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XPS study on double glow plasma corrosion-resisting surface alloying layer
Applied Surface Science 206 (2003) 230±236
XPS study on double glow plasma corrosion-resisting surface alloying layer Jiahe Aia,*, Jiang Xua, Fei Heb, Xishan Xiea, Zhong Xuc a
School of Materials Science and Engineering, University of Science and Technology, Beijing 100083, China b State Key Laboratory of C1 Chemistry and Technology, Tianjin University, Tianjin 300072, China c Institute of Surface Engineering, Taiyuan University of Technology, Taiyuan 030024, China Received 3 July 2002; received in revised form 3 July 2002; accepted 2 November 2002
Abstract Double glow plasma corrosion-resisting surface alloying layer (SAL) formed on low carbon steel 1020 was studied by X-ray photoelectron spectroscopy (XPS) and other means. Results show that the passive ®lm of the surface alloying layer after electrochemical test in 3.5% NaCl solution consists of Cr and Fe oxide such as CrO3, Cr2O3, Fe2O3 and FeO and metallic Ni and Mo, and it attributes to the fact that a continuous and compact corrosion-resisting surface alloying layer with rich Cr, Ni and Mo was formed on the surface of steel 1020 so as to increase its corrosion resistance greatly. Therefore, double glow plasma technique will be widely used in corrosion-resisting surface science. # 2002 Elsevier Science B.V. All rights reserved. PACS: 8165 Keywords: XPS; Double glow plasma technique; Surface alloying layer; Passive ®lm; Corrosion resistance
1. Introduction Double glow plasma surface alloying technique, Xu-Tech Process, was invented by Prof. Zhong [1,2,8]. By using this technique, surface alloying layers (SALs) with various alloying elements such as Ni, Cr, Mo, W, Ta, Al and Ti, various depth from several microns to 500 mm and special physical, chemical and mechanical properties can be formed on the surface of metallic materials [3±6]. A double glow plasma corrosion-resisting surface alloying layer was formed on low carbon steel 1020, and X-ray *
Corresponding author. Tel.: 86-101-3661251/810; fax: 86-106-2332884. E-mail address: [email protected] (J. Ai).
photoelectron spectroscopy (XPS) and other means were used to analyze this surface alloying layer. 2. Experimental methods The double glow plasma surface alloying experiment was conducted in a self-made furnace. Source material was Hastelloy C-2000 alloy (Ni 59 wt.%, Mo 16 wt.%, Cr 23 wt.%, Cu 1.6 wt.%, C 0:01) with a size of 130 mm 50 mm 4 mm, and substrate material was low carbon steel 1020 with a size of 80 mm 25 mm 3 mm. During the experiment, an impulse discharge was used, where the source electrode was 1050 V direct current source, and the work piece electrode was 250 V impulse current
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 1 2 2 2 - 9
J. Ai et al. / Applied Surface Science 206 (2003) 230±236
source. The vacuum in the furnace was 35 Pa. The inter-electrode distance was 15 mm. Experimental time was 3 h [7]. The corrosion resistance of the surface alloying layer was measured by M351 electrochemical testing system, where the corrodent was 3.5% NaCl solution; the comparison electrode was a saturation calomel electrode and the scanning speed was 20 mV/min. In order to analyze the passive ®lm of the surface alloying layer after electrochemical test in 3.5% NaCl solution, F-1600 XPS was used and its experimental parameters were as follows: an Mg Ka target with power 300 W, pass energy 187.85 eV, analysis area, approximately, 0.8 mm2 and vacuum in the test chamber 2 10 8 Pa. During depth pro®ling, beam voltage of ion gun was 4 kV with a scanning area of 1 1 mm2, and depth calibration was calculated on the basis of average sputtering rate 30 nm/min [9]. The sputtering time was 10min. The binding energy was corrected by taking C1s, 284.6 eV as a criterion. Before XPS experiment, the specimen was cleaned in acetone so as to remove the contamination. 3. Experimental results and discussion The microstructure of the surface alloying layer observed by SEM is shown in Fig. 1. It can be seen from Fig. 1 that the surface alloying layer is continuous and compact, and the precipitated phases include m-phase and M6C [7]. EDX line scanning
231
Fig. 2. Distribution of Ni, Cr, Mo and Cu vs. depth.
