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Magnetic field effect on the anodic behaviour of a ferromagnetic electrode in acidic solutions
Electrochimica Acta 54 (2009) 2229–2233
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Magnetic field effect on the anodic behaviour of a ferromagnetic electrode in acidic solutions R. Sueptitz ∗ , J. Koza, M. Uhlemann, A. Gebert, L. Schultz Leibniz Institute for Solid State and Materials Research IFW Dresden, Helmholtzstr. 20, D-01069 Dresden, Germany
a r t i c l e
i n f o
Article history: Received 9 June 2008 Received in revised form 15 October 2008 Accepted 16 October 2008 Available online 6 November 2008 Keywords: Magnetic field Iron Corrosion Passivation Field gradient force
a b s t r a c t The magnetization of a ferromagnetic electrode in an external homogeneous magnetic field leads to a stray field in front of the electrode. This stray and its gradients can alter the anodic behaviour of the electrode significantly. Potentiodynamic polarisation measurements of an iron wire in a 0.5 M sulfuric acid solution (pH 0.25) and in a 0.5 M phthalate buffer solution (pH 5) without and with applied magnetic fields up to 0.6 T in different orientations to the electrode surface were performed. In sulfuric acid solution an increase of the diffusion-limited dissolution current density and a shift of the active–passive transition potential to more noble potentials was observed when the magnetic field was applied parallel to the electrode surface. In contrast, in perpendicular field configuration the diffusion-limited current density is lowered and the active–passive transition potential is shifted to less noble values. In phthalate buffer no significant influence of the magnetic field on the current density was observed in the active region, but a shift of the active–passive transition to less noble potentials occurred irrespective of the magnetic field configuration. The observed effects of a superimposed magnetic field on the anodic behaviour of iron are discussed with respect to an increase of the mass transport due to the Lorentz-force-driven magnetohydrodynamic (MHD) effect, the magnetic field gradient force and its interaction with the paramagnetic iron ions. The results of this paper show that the effect of the field gradient force can become very important due to the high magnetic field gradient at ferromagnetic electrodes. © 2008 Elsevier Ltd. All rights reserved.
1. Introduction The main effect of a magnetic field applied on an electrochemical system is the introduction of additional forces on the ions in the electrolyte [1]. Generally accepted is the Lorentz force. It acts on moving charges by accelerating them in the direction perpendicular to the current and the flux density and thus leads to a stirring of the electrolyte. This effect, which can enhance the mass transport significantly [1–4], is the so-called MHD effect. Another force is the field gradient force. It pulls paramagnetic ions in regions of high flux density and diamagnetic ions in regions of low flux density, respectively [1]. While it is negligible in homogeneous magnetic fields, its influence may overcome that of the Lorentz force when high gradients of the magnetic flux density are present [5,6]. A still controversially discussed force is the paramagnetic concentration gradient force. It is expected to balance the concentration of paramagnetic ions in a homogeneous magnetic field and thus, is directed towards higher concentration gradients and points in the same direction as diffusion [7,8]. However, some
∗ Corresponding author. Tel.: +49 351 4659 715. E-mail addresses: [email protected], [email protected] (R. Sueptitz). 0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2008.10.055
authors pointed out that this force, if it exists at all, will not show an influence on electrochemical processes [1,9,10]. The influence of magnetic fields on corrosion processes is of high technical importance but not fully understood until now. In the case of iron some fundamental studies on the effect of a magnetic field on its anodic polarisation behaviour in strongly acidic solutions [11,12] and weakly alkaline solutions [13] and neutral solutions [14] have been carried out. In weakly acidic buffered solutions no investigations are reported yet. It was shown [11–13] that the passivation potential shifts to a more noble value with an applied magnetic field, especially with the magnetic flux perpendicular to the current flow. The common explanation of this phenomenon is the MHD effect driven by the Lorentz force. However, due to the ferromagnetic nature of iron it becomes magnetized in a magnetic field. The magnetization leads to a strongly inhomogeneous stray field over the surface with a flux density which can overcome the external one. Ragsdale et al. [5] demonstrated, that paramagnetic molecules are concentrated at iron electrodes due to the magnetic field gradient force. This increased concentration at the electrode surface can become important during the dissolution and passivation of a ferromagnetic electrode itself, especially when the effect of the Lorentz force
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is minimized. The minimization of the Lorentz force influence was achieved in this study by applying the magnetic field parallel to the current flow and by lowering the current density in the active region using a weakly acidic buffered electrolyte. The maximum of the gradient of the magnetic flux density is achieved when the ferromagnetic sample is magnetized completely (saturated state). Due to the magnetic shape anisotropy of a cylindrical shaped sample it is easy to magnetize a wire electrode in the direction of the wire axis (minimized Lorentz force configuration) and hard to magnetize it perpendicular to the wire axis. Thus, it is possible to divide effects of the Lorentz force and the field gradient force qualitatively. Aim of this work was to show that the field gradient force can alter the anodic behaviour of a ferromagnetic electrode significantly, depending on the orientation of the applied magnetic field. 2. Experimental The electrochemical experiments were carried out in a three electrode Teflon® cell schematically drawn in Fig. 1. An iron wire (purity 99.9%) with a diameter of 0.8 mm, embedded in epoxy resign, and a Pt sheet were used as working electrode and counter electrode, respectively. The reference electrode was a Hg/HgSO4 /K2 SO4 (sat.) electrode with 655 mV vs. SHE. All potentials in the paper are referred to SHE. The working electrode was ground with abrasive paper up to grid 4000 and polished with 1 m diamond suspension before each experiment. The electrolytes, 0.5 M sulfuric acid solution (pH 0.25) and 0.5 M phthalate buffer (pH 5), were prepared using analytical grade chemicals and they were purged with nitrogen for 1 h prior each measurement. The electrochemical cell was connected to a ‘SI 1287 Electrochemical Interface’ (Solartron). After monitoring the open circuit potential (OCP) for 10 min anodic potentiodynamic polarisation measurements were performed with a scan rate of 0.5 mV/s. The potential range was selected starting at −100 mV vs. OCP and it was scanned to 1655 mV vs. SHE. A homogeneous magnetic field up to 0.6 T (HV7, Walker Scientific) has been superimposed during the electrochemical investigations in two different configurations, i.e. parallel and perpendicular to the vertical electrode surface. All experiments were carried out at room temperature. With repeating the experiments a good reproducibility was achieved. Simulations of the flux density distribution in front of the electrode were done by the numerical three dimensional magnetostatic field solver ‘Amperes 6.0’ (Enginia Research Inc.).
