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Surface oxide reduction by hydrogen permeation through iron foil detected using a scanning Kelvin probe
Electrochemistry Communications 27 (2013) 144–147
Contents lists available at SciVerse ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Surface oxide reduction by hydrogen permeation through iron foil detected using a scanning Kelvin probe G. Williams a,⁎, H.N. McMurray a, R.C. Newman b a b
Materials Research Centre, College of Engineering, Swansea University, Singleton Park, Swansea, SA2 8PP, UK Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto, Ontario, Canada M5S 3E5
a r t i c l e
i n f o
Article history: Received 21 September 2012 Received in revised form 6 November 2012 Accepted 19 November 2012 Available online 24 November 2012 Keywords: Scanning Kelvin probe Hydrogen Diffusion Iron Volta potential
a b s t r a c t The interaction of atomic hydrogen with the air-formed surface oxide film of an iron foil, following cathodic hydrogen evolution on the reverse face, is shown to produce significant Volta potential changes which can be mapped using a scanning Kelvin probe (SKP). The magnitude of the Volta potential depression decreases with holding time following hydrogen charging and exhibits a dependence upon the oxygen partial pressure of the holding environment. It is proposed that the Volta potential depression observed in the area of hydrogen emergence results from a change in the ratio of FeIII/Fe II states within the surface oxide. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The subject of hydrogen permeation in steel has received significant attention for many decades, since hydrogen, formed as a cathodic product of corrosion under conditions of low pH or oxygen partial pressure can be absorbed into the bulk metal and cause embrittlement [1]. Using 304 grade stainless steel undergoing stress corrosion cracking, Masuda recently demonstrated, by scanning Kelvin force microscopic (SKPFM) Volta potential imaging, that regions in the vicinity of individual crack tips show significantly different potentials to the unaffected surface [2]. The observed depression of potential was ascribed to the presence of atomic hydrogen in these regions. Senöz et al. [3] used the same technique to show that significant Volta potential differences could be observed between austenitic and ferritic phases in duplex steel through which hydrogen was actively permeating. These observations suggest that Volta potential mapping may be a novel method of detecting hydrogen emergence from ferrous-metal based components such as tubes or pipes, and as such provide early warning of hidden corrosion damage. In this work we use a conventional scanning Kelvin probe (SKP), operating on a macroscopic scale, to elucidate the source of Volta potential changes produced by the emergence of hydrogen at the oxide covered surface of an iron substrate. Previous work using palladium (Pd) foil [3,4] and Pd-coated iron [4] has already demonstrated that significant hydrogen-induced Volta potential changes are produced on the exit face of a sample undergoing cathodic ⁎ Corresponding author. Tel.: +44 1792 295589, fax: +44 1792 295676. E-mail address: [email protected] (G. Williams). 1388-2481/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elecom.2012.11.022
hydrogen charging on the entry side. Furthermore, fully quantitative measurement of hydrogen permeation even down to extremely low levels has been demonstrated [4], underlining the powerful capability of this new methodology. The approach used here involves the initial evolution of hydrogen (H2) on an iron foil by cathodic polarisation of a circular working electrode area to allow ingress of atomic hydrogen into the bulk metal. After stopping cathodic H2 charging, SKP Volta potential mapping is carried out over the native oxide-covered reverse face under conditions of varying oxygen partial pressure, in order to elucidate the mechanism which gives rise to the observed potential differences. 2. Experimental details Iron foil substrates (5 × 5 cm) of 0.1 mm thickness and 99.99% purity were supplied by Goodfellow Ltd. The face subjected to cathodic H2 charging was abrasively cleaned using an aqueous slurry of polishing alumina followed by rinsing with water and ethanol, while the reverse face of the iron foil was only degreased using acetone. Initial cathodic polarisation of the iron foil was carried out in an electrochemical cell which allowed a circular area of 5 mm diameter to be galvanostatically polarised at − 50 mA cm −2 in 0.1 mol dm −3 Na2SO4 (aq) solution. The iron foil was clamped flat to the cell by means of a polycarbonate back-plate. The cell design also enabled vigorous stirring of the electrolyte during cathodic H2 charging to avoid excessive bubble formation blocking the working electrode area. Volta potential mapping of the reverse face of the iron substrates was carried out ex-situ using a scanning Kelvin probe (SKP) employing
G. Williams et al. / Electrochemistry Communications 27 (2013) 144–147
