Study of passive films formed on mild steel in alkaline media by the application of anodic potentials

Materials Chemistry and Physics 114 (2009) 962–972

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Study of passive films formed on mild steel in alkaline media by the application of anodic potentials L. Freire a,∗ , X.R. Nóvoa a , M.F. Montemor b , M.J. Carmezim b,c a

Universidade de Vigo, E.T.S.E.I., Campus Universitario, 36310 Vigo, Spain ICEMS – Instituto Superior Técnico, Universidade Técnica de Lisboa, Av. Rovisco Pais, 1049 – 001 Lisboa, Portugal c EST Setúbal, DEM, Instituto Politécnico de Setúbal, Campus IPS, 2910 Setúbal, Portugal b

a r t i c l e

i n f o

Article history: Received 4 May 2008 Received in revised form 13 October 2008 Accepted 3 November 2008 Keywords: Mild steel Passivity Cathodic protection Alkaline media

a b s t r a c t In this paper, iron oxide thin layers formed on mild steel substrates in alkaline media by the application of different anodic potentials were studied in order to characterize their morphology, composition and electrochemical behaviour, in particular under conditions of cathodic protection. The surface composition was evaluated by X-Ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES). The morphology of the surface oxides was studied via Atomic Force Microscopy (AFM). The electrochemical behaviour of the surface oxides was studied using Electrochemical Impedance Spectroscopy (EIS). The results showed that the surface film is composed by Fe2+ oxides and Fe3+ oxides and/or hydroxides. The contribution of Fe2+ species vanishes when the potential of film formation increases in the passive domain. Two distinct phases were differentiated in the outer layers of the surface film, which proves that film growing is topotactic in nature. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Steel passivation in alkaline environments is due to the formation of a very thin, but highly protective oxide/hydroxide layer. This layer plays an important role, for example in corrosion protection of reinforcing steel in concrete. Furthermore, the composition and morphological changes of this layer can affect the electrochemical behaviour of reinforcing steel under cathodic protection. During cathodic protection operation a cathodic current is applied, usually to pre-oxidized structures and the composition and morphology of these pre-existing oxides may influence the current distribution and, consequently the efficiency of the cathodic protection process. Therefore, the characterization of the oxides formed on steel under alkaline conditions and the understanding their electrochemical behaviour are an important contribution for the definition of the relevant operating parameters regarding the efficiency of cathodic protection systems applied to reinforcing steel. Corrosion of steel structures at ambient temperature in neutral to alkaline media results in the formation of iron oxo-hydroxides like goethite (␣-FeOOH), lepidocrocite (␥-FeOOH) and akagonite

∗ Corresponding author. E-mail address: [email protected] (L. Freire). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.11.012

(␤-FeOOH) below which more protective forms of iron oxides including magnetite (Fe3 O4 ), maghemite (␥-Fe2 O3 ) and hematite (␣-Fe2 O3 ) are present [1,2]. Depending on the external conditions, the outer iron oxide layer has variable stoichiometry and atomic density, due to the presence of different chemical species. In highly alkaline media, Fe3 O4 oxide forms at low electrode potentials (−0.8 V) and Fe2 O3 oxide, at more positive potential values [3]. Moreover, hydrated species are present all over the potential domain, but the passive film tends to dehydration as the potential increases [4]. This fact can explain why steel in concrete becomes more resistant to chlorides when the potential in the passive range decreases [5]. As the potential becomes more negative, the hydration degree increases, reducing the local [Cl− ]/[OH− ] ratio and consequently pitting potential becomes higher than that corresponding to the nominal Cl− content in solution. Magnetite is of particular importance for the understanding of the electrochemical behaviour of steel in concrete because it plays an essential role in the evolution of the passive layer [6]. Although the structure and composition of passive films formed on steel substrates is still a controversial subject, it has been proved that, in alkaline media, the film corresponds basically to a double-layer model consisting of an inner magnetite and an outer ferric oxide according to a Fe3 O4 /Fe3+ structure [7–12]. The most internal layer is composed of Fe2+ oxides in contact with the substrate. The thermodynamic instability of both Fe2+ oxides and magnetite in the presence of oxygen leads to the formation of an outer layer of Fe3+