Fig. 1. Experimental setup; electrolyte volume: ∼16 ml; 1: body (Teflon), 2: working electrode (iron wire), 3: counter electrode (Pt sheet), 4: reference electrode (Hg/HgSO4 /K2 SO4. (sat.)) 5: Luggin capillary.
3. Results The potentiodynamic polarisation curves recorded for the iron wire in sulfuric acid solution (pH 0.25) without and with a magnetic field applied parallel to the electrode surface are shown in Fig. 2. The Tafel region is followed by a transition region entering into a diffusion-controlled region with almost constant current density. This region ends with an abrupt decrease of the current density, the active–passive transmission, which is followed by a stable passive state. There is no visible influence of the superimposed magnetic field on the OCP and the Tafel region. With increasing magnetic flux density the current density in the diffusion-controlled range (∼0–600 mVSHE at 0 T) increases from imax = 120 mA/cm2 without applied magnetic field to imax = 540 mA/cm2 at a flux density of 0.6 T and the potential of the active–passive transition is shifted to more noble values, i.e. 700 mV at 0.3 T and 1250 mV at 0.6 T, respectively. The passive current density increases with rising flux density from 180 A/cm2 without magnetic field to 680 A/cm2 at 0.6 T. The detected effects of an increased diffusion-controlled current density and a shift of the active–passive transmission to more noble potentials with increasing magnetic flux density confirm observations which were reported by Lu et al. [12]. The results were explained by an additional convection of the electrolyte due to the acting Lorentz force, which has its maximum in this field configuration [15]. The potentiodynamic polarisation curves obtained in sulfuric acid with the magnetic field applied in the perpendicular to the electrode configuration are shown in Fig. 3. Until a potential of about 700 mV no significant difference between the curves recorded in a flux density of 0.3 T and 0.6 T is visible. In contrast to the measurements without applied field, a maximum in the current density is reached at −100 mV followed by a sudden decrease of the current density by more than one order of magnitude. After a sharp minimum followed by a rapid increase the current density remains constant, but still in the order of about one tenth of the limiting current density that is reached without applied magnetic field. Furthermore, it is apparent that the active–passive transition occurs earlier, i.e. at more negative potentials (by ∼50 mV) and is less sharp. The passive current density is not significantly affected by the magnetic field. The anodic behaviour of iron in phthalate buffer differs strongly from the behaviour observed in the sulfuric acid solution, as shown
Fig. 2. Current density–potential curves of iron in sulfuric acid solution (pH 0.25) without and with a magnetic field applied parallel to the electrode surface (scan rate of 0.5 mV/s).
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Fig. 3. Current density–potential curves of iron in 0.5 M sulfuric acid solution (pH 0.25) without and with magnetic field applied perpendicular to the electrode surface (scan rate of 0.5 mV/s).
Fig. 5. Current density–potential curves of iron in phthalate buffer (pH 5) without and with applied magnetic field perpendicular to the electrode surface (scan rate of 0.5 mV/s).
in Fig. 4 for the parallel field configuration. The free corrosion potential adjusts at less noble values and the free corrosion current density is about one order of magnitude lower. Also in this environment the free corrosion parameters are not magnetic field dependent. Between the anodic Tafel region and the active–passive transition the current density increases continuously, reaching a maximum value of 50 mA/cm2 , which is about one order of magnitude lower than in pH 0.25. At these strongly reduced dissolution rates no diffusion limits the reaction and thus, a Lorentz-forcedriven convection of the electrolyte shows no significant effect on the current density. Furthermore, it is visible, that with increasing flux density the active–passive transition potential is shifted to less noble values (by 100 mV at 0.6 T) and the passive current density is decreased. The current density in the transpassive region is not effected by the magnetic field. Especially the negative shift of the active–passive transition potential and the decreased passive current density cannot be explained by a MHD effect. In phthalate buffer a magnetic field applied perpendicular to the surface (Fig. 5) leads to a slightly increased current density in the active dissolution range and a shift of the active–passive transition
potential of 300 mV in the negative direction. At low passive potentials (∼750 mV), the passive current density is lowered to about one tenth of its value at 0 T, but it converges to the value measured without magnetic field with rising potential. As it was found in sulfuric acid solution with the magnetic field applied perpendicular to the electrode surface (Fig. 3) an increase of the flux density from 0.3 T to 0.6 T does not further change the observed effects.
Fig. 4. Current density–potential curves of iron in phthalate buffer (pH 5) without and with applied magnetic field parallel to the electrode surface (scan rate of 0.5 mV/s).
