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Response of surface mechanical properties to electrochemical dissolution determined by in situ nanoindentation technique
Electrochemistry Communications 8 (2006) 1092–1098 www.elsevier.com/locate/elecom
Response of surface mechanical properties to electrochemical dissolution determined by in situ nanoindentation technique H.X. Guo, B.T. Lu, J.L. Luo
*
Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alta., Canada T6E 2G6 Received 22 March 2006; accepted 25 April 2006 Available online 6 June 2006
Abstract An in situ nanoindentation technique was used to study the mechanical property degradation in the surface layer induced by electrochemical dissolution. The load–depth curves of indentation were measured using pure iron immersed in electrolyte. The experimental result showed that the anodic currents reduced both hardness and elastic modulus of the material surface layer, and the degradation effect declined with increasing distance from the surface. The electrochemical dissolution-induced degradation was explained by the generation of non-equilibrium vacancies in the metal surface layer during dissolution process. Ó 2006 Elsevier B.V. All rights reserved. Keywords: In situ nanoindentation; Hardness; Modulus; Electrochemical dissolution; Iron
1. Introduction In industries, many failures of engineering components are related to the synergistic effect between mechanical and electrochemical factors. The examples include erosion–corrosion, stress corrosion crack and corrosion fatigue [1–4]. Theoretically, failure process or property degradation of materials is often associated to irreversible thermodynamic processes. In the corrosion-related processes, at least two irreversible fluxes are involved, the electrochemical corrosion and the plastic deformation. In line with non-equilibrium thermodynamics, the irreversible processes will interact with each other [5–7]. Experimental observations have indicated that deformation can promote corrosion and the corrosion, in turn, can degrade the mechanical properties of materials [7–10]. The corrosioninduced degradation of mechanical properties of metallic materials has been demonstrated by the accelerated creep
*
Corresponding author. Tel.: +1 780 492 2232; fax: +1 780 492 2881. E-mail address: [email protected] (J.L. Luo).
1388-2481/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2006.04.020
rate and the hardness degradation resulting from anodic dissolution [8–13]. Since corrosion occurs only on material surface, the corrosion-induced degradation of mechanical properties is expected to occur within a very thin surface layer [12,13]. To further understand the response of mechanical properties to corrosion, a technique for mechanical property evaluation in micro or nanometric scale is needed. Nanoindentation provides a good approach for this intention. Conventionally, the nanoindentation tests were performed in air [14,15]. Recently Seo and Chiba [16–18] developed an in situ nanoindentation technique to study the mechanical properties of a passive film during electrochemical process. This method combines the nanoindentation technique and electrochemical measurement, thereby being able to determine the instantaneous response of surface mechanical properties to electrochemical dissolution. In the present work, the in situ nanoindentation technique was used to investigate the influence of electrochemical dissolution process on the mechanical properties of the material surface layer.
H.X. Guo et al. / Electrochemistry Communications 8 (2006) 1092–1098
Test material was commercial pure iron (wt%: C, 0.003; Si, 0.01; Mn, 0.12; P, 0.006; Cr, 0.01; B, 0.009; Al, 0.32 and Fe balance). Specimens were in the shape of disks with 16 mm in diameter and 3.5 mm in height. Before tests, the specimens were mechanically ground with SiC abrasive paper to grit 600 and then polished with diamond paste of 6 lm and alumina particles of 0.05 lm. The electrolyte employed for experiments was 0.01 M Na2SO4 solution and 0.1 M H2SO4 solution. The in situ indentation tests were carried out as a constant current was applied. The nanoindentation apparatus (Hysitron Co., Ltd., TriboscopeÒ) was combined with AFM (Digital Instruments, Nanoscope E) to control the positioning and displacement of indenter on specimen surface. A power supply and an amperometer were used to control the current. The indenter, designed specially for tests in liquid, was a diamond cube corner indenter. A small electrochemical cell of polymethyl methacrylate was similar to that used by Seo and Chiba [16–18]. The counter electrode was a platinum ring. A triangular load function for the indentation consisting of a 5-s loading segment with a peak value of 200–2000 lN and a 5-s unloading segment was employed. Fig. 1 shows the typical indentation images obtained in air as the peak load was 200, 600 and 800 lN. The craters produced by the indentation are much smaller than the average grain size of the sample, which was estimated to be larger than 50 lm. The mechanical properties such as hardness and elastic modulus can be evaluated by analyzing the load–depth data. In this work the analysis method developed by Oliver and Pharr [19,20] was used. When the modulus of the specimen was calculated from the measured data, the Poisson’s ratio for the specimen was taken as 0.29, and the modulus and Poisson’s ratio for the tip were taken as 1140 GPa and 0.07, respectively.