(just detecting Ni, Cr, Mo and Cu) indicates that in the surface alloying layer the distribution of Ni, Cr, Mo and Cu versus depth is shown in Fig. 2. Of course, there are Fe and other elements in the alloying layer besides the above elements. Ni, Cr, Mo and Cu were obtained from the source material Hastelloy C-2000. As a result of the high temperature 950 8C produced by double glow plasma discharge, Fe diffused from substrate material steel 1020, so there exists Fe in the surface alloying layer. The electrochemical results of the surface alloying layer, 304 stainless steel and Hastelloy C-2000 in 3.5% NaCl solution are shown in Table 1. Comparison of these electrochemical results shows that although the corrosion resistance of SAL is inferior to that of Hastelloy C-2000, the pitting potential of SAL approaches that of 304 stainless steel, and the passive current density and corrosion rate of SAL are nearly half of that of 304 stainless steel. Observed by SEM, the surface morphology of the alloying layer after the electrochemical test mainly consists of corrosive pits, coupled with some inter-crystalline Table 1 Electrochemical results in 5% NaCl solution
Fig. 1. The microstructure of the surface alloying layer.
Specimen
Pitting potential (mV)
Passive Corrosion rate current (mm per year) density (mA/cm2)
SAL 304 steel Hastelloy C-2000
190 210 600
31.622 56.788 10.000
0.3406 0.6767 0.1076
232
J. Ai et al. / Applied Surface Science 206 (2003) 230±236
Fig. 3. XPS survey spectra after sputtering for 2 min.
corrosion. The corrosive pits are related to the formation of the detrimental phases such as m-phase and M6C [7]. An XPS survey spectra of some plain areas on the alloying layer surface after electrochemical test in 3.5% NaCl solution exhibits proofs for that there are Cr, Ni, Fe, Mo, O and C after sputtering for 2 min specimen (shown in Fig. 3), and the disappearance of copper is being investigated. Depth pro®ling on the surface alloying layer indicates that the chemical composition versus sputter time is shown in Table 1 and Fig. 4. Table 2 shows that before sputtering, the outermost surface has been considerably contaminated by carbon and C content amounts to 14.58 at.%; C content decreases with the increase of sputter time, and it falls to 8.85 and 4.78 at.%, respectively, after sputtering for 2 and 10 min; O content also decreases with sputter time, and it falls to 8.34 from 19.73 at.% after sputtering for 10 min; with the increase of sputter time, Cr content ®rstly increases and then decreases slowly, and Fe content ®rstly increases and then almost keeps 30 at.%; before sputtering, Ni and Mo contents
amount to 24.46 and 8.56 at.%, respectively, and after sputtering for 10 min, they always increase to 33.70 and 13.08 at.%, respectively. The valence state spectra of Cr, Fe, Ni and Mo after sputtering for 2 min were ®tted, and a valence state analysis was conducted. Curve ®tting of Cr2p, Fe2p, Ni2p and Mo3d high-resolution spectrum after sputtering for 2 min are shown in Figs. 5±8, respectively. It can be seen from Figs. 5±8 that even after sputtering 2 min, quite a number of Cr and Fe was oxidized into their oxides; nearly 50% of Cr was oxidized into CrO3 and Cr2O3, and they amount to 16.40 and 33.38%, respectively (shown in Fig. 5); nearly 60% of Fe was oxidized into Fe2O3 and FeO, and they amount to 22.2 and 37.74%, respectively (shown in Fig. 6); but all Ni and Mo remain as metallic Ni and Mo (shown in Figs. 7 and 8). Cr2p and Fe2p spectra versus sputter time indicate that the shape and position of their spectra change a little during the ®rst sputter time 7 min. Curve ®ttings of Cr2p and Fe2p spectra at sputter time 7 min show that Cr or Fe oxide content does not decrease much,
J. Ai et al. / Applied Surface Science 206 (2003) 230±236
Fig. 4. Chemical composition versus sputter time.