a 100 micron diameter gold wire reference probe. Full details of the design, operation and calibration of the SKP instrumentation used in this work are given elsewhere [5,6]. The SKP environment chamber enabled humidified nitrogen–oxygen gas mixtures of varying composition to be admitted. All Volta potential scans were carried out at room temperature at a fixed relative humidity of 95%, using a 1 cm2 square scan area on the reverse side of the iron foil centred on the 5 mm diameter circular region which had previously been subjected to cathodic hydrogen evolution. SKP Volta potential mapping typically commenced within 15 min of stopping the hydrogen evolution process, using a data point density of 10 points/mm and a probe-to-sample distance of 100 μm. Repeat scans were carried out at 120 min intervals over a 24 h period to study the time-dependence of the Volta potential difference distribution over the scanned surface. 3. Results Fig. 1 shows a series of greyscale Volta potential maps, recorded in air at a relative humidity of 95%, over a 24 h period following cathodic H2 evolution on a circular working electrode area on the reverse face of an iron foil substrate. Since it is widely accepted that Volta potential differences measured on metal surfaces in the presence of thin humidity layers may be interpreted in terms of electrochemical potentials [7,8], SKP calibration has been used to present the data as maps of distributions of potential versus the standard hydrogen
145
electrode (ESHE). From Fig. 1 it is clear that SKP is able to reveal a circular feature of markedly depressed potential which has similar dimensions to the electrolyte contacting area on the reverse face of the iron foil. Local ESHE values in this circular region increase with post H2-evolution holding time, with the result that the contrast between the hydrogen affected zone and the surrounding iron surface becomes less distinct. In addition, there also appear to be local variations in ESHE values within the circular zone, where potentials in the central portion of the circle are up to 50 mV more positive than those measured close to the perimeter. Repeat experiments carried out using varying cathodic charging periods in the range 5–60 min did not show appreciable ESHE contrast differences between the H2-affected zone and the unaffected Fe surface. In a separate experiment, ESHE versus distance profiles were recorded across a circular region of depressed ESHE at 60 min following cathodic H2 charging, in environments of varying oxygen partial pressure (PO2) at a fixed relative humidity of 95%. From the results given in Fig. 2a, it is evident that ESHE in this region shows a dependence on PO2, with the maximum ESHE difference between the H2-affected zone and the intact iron surface observed in the absence of oxygen. As PO2 in the holding environment is increased, a corresponding rise in ESHE is observed. Fig. 2b shows that upon repeatedly cycling the holding environment from N2 to air, a consistent and reversible change in ESHE of ca. 150 mV is obtained, giving relatively constant ESHE values within ca. 30 min of changing PO2. Additionally, it is evident that the time
Fig. 1. Greyscale interpolated ESHE maps of the reverse face of a cathodised Fe foil recorded in humidified synthetic air at 95% rh (a) 60, (b) 200, (c) 480, and (d) 1440 min after halting galvanostatic polarisation. A current density of −50 mA cm−2 was applied for 15 min in vigorously stirred 0.1 mol dm−3 Na2SO4 at pH 7.
146
G. Williams et al. / Electrochemistry Communications 27 (2013) 144–147
which diffuses into the polycarbonate back-plate. The processes of H + discharge, H diffusion through the iron and H2 diffusion through the polymer back-plate appear to reach a steady state after a short period of electrolysis (≤ 5 min). This steady state flux distribution results in the size, shape and ESHE contrast of the reverse-side potential reduction detected by SKP being independent of electrolysis time for periods of between 5 and 60 min. The slightly more positive ESHE values measured in the centre of the circular feature, giving rise to a “halo” effect in Fig. 1 is thought to arise from a thinning of the oxide layer on the hydrogen entry side as a consequence of cathodic polarisation. This in turn will favour emergence of hydrogen from the bulk iron at the entry side when cathodisation is halted, since for the experiments outlined here, atomic H which has entered the metal foil is free to emerge at both faces. On contact with atmospheric oxygen, surface Fe II states will be re-oxidised to Fe III, according to a scheme proposed elsewhere [11,14], and subsurface Fe II in the oxide layer becomes re-oxidised through Fe III–Fe II electron exchange. A Fe III/Fe II ratio profile will then become established in the oxide film as shown schematically in Fig. 3a. Eventually the Fe III/Fe II ratio will come into equilibrium with atmospheric oxygen throughout the oxide film as shown in Fig. 3b, though this process may take some time. This argument is immediately consistent with the observed SKP findings under conditions of varying PO2, given in Fig. 2a, where ESHE in the H2-affected zone progressively becomes more negative as PO2 is systematically decreased. Under N2, where only a trace of adventitious oxygen will
Fig. 2. (a) SKP-derived ESHE-distance profiles taken across a circular feature on the reverse face of a previously cathodised Fe foil sample in (i) N2, (ii) 5% O2 in N2, (iii) air and (iv) pure O2 at 95% rh ca. 90 min after stopping cathodic hydrogen charging. (b) Plot of ESHE time-dependence as the holding environment is cycled between N2 and air at 95% rh.