In addition, the Vickers micro-hardness of samples was also measured with a Shimazhu micro-hardness tester. The indenting load was 50 g and indenting time was 15 s. 3. Results and discussion Fig. 2(a) shows the load–depth curves measured in air with the peak loads in the range of 200–2000 lN. With the analysis method mentioned above, the hardness data can be determined, as presented in Fig. 2(b). It is found that the hardness tended to decrease with the indentation depth. Similar results have also been reported for single crystal copper and silver [21,22], and this phenomenon has been known as indentation size effect [23,24]. Before carrying out in situ nanoindentation tests, the polarization behaviour of iron in 0.01 M Na2SO4 solution was investigated first. In anodic polarization zone the pure iron specimen exhibited active dissolution behaviour. Hence, it is possible to control the anodic dissolution rate by applying an impressed current. Fig. 3 shows the effect
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Fig. 2. Hardness of iron measured in air with the maximum load of 200– 2000 lN: (a) load–depth curves and (b) dependence of hardness on the contact depth.
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penetration depth of the indenter increased as the anodic current was applied to the specimen. Fig. 4 shows the hardness of the pure iron at various anodic dissolution densities. Although the experimental data were quite scattered, it can still be seen that the hardness degraded with an increase in the anodic dissolution rate. In addition, as the penetration depth and the contact area increase, the modulus of the pure iron will decrease slightly. Fig. 5 shows the effect of the anodic current on the modulus of specimens. As expected, the modulus of the specimens decreased slightly with the applied anodic current. Before any analysis associated with the above experimental phenomenon is carried out, the influence from corrosion product and/or surface change should be considered, because it is possible that hardness change in Fig. 4 arises from the corrosion product deposited on the sample surface or other surface change, such as roughness.
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of the anodic dissolution on the load–depth curves under the applied load with peak value of 500, 1200 and 1800 lN. It is clearly seen that under the same load, the
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Fig. 5. Effect of anodic current on the elastic modulus of iron.
H.X. Guo et al. / Electrochemistry Communications 8 (2006) 1092–1098
To separate this effect, after the hardness with applied current was measured, the anodic current was shut off and nanoindentation measurements were conducted on the corroded specimen surface. In this case, the hardness of specimen was measured after corrosion, which can be regarded as ex situ test. Fig. 6(a) shows the comparison of the in situ and ex situ load–depth curves while the in situ curve was measured at the current density of 1 mA/cm2. Fig. 6(b) shows the resulted surface hardness as a function of the contact depth. Apparently, after anodic dissolution at 1 mA/cm2 and then the applied current was turned off, the hardness of the material did not recover to its original value, suggesting the possible effect of corrosion product on the surface hardness. Nevertheless, the hardness with applied current was still much lower than the corroded surface. It suggests that only a small part of the hardness degradation, if any, is attributed to the corrosion products and/or surface change. Thus, the results in Fig. 6 confirm the anodic dissolution-induced hardness degradation. The contribution of the corrosion product and/or surface
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change to hardness degradation is hard to be determined at this moment. In fact, it is not very accurate to ascribe the hardness degradation determined in ex situ tests to the corrosion products and/or surface change entirely, because this kind of decrease could also be possibly caused by the hysteretic effect of anodic dissolution, which will be discussed further in the following section. In despite of some uncertain due to corrosion products, etc., the above experimental results show clearly that the presence of anodic current will cause the degradation of mechanical properties in the surface layer. This phenomenon could be explained by the electrochemical dissolution process. Although the most energetically favourable dissolution atoms are from kink sites, the anodic overvoltage could favour dissolution directly from ledge sites [3]. This nonuniform dissolution might inject vacancies into the subsurface of materials. Then, supersaturation of vacancies could form in the subsurface, as demonstrated by Jones et al. [25–27], who investigated the room-temperature diffusion in Cu–Ag thin-film couples promoted by anodic dissolution. As a large amount of vacancies generate in the subsurface, the inter-atomic bonds of the metal surface layer could be attenuated. Such changes will deteriorate the mechanical properties of material, such as tensile strength, modulus and fatigue life [28–31]. Also, Meletis et al. [32] calculated the binding energies between vacancies and dislocations and proposed that the vacancies generated during electrochemical dissolution are attracted to dislocations and increase their mobility. Accordingly, the resistance of material to plastic deformation would be reduced, thus resulting in low hardness. In fact, the generation of vacancies induced by anodic dissolution has been employed to explain several experimental phenomena, although some controversies still exist. Pickering et al. [33,34] proposed that the vacancies, especially divacancies which have much more rapid diffusion rate, were generated during the preferential dissolution of Cu in Cu–Au alloys and Zn in brass. Revie and Uhlig [13] ascribed the increased creep rates of Cu and Fe with presence of anodic current to the supersaturation of vacancies. In stress corrosion cracking study, Jones [3,27] proposed localized surface plasticity (LSP), Magnin et al. [35] developed corrosion-enhanced plasticity model, and Galvele [36] proposed surface-mobility mechanism. All these three mechanisms involved the generation of vacancies during electrochemical dissolution. Moreover, Seo and Sato [37] employed the concept of ‘excess divacancies with high diffusivity’ in illustrating the selective dissolution of binary alloys and the passivation process. Although the generation of vacancies was employed to understand the effect of electrochemical dissolution on the degradation of the material mechanical properties, the analysis on the diffusion kinetics is still unknown. In the creep tests, Revie and Uhlig reported a lag time with magnitude of several to tens of minutes [13]. In order to understand this phenomenon further, the hysteretic effect in hardness test was also investigated in the present work.