Fig. 5. Curve ®tting of Cr2p spectrum after sputtering 2 min.
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J. Ai et al. / Applied Surface Science 206 (2003) 230±236
Fig. 6. Curve ®tting of Fe2p spectrum after sputtering 2 min.
Fig. 7. Ni2p spectrum after sputtering 2 min.
J. Ai et al. / Applied Surface Science 206 (2003) 230±236
235
Fig. 8. Mo3d spectrum after sputtering 2 min.
and quite a number of Cr or Fe oxide still exists even after sputtering for 7 min. Ni2p and Mo3d spectra versus sputter time show all Ni and Mo always remain as metallic Ni and Mo during the ®rst sputter time 7 min. The formation of such passive ®lm in 3.5% NaCl solution attributes to the fact that there are high concentration Ni, Cr and Mo on the surface of the alloying layer. Such Cr and Fe oxides in the passive ®lm as CrO3, Cr2O3, Fe2O3 and FeO are barrier against further corrosion; Ni and Mo are also corrosion-resisting elements.
Table 2 Chemical composition (at.%) versus sputter time Time (min)
C1s
O1s
Cr2p
Fe2p
Ni2p
Mo3d
0 2 4 6 8 10
14.58 8.85 6.06 5.55 5.12 4.78
19.73 15.24 11.98 10.60 9.51 8.34
8.19 9.03 10.17 10.76 10.68 9.87
24.46 28.43 30.02 29.90 30.05 30.22
24.46 28.51 30.67 31.49 32.37 33.70
8.58 9.93 11.09 11.69 12.28 13.08
4. Summary and conclusions A continuous and compact corrosion-resisting surface alloying layer with rich Cr, Ni and Mo was formed on the surface of low carbon steel 1020 by using double glow plasma technique, and it improved greatly the corrosion resistance of low carbon steel 1020 in 3.5% NaCl solution as a result of the formation of a passive ®lm. The passive ®lm consists of the oxides such as CrO3, Cr2O3, Fe2O3, FeO and metallic elements such as Ni, Mo, Cr and Fe. Double glow plasma technique can be used in corrosion-resisting surface science. References [1] Zhong Xu, Double glow surface alloying process [A], in: Proceedings of the Third Paci®c Rim International Conference on Advanced Materials and Processing [C], vol. 6, Hawaii, 1998, pp. 1969±1978. [2] Zhong Xu, Congzeng Wang, Yongan Su, Sawing and cutting tool double glow surface alloying technique [P], CN871043580, 1986.
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J. Ai et al. / Applied Surface Science 206 (2003) 230±236
[3] Yuan Gao, Zhiyong Huo, JinXiang Zhao, Microstructure in the surface hardened layer of power hacksaw blade made of Steel 20CrV, Mater. Sci. Technol. (Chinese Edition) 3 (3) (1995) 62±66. [4] Yuan Gao, A new type high speed steel surface alloying, J. Taiyuan Univ. Technol. 27 (1) (1996) 33±35. [5] Yuan Gao, Xiaoping Liu, Zhiyong Huo, Zhong Xu, Alloying elements in W±Mo ionized alloy hacksaw blades and their cutting performance, Acta Armamentarii (Chinese Edition) 19 (4) (1998) 331±335.
[6] Yuan Gao, Zhiyong Huo et al., Research on contents of alloy elements in plasma alloying layer of hacksaw blade, Mater. Sci. Technol. (Chinese Edition) 5 (3) (1997) 97±101. [7] Jiang Xu, XiShan Xie, Zhong Xu, Ni-base corrosion-resist alloy surface metallurgy study by the technology of double glow plasma, Trans. Mater. Heat Treatment (Chinese Edition) 1 (2002) 28±31. [8] Zhong Xu, US Patent 4,731,539. [9] Yunxin Qi, Anal. Chem. (Chinese Edition) 18 (5) (1990) 401±404.