4. Discussion The magnetic behaviour of the ferromagnetic iron wire in an external applied magnetic flux density Bappl. of 0.3 T and 0.6 T, respectively was simulated numerically to evaluate the flux density distribution near the electrode surface. In Fig. 6 the distribution of the flux density Bsurf. on a plane, which is parallel to the electrode surface at a distance of 5 m, is shown for the case of an applied flux density of 0.3 T. Fig. 7 shows the magnitude of the flux density along the coordinate (marked in Fig. 6) at distances of 0 and 100 m to the electrode surface for applied flux densities of 0.3 T and 0.6 T, respectively. It is apparent in Fig. 6a, that the flux density is concentrated at the rim regions perpendicular to the external field if the field is applied parallel to the electrode surface. In contrast, if the external field is applied perpendicular (Fig. 6b), the flux density is homogeneously in a maximum state all along the electrode rim. It is remarkable, that the simulated flux density distribution for the case of a parallel applied field corresponds to the shape of the corrosion pattern on an iron wire electrode surface observed by Lu et al. [12] after potentiostatic polarisation in the active region with an applied magnetic field of 0.4 T. If the external field is applied parallel to the surface, the gradient of the flux density is increased as well when the applied flux density is increased (Fig. 7a) due to a higher magnetization grade of the iron. In contrast, if the external field is applied perpendicular to the surface, the gradient of the flux density is only slightly increased because the iron is already magnetically saturated in an external field of 0.3 T (Fig. 7b). In the following the simulated flux density distributions will be used as basis for the discussion of the observed field effects on the polarisation behaviour. Under the used experimental conditions no significant effect of an external applied magnetic field on the free corrosion potential and the free corrosion current density was found. However, the variations of the free corrosion potential in different tests are
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Fig. 6. (a) Flux density distribution (magnitude) Bsurf. on a plane perpendicular to the electrode surface in a distance of 5 m with an applied flux density Bappl. of 0.3 T parallel to the surface and the distance coordinate. (b) Flux density distribution (magnitude) Bsurf. on a plane perpendicular to the electrode surface in a distance of 5 m with an applied flux density Bappl. of 0.3 T perpendicular to the surface.
higher then the observed shifts of several mV in the noble direction reported by Lu and Yang [16] and Rhen et al. [17]. In the region where the electron transfer is the rate determining step, e.g. in the Tafel region, no influence of the magnetic field was found as well. In the active region a maximum–minimum behaviour may occur due to a temporary coverage of the electrode with a slightly soluble species, which acts as a membrane inhibitor on the active dissolution [18,19]. Depending on the experimental condition it is not visible in the polarisation curve (Fig. 2), it is visible as a current density shoulder (Fig. 4, −250 mV) or is visible as a small current density maximum followed by a minimum (Fig. 5, −250 mV and Fig. 3, −100 mV). According to [18,19] the adsorbed species is suggested to be Fe[(OH)2 ]ads and its fraction is dependent on the pH value, the type of anions in the solution and their concentration and the concentration of Fe2+ ions. In particular, the polarisation curves obtained in sulfuric acid solution with the magnetic field superimposed perpendicular to the electrode surface generating a homogeneous flux density maximum all over the electrode rim (Fig. 3) show a maximum followed by a minimum, which are both much more pronounced than in the 0 T case or in the phthalate buffer.
Fig. 7. (a) Flux density distribution Bsurf. (magnitude) parallel to the electrode surface along the coordinate in Fig. 6a in a distance of 0 and 100 m with an applied flux density Bappl. of 0.3 T and 0.6 T parallel to the surface. (b) Flux density distribution Bsurf. (magnitude) parallel to the electrode surface along the coordinate in Fig. 6b in a distance of 0 and 100 m with an applied flux density Bappl. of 0.3 T and 0.6 T perpendicular to the surface.
It is possible that the field gradient force concentrates the paramagnetic Fe2+ ions at the surface all over the rim and thus, hampers the dissolution of the inhibiting film, promotes its formation or makes it less porous. As soon as the current density dropped due to the inhibitor film formation, less new Fe2+ ions are produced by dissolution of the metal and the film destabilizes. The current density increases again until it is limited by diffusion. The diffusionlimited current density is lowered due to the field gradient force pointing in the opposite direction than diffusion (Fig. 3). The field gradient reaches its maximum homogeneously along the rim of the wire (Fig. 6b) and thus, the concentration of Fe2+ ions reaches its maximum at the rim as well. With the magnetic field applied the passivation starts at 500 mV and the current density decreases comparatively slowly with rising potential. From 600 mV – the passivation potential without magnetic field – the current density is independent of the magnetic flux density. Thus, it is suggested that at 500 mV the electrode under field influence is partially (at the rim) covered with a protecting layer in result of the local Fe2+ ion enrichment at the rim surface. This layer grows with progressing polarisation to the centre of the surface, where the gradient in magnetic flux density is small compared to the rim. An increase of the applied magnetic field from 0.3 T to 0.6 T does not alter
R. Sueptitz et al. / Electrochimica Acta 54 (2009) 2229–2233