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Considering the difficulty to measure the hardness with nanoindentation technique at controlled time because of the tip engagement process, micro-hardness tests were carried out for this intention. Before anodic dissolution, the original hardness, H, was measured first with Vickers hardness tester. Then, an anodic current of 10 mA/cm2 was applied to the sample for 20 min in 0.1 M H2SO4 solution. After corrosion, the hardness of the sample as a function of time, Ht, was immediately measured in air. Fig. 7 shows the dependence of the ratio Ht/H on time for five parallel samples. Due to the low indentation load, it can be observed that the data were rather scattered. However, a hysteretic effect with a lag time around 10 min can still be found, suggesting there is a diffusion process of vacancies and that the diffusion rate of vacancies could be much higher than the normal one. However, more investigation is needed to explore the exact mechanism for the generation and diffusion of vacancies during the anodic dissolution. For the nanoindentation tests, the data measured with and after corrosion could be regarded as the stable values owing to the tip engagement process. Therefore, the decreased hardness in Fig. 6 between the ones measured in air and after corrosion might be mainly attributed to the corrosion products and/or other surface changes resulted from anodic dissolution. In addition of the generation of vacancies discussed above, the chemo-mechanical effect proposed by Gutman [7] may also play a role in the degradation of a material surface layer caused by the applied anodic current. According to this theory, the weakening inter-atomic bonds as a result of the surface dissolution can lead to a reduction in the chemical potential of dislocations within the surface layer, and thereby inducing a chemical potential gradient between the surface layer and bulk material. As a result, an anodic dissolution-induced dislocation flux will appear in the surface layer, and then the resistance of the surface
layer against plastic deformation will degrade correspondingly because of the enhanced mobility of dislocations. Owing to the fact that anodic dissolution occurs on material surface, it is reasonable to expect that its influence on material properties is only limited within a thin surface layer. The concentration of non-equilibrium vacancies is expected to decrease with depth because it is directly related to the anodic dissolution of atoms. From the dislocation analysis, on the other hand, only the dislocations that situated within a depth less than or equal to the mean free path of dislocation movement can reach the surface. This suggests that the influence of anodic current on the mechanical properties measured by the nanoindentation technique will decline with the indentation depth, as indicated by the curves in Figs. 8 and 9. The hardness and modulus data in these two figures were obtained from fitting the curves in Figs. 4 and 5. In Fig. 8, the dependence of hardness on anodic current density is characterized with the hardness ratio H 0 /H and the hardness difference H H 0 as a function of indentation depth, where H and H 0 are the hardness measured in air and at anodic current, respectively. The similar comparison for modulus is shown in Fig. 9. It is clear seen that both the hardness difference and the modulus difference decreased with depth and consistently both the hardness ratio and the modulus ratio increased with the indentation depth. In addition, the dependence of the hardness degradation on the indentation depth is further confirmed by the comparison between the hardness measured with nanoindentation and with micro-hardness techniques. Fig. 8 indicates that the maximum hardness degradation measured with nanoindentation technique is more than 30% whereas the hardness degradation measured with the micro-hardness technique was only 10–15% [11]. For comparison, the effect of cathodic current on surface hardness was also investigated. The cathodic current
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H.X. Guo et al. / Electrochemistry Communications 8 (2006) 1092–1098 1
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(2) The presence of anodic dissolution reduces the hardness and elastic modulus of iron in the surface layer. This effect declines with increasing distance from the surface. (3) The hardness degradation resulting from anodic dissolution could be attributed to the generation of vacancies in the metal surface layer. (4) Cathodic current from the reduction of oxygen does not influence the surface hardness of iron for the present system.
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This work was supported by Natural Sciences and Engineering Research Council of Canada.
Fig. 10. The effect of cathodic current on the hardness of iron.
References density corresponding to potential 0.85 V (SCE) was applied to the iron sample and at the same time the surface hardness was measured and the results are shown in Fig. 10. It is observed that the surface hardness is independent of the applied cathodic potential. In neutral solution, the cathodic reaction under cathodic protection potential is the reduction of the dissolved oxygen. For this process the metallic electrode only provides the locations for reaction to take place, rather than participates in reaction. The cathode reaction does not affect the pack order of the metal atoms or the inter-atom bonds, therefore, the mechanical properties of the surface layer. 4. Conclusions (1) In situ nanoindentation technique is a useful tool to study the surface mechanical property degradation caused by corrosion.