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oxides and continuous exposure to oxygen and humidity promotes film growth and a progressive enrichment of the passive film in Fe3+ , either in oxide form or oxo-hydroxide form, depending on the potential. The analysis of the extent to which this picture of homogeneous growth corresponds to the actual behaviour is one of the objectives of the present study. The present work also aims at obtaining more detailed knowledge of the initial stages of mild steel passivation at different potentials in alkaline media. The composition and structure of these passive films will be correlated with their electrochemical behaviour under applied cathodic potentials. 2. Experimental 2.1. The electrolyte The measurements were carried out in 0.1 M NaOH + 0.1 M KOH solution, pH 13.0, prepared with grade chemical reagents and distilled water. The measured pH was 13.0. This solution was chosen in order to simulate the pH of concrete pores’ solution [13]. 2.2. The tested material The substrate was mild steel, type AISI 1040, which presents as main alloying elements C: 0.37% (w/w) and Mn: 0.45% (w/w). The working electrode surface was abraded with wet SiC paper of decreasing grit size (200, 500, 1200) and then freshly polished with alumina (1 ␮m) to eliminate the scratches caused by the mechanical polishing treatment. After polishing the steel samples were degreased with acetone and rinsed with distilled water. Finally, they were dried in a hot air current (120 ◦ C) during 30 s. No oxidation is expected to occur at this stage. 2.3. Electrochemical measurements All electrochemical experiments were performed in a conventional threeelectrode electrochemical cell arrangement, where the working electrode was a 1 cm × 1 cm sheet of carbon steel. An Hg/HgO KOH 1 mol L−1 (E◦ = +0.86 V vs. SHE) was employed as reference electrode in order to avoid chlorides contamination of the alkaline solution. The counter-electrode was a platinum gauze of large surface (50 cm2 ). All potentials in this study were converted to the saturated calomel electrode (SCE, E◦ = +0.24 V vs. SHE), the reference electrode mostly referred in the literature. The experiments were made at room temperature (20◦ C), using an AUTOLAB 30 Potentiostat (from EcoChemie, NL). Two different electrochemical techniques were employed: D.C. Polarization and Electrochemical Impedance Spectroscopy (EIS). Before potentiostatic polarization tests the samples were submitted to cathodic cleaning (−1.2 V) for 600 s in order to eliminate possible surface oxides and produce reproducible surfaces. EIS tests were carried out on the films formed at different potential for 1 h. The frequency was scanned from 10 kHz down to 1 mHz, and a 10 mV r.m.s. signal was applied. Galvanic current measurements under conditions of cathodic protection were performed using an ACM (Applied Corrosion Monitoring, UK) multi-zero resistance ammeter.

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and phase of photocurrent at different wavelengths were measured by means of a two-phase lock-in amplifier (EG&G 5210) coupled to the chopper (frequency 31 Hz) and to a potentiostat (EG&G 273A). The photocurrent spectra were obtained on a sample polarized at +200 mV vs. SCE, by scanning the wavelength of the light in steps of 10 nm from 350 nm to 700 nm. Intensities were normalized with respect to the photon emission of the light source.

3. Results and discussion 3.1. Cyclic voltammetry and D.C. polarization Fig. 1 gives a general view of the electrochemical behaviour of the steel tested in alkaline media. The behaviour is similar to that reported [11] for iron in a similar medium. The magnetite formation peak appears close to −0.8 V, and the passive region corresponds roughly to the −0.3 V to +0.5 V range. D.C. polarization on mild steel samples was performed at different potentials from magnetite formation to passivity domain, i.e., from −0.8 V to +0.3 V in steps of 0.1 V. All potentials were applied for 1 h in order to reach steady state conditions for the passive layer formation. The evolution of the current measured during the application of different potentials (−0.2 V; 0 mV and +0.2 V) in the passive domain was recorded. The initial charging peak was followed by a plateau, at low and constant residual current density that defines the passive steady state. The steady state current density was nearly 1 ␮A cm−2 (Fig. 2). 3.2. Surface analysis 3.2.1. XPS The chemical composition and the oxidation state of iron in the passive films formed at different potentials were assessed by XPS analysis. Energy windows for the Fe2p (Figs. 3 and 4) and O1s ionization (Fig. 5) were recorded. The Fe2p3 ionization was deconvoluted in three different contributions as depicted in Fig. 3. These contributions were assigned to the presence of (i) Fe2+ (Eb ∼708–708.5 eV) and Fe3+ in the oxide form (Eb ∼710.5–711 eV) and in the hydroxide form (Eb ∼711.5–712 eV). It’s not easy for XPS to distinguish the Fe3+ in Fe2 O3 from Fe3+ in Fe hydroxide, as many elements can make the binding energy shift a little [14]. The binding energy for the peak at 708 eV is too high for the metal contribution, which is, generally below 707 eV. However, it is also lower than the energy typically expected for Fe2+ , which is