the observed effects any further. In a field of 0.3 T the iron wire is already magnetically saturated and an increase of the external field does not increase the gradient in the flux density any more (Fig. 7b). In contrast, if the magnetic field is applied parallel to the electrode surface (Fig. 2), an additional convection of the electrolyte is driven by the Lorentz force. This causes a reduction of the diffusion layer thickness and thus, accelerates the removal of the Fe2+ ions away from the surface. Additionally, the gradient in the magnetic flux density at the electrode surface is locally restricted to selected rim regions (Fig. 6a). In consequence of both aspects, a temporary inhibition film formation corresponding to a local maximum–minimum in the polarisation curve as explained above is not visible. Also, the diffusion-limited current density is increased. Due to the continuous removal of the Fe2+ ions from the surface near regions the active–passive transition can take place only at much more noble potentials. These observed effects are in good agreement with results reported by Lu et al. [12]. In the potentiodynamic polarisation curves obtained in phthalate buffer (Figs. 4 and 5) no influence of the superimposed magnetic field on the free corrosion potential and the Tafel region was observed. In the perpendicular field configuration an increased current density was found in the higher active region (0–300 mV), which cannot be explained yet. An influence of a Lorentz-forcedriven convection can be mainly excluded because an increased current density was not found in the parallel field configuration (Fig. 5), in which the Lorentz force is principally expected to be higher. However, in contrast to the anodisation in sulfuric acid, in weakly acidic phthalate buffer (pH 5) the maximum dissolution current density level remains below 100 mA/cm2 under all applied field configurations and diffusion is not predominant. Thus, the effect of the Lorentz force to the current density is not significant. In the perpendicular field configuration the active–passive transition occurs at a 300 mV less noble potential if a magnetic field of 0.3 or 0.6 T is applied. It can be assumed as it has been discussed before, that the magnetic field gradient force concentrates Fe2+ ions at the surface, especially along the whole rim region where the gradient is maximal, and thus, triggers the passivation starting from the rim towards the centre of the wire. An increase of the applied flux density from 0.3 T to 0.6 T does not alter this effect any more because the sample is already saturated at 0.3 T (Fig. 7b). If the superimposed magnetic field is oriented parallel to the surface the effect of the field gradient force is reduced because the gradient in flux density is smaller and restricted to smaller rim regions (Fig. 6a) and disturbed by the Lorentz-force-driven convection. Thus, the negative shift of the active–passive transition potential is less pronounced (100 mV at 0.6 T). If the external field is increased from 0.3 T to 0.6 T the effect is enhanced because the wire is not yet completely magnetized at an external flux density of 0.3 T, i.e. an increase of the external field leads to an increase of the gradient in the flux density at the wire surface (Fig. 7a). Furthermore, it is obvious that the superimposed magnetic field reduces the passive current density in both field configurations in phthalate buffer. At the initial stage of the passive region (∼700 mV) the passive current density is lowered by about one order of magnitude (0.6 T applied perpendicular to the surface), but with increasing potential it converges to the value obtained without applied magnetic field. It is proposed that the formation of ferrimagnetic Fe3 O4 is initially favoured due to the high flux density at the electrode surface [20], but with rising potential the field influence on the layer properties becomes smaller.
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5. Conclusions The Lorentz force only alters the dissolution and passivation behaviour of an iron wire in acidic environment when the anodic reaction is diffusion-controlled in the active range. It elevates the diffusion-limited current density and causes a shift of the active–passive transition potential to more noble values. Due to the ferromagnetic properties of iron, a high gradient of the magnetic flux density occurs at the rim of the wire electrode. A homogeneous flux density maximum exists all over the rim if the external magnetic field is applied perpendicular to the wire cross section. In this field configuration the effect of the field gradient force can overcome the effect of the Lorentz force even if the reaction is diffusion-limited. A dominance of the field gradient force leads to a decrease of the diffusion-limited current density and a shift of the active–passive transition potential to less noble values. This is presumably due to a favoured passive layer formation at the rim regions, which expands to the centre region with progressing passivation. Further investigations are in progress addressing the effect of the size of the electrode on this behaviour. It can be expected that the Lorentz force will dominate at higher electrode surfaces. In phthalate buffer the anodic dissolution reaction rate is lower and is not diffusion-limited. Thus, the anodic behaviour of the iron electrode is only affected by the field gradient force. The passivation potential is shifted to less noble values due to a concentration of Fe2+ ions at the surface rim region. This effect is particularly pronounced when the external field is applied perpendicular to the electrode surface yielding the highest gradient in flux density and a homogeneous flux density distribution maximum all over the rim. The passive current density is decreased especially at low passive potentials. Effects of an applied magnetic field on the structure and composition of the formed passive layers will be in focus of further investigations. Acknowledgements S. Roth and J. Buschbeck are acknowledged for fruitful discussions. The German Research Foundation (DFG) is gratefully acknowledged for the support of this work under grant GE 1106/6 “Corrosion influenced by magnetic fields.” References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