taken to attain 50% of the final ESHE values depends upon whether the holding environment is changed from low to high PO2 (ca. 150 s) or vice versa (ca. 600 s). 4. Discussion In the preliminary cathodic charging process, atomic hydrogen is generated by H + discharge onto the working electrode, subsequently entering the bulk metal and diffusing through the iron foil. Upon reaching the reverse side, atomic hydrogen will reduce Fe III states to Fe II in the native oxide layer on the iron surface [9–11]. This will appear as an area of reduced potential to the SKP, in this case reflecting the working area over which hydrogen enters the metal foil, for which the potential is predictable by the Nernst equation below, h i III Fe RT E ¼ E þ 2:303 log10 II nF Fe
ð1Þ
where a 59 mV shift per 10 fold change in Fe III/Fe II ratio will be observed [8,12,13]. Any atomic hydrogen reaching the reverse side and not consumed in reducing Fe III states will recombine to form H2
Fig. 3. Schematic representation of (a) interfacial electron transfer reactions occurring in the vicinity of the surface oxide and (b) the influence of oxygen partial pressure on potential.
G. Williams et al. / Electrochemistry Communications 27 (2013) 144–147
be present, the region of reverse-side depressed potential contrast detected by SKP is most intense, as predicted for a low Fe III/Fe II ratio by Eq. (1). Whether the regions of persistent low potential detected by SKP reflect a continuing presence of un-recombined H retained within the iron foil is a matter of conjecture. The rapid establishment of a steady state in H/H2 flux during the cathodic charging process suggests fairly rapid kinetics of H recombination on the reverse-side oxide-covered iron surface. It may be that the persistence of low potentials over a 24 h holding period results from slow kinetics in the re-oxidation of surface oxide Fe II states to Fe III by atmospheric oxygen, although the reversibility of ESHE changes observed upon cycling between two different PO2 levels, demonstrated in Fig. 2b, is not immediately consistent with this hypothesis. Indeed the different time-dependence of ESHE observed upon changing from low to high PO2 and vice-versa suggests that Fe II re-oxidation is more rapid than the corresponding reduction of Fe III by atomic hydrogen. However, care should be exercised in the interpretation of kinetic data due to the proposed Nernstian relationship of E with the Fe III/Fe II ratio. Published diffusion coefficients in the range 5 × 10−5 to 3 × 10−7 cm2 s −1 for atomic hydrogen in annealed iron [15,16] are immediately consistent with the rapid (b5 min) appearance of low potentials on the reverse side of the cathodised sample. However, they are too high to account for the observed persistence of the same low potentials over a period of up to 24 h as seen in Fig. 1. It would therefore seem likely that if uncombined atomic hydrogen is retained within the foil, it is by virtue of the surface oxide acting as a barrier to its emergence [17]. 5. Conclusions The findings presented in this paper suggest that SKP and higher spatial resolution SKPFM are useful techniques for determining the physical distribution of hydrogen emerging through the surface of corroding ferrous alloys. Atomic hydrogen diffusing to the surface of an iron foil is shown to interact with the surface oxide layer to
147
produce marked Volta potential changes which are detectable by SKP. An observed dependence of the magnitude of the Volta potential depression with oxygen partial pressure is consistent with the reduction of Fe III surface states by atomic hydrogen at the metal–oxide interface and subsequent re-oxidation of Fe II by atmospheric oxygen at the oxide–gas interface. Under conditions of constant oxygen partial pressure and atomic hydrogen activity at the metal–oxide interface, a local equilibrium condition is reached where the magnitude of the Volta potential change is governed by the Fe III/Fe II ratio in the surface oxide.