2.4. Surface analysis X-Ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES) measurements were performed using a Microlab 310 F (Thermo Electron—former VG Scientific). The AES spectra were taken using an electron beam accelerated at 10 keV and a target current of 5 nA. Auger depth profiling was performed using a differentially pumped Ar+ ion gun. The XPS analysis was performed under pressures below 5 × 10−9 mbar, using the Al radiation (no-monochromator). The spectra were obtained in constant analyser mode CAE = 30 eV and accelerating voltage of 15 kV. The quantification was performed after peak fitting. The peak fitting function used was a Gaussian-Lorentzian product function and the algorithm was based on the Simplex optimization as used in the Avantage® software. AFM mapping of the samples was made in the tapping mode, using a Topometrix 2010 “Discoverer” equipment with a silicon nitride tip. The images of the film formed on mild steel at −0.2 V and +0.2 V vs. SCE were obtained on a 50 ␮m × 50 ␮m area. 2.5. Photoelectrochemistry Photoelectrochemical measurements were performed by using a 150 W Xenon arc lamp (Oriel 6254), a grating monochromator (Oriel 77200), a mechanical chopper and a photo detector (Oriel 7183). The lock-in technique was applied, where intensity

Fig. 1. First an eighth cyclic voltammograms obtained for a mild steel sample in NaOH 0.1 M + KOH 0.1 M from −1.4 V to +0.6 V vs. SCE at 2 mV s−1 scan rate.

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Fig. 2. D.C. polarization curves on mild steel at different potentials for 1 h to obtain a steady current and to ensure a passive film formation.

around 709 eV. The expected range of binding energies for Fe3 O4 is wide and changes according literature. Some works [15] report the presence of Fe3 O4 at 708.3 eV, in the same range observed in this work. Asami [16] assigns the presence of Fe2+ at 708.5 eV. Another interpretation proposed in literature [17] reports that the presence of a peak at 707.9 eV can be due to the presence of metallic iron strongly interacting with the oxide. In this work we observed that the binding energy of the Fe2+ peak slightly increased for more negative potentials. For 0.2 V the binding energy was 708 eV, but for potentials of −0.8 V (potential where Fe3 O4 is stable) the binding energy was 708.5 eV. This trend confirms that the low binding energy peak is likely to be associated with Fe3 O4 formation, rather than a metallic contribution. From the O1s spectra depicted in Fig. 5 it can be verified that the iron species can exist in the form of oxides or hydroxides. A small shoulder, assigned to water contribution was also detected. The dominant peak is that associated with Fe3+ species. As the applied potential shifts in the more positive direction the shoulder corresponding to Fe2+ tends to disappear. For potentials above +0.2 V the peak vanishes, revealing that the outer layer is mainly Fe3+ oxides and hydroxides. This result can be due either to the for-

Fig. 3. Fe2p peak after peak fitting. Three contributions can be seen: the Fe2+ and Fe3+ in the form of oxides and hydroxides. Spectra for the film formed at −0.8 V.

Fig. 4. XPS spectra for Fe2p ionization of the passive film formed at different potentials.

mation of a thicker outer layer that hinders the signal coming out from the inner Fe2+ layer or it can be due to the formation of an increasing degree of surface coverage by Fe3+ species. The ratio Fe2+ /Fe3+ was plotted as a function of the applied potential in Fig. 6. This ratio attained the lowest value for the films formed at +0.3 V and started to increase as the applied potential

Fig. 5. XPS spectra for O1s ionization of the passive film formed at −0.8 V.