G. Hinds, J.M.D. Coey, M.E.G. Lyons, Electrochem. Commun. 3 (2001) 215. R.A. Tacken, L.J.J. Jansen, J. Appl. Electrochem. 25 (1995) 1. J.M.D. Coey, G. Hinds, J. Alloys Compds. 326 (2001) 238. T.Z. Fahidy, J. Appl. Electrochem. 13 (1983) 553. S.R. Ragsdale, K.M. Grant, H.S. White, J. Am. Chem. Soc. 120 (1998) 13461. M.D. Pullins, K.M. Grant, H.S. White, J. Phys. Chem. B 105 (2001) 8989. R.N. O’Brien, K.S.V. Santhanam, J. Appl. Electrochem. 27 (1997) 573. N. Leventis, A. Dass, J. Am. Chem. Soc. 127 (2005) 4988. T. Weier, K. Eckert, S. Mühlenhoff, C. Cierpka, A. Bund, M. Uhlemann, Electrochem. Commun. 9 (2007) 2479. J.M.D. Coey, F.M.F. Rhen, P. Dunne, S. McMurry, J. Solid State Electrochem. 11 (2007) 711. Y.C. Tang, A.J. Davenport, J. Electrochem. Soc. 154 (2007) C362. Z. Lu, D. Huang, W. Yang, J. Congleton, Corros. Sci. 45 (2003) 2233. Z. Lu, C. Huang, D. Huang, W. Yang, Corros. Sci. 48 (2006) 3049. Z. Lu, J.-M. Chen, Br. Corros. J. 45 (3) (2000) 224. Y.C. Tang, M. Gonzalez-Torreira, S. Yang, A.J. Davenport, JCSE 6 (2007) 46. Z. Lu, W. Yang, Corros. Sci. 50 (2008) 510. F.M.F. Rhen, D. Fernandez, G. Hinds, J.M.D. Coey, J. Electrochem. Soc. 153 (1) (2006) J1. J. Bessone, L. Karakaya, P. Lorbeer, W.J. Lorenz, Electrochim. Acta 22 (1977) 1147. A.A. El Miligy, D. Geana, W.J. Lorenz, Electrochim. Acta 20 (1975) 273. T. Peev, B. Mandjukova, I. Mandjukov, V. Rusanov, Mater. Corros. 42 (1991) 90.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Magnetic field effect on the anodic behaviour of a ferromagnetic electrode in acidic solutions R. Sueptitz ∗ , J. Koza, M. Uhlemann, A. Gebert, L. Schultz Leibniz Institute for Solid State and Materials Research IFW Dresden, Helmholtzstr. 20, D-01069 Dresden, Germany
a r t i c l e
i n f o
Article history: Received 9 June 2008 Received in revised form 15 October 2008 Accepted 16 October 2008 Available online 6 November 2008 Keywords: Magnetic field Iron Corrosion Passivation Field gradient force
a b s t r a c t The magnetization of a ferromagnetic electrode in an external homogeneous magnetic field leads to a stray field in front of the electrode. This stray and its gradients can alter the anodic behaviour of the electrode significantly. Potentiodynamic polarisation measurements of an iron wire in a 0.5 M sulfuric acid solution (pH 0.25) and in a 0.5 M phthalate buffer solution (pH 5) without and with applied magnetic fields up to 0.6 T in different orientations to the electrode surface were performed. In sulfuric acid solution an increase of the diffusion-limited dissolution current density and a shift of the active–passive transition potential to more noble potentials was observed when the magnetic field was applied parallel to the electrode surface. In contrast, in perpendicular field configuration the diffusion-limited current density is lowered and the active–passive transition potential is shifted to less noble values. In phthalate buffer no significant influence of the magnetic field on the current density was observed in the active region, but a shift of the active–passive transition to less noble potentials occurred irrespective of the magnetic field configuration. The observed effects of a superimposed magnetic field on the anodic behaviour of iron are discussed with respect to an increase of the mass transport due to the Lorentz-force-driven magnetohydrodynamic (MHD) effect, the magnetic field gradient force and its interaction with the paramagnetic iron ions. The results of this paper show that the effect of the field gradient force can become very important due to the high magnetic field gradient at ferromagnetic electrodes. © 2008 Elsevier Ltd. All rights reserved.
1. Introduction The main effect of a magnetic field applied on an electrochemical system is the introduction of additional forces on the ions in the electrolyte [1]. Generally accepted is the Lorentz force. It acts on moving charges by accelerating them in the direction perpendicular to the current and the flux density and thus leads to a stirring of the electrolyte. This effect, which can enhance the mass transport significantly [1–4], is the so-called MHD effect. Another force is the field gradient force. It pulls paramagnetic ions in regions of high flux density and diamagnetic ions in regions of low flux density, respectively [1]. While it is negligible in homogeneous magnetic fields, its influence may overcome that of the Lorentz force when high gradients of the magnetic flux density are present [5,6]. A still controversially discussed force is the paramagnetic concentration gradient force. It is expected to balance the concentration of paramagnetic ions in a homogeneous magnetic field and thus, is directed towards higher concentration gradients and points in the same direction as diffusion [7,8]. However, some
∗ Corresponding author. Tel.: +49 351 4659 715. E-mail addresses: [email protected], [email protected] (R. Sueptitz). 0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2008.10.055
authors pointed out that this force, if it exists at all, will not show an influence on electrochemical processes [1,9,10]. The influence of magnetic fields on corrosion processes is of high technical importance but not fully understood until now. In the case of iron some fundamental studies on the effect of a magnetic field on its anodic polarisation behaviour in strongly acidic solutions [11,12] and weakly alkaline solutions [13] and neutral solutions [14] have been carried out. In weakly acidic buffered solutions no investigations are reported yet. It was shown [11–13] that the passivation potential shifts to a more noble value with an applied magnetic field, especially with the magnetic flux perpendicular to the current flow. The common explanation of this phenomenon is the MHD effect driven by the Lorentz force. However, due to the ferromagnetic nature of iron it becomes magnetized in a magnetic field. The magnetization leads to a strongly inhomogeneous stray field over the surface with a flux density which can overcome the external one. Ragsdale et al. [5] demonstrated, that paramagnetic molecules are concentrated at iron electrodes due to the magnetic field gradient force. This increased concentration at the electrode surface can become important during the dissolution and passivation of a ferromagnetic electrode itself, especially when the effect of the Lorentz force