[1] Y. Li, G.T. Burstein, I.M. Hutchings, Wear 186 (2) (1995) 515. [2] Z. Yue, P. Zhou, J. Shi, Wear of Materials, ASME, New York, 1987, p. 763. [3] A. Jones, Metall. Trans. A 16 (1985) 1133. [4] T. Magnin, R. Chieragatti, R. Oltra, Acta Metall. Mater. 38 (1990) 1313. [5] B.T. Lu, J.L. Luo, J. Phys. Chem. B 110 (9) (2006) 4217. [6] S. Groot, P. Mazur, Non-equilibrium Thermodynamics, Dover Publications, New York, 1984. [7] E.M. Gutman, Mechanochemistry of Materials, Cambridge International Science Publishing, Great Abington, Cambridge, UK, 1998. [8] J.H. Xie, A.T. Alpas, D.O. Northwood, J. Mater. Sci. 38 (2003) 4849. [9] S. Yin, D.Y. Li, Mater. Sci. Eng. A 394 (2005) 266. [10] M. Ocampo, Corrosion 58 (2002) 601. [11] H.X. Guo, B.T. Lu, J.L. Luo, Electrochim. Acta 51 (2005) 315. [12] B. Gu, W.Y. Chu, L.J. Qiao, et al., Corros. Sci. 36 (8) (1994) 1437. [13] R.W. Revie, H.H. Uhlig, Acta Metall. 22 (1974) 619. [14] W.D. Nix, Mater. Sci. Eng. A 234 (1997) 37. [15] B. Bhushan (Ed.), Handbook of Micro/Nano Tribology, CRC Press, New York, 1995, p. 321. [16] M. Chiba, M. Seo, Corros. Sci. 44 (10) (2002) 2379. [17] M. Seo, M. Chiba, Electrochim. Acta 47 (1–2) (2001) 319.
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[18] M. Seo, M. Chiba, K. Suzuki, J. Electroanal. Chem. 473 (1–2) (1999) 49. [19] W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (6) (1992) 1564. [20] W.C. Oliver, G.M. Pharr, J. Mater. Res. 19 (1) (2004) 3. [21] Q. Ma, D.R. Clarke, J. Mater. Res. 10 (1995) 853. [22] K.W. McElhaney, J.J. Vlassak, W.D. Nix, J. Mater. Res. 13 (5) (1998) 1300. [23] N.A. Flick, J.W. Hutchinson, J. Mech. Phys. Solid. 41 (1993) 1825. [24] N.A. Flick, G.A. Muller, M.F. Ashby, et al., Acta Metall. Mater. 42 (1994) 475. [25] D.A. Jones, A.F. Jankowski, Scripta Metall. Mater. 29 (1993) 701. [26] D.A. Jones, A.F. Jankowski, G.A. Davidson, Metall. Mater. Trans. 28A (1997) 843.
[27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37]
D.A. Jones, Corrosion 52 (5) (1996) 356. C.H. Caceres, B.I. Selling, Mater. Sci. Eng. A 220 (1996) 109. N. Chawla, X. Deng, Mater. Sci. Eng. A 390 (2005) 98. A. Hadrboletz, B. Weiss, Int. Mater. Rev. 42 (1997) 1. C. Tekmen, I. Ozdemir, U. Cocen, et al., Mater. Sci. Eng. A 360 (2003) 365. E.I. Meletis, K. Lian, W. Huang, in: Proceedings of the Corrosion– Deformation Interactions, Les Ulis, France, 1993, p. 69. H. Pickering, J. Electrochem. Soc. 117 (1970) 8. H. Pickering, C. Wagner, J. Electrochem. Soc. 114 (1967) 698. T. Magnin, A. Chambreuil, B. Bayle, Acta Mater. 44 (4) (1996) 1457. J.R. Galvele, Corros. Sci. 35 (1993) 419. M. Seo, N. Sato, Langmuir 3 (6) (1987) 917.