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Fig. 6. Evolution of the Fe2+ /Fe3+ ratio calculated from the XPS spectra with the applied potential.

moves towards more negative values. The results show clearly an increase of the Fe2+ contribution up to −0.2 to −0.3 V. For the most negative potential value, −0.8 V, the Fe2+ contribution is the highest since it corresponds to the potential of formation of magnetite, although the 0.5 stochiometria ratio is not reached, which indicates that some Fe3+ remains stable and not reduced. 3.2.2. Auger Fig. 7 depicts the Auger depth profiles obtained in the passive films formed at −0.2 V and +0.2 V (vs. SCE) for 3600 s. The amount of oxygen at the outermost level is around 70% atomic and the content of iron about half of that amount. Gradually, the oxygen contribution decreases and the iron contribution starts to increase. However,

Fig. 7. Auger profiles of the films obtained on the films formed at −0.2 V and +0.2 V vs. SCE respectively.

after approximately 100 s of etching, both oxygen and iron contents become constant, revealing the presence of an inner level depleted in oxygen comparatively to the outermost level. The Auger results clearly distinguish two different regions in the passive film, being consistent with the double layer model described in literature [7–12]. Although both profiles in Fig. 7 are very similar, the film formed at −0.2 V presents higher oxygen content at the outer layers, which may correspond to higher contribution from hydrated species on the Fe3+ -rich level, in agreement with previous results [11].

Fig. 8. AFM images (tapping mode) of the film formed at −0.2 V vs. SCE in 0.1 M NaOH + 0.1 M KOH for 1 h. (a) topographical image (under layer regions are marked). (b) Phase image. (c) Amplitude image.

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Fig. 9. AFM images (tapping mode) of the film formed at +0.2 V vs. SCE in 0.1 M NaOH + 0.1 M KOH for 1 h. (a) Topographical image. (b) Phase image. (c) Amplitude image.

Fig. 7 shows also that the inner level is thinner at −0.2 V, which can be relevant regarding the amount of charge in the passive film accessible to a redox reversible process as discussed below in Section 3.4. 3.2.3. AFM Topographic data was obtained by AFM to investigate the characteristics of the outer film layers formed on the metal surfaces after polarization for 1 h at −0.2 V and +0.2 V. The AFM data clearly show

a rough, compact and continuous film but it also reveals a nonuniform distribution. Some topographical features are observed in AFM images (Figs. 8 and 9) for the films formed at different potentials. For the films formed at −0.2 V the topographic and the phase images (Figs. 8a and b) reveal a dominant phase together with some localized regions (darker colour regions indicated in Fig. 8a) which suggest the existence of a different inner phase. The dominant outer phase, more elastic, could be the outermost Fe3+ hydrated oxide

Fig. 10. Plots of Iph vs.  and (h)0.5 vs. h for a passive film formed on a mild steel at +0.2 V vs. SCE.

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Fig. 11. (A) Nyquist and (B) Bode plots for the tested steel in 0.1 M Na OH + 0.1 M KOH solution polarized at −0.2 V during 64 h. In the Nyquist plot the fitted data are also depicted (see text).

layer, as the previous XPS and Auger results suggest. The presence of uncovered inner oxide phase regions proves that the film formation is a topotactic process. The film formed at +0.2 V shows again two well-differentiated phases (Fig. 9). Large islands (lighter colour in the image) are dispersed in a continuous darker phase (Fig. 9b). According to the topographic image (Fig. 9a), these localized sites correspond to the outermost layer. At this anodic potential, the oxidation reaction seems to be complete and the dehydration process likely to begin. This film is mainly composed of Fe3+ species, so these local areas shall correspond to Fe2 O3 and Fe(OH)3 with increasing Fe2 O3 coverage as the potential moves towards the more anodic values. 3.3. Electrochemical measurements 3.3.1. Photoelectrochemistry The quantum efficiency , defined as the ratio between the photocurrent Iph and the incident photon flux 0 of energy h, is given by: =

Iph 0

= eAw

(h − Eg ) h

n

(1)

where A is a constant, e the elementary charge, w the thickness of space charge layer and Eg the band gap energy. The optical band gap energy Eg , of the passive film has been determined from the photocurrent measurements at constant potential, by applying the

Gärtner model [18], plotting (h)1/n vs. h and extrapolating to zero quantum efficiency. For a crystalline semiconductor, a direct photo transition corresponds to n = 1, and an indirect photo transition to n = 4. In this case, the value of n = 4 gives the best fit of the photocurrent spectra obtained for the passive film at +0.2 V (Fig. 10). This value of n corresponds to an indirect transition in crystalline materials and also to an indirect transition in amorphous solids. The band gap energy determined is 2.5 eV, close to the 1.9–2.1 eV energy range reported in literature [19–21] for the n-type anodic films formed on iron and on carbon steel. For the passive film formed at −200 mV it was not possible to obtain an accurate photocurrent signal due to the high conductivity of the passive layer, which is in agreement with the above discussed magnetite transformation degree. 3.3.2. Electrochemical Impedance Spectroscopy (EIS) The results discussed above evidence the presence of a passive film of semiconducting character which composition depends on the electrode potential. Moreover, the changes induced upon potential change are of topotactic nature. In this section the electrochemical behaviour of the passive films formed at selected potentials (−0.2 V and +0.2 V) was followed by EIS for 3 days of immersion. This period of time was chosen because it is similar to the one that has been reported as necessary to reach steady state conditions in the concrete pore solution [13].