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is minimized. The minimization of the Lorentz force influence was achieved in this study by applying the magnetic field parallel to the current flow and by lowering the current density in the active region using a weakly acidic buffered electrolyte. The maximum of the gradient of the magnetic flux density is achieved when the ferromagnetic sample is magnetized completely (saturated state). Due to the magnetic shape anisotropy of a cylindrical shaped sample it is easy to magnetize a wire electrode in the direction of the wire axis (minimized Lorentz force configuration) and hard to magnetize it perpendicular to the wire axis. Thus, it is possible to divide effects of the Lorentz force and the field gradient force qualitatively. Aim of this work was to show that the field gradient force can alter the anodic behaviour of a ferromagnetic electrode significantly, depending on the orientation of the applied magnetic field. 2. Experimental The electrochemical experiments were carried out in a three electrode Teflon® cell schematically drawn in Fig. 1. An iron wire (purity 99.9%) with a diameter of 0.8 mm, embedded in epoxy resign, and a Pt sheet were used as working electrode and counter electrode, respectively. The reference electrode was a Hg/HgSO4 /K2 SO4 (sat.) electrode with 655 mV vs. SHE. All potentials in the paper are referred to SHE. The working electrode was ground with abrasive paper up to grid 4000 and polished with 1 m diamond suspension before each experiment. The electrolytes, 0.5 M sulfuric acid solution (pH 0.25) and 0.5 M phthalate buffer (pH 5), were prepared using analytical grade chemicals and they were purged with nitrogen for 1 h prior each measurement. The electrochemical cell was connected to a ‘SI 1287 Electrochemical Interface’ (Solartron). After monitoring the open circuit potential (OCP) for 10 min anodic potentiodynamic polarisation measurements were performed with a scan rate of 0.5 mV/s. The potential range was selected starting at −100 mV vs. OCP and it was scanned to 1655 mV vs. SHE. A homogeneous magnetic field up to 0.6 T (HV7, Walker Scientific) has been superimposed during the electrochemical investigations in two different configurations, i.e. parallel and perpendicular to the vertical electrode surface. All experiments were carried out at room temperature. With repeating the experiments a good reproducibility was achieved. Simulations of the flux density distribution in front of the electrode were done by the numerical three dimensional magnetostatic field solver ‘Amperes 6.0’ (Enginia Research Inc.).
Fig. 1. Experimental setup; electrolyte volume: ∼16 ml; 1: body (Teflon), 2: working electrode (iron wire), 3: counter electrode (Pt sheet), 4: reference electrode (Hg/HgSO4 /K2 SO4. (sat.)) 5: Luggin capillary.
3. Results The potentiodynamic polarisation curves recorded for the iron wire in sulfuric acid solution (pH 0.25) without and with a magnetic field applied parallel to the electrode surface are shown in Fig. 2. The Tafel region is followed by a transition region entering into a diffusion-controlled region with almost constant current density. This region ends with an abrupt decrease of the current density, the active–passive transmission, which is followed by a stable passive state. There is no visible influence of the superimposed magnetic field on the OCP and the Tafel region. With increasing magnetic flux density the current density in the diffusion-controlled range (∼0–600 mVSHE at 0 T) increases from imax = 120 mA/cm2 without applied magnetic field to imax = 540 mA/cm2 at a flux density of 0.6 T and the potential of the active–passive transition is shifted to more noble values, i.e. 700 mV at 0.3 T and 1250 mV at 0.6 T, respectively. The passive current density increases with rising flux density from 180 A/cm2 without magnetic field to 680 A/cm2 at 0.6 T. The detected effects of an increased diffusion-controlled current density and a shift of the active–passive transmission to more noble potentials with increasing magnetic flux density confirm observations which were reported by Lu et al. [12]. The results were explained by an additional convection of the electrolyte due to the acting Lorentz force, which has its maximum in this field configuration [15]. The potentiodynamic polarisation curves obtained in sulfuric acid with the magnetic field applied in the perpendicular to the electrode configuration are shown in Fig. 3. Until a potential of about 700 mV no significant difference between the curves recorded in a flux density of 0.3 T and 0.6 T is visible. In contrast to the measurements without applied field, a maximum in the current density is reached at −100 mV followed by a sudden decrease of the current density by more than one order of magnitude. After a sharp minimum followed by a rapid increase the current density remains constant, but still in the order of about one tenth of the limiting current density that is reached without applied magnetic field. Furthermore, it is apparent that the active–passive transition occurs earlier, i.e. at more negative potentials (by ∼50 mV) and is less sharp. The passive current density is not significantly affected by the magnetic field. The anodic behaviour of iron in phthalate buffer differs strongly from the behaviour observed in the sulfuric acid solution, as shown
Fig. 2. Current density–potential curves of iron in sulfuric acid solution (pH 0.25) without and with a magnetic field applied parallel to the electrode surface (scan rate of 0.5 mV/s).
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Fig. 3. Current density–potential curves of iron in 0.5 M sulfuric acid solution (pH 0.25) without and with magnetic field applied perpendicular to the electrode surface (scan rate of 0.5 mV/s).