XPS study on double glow plasma corrosion-resisting surface alloying layer Jiahe Aia,*, Jiang Xua, Fei Heb, Xishan Xiea, Zhong Xuc a
School of Materials Science and Engineering, University of Science and Technology, Beijing 100083, China b State Key Laboratory of C1 Chemistry and Technology, Tianjin University, Tianjin 300072, China c Institute of Surface Engineering, Taiyuan University of Technology, Taiyuan 030024, China Received 3 July 2002; received in revised form 3 July 2002; accepted 2 November 2002
Abstract Double glow plasma corrosion-resisting surface alloying layer (SAL) formed on low carbon steel 1020 was studied by X-ray photoelectron spectroscopy (XPS) and other means. Results show that the passive ®lm of the surface alloying layer after electrochemical test in 3.5% NaCl solution consists of Cr and Fe oxide such as CrO3, Cr2O3, Fe2O3 and FeO and metallic Ni and Mo, and it attributes to the fact that a continuous and compact corrosion-resisting surface alloying layer with rich Cr, Ni and Mo was formed on the surface of steel 1020 so as to increase its corrosion resistance greatly. Therefore, double glow plasma technique will be widely used in corrosion-resisting surface science. # 2002 Elsevier Science B.V. All rights reserved. PACS: 8165 Keywords: XPS; Double glow plasma technique; Surface alloying layer; Passive ®lm; Corrosion resistance
1. Introduction Double glow plasma surface alloying technique, Xu-Tech Process, was invented by Prof. Zhong [1,2,8]. By using this technique, surface alloying layers (SALs) with various alloying elements such as Ni, Cr, Mo, W, Ta, Al and Ti, various depth from several microns to 500 mm and special physical, chemical and mechanical properties can be formed on the surface of metallic materials [3±6]. A double glow plasma corrosion-resisting surface alloying layer was formed on low carbon steel 1020, and X-ray *
Corresponding author. Tel.: 86-101-3661251/810; fax: 86-106-2332884. E-mail address: [email protected] (J. Ai).
photoelectron spectroscopy (XPS) and other means were used to analyze this surface alloying layer. 2. Experimental methods The double glow plasma surface alloying experiment was conducted in a self-made furnace. Source material was Hastelloy C-2000 alloy (Ni 59 wt.%, Mo 16 wt.%, Cr 23 wt.%, Cu 1.6 wt.%, C 0:01) with a size of 130 mm 50 mm 4 mm, and substrate material was low carbon steel 1020 with a size of 80 mm 25 mm 3 mm. During the experiment, an impulse discharge was used, where the source electrode was 1050 V direct current source, and the work piece electrode was 250 V impulse current
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 1 2 2 2 - 9
J. Ai et al. / Applied Surface Science 206 (2003) 230±236
source. The vacuum in the furnace was 35 Pa. The inter-electrode distance was 15 mm. Experimental time was 3 h [7]. The corrosion resistance of the surface alloying layer was measured by M351 electrochemical testing system, where the corrodent was 3.5% NaCl solution; the comparison electrode was a saturation calomel electrode and the scanning speed was 20 mV/min. In order to analyze the passive ®lm of the surface alloying layer after electrochemical test in 3.5% NaCl solution, F-1600 XPS was used and its experimental parameters were as follows: an Mg Ka target with power 300 W, pass energy 187.85 eV, analysis area, approximately, 0.8 mm2 and vacuum in the test chamber 2 10 8 Pa. During depth pro®ling, beam voltage of ion gun was 4 kV with a scanning area of 1 1 mm2, and depth calibration was calculated on the basis of average sputtering rate 30 nm/min [9]. The sputtering time was 10min. The binding energy was corrected by taking C1s, 284.6 eV as a criterion. Before XPS experiment, the specimen was cleaned in acetone so as to remove the contamination. 3. Experimental results and discussion The microstructure of the surface alloying layer observed by SEM is shown in Fig. 1. It can be seen from Fig. 1 that the surface alloying layer is continuous and compact, and the precipitated phases include m-phase and M6C [7]. EDX line scanning
231
Fig. 2. Distribution of Ni, Cr, Mo and Cu vs. depth.