Response of surface mechanical properties to electrochemical dissolution determined by in situ nanoindentation technique H.X. Guo, B.T. Lu, J.L. Luo
*
Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alta., Canada T6E 2G6 Received 22 March 2006; accepted 25 April 2006 Available online 6 June 2006
Abstract An in situ nanoindentation technique was used to study the mechanical property degradation in the surface layer induced by electrochemical dissolution. The load–depth curves of indentation were measured using pure iron immersed in electrolyte. The experimental result showed that the anodic currents reduced both hardness and elastic modulus of the material surface layer, and the degradation effect declined with increasing distance from the surface. The electrochemical dissolution-induced degradation was explained by the generation of non-equilibrium vacancies in the metal surface layer during dissolution process. Ó 2006 Elsevier B.V. All rights reserved. Keywords: In situ nanoindentation; Hardness; Modulus; Electrochemical dissolution; Iron
1. Introduction In industries, many failures of engineering components are related to the synergistic effect between mechanical and electrochemical factors. The examples include erosion–corrosion, stress corrosion crack and corrosion fatigue [1–4]. Theoretically, failure process or property degradation of materials is often associated to irreversible thermodynamic processes. In the corrosion-related processes, at least two irreversible fluxes are involved, the electrochemical corrosion and the plastic deformation. In line with non-equilibrium thermodynamics, the irreversible processes will interact with each other [5–7]. Experimental observations have indicated that deformation can promote corrosion and the corrosion, in turn, can degrade the mechanical properties of materials [7–10]. The corrosioninduced degradation of mechanical properties of metallic materials has been demonstrated by the accelerated creep
*
Corresponding author. Tel.: +1 780 492 2232; fax: +1 780 492 2881. E-mail address: [email protected] (J.L. Luo).
1388-2481/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2006.04.020
rate and the hardness degradation resulting from anodic dissolution [8–13]. Since corrosion occurs only on material surface, the corrosion-induced degradation of mechanical properties is expected to occur within a very thin surface layer [12,13]. To further understand the response of mechanical properties to corrosion, a technique for mechanical property evaluation in micro or nanometric scale is needed. Nanoindentation provides a good approach for this intention. Conventionally, the nanoindentation tests were performed in air [14,15]. Recently Seo and Chiba [16–18] developed an in situ nanoindentation technique to study the mechanical properties of a passive film during electrochemical process. This method combines the nanoindentation technique and electrochemical measurement, thereby being able to determine the instantaneous response of surface mechanical properties to electrochemical dissolution. In the present work, the in situ nanoindentation technique was used to investigate the influence of electrochemical dissolution process on the mechanical properties of the material surface layer.
H.X. Guo et al. / Electrochemistry Communications 8 (2006) 1092–1098
Test material was commercial pure iron (wt%: C, 0.003; Si, 0.01; Mn, 0.12; P, 0.006; Cr, 0.01; B, 0.009; Al, 0.32 and Fe balance). Specimens were in the shape of disks with 16 mm in diameter and 3.5 mm in height. Before tests, the specimens were mechanically ground with SiC abrasive paper to grit 600 and then polished with diamond paste of 6 lm and alumina particles of 0.05 lm. The electrolyte employed for experiments was 0.01 M Na2SO4 solution and 0.1 M H2SO4 solution. The in situ indentation tests were carried out as a constant current was applied. The nanoindentation apparatus (Hysitron Co., Ltd., TriboscopeÒ) was combined with AFM (Digital Instruments, Nanoscope E) to control the positioning and displacement of indenter on specimen surface. A power supply and an amperometer were used to control the current. The indenter, designed specially for tests in liquid, was a diamond cube corner indenter. A small electrochemical cell of polymethyl methacrylate was similar to that used by Seo and Chiba [16–18]. The counter electrode was a platinum ring. A triangular load function for the indentation consisting of a 5-s loading segment with a peak value of 200–2000 lN and a 5-s unloading segment was employed. Fig. 1 shows the typical indentation images obtained in air as the peak load was 200, 600 and 800 lN. The craters produced by the indentation are much smaller than the average grain size of the sample, which was estimated to be larger than 50 lm. The mechanical properties such as hardness and elastic modulus can be evaluated by analyzing the load–depth data. In this work the analysis method developed by Oliver and Pharr [19,20] was used. When the modulus of the specimen was calculated from the measured data, the Poisson’s ratio for the specimen was taken as 0.29, and the modulus and Poisson’s ratio for the tip were taken as 1140 GPa and 0.07, respectively.
In addition, the Vickers micro-hardness of samples was also measured with a Shimazhu micro-hardness tester. The indenting load was 50 g and indenting time was 15 s. 3. Results and discussion Fig. 2(a) shows the load–depth curves measured in air with the peak loads in the range of 200–2000 lN. With the analysis method mentioned above, the hardness data can be determined, as presented in Fig. 2(b). It is found that the hardness tended to decrease with the indentation depth. Similar results have also been reported for single crystal copper and silver [21,22], and this phenomenon has been known as indentation size effect [23,24]. Before carrying out in situ nanoindentation tests, the polarization behaviour of iron in 0.01 M Na2SO4 solution was investigated first. In anodic polarization zone the pure iron specimen exhibited active dissolution behaviour. Hence, it is possible to control the anodic dissolution rate by applying an impressed current. Fig. 3 shows the effect
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Fig. 2. Hardness of iron measured in air with the maximum load of 200– 2000 lN: (a) load–depth curves and (b) dependence of hardness on the contact depth.