References [1] In: L.L. Shreir, R.A. Jarman, G.T. Burstein (Eds.), Corrosion, 3rd edition, ButterworthHeinemann, Oxford, UK, 1994. [2] H. Masuda, Corrosion Science 49 (2007) 120. [3] C. Senöz, S. Evers, M. Stratmann, M. Rohwerder, Electrochemistry Communications 13 (2011) 1542. [4] S. Evers, M. Rohwerder, Electrochemistry Communications 24 (2012) 85. [5] G. Williams, H.N. McMurray, D.A. Worsley, Journal of Forensic Sciences 46 (2001) 1085. [6] G. Williams, H.N. McMurray, Journal of the Electrochemical Society 148 (2001) B337. [7] S. Yee, R.A. Oriani, M. Stratmann, Journal of the Electrochemical Society 138 (1991) 55. [8] M. Stratmann, H. Streckel, Corrosion Science 30 (1990) 681. [9] T. Casanova, J. Crousier, Corrosion Science 38 (1996) 1535. [10] S. Modianoa, J.A.V. Carreno, C.S. Fugivara, R.M. Torresi, V. Vivier, A.V. Benedetti, O.R. Mattos, Electrochimica Acta 53 (2008) 3670. [11] M. Krasemann, H. Streckel, K. Hoffmann, H.J. Grabke, M. Stratmann, Proceedings of the Electrochemical Society 97 (1997) 207. [12] M. Stratmann, H. Streckel, Corrosion Science 30 (1990) 697. [13] M. Stratmann, H. Streckel, K.T. Kim, S. Crockett, Corrosion Science 30 (1990) 715. [14] M. Stratmann, K. Hoffman, Corrosion Science 29 (1989) 1329. [15] M.A.V. Devanathan, Z. Stachurski, Journal of the Electrochemical Society 111 (1964) 619. [16] T.Y. Zhang, Y.P. Zheng, Q.-Y. Wu, Journal of the Electrochemical Society 146 (1999) 1741. [17] P. Manolatos, M. Jerome, C. Duret-Thuali, J. Le Coze, Corrosion Science 37 (1995) 1773.
Contents lists available at SciVerse ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Surface oxide reduction by hydrogen permeation through iron foil detected using a scanning Kelvin probe G. Williams a,⁎, H.N. McMurray a, R.C. Newman b a b
Materials Research Centre, College of Engineering, Swansea University, Singleton Park, Swansea, SA2 8PP, UK Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto, Ontario, Canada M5S 3E5
a r t i c l e
i n f o
Article history: Received 21 September 2012 Received in revised form 6 November 2012 Accepted 19 November 2012 Available online 24 November 2012 Keywords: Scanning Kelvin probe Hydrogen Diffusion Iron Volta potential
a b s t r a c t The interaction of atomic hydrogen with the air-formed surface oxide film of an iron foil, following cathodic hydrogen evolution on the reverse face, is shown to produce significant Volta potential changes which can be mapped using a scanning Kelvin probe (SKP). The magnitude of the Volta potential depression decreases with holding time following hydrogen charging and exhibits a dependence upon the oxygen partial pressure of the holding environment. It is proposed that the Volta potential depression observed in the area of hydrogen emergence results from a change in the ratio of FeIII/Fe II states within the surface oxide. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The subject of hydrogen permeation in steel has received significant attention for many decades, since hydrogen, formed as a cathodic product of corrosion under conditions of low pH or oxygen partial pressure can be absorbed into the bulk metal and cause embrittlement [1]. Using 304 grade stainless steel undergoing stress corrosion cracking, Masuda recently demonstrated, by scanning Kelvin force microscopic (SKPFM) Volta potential imaging, that regions in the vicinity of individual crack tips show significantly different potentials to the unaffected surface [2]. The observed depression of potential was ascribed to the presence of atomic hydrogen in these regions. Senöz et al. [3] used the same technique to show that significant Volta potential differences could be observed between austenitic and ferritic phases in duplex steel through which hydrogen was actively permeating. These observations suggest that Volta potential mapping may be a novel method of detecting hydrogen emergence from ferrous-metal based components such as tubes or pipes, and as such provide early warning of hidden corrosion damage. In this work we use a conventional scanning Kelvin probe (SKP), operating on a macroscopic scale, to elucidate the source of Volta potential changes produced by the emergence of hydrogen at the oxide covered surface of an iron substrate. Previous work using palladium (Pd) foil [3,4] and Pd-coated iron [4] has already demonstrated that significant hydrogen-induced Volta potential changes are produced on the exit face of a sample undergoing cathodic ⁎ Corresponding author. Tel.: +44 1792 295589, fax: +44 1792 295676. E-mail address: [email protected] (G. Williams). 1388-2481/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elecom.2012.11.022