Fig. 12. (A) Nyquist plot and (B) Bode plot for the tested steel in 0.1 M Na OH + 0.1 M KOH solution polarized at +0.2 V during 64 h. In the Nyquist plot the fitted data are also depicted (see text).

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Fig. 13. Best fitting high frequency parameters (R1 , C1 in Eq. (2)) for the impedance data given in Figs. 10 and 11.

Fig. 14. Best fitting low frequency parameters (R2 , C2 in Eq. (2)) for the impedance data given in Figs. 10 and 11.

Figs. 11 and 12 show that the impedance of the system increases with time in agreement with reported data [22,23]. After 3 days, stabilization is reached for the film formed at +0.2 V (Fig. 12), but the impedance is still increasing for the film formed at −0.2 V (Fig. 11). Figs. 11 and 12 show that the conductivity of the passive film is roughly one order of magnitude higher for the film formed at −0.2 V, in agreement with the photoelectrochemical results. The presence of two time constants was observed for all the impedance spectra, in agreement with literature [23,24]. EIS results have been successfully fitted (see Figs. 11A and 12A) using an equivalent circuit composed of two RC hierarchically distributed time constants according to the impedance function given in Eq. (2). This circuit has been already reported to model the behaviour of steel in alkaline media [6,11]. The high frequency constant time, R1 C1 , is associated to the charge transfer with the solution trough the double layer capacitance C1 . The low frequency time constant,

R2 C2 , is related to the redox process occurring in the passive film. A detailed description of these parameters can be found in literature [11,23] Z(ω) = Re +

(jωR1 C1 )

being Z2 (ω) =

˛1

R1 + 1/(1 + (Z2 (ω)/R1 ))

R2 ˛ 1 + (jωR2 C2 ) 2

(2)

The redox processes present in the passive film depend on the electrode potential and are summarized in Eqs. (3)–(6). For −0.2 V and +0.2 V only the processes in Eqs. (5) and (6) are of interest. At −0.9 V vs. SCE the Fe0 oxidation occurs according to the Eq. (3): Fe + 2OH−  Fe(OH)2

Fig. 15. Nyquist (A) and Bode (B) plots for a sample obtained at −0.8 V for a carbon steel plate passivated for 60 min in alkaline solution at +0.2 V.

(3)

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Fig. 16. Nyquist (A) and Bode (B) plots for a sample obtained at −0.6 V for a carbon steel plate passivated for 60 min in alkaline solution at +0.2 V.

At E ∼−0.8 V the magnetite formation takes place according to the oxidation reaction of Eq. (4): 3Fe(OH)2 + 2OH−  Fe3 O4 + 4H2 O + 2e−

(4)

Finally, at more anodic potentials, magnetite oxidation occurs to form Fe3+ oxides or hydroxides depending on the H2 O availability: Fe3 O4 + OH− + H2 O  3 − FeOOH + e− −

(5) −

2Fe3 O4 + 2OH + H2 O  3 − Fe2 O3 + 2e

(6)

The fitting parameters of the data presented in Figs. 11 and 12 are given in Figs. 13 and 14. Fig. 13A shows that for the film formed at +0.2 V the charge transfer resistance is about one order of magnitude higher at +0.2 V than at −0.2 V, which again confirms the poorer conductivity of the film formed at +0.2 V. The capacitance data presented in Fig. 13B suggest that poorer conductivity is due to the formation of an outer insulating layer, probably goethite (␣-FeOOH) and/or lepidocrocite (␥-FeOOH). The fact that the film formed at +0.2 V shows capacitance values lower than the film formed at −0.2 V indicates the presence of a series contribution to the double layer capacitance (the dielectric capacitance of the insulating outer level). Considering the average C1 values of 55 ␮F cm−2 at −0.2 V and 22 ␮F cm−2 at +0.2 V, the capacitance of the insulating layer, Cfilm , can be