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penetration depth of the indenter increased as the anodic current was applied to the specimen. Fig. 4 shows the hardness of the pure iron at various anodic dissolution densities. Although the experimental data were quite scattered, it can still be seen that the hardness degraded with an increase in the anodic dissolution rate. In addition, as the penetration depth and the contact area increase, the modulus of the pure iron will decrease slightly. Fig. 5 shows the effect of the anodic current on the modulus of specimens. As expected, the modulus of the specimens decreased slightly with the applied anodic current. Before any analysis associated with the above experimental phenomenon is carried out, the influence from corrosion product and/or surface change should be considered, because it is possible that hardness change in Fig. 4 arises from the corrosion product deposited on the sample surface or other surface change, such as roughness.
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400
1 mA/cm2
350
E (GPa)
500
0 0 (c)
100
100
200 h (nm)
300
400
Fig. 3. Effect of anodic current on load–depth curves of iron: (a) Pmax = 500 lN; (b) Pmax = 1200 lN and (c) Pmax = 1800 lN.
300 250 200 150 100 50 0
of the anodic dissolution on the load–depth curves under the applied load with peak value of 500, 1200 and 1800 lN. It is clearly seen that under the same load, the
0
100
200 300 hc (nm)
400
500
Fig. 5. Effect of anodic current on the elastic modulus of iron.
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To separate this effect, after the hardness with applied current was measured, the anodic current was shut off and nanoindentation measurements were conducted on the corroded specimen surface. In this case, the hardness of specimen was measured after corrosion, which can be regarded as ex situ test. Fig. 6(a) shows the comparison of the in situ and ex situ load–depth curves while the in situ curve was measured at the current density of 1 mA/cm2. Fig. 6(b) shows the resulted surface hardness as a function of the contact depth. Apparently, after anodic dissolution at 1 mA/cm2 and then the applied current was turned off, the hardness of the material did not recover to its original value, suggesting the possible effect of corrosion product on the surface hardness. Nevertheless, the hardness with applied current was still much lower than the corroded surface. It suggests that only a small part of the hardness degradation, if any, is attributed to the corrosion products and/or surface change. Thus, the results in Fig. 6 confirm the anodic dissolution-induced hardness degradation. The contribution of the corrosion product and/or surface
800 In air 1 mA/cm2 After corrosion
700 600
P (uN)
500 400 300 200 100 0 0
50
100
150
h (nm)
(a)
7 6
In air 1mA/cm2
5
H (GPa)
After corrosion 4 3 2 1 0 0 (b)
100
200 hc (nm)
300
400
Fig. 6. Comparison of the hardness of iron tested by in situ and ex situ methods: (a) load–depth curves and (b) hardness vs. contact depth.
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change to hardness degradation is hard to be determined at this moment. In fact, it is not very accurate to ascribe the hardness degradation determined in ex situ tests to the corrosion products and/or surface change entirely, because this kind of decrease could also be possibly caused by the hysteretic effect of anodic dissolution, which will be discussed further in the following section. In despite of some uncertain due to corrosion products, etc., the above experimental results show clearly that the presence of anodic current will cause the degradation of mechanical properties in the surface layer. This phenomenon could be explained by the electrochemical dissolution process. Although the most energetically favourable dissolution atoms are from kink sites, the anodic overvoltage could favour dissolution directly from ledge sites [3]. This nonuniform dissolution might inject vacancies into the subsurface of materials. Then, supersaturation of vacancies could form in the subsurface, as demonstrated by Jones et al. [25–27], who investigated the room-temperature diffusion in Cu–Ag thin-film couples promoted by anodic dissolution. As a large amount of vacancies generate in the subsurface, the inter-atomic bonds of the metal surface layer could be attenuated. Such changes will deteriorate the mechanical properties of material, such as tensile strength, modulus and fatigue life [28–31]. Also, Meletis et al. [32] calculated the binding energies between vacancies and dislocations and proposed that the vacancies generated during electrochemical dissolution are attracted to dislocations and increase their mobility. Accordingly, the resistance of material to plastic deformation would be reduced, thus resulting in low hardness. In fact, the generation of vacancies induced by anodic dissolution has been employed to explain several experimental phenomena, although some controversies still exist. Pickering et al. [33,34] proposed that the vacancies, especially divacancies which have much more rapid diffusion rate, were generated during the preferential dissolution of Cu in Cu–Au alloys and Zn in brass. Revie and Uhlig [13] ascribed the increased creep rates of Cu and Fe with presence of anodic current to the supersaturation of vacancies. In stress corrosion cracking study, Jones [3,27] proposed localized surface plasticity (LSP), Magnin et al. [35] developed corrosion-enhanced plasticity model, and Galvele [36] proposed surface-mobility mechanism. All these three mechanisms involved the generation of vacancies during electrochemical dissolution. Moreover, Seo and Sato [37] employed the concept of ‘excess divacancies with high diffusivity’ in illustrating the selective dissolution of binary alloys and the passivation process. Although the generation of vacancies was employed to understand the effect of electrochemical dissolution on the degradation of the material mechanical properties, the analysis on the diffusion kinetics is still unknown. In the creep tests, Revie and Uhlig reported a lag time with magnitude of several to tens of minutes [13]. In order to understand this phenomenon further, the hysteretic effect in hardness test was also investigated in the present work.