Fig. 5. Current density–potential curves of iron in phthalate buffer (pH 5) without and with applied magnetic field perpendicular to the electrode surface (scan rate of 0.5 mV/s).
in Fig. 4 for the parallel field configuration. The free corrosion potential adjusts at less noble values and the free corrosion current density is about one order of magnitude lower. Also in this environment the free corrosion parameters are not magnetic field dependent. Between the anodic Tafel region and the active–passive transition the current density increases continuously, reaching a maximum value of 50 mA/cm2 , which is about one order of magnitude lower than in pH 0.25. At these strongly reduced dissolution rates no diffusion limits the reaction and thus, a Lorentz-forcedriven convection of the electrolyte shows no significant effect on the current density. Furthermore, it is visible, that with increasing flux density the active–passive transition potential is shifted to less noble values (by 100 mV at 0.6 T) and the passive current density is decreased. The current density in the transpassive region is not effected by the magnetic field. Especially the negative shift of the active–passive transition potential and the decreased passive current density cannot be explained by a MHD effect. In phthalate buffer a magnetic field applied perpendicular to the surface (Fig. 5) leads to a slightly increased current density in the active dissolution range and a shift of the active–passive transition
potential of 300 mV in the negative direction. At low passive potentials (∼750 mV), the passive current density is lowered to about one tenth of its value at 0 T, but it converges to the value measured without magnetic field with rising potential. As it was found in sulfuric acid solution with the magnetic field applied perpendicular to the electrode surface (Fig. 3) an increase of the flux density from 0.3 T to 0.6 T does not further change the observed effects.
Fig. 4. Current density–potential curves of iron in phthalate buffer (pH 5) without and with applied magnetic field parallel to the electrode surface (scan rate of 0.5 mV/s).
4. Discussion The magnetic behaviour of the ferromagnetic iron wire in an external applied magnetic flux density Bappl. of 0.3 T and 0.6 T, respectively was simulated numerically to evaluate the flux density distribution near the electrode surface. In Fig. 6 the distribution of the flux density Bsurf. on a plane, which is parallel to the electrode surface at a distance of 5 m, is shown for the case of an applied flux density of 0.3 T. Fig. 7 shows the magnitude of the flux density along the coordinate (marked in Fig. 6) at distances of 0 and 100 m to the electrode surface for applied flux densities of 0.3 T and 0.6 T, respectively. It is apparent in Fig. 6a, that the flux density is concentrated at the rim regions perpendicular to the external field if the field is applied parallel to the electrode surface. In contrast, if the external field is applied perpendicular (Fig. 6b), the flux density is homogeneously in a maximum state all along the electrode rim. It is remarkable, that the simulated flux density distribution for the case of a parallel applied field corresponds to the shape of the corrosion pattern on an iron wire electrode surface observed by Lu et al. [12] after potentiostatic polarisation in the active region with an applied magnetic field of 0.4 T. If the external field is applied parallel to the surface, the gradient of the flux density is increased as well when the applied flux density is increased (Fig. 7a) due to a higher magnetization grade of the iron. In contrast, if the external field is applied perpendicular to the surface, the gradient of the flux density is only slightly increased because the iron is already magnetically saturated in an external field of 0.3 T (Fig. 7b). In the following the simulated flux density distributions will be used as basis for the discussion of the observed field effects on the polarisation behaviour. Under the used experimental conditions no significant effect of an external applied magnetic field on the free corrosion potential and the free corrosion current density was found. However, the variations of the free corrosion potential in different tests are
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Fig. 6. (a) Flux density distribution (magnitude) Bsurf. on a plane perpendicular to the electrode surface in a distance of 5 m with an applied flux density Bappl. of 0.3 T parallel to the surface and the distance coordinate. (b) Flux density distribution (magnitude) Bsurf. on a plane perpendicular to the electrode surface in a distance of 5 m with an applied flux density Bappl. of 0.3 T perpendicular to the surface.
higher then the observed shifts of several mV in the noble direction reported by Lu and Yang [16] and Rhen et al. [17]. In the region where the electron transfer is the rate determining step, e.g. in the Tafel region, no influence of the magnetic field was found as well. In the active region a maximum–minimum behaviour may occur due to a temporary coverage of the electrode with a slightly soluble species, which acts as a membrane inhibitor on the active dissolution [18,19]. Depending on the experimental condition it is not visible in the polarisation curve (Fig. 2), it is visible as a current density shoulder (Fig. 4, −250 mV) or is visible as a small current density maximum followed by a minimum (Fig. 5, −250 mV and Fig. 3, −100 mV). According to [18,19] the adsorbed species is suggested to be Fe[(OH)2 ]ads and its fraction is dependent on the pH value, the type of anions in the solution and their concentration and the concentration of Fe2+ ions. In particular, the polarisation curves obtained in sulfuric acid solution with the magnetic field superimposed perpendicular to the electrode surface generating a homogeneous flux density maximum all over the electrode rim (Fig. 3) show a maximum followed by a minimum, which are both much more pronounced than in the 0 T case or in the phthalate buffer.
Fig. 7. (a) Flux density distribution Bsurf. (magnitude) parallel to the electrode surface along the coordinate in Fig. 6a in a distance of 0 and 100 m with an applied flux density Bappl. of 0.3 T and 0.6 T parallel to the surface. (b) Flux density distribution Bsurf. (magnitude) parallel to the electrode surface along the coordinate in Fig. 6b in a distance of 0 and 100 m with an applied flux density Bappl. of 0.3 T and 0.6 T perpendicular to the surface.