(just detecting Ni, Cr, Mo and Cu) indicates that in the surface alloying layer the distribution of Ni, Cr, Mo and Cu versus depth is shown in Fig. 2. Of course, there are Fe and other elements in the alloying layer besides the above elements. Ni, Cr, Mo and Cu were obtained from the source material Hastelloy C-2000. As a result of the high temperature 950 8C produced by double glow plasma discharge, Fe diffused from substrate material steel 1020, so there exists Fe in the surface alloying layer. The electrochemical results of the surface alloying layer, 304 stainless steel and Hastelloy C-2000 in 3.5% NaCl solution are shown in Table 1. Comparison of these electrochemical results shows that although the corrosion resistance of SAL is inferior to that of Hastelloy C-2000, the pitting potential of SAL approaches that of 304 stainless steel, and the passive current density and corrosion rate of SAL are nearly half of that of 304 stainless steel. Observed by SEM, the surface morphology of the alloying layer after the electrochemical test mainly consists of corrosive pits, coupled with some inter-crystalline Table 1 Electrochemical results in 5% NaCl solution
Fig. 1. The microstructure of the surface alloying layer.
Specimen
Pitting potential (mV)
Passive Corrosion rate current (mm per year) density (mA/cm2)
SAL 304 steel Hastelloy C-2000
190 210 600
31.622 56.788 10.000
0.3406 0.6767 0.1076
232
J. Ai et al. / Applied Surface Science 206 (2003) 230±236
Fig. 3. XPS survey spectra after sputtering for 2 min.
corrosion. The corrosive pits are related to the formation of the detrimental phases such as m-phase and M6C [7]. An XPS survey spectra of some plain areas on the alloying layer surface after electrochemical test in 3.5% NaCl solution exhibits proofs for that there are Cr, Ni, Fe, Mo, O and C after sputtering for 2 min specimen (shown in Fig. 3), and the disappearance of copper is being investigated. Depth pro®ling on the surface alloying layer indicates that the chemical composition versus sputter time is shown in Table 1 and Fig. 4. Table 2 shows that before sputtering, the outermost surface has been considerably contaminated by carbon and C content amounts to 14.58 at.%; C content decreases with the increase of sputter time, and it falls to 8.85 and 4.78 at.%, respectively, after sputtering for 2 and 10 min; O content also decreases with sputter time, and it falls to 8.34 from 19.73 at.% after sputtering for 10 min; with the increase of sputter time, Cr content ®rstly increases and then decreases slowly, and Fe content ®rstly increases and then almost keeps 30 at.%; before sputtering, Ni and Mo contents
amount to 24.46 and 8.56 at.%, respectively, and after sputtering for 10 min, they always increase to 33.70 and 13.08 at.%, respectively. The valence state spectra of Cr, Fe, Ni and Mo after sputtering for 2 min were ®tted, and a valence state analysis was conducted. Curve ®tting of Cr2p, Fe2p, Ni2p and Mo3d high-resolution spectrum after sputtering for 2 min are shown in Figs. 5±8, respectively. It can be seen from Figs. 5±8 that even after sputtering 2 min, quite a number of Cr and Fe was oxidized into their oxides; nearly 50% of Cr was oxidized into CrO3 and Cr2O3, and they amount to 16.40 and 33.38%, respectively (shown in Fig. 5); nearly 60% of Fe was oxidized into Fe2O3 and FeO, and they amount to 22.2 and 37.74%, respectively (shown in Fig. 6); but all Ni and Mo remain as metallic Ni and Mo (shown in Figs. 7 and 8). Cr2p and Fe2p spectra versus sputter time indicate that the shape and position of their spectra change a little during the ®rst sputter time 7 min. Curve ®ttings of Cr2p and Fe2p spectra at sputter time 7 min show that Cr or Fe oxide content does not decrease much,
J. Ai et al. / Applied Surface Science 206 (2003) 230±236
Fig. 4. Chemical composition versus sputter time.