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Considering the difficulty to measure the hardness with nanoindentation technique at controlled time because of the tip engagement process, micro-hardness tests were carried out for this intention. Before anodic dissolution, the original hardness, H, was measured first with Vickers hardness tester. Then, an anodic current of 10 mA/cm2 was applied to the sample for 20 min in 0.1 M H2SO4 solution. After corrosion, the hardness of the sample as a function of time, Ht, was immediately measured in air. Fig. 7 shows the dependence of the ratio Ht/H on time for five parallel samples. Due to the low indentation load, it can be observed that the data were rather scattered. However, a hysteretic effect with a lag time around 10 min can still be found, suggesting there is a diffusion process of vacancies and that the diffusion rate of vacancies could be much higher than the normal one. However, more investigation is needed to explore the exact mechanism for the generation and diffusion of vacancies during the anodic dissolution. For the nanoindentation tests, the data measured with and after corrosion could be regarded as the stable values owing to the tip engagement process. Therefore, the decreased hardness in Fig. 6 between the ones measured in air and after corrosion might be mainly attributed to the corrosion products and/or other surface changes resulted from anodic dissolution. In addition of the generation of vacancies discussed above, the chemo-mechanical effect proposed by Gutman [7] may also play a role in the degradation of a material surface layer caused by the applied anodic current. According to this theory, the weakening inter-atomic bonds as a result of the surface dissolution can lead to a reduction in the chemical potential of dislocations within the surface layer, and thereby inducing a chemical potential gradient between the surface layer and bulk material. As a result, an anodic dissolution-induced dislocation flux will appear in the surface layer, and then the resistance of the surface
layer against plastic deformation will degrade correspondingly because of the enhanced mobility of dislocations. Owing to the fact that anodic dissolution occurs on material surface, it is reasonable to expect that its influence on material properties is only limited within a thin surface layer. The concentration of non-equilibrium vacancies is expected to decrease with depth because it is directly related to the anodic dissolution of atoms. From the dislocation analysis, on the other hand, only the dislocations that situated within a depth less than or equal to the mean free path of dislocation movement can reach the surface. This suggests that the influence of anodic current on the mechanical properties measured by the nanoindentation technique will decline with the indentation depth, as indicated by the curves in Figs. 8 and 9. The hardness and modulus data in these two figures were obtained from fitting the curves in Figs. 4 and 5. In Fig. 8, the dependence of hardness on anodic current density is characterized with the hardness ratio H 0 /H and the hardness difference H H 0 as a function of indentation depth, where H and H 0 are the hardness measured in air and at anodic current, respectively. The similar comparison for modulus is shown in Fig. 9. It is clear seen that both the hardness difference and the modulus difference decreased with depth and consistently both the hardness ratio and the modulus ratio increased with the indentation depth. In addition, the dependence of the hardness degradation on the indentation depth is further confirmed by the comparison between the hardness measured with nanoindentation and with micro-hardness techniques. Fig. 8 indicates that the maximum hardness degradation measured with nanoindentation technique is more than 30% whereas the hardness degradation measured with the micro-hardness technique was only 10–15% [11]. For comparison, the effect of cathodic current on surface hardness was also investigated. The cathodic current
1.2
0.9
5
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4
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H'/H
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Sample #1 Sample #2 Sample #3 Sample #4 Sample #5
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for 1
2
H-H' (GPa)
Ht /H
1
35
t (min) Fig. 7. Hardness degradation as a function of time after electrochemical dissolution.
0
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200
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400
0 500
hc (nm)
Fig. 8. Hardness ratio and difference as a function of contact depth (data were obtained from fitting the data in Fig. 4).
H.X. Guo et al. / Electrochemistry Communications 8 (2006) 1092–1098 1
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60
0.95
50
0.9 40
E'/E
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30
0.75
E-E' (GPa)
0.85
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0.7 10
0.65 0.6
0 0
100
200 hc (nm)
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400
Fig. 9. Modulus ratio and difference as a function of contact depth. The applied current is 1 mA/cm2 (data were obtained from fitting the data in Fig. 5).
7
(2) The presence of anodic dissolution reduces the hardness and elastic modulus of iron in the surface layer. This effect declines with increasing distance from the surface. (3) The hardness degradation resulting from anodic dissolution could be attributed to the generation of vacancies in the metal surface layer. (4) Cathodic current from the reduction of oxygen does not influence the surface hardness of iron for the present system.