hydrogen charging on the entry side. Furthermore, fully quantitative measurement of hydrogen permeation even down to extremely low levels has been demonstrated [4], underlining the powerful capability of this new methodology. The approach used here involves the initial evolution of hydrogen (H2) on an iron foil by cathodic polarisation of a circular working electrode area to allow ingress of atomic hydrogen into the bulk metal. After stopping cathodic H2 charging, SKP Volta potential mapping is carried out over the native oxide-covered reverse face under conditions of varying oxygen partial pressure, in order to elucidate the mechanism which gives rise to the observed potential differences. 2. Experimental details Iron foil substrates (5 × 5 cm) of 0.1 mm thickness and 99.99% purity were supplied by Goodfellow Ltd. The face subjected to cathodic H2 charging was abrasively cleaned using an aqueous slurry of polishing alumina followed by rinsing with water and ethanol, while the reverse face of the iron foil was only degreased using acetone. Initial cathodic polarisation of the iron foil was carried out in an electrochemical cell which allowed a circular area of 5 mm diameter to be galvanostatically polarised at − 50 mA cm −2 in 0.1 mol dm −3 Na2SO4 (aq) solution. The iron foil was clamped flat to the cell by means of a polycarbonate back-plate. The cell design also enabled vigorous stirring of the electrolyte during cathodic H2 charging to avoid excessive bubble formation blocking the working electrode area. Volta potential mapping of the reverse face of the iron substrates was carried out ex-situ using a scanning Kelvin probe (SKP) employing
G. Williams et al. / Electrochemistry Communications 27 (2013) 144–147
a 100 micron diameter gold wire reference probe. Full details of the design, operation and calibration of the SKP instrumentation used in this work are given elsewhere [5,6]. The SKP environment chamber enabled humidified nitrogen–oxygen gas mixtures of varying composition to be admitted. All Volta potential scans were carried out at room temperature at a fixed relative humidity of 95%, using a 1 cm2 square scan area on the reverse side of the iron foil centred on the 5 mm diameter circular region which had previously been subjected to cathodic hydrogen evolution. SKP Volta potential mapping typically commenced within 15 min of stopping the hydrogen evolution process, using a data point density of 10 points/mm and a probe-to-sample distance of 100 μm. Repeat scans were carried out at 120 min intervals over a 24 h period to study the time-dependence of the Volta potential difference distribution over the scanned surface. 3. Results Fig. 1 shows a series of greyscale Volta potential maps, recorded in air at a relative humidity of 95%, over a 24 h period following cathodic H2 evolution on a circular working electrode area on the reverse face of an iron foil substrate. Since it is widely accepted that Volta potential differences measured on metal surfaces in the presence of thin humidity layers may be interpreted in terms of electrochemical potentials [7,8], SKP calibration has been used to present the data as maps of distributions of potential versus the standard hydrogen
145
electrode (ESHE). From Fig. 1 it is clear that SKP is able to reveal a circular feature of markedly depressed potential which has similar dimensions to the electrolyte contacting area on the reverse face of the iron foil. Local ESHE values in this circular region increase with post H2-evolution holding time, with the result that the contrast between the hydrogen affected zone and the surrounding iron surface becomes less distinct. In addition, there also appear to be local variations in ESHE values within the circular zone, where potentials in the central portion of the circle are up to 50 mV more positive than those measured close to the perimeter. Repeat experiments carried out using varying cathodic charging periods in the range 5–60 min did not show appreciable ESHE contrast differences between the H2-affected zone and the unaffected Fe surface. In a separate experiment, ESHE versus distance profiles were recorded across a circular region of depressed ESHE at 60 min following cathodic H2 charging, in environments of varying oxygen partial pressure (PO2) at a fixed relative humidity of 95%. From the results given in Fig. 2a, it is evident that ESHE in this region shows a dependence on PO2, with the maximum ESHE difference between the H2-affected zone and the intact iron surface observed in the absence of oxygen. As PO2 in the holding environment is increased, a corresponding rise in ESHE is observed. Fig. 2b shows that upon repeatedly cycling the holding environment from N2 to air, a consistent and reversible change in ESHE of ca. 150 mV is obtained, giving relatively constant ESHE values within ca. 30 min of changing PO2. Additionally, it is evident that the time
Fig. 1. Greyscale interpolated ESHE maps of the reverse face of a cathodised Fe foil recorded in humidified synthetic air at 95% rh (a) 60, (b) 200, (c) 480, and (d) 1440 min after halting galvanostatic polarisation. A current density of −50 mA cm−2 was applied for 15 min in vigorously stirred 0.1 mol dm−3 Na2SO4 at pH 7.