Fig. 5. Curve ®tting of Cr2p spectrum after sputtering 2 min.
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J. Ai et al. / Applied Surface Science 206 (2003) 230±236
Fig. 6. Curve ®tting of Fe2p spectrum after sputtering 2 min.
Fig. 7. Ni2p spectrum after sputtering 2 min.
J. Ai et al. / Applied Surface Science 206 (2003) 230±236
235
Fig. 8. Mo3d spectrum after sputtering 2 min.
and quite a number of Cr or Fe oxide still exists even after sputtering for 7 min. Ni2p and Mo3d spectra versus sputter time show all Ni and Mo always remain as metallic Ni and Mo during the ®rst sputter time 7 min. The formation of such passive ®lm in 3.5% NaCl solution attributes to the fact that there are high concentration Ni, Cr and Mo on the surface of the alloying layer. Such Cr and Fe oxides in the passive ®lm as CrO3, Cr2O3, Fe2O3 and FeO are barrier against further corrosion; Ni and Mo are also corrosion-resisting elements.
Table 2 Chemical composition (at.%) versus sputter time Time (min)
C1s
O1s
Cr2p
Fe2p
Ni2p
Mo3d
0 2 4 6 8 10
14.58 8.85 6.06 5.55 5.12 4.78
19.73 15.24 11.98 10.60 9.51 8.34
8.19 9.03 10.17 10.76 10.68 9.87
24.46 28.43 30.02 29.90 30.05 30.22
24.46 28.51 30.67 31.49 32.37 33.70
8.58 9.93 11.09 11.69 12.28 13.08
4. Summary and conclusions A continuous and compact corrosion-resisting surface alloying layer with rich Cr, Ni and Mo was formed on the surface of low carbon steel 1020 by using double glow plasma technique, and it improved greatly the corrosion resistance of low carbon steel 1020 in 3.5% NaCl solution as a result of the formation of a passive ®lm. The passive ®lm consists of the oxides such as CrO3, Cr2O3, Fe2O3, FeO and metallic elements such as Ni, Mo, Cr and Fe. Double glow plasma technique can be used in corrosion-resisting surface science. References [1] Zhong Xu, Double glow surface alloying process [A], in: Proceedings of the Third Paci®c Rim International Conference on Advanced Materials and Processing [C], vol. 6, Hawaii, 1998, pp. 1969±1978. [2] Zhong Xu, Congzeng Wang, Yongan Su, Sawing and cutting tool double glow surface alloying technique [P], CN871043580, 1986.
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J. Ai et al. / Applied Surface Science 206 (2003) 230±236
[3] Yuan Gao, Zhiyong Huo, JinXiang Zhao, Microstructure in the surface hardened layer of power hacksaw blade made of Steel 20CrV, Mater. Sci. Technol. (Chinese Edition) 3 (3) (1995) 62±66. [4] Yuan Gao, A new type high speed steel surface alloying, J. Taiyuan Univ. Technol. 27 (1) (1996) 33±35. [5] Yuan Gao, Xiaoping Liu, Zhiyong Huo, Zhong Xu, Alloying elements in W±Mo ionized alloy hacksaw blades and their cutting performance, Acta Armamentarii (Chinese Edition) 19 (4) (1998) 331±335.
[6] Yuan Gao, Zhiyong Huo et al., Research on contents of alloy elements in plasma alloying layer of hacksaw blade, Mater. Sci. Technol. (Chinese Edition) 5 (3) (1997) 97±101. [7] Jiang Xu, XiShan Xie, Zhong Xu, Ni-base corrosion-resist alloy surface metallurgy study by the technology of double glow plasma, Trans. Mater. Heat Treatment (Chinese Edition) 1 (2002) 28±31. [8] Zhong Xu, US Patent 4,731,539. [9] Yunxin Qi, Anal. Chem. (Chinese Edition) 18 (5) (1990) 401±404.