Under CP 6
In air
H (GPa)
5 4 3 2 1
Acknowledgement 0 0
50
100
150 hc (nm)
200
250
300
This work was supported by Natural Sciences and Engineering Research Council of Canada.
Fig. 10. The effect of cathodic current on the hardness of iron.
References density corresponding to potential 0.85 V (SCE) was applied to the iron sample and at the same time the surface hardness was measured and the results are shown in Fig. 10. It is observed that the surface hardness is independent of the applied cathodic potential. In neutral solution, the cathodic reaction under cathodic protection potential is the reduction of the dissolved oxygen. For this process the metallic electrode only provides the locations for reaction to take place, rather than participates in reaction. The cathode reaction does not affect the pack order of the metal atoms or the inter-atom bonds, therefore, the mechanical properties of the surface layer. 4. Conclusions (1) In situ nanoindentation technique is a useful tool to study the surface mechanical property degradation caused by corrosion.
[1] Y. Li, G.T. Burstein, I.M. Hutchings, Wear 186 (2) (1995) 515. [2] Z. Yue, P. Zhou, J. Shi, Wear of Materials, ASME, New York, 1987, p. 763. [3] A. Jones, Metall. Trans. A 16 (1985) 1133. [4] T. Magnin, R. Chieragatti, R. Oltra, Acta Metall. Mater. 38 (1990) 1313. [5] B.T. Lu, J.L. Luo, J. Phys. Chem. B 110 (9) (2006) 4217. [6] S. Groot, P. Mazur, Non-equilibrium Thermodynamics, Dover Publications, New York, 1984. [7] E.M. Gutman, Mechanochemistry of Materials, Cambridge International Science Publishing, Great Abington, Cambridge, UK, 1998. [8] J.H. Xie, A.T. Alpas, D.O. Northwood, J. Mater. Sci. 38 (2003) 4849. [9] S. Yin, D.Y. Li, Mater. Sci. Eng. A 394 (2005) 266. [10] M. Ocampo, Corrosion 58 (2002) 601. [11] H.X. Guo, B.T. Lu, J.L. Luo, Electrochim. Acta 51 (2005) 315. [12] B. Gu, W.Y. Chu, L.J. Qiao, et al., Corros. Sci. 36 (8) (1994) 1437. [13] R.W. Revie, H.H. Uhlig, Acta Metall. 22 (1974) 619. [14] W.D. Nix, Mater. Sci. Eng. A 234 (1997) 37. [15] B. Bhushan (Ed.), Handbook of Micro/Nano Tribology, CRC Press, New York, 1995, p. 321. [16] M. Chiba, M. Seo, Corros. Sci. 44 (10) (2002) 2379. [17] M. Seo, M. Chiba, Electrochim. Acta 47 (1–2) (2001) 319.
1098
H.X. Guo et al. / Electrochemistry Communications 8 (2006) 1092–1098
[18] M. Seo, M. Chiba, K. Suzuki, J. Electroanal. Chem. 473 (1–2) (1999) 49. [19] W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (6) (1992) 1564. [20] W.C. Oliver, G.M. Pharr, J. Mater. Res. 19 (1) (2004) 3. [21] Q. Ma, D.R. Clarke, J. Mater. Res. 10 (1995) 853. [22] K.W. McElhaney, J.J. Vlassak, W.D. Nix, J. Mater. Res. 13 (5) (1998) 1300. [23] N.A. Flick, J.W. Hutchinson, J. Mech. Phys. Solid. 41 (1993) 1825. [24] N.A. Flick, G.A. Muller, M.F. Ashby, et al., Acta Metall. Mater. 42 (1994) 475. [25] D.A. Jones, A.F. Jankowski, Scripta Metall. Mater. 29 (1993) 701. [26] D.A. Jones, A.F. Jankowski, G.A. Davidson, Metall. Mater. Trans. 28A (1997) 843.
[27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37]
D.A. Jones, Corrosion 52 (5) (1996) 356. C.H. Caceres, B.I. Selling, Mater. Sci. Eng. A 220 (1996) 109. N. Chawla, X. Deng, Mater. Sci. Eng. A 390 (2005) 98. A. Hadrboletz, B. Weiss, Int. Mater. Rev. 42 (1997) 1. C. Tekmen, I. Ozdemir, U. Cocen, et al., Mater. Sci. Eng. A 360 (2003) 365. E.I. Meletis, K. Lian, W. Huang, in: Proceedings of the Corrosion– Deformation Interactions, Les Ulis, France, 1993, p. 69. H. Pickering, J. Electrochem. Soc. 117 (1970) 8. H. Pickering, C. Wagner, J. Electrochem. Soc. 114 (1967) 698. T. Magnin, A. Chambreuil, B. Bayle, Acta Mater. 44 (4) (1996) 1457. J.R. Galvele, Corros. Sci. 35 (1993) 419. M. Seo, N. Sato, Langmuir 3 (6) (1987) 917.