146
G. Williams et al. / Electrochemistry Communications 27 (2013) 144–147
which diffuses into the polycarbonate back-plate. The processes of H + discharge, H diffusion through the iron and H2 diffusion through the polymer back-plate appear to reach a steady state after a short period of electrolysis (≤ 5 min). This steady state flux distribution results in the size, shape and ESHE contrast of the reverse-side potential reduction detected by SKP being independent of electrolysis time for periods of between 5 and 60 min. The slightly more positive ESHE values measured in the centre of the circular feature, giving rise to a “halo” effect in Fig. 1 is thought to arise from a thinning of the oxide layer on the hydrogen entry side as a consequence of cathodic polarisation. This in turn will favour emergence of hydrogen from the bulk iron at the entry side when cathodisation is halted, since for the experiments outlined here, atomic H which has entered the metal foil is free to emerge at both faces. On contact with atmospheric oxygen, surface Fe II states will be re-oxidised to Fe III, according to a scheme proposed elsewhere [11,14], and subsurface Fe II in the oxide layer becomes re-oxidised through Fe III–Fe II electron exchange. A Fe III/Fe II ratio profile will then become established in the oxide film as shown schematically in Fig. 3a. Eventually the Fe III/Fe II ratio will come into equilibrium with atmospheric oxygen throughout the oxide film as shown in Fig. 3b, though this process may take some time. This argument is immediately consistent with the observed SKP findings under conditions of varying PO2, given in Fig. 2a, where ESHE in the H2-affected zone progressively becomes more negative as PO2 is systematically decreased. Under N2, where only a trace of adventitious oxygen will
Fig. 2. (a) SKP-derived ESHE-distance profiles taken across a circular feature on the reverse face of a previously cathodised Fe foil sample in (i) N2, (ii) 5% O2 in N2, (iii) air and (iv) pure O2 at 95% rh ca. 90 min after stopping cathodic hydrogen charging. (b) Plot of ESHE time-dependence as the holding environment is cycled between N2 and air at 95% rh.
taken to attain 50% of the final ESHE values depends upon whether the holding environment is changed from low to high PO2 (ca. 150 s) or vice versa (ca. 600 s). 4. Discussion In the preliminary cathodic charging process, atomic hydrogen is generated by H + discharge onto the working electrode, subsequently entering the bulk metal and diffusing through the iron foil. Upon reaching the reverse side, atomic hydrogen will reduce Fe III states to Fe II in the native oxide layer on the iron surface [9–11]. This will appear as an area of reduced potential to the SKP, in this case reflecting the working area over which hydrogen enters the metal foil, for which the potential is predictable by the Nernst equation below, h i III Fe RT E ¼ E þ 2:303 log10 II nF Fe
ð1Þ
where a 59 mV shift per 10 fold change in Fe III/Fe II ratio will be observed [8,12,13]. Any atomic hydrogen reaching the reverse side and not consumed in reducing Fe III states will recombine to form H2
Fig. 3. Schematic representation of (a) interfacial electron transfer reactions occurring in the vicinity of the surface oxide and (b) the influence of oxygen partial pressure on potential.
