 284.8eV) were detected (Fig. 5a). This is due to existence of a loose scale-like layer, which is confirmed by the SEM picture (Fig. 3a). After 4 minutes of surface etching, which is equivalent to 20nm etched thickness, the characteristic peaks of nickel (Ni 2p at 871.2eV and 853.6eV, Ni LMM at 642.4eV), iron (Fe LMM at 784eV, Fe 2p at 721.6eV and 708eV) and molybdenum (Mo 2p at 412.8eV and 394.4eV, Mo 3d at 231.2eV) were detected besides peaks of TABLE 1 EDX Results of Corroded Surface of the Untreated and Alloyed Samples (wt. %) Type
O
Si
Cr
Cl
Mo
Fe
Ni
Cu
Untreated Alloyed
6.34 3.43
3.99 0.79
3.92 2.94
— 1.79
4.81 3.12
46.93 33.66
33.99 11.42
— 42.85
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Figure 5. XPS spectra (a) survey spectrum at depth of 20 nm from the corroded surface of the untreated sample, (b) survey spectrum obtained from the corroded surface of the alloyed sample and (c) Cu 2p spectra of the alloyed sample.
oxygen and carbon (see Fig. 5a). These results show that Ni, Fe and Mo are in oxide states. Therefore, the corrosion behavior of the untreated sample can be related to formation of a layer of the metallic oxides, which are responsible for the good corrosion resistance of high nickel ductile iron. In Fig. 5(b), the XPS survey spectrum obtained from the eroded surface of the laser-surface-alloyed sample shows clear Cu peaks (Cu 2p3/2 at 932.8eV and Cu 2p1/2 at 952.8eV, Cu LMM at 569.6eV, the small Cu 3p at 75eV), oxygen peak (O 1s at 530.4eV) and carbon peak, and no signal of nickel and iron were evident. The results of XPS analysis indicate that besides O and C, the eroded surface contains Cu only, which plays an important effect on the corrosion resistance improvement of high nickel ductile iron by LSA. Further, the Cu and O peaks obtained are in considerable agreement with the XPS data of Cu2O that have been reported by Vasquez (Cu 2p3/2 at 932.5eV, Cu LMM at 569.7eV and O 1s at 530.3eV) [12]. This means that Cu2O oxide film was formed on the alloyed surface in the chloride solution. Cu 2p spectra of not-eroded and eroded surface of laser-surface-alloyed sample are presented in Fig. 5(c). For both samples, peaks clearly appeared at the same binding energies of 932.8eV and 952.8eV, which corresponded to Cu 2p3/2 and Cu 2p1/2 respectively. This substantiates the conclusion that the corrosion product of laser-surface-alloyed sample is Cu2O, because the binding energies of Cu 2p3/2 and Cu 2p1/2 of Cu2O are very close to the values of metallic Cu [13]. On the other hand, small satellite peaks were clearly observed at the higher binding energy side of the main peak. This is another proof of presence of Cu2O oxide film on the eroded surface of laser-surface-alloyed sample. Therefore, a compact Cu2O passive oxide film on the alloyed surface in the chloride solution is mainly responsible
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for the enhancement of corrosion resistance for the high nickel ductile iron by LSA with pure copper due to its low conductivity, as already reported by several authors [8 –11]. Further, Barbucci and co-workers [11] reported that Ni could enter the defective structure of Cu2O, reducing its conductivity and hence, enhancing corrosion inhibition. In this study, however, no nickel existed in Cu2O. The reason is not clear and needs to be studied further. Conclusions The corrosion resistance of the high nickel ductile iron was improved after laser surface alloying with pure copper. XPS analysis results for the corrosion products of the laser-surface-alloyed sample show that a compact Cu2O oxide passive film on the alloyed surface in the chloride solution is formed. Due to its low electric conductivity, the compact Cu2O oxide passive film on the alloyed surface is mainly responsible for the enhancement of corrosion resistance of the high nickel ductile iron by LSA with pure copper. Acknowledgments Financial supports by Foundation of Huazhong University of Science and Technology (HUST), Talent Foundation of HUST and Foundation of the State Key Laboratory of Plastic Forming Simulation and Mould Technology are gratefully acknowledged. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
C. Labrecque and M. Gagne´, Can. Metall. Q. 37, 343 (1998). J. R. Davis, ASM Specialty Handbook: Cast Irons, p. 437, ASM International, Materials Park, OH (1996). H. M. Shalaby, S. Attari, W. T. Riad, and V. K. Gouda, Corrosion. 48, 206 (1992). A. Belfrouh, C. Masson, D. Vouagner, A. M. De Becdelievre, N. S. Prakash, and J. P. Audouard, Corros. Sci. 38, 1639 (1996). J. G. Pearce and K. Bromage, Copper in Cast Iron, Hutchinson, London (1964). K. K. Prashanth, K. Chattopadhyay, and J. Mazumder, J. Mater. Sci. 34, 3437 (1999). D. Zeng, C. Xie, Q. Ho, and K. C. Yung, Scripta Mater. submitted for publication. R. G. Blundy and M. J. Pryor, Corros. Sci. 12, 65 (1972). R. F. North and M. J. Pryor, Corros. Sci. 10, 297 (1970). I. Milosev and M. Metikos-Hukovic, Electrochim. Acta, 42, 1537 (1997). A. Barbucci, G. Farne`, P. Matteazzi, R. Riccieri, and G. Cerisola, Corros. Sci. 41, 463 (1999). R. P. Vasquez, Surf. Sci. Spectra, 5, 257 (1998).