It is possible that the field gradient force concentrates the paramagnetic Fe2+ ions at the surface all over the rim and thus, hampers the dissolution of the inhibiting film, promotes its formation or makes it less porous. As soon as the current density dropped due to the inhibitor film formation, less new Fe2+ ions are produced by dissolution of the metal and the film destabilizes. The current density increases again until it is limited by diffusion. The diffusionlimited current density is lowered due to the field gradient force pointing in the opposite direction than diffusion (Fig. 3). The field gradient reaches its maximum homogeneously along the rim of the wire (Fig. 6b) and thus, the concentration of Fe2+ ions reaches its maximum at the rim as well. With the magnetic field applied the passivation starts at 500 mV and the current density decreases comparatively slowly with rising potential. From 600 mV – the passivation potential without magnetic field – the current density is independent of the magnetic flux density. Thus, it is suggested that at 500 mV the electrode under field influence is partially (at the rim) covered with a protecting layer in result of the local Fe2+ ion enrichment at the rim surface. This layer grows with progressing polarisation to the centre of the surface, where the gradient in magnetic flux density is small compared to the rim. An increase of the applied magnetic field from 0.3 T to 0.6 T does not alter
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the observed effects any further. In a field of 0.3 T the iron wire is already magnetically saturated and an increase of the external field does not increase the gradient in the flux density any more (Fig. 7b). In contrast, if the magnetic field is applied parallel to the electrode surface (Fig. 2), an additional convection of the electrolyte is driven by the Lorentz force. This causes a reduction of the diffusion layer thickness and thus, accelerates the removal of the Fe2+ ions away from the surface. Additionally, the gradient in the magnetic flux density at the electrode surface is locally restricted to selected rim regions (Fig. 6a). In consequence of both aspects, a temporary inhibition film formation corresponding to a local maximum–minimum in the polarisation curve as explained above is not visible. Also, the diffusion-limited current density is increased. Due to the continuous removal of the Fe2+ ions from the surface near regions the active–passive transition can take place only at much more noble potentials. These observed effects are in good agreement with results reported by Lu et al. [12]. In the potentiodynamic polarisation curves obtained in phthalate buffer (Figs. 4 and 5) no influence of the superimposed magnetic field on the free corrosion potential and the Tafel region was observed. In the perpendicular field configuration an increased current density was found in the higher active region (0–300 mV), which cannot be explained yet. An influence of a Lorentz-forcedriven convection can be mainly excluded because an increased current density was not found in the parallel field configuration (Fig. 5), in which the Lorentz force is principally expected to be higher. However, in contrast to the anodisation in sulfuric acid, in weakly acidic phthalate buffer (pH 5) the maximum dissolution current density level remains below 100 mA/cm2 under all applied field configurations and diffusion is not predominant. Thus, the effect of the Lorentz force to the current density is not significant. In the perpendicular field configuration the active–passive transition occurs at a 300 mV less noble potential if a magnetic field of 0.3 or 0.6 T is applied. It can be assumed as it has been discussed before, that the magnetic field gradient force concentrates Fe2+ ions at the surface, especially along the whole rim region where the gradient is maximal, and thus, triggers the passivation starting from the rim towards the centre of the wire. An increase of the applied flux density from 0.3 T to 0.6 T does not alter this effect any more because the sample is already saturated at 0.3 T (Fig. 7b). If the superimposed magnetic field is oriented parallel to the surface the effect of the field gradient force is reduced because the gradient in flux density is smaller and restricted to smaller rim regions (Fig. 6a) and disturbed by the Lorentz-force-driven convection. Thus, the negative shift of the active–passive transition potential is less pronounced (100 mV at 0.6 T). If the external field is increased from 0.3 T to 0.6 T the effect is enhanced because the wire is not yet completely magnetized at an external flux density of 0.3 T, i.e. an increase of the external field leads to an increase of the gradient in the flux density at the wire surface (Fig. 7a). Furthermore, it is obvious that the superimposed magnetic field reduces the passive current density in both field configurations in phthalate buffer. At the initial stage of the passive region (∼700 mV) the passive current density is lowered by about one order of magnitude (0.6 T applied perpendicular to the surface), but with increasing potential it converges to the value obtained without applied magnetic field. It is proposed that the formation of ferrimagnetic Fe3 O4 is initially favoured due to the high flux density at the electrode surface [20], but with rising potential the field influence on the layer properties becomes smaller.
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5. Conclusions The Lorentz force only alters the dissolution and passivation behaviour of an iron wire in acidic environment when the anodic reaction is diffusion-controlled in the active range. It elevates the diffusion-limited current density and causes a shift of the active–passive transition potential to more noble values. Due to the ferromagnetic properties of iron, a high gradient of the magnetic flux density occurs at the rim of the wire electrode. A homogeneous flux density maximum exists all over the rim if the external magnetic field is applied perpendicular to the wire cross section. In this field configuration the effect of the field gradient force can overcome the effect of the Lorentz force even if the reaction is diffusion-limited. A dominance of the field gradient force leads to a decrease of the diffusion-limited current density and a shift of the active–passive transition potential to less noble values. This is presumably due to a favoured passive layer formation at the rim regions, which expands to the centre region with progressing passivation. Further investigations are in progress addressing the effect of the size of the electrode on this behaviour. It can be expected that the Lorentz force will dominate at higher electrode surfaces. In phthalate buffer the anodic dissolution reaction rate is lower and is not diffusion-limited. Thus, the anodic behaviour of the iron electrode is only affected by the field gradient force. The passivation potential is shifted to less noble values due to a concentration of Fe2+ ions at the surface rim region. This effect is particularly pronounced when the external field is applied perpendicular to the electrode surface yielding the highest gradient in flux density and a homogeneous flux density distribution maximum all over the rim. The passive current density is decreased especially at low passive potentials. Effects of an applied magnetic field on the structure and composition of the formed passive layers will be in focus of further investigations. Acknowledgements S. Roth and J. Buschbeck are acknowledged for fruitful discussions. The German Research Foundation (DFG) is gratefully acknowledged for the support of this work under grant GE 1106/6 “Corrosion influenced by magnetic fields.” References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
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