G. Williams et al. / Electrochemistry Communications 27 (2013) 144–147
be present, the region of reverse-side depressed potential contrast detected by SKP is most intense, as predicted for a low Fe III/Fe II ratio by Eq. (1). Whether the regions of persistent low potential detected by SKP reflect a continuing presence of un-recombined H retained within the iron foil is a matter of conjecture. The rapid establishment of a steady state in H/H2 flux during the cathodic charging process suggests fairly rapid kinetics of H recombination on the reverse-side oxide-covered iron surface. It may be that the persistence of low potentials over a 24 h holding period results from slow kinetics in the re-oxidation of surface oxide Fe II states to Fe III by atmospheric oxygen, although the reversibility of ESHE changes observed upon cycling between two different PO2 levels, demonstrated in Fig. 2b, is not immediately consistent with this hypothesis. Indeed the different time-dependence of ESHE observed upon changing from low to high PO2 and vice-versa suggests that Fe II re-oxidation is more rapid than the corresponding reduction of Fe III by atomic hydrogen. However, care should be exercised in the interpretation of kinetic data due to the proposed Nernstian relationship of E with the Fe III/Fe II ratio. Published diffusion coefficients in the range 5 × 10−5 to 3 × 10−7 cm2 s −1 for atomic hydrogen in annealed iron [15,16] are immediately consistent with the rapid (b5 min) appearance of low potentials on the reverse side of the cathodised sample. However, they are too high to account for the observed persistence of the same low potentials over a period of up to 24 h as seen in Fig. 1. It would therefore seem likely that if uncombined atomic hydrogen is retained within the foil, it is by virtue of the surface oxide acting as a barrier to its emergence [17]. 5. Conclusions The findings presented in this paper suggest that SKP and higher spatial resolution SKPFM are useful techniques for determining the physical distribution of hydrogen emerging through the surface of corroding ferrous alloys. Atomic hydrogen diffusing to the surface of an iron foil is shown to interact with the surface oxide layer to
147
produce marked Volta potential changes which are detectable by SKP. An observed dependence of the magnitude of the Volta potential depression with oxygen partial pressure is consistent with the reduction of Fe III surface states by atomic hydrogen at the metal–oxide interface and subsequent re-oxidation of Fe II by atmospheric oxygen at the oxide–gas interface. Under conditions of constant oxygen partial pressure and atomic hydrogen activity at the metal–oxide interface, a local equilibrium condition is reached where the magnitude of the Volta potential change is governed by the Fe III/Fe II ratio in the surface oxide.
References [1] In: L.L. Shreir, R.A. Jarman, G.T. Burstein (Eds.), Corrosion, 3rd edition, ButterworthHeinemann, Oxford, UK, 1994. [2] H. Masuda, Corrosion Science 49 (2007) 120. [3] C. Senöz, S. Evers, M. Stratmann, M. Rohwerder, Electrochemistry Communications 13 (2011) 1542. [4] S. Evers, M. Rohwerder, Electrochemistry Communications 24 (2012) 85. [5] G. Williams, H.N. McMurray, D.A. Worsley, Journal of Forensic Sciences 46 (2001) 1085. [6] G. Williams, H.N. McMurray, Journal of the Electrochemical Society 148 (2001) B337. [7] S. Yee, R.A. Oriani, M. Stratmann, Journal of the Electrochemical Society 138 (1991) 55. [8] M. Stratmann, H. Streckel, Corrosion Science 30 (1990) 681. [9] T. Casanova, J. Crousier, Corrosion Science 38 (1996) 1535. [10] S. Modianoa, J.A.V. Carreno, C.S. Fugivara, R.M. Torresi, V. Vivier, A.V. Benedetti, O.R. Mattos, Electrochimica Acta 53 (2008) 3670. [11] M. Krasemann, H. Streckel, K. Hoffmann, H.J. Grabke, M. Stratmann, Proceedings of the Electrochemical Society 97 (1997) 207. [12] M. Stratmann, H. Streckel, Corrosion Science 30 (1990) 697. [13] M. Stratmann, H. Streckel, K.T. Kim, S. Crockett, Corrosion Science 30 (1990) 715. [14] M. Stratmann, K. Hoffman, Corrosion Science 29 (1989) 1329. [15] M.A.V. Devanathan, Z. Stachurski, Journal of the Electrochemical Society 111 (1964) 619. [16] T.Y. Zhang, Y.P. Zheng, Q.-Y. Wu, Journal of the Electrochemical Society 146 (1999) 1741. [17] P. Manolatos, M. Jerome, C. Duret-Thuali, J. Le Coze, Corrosion Science 37 (1995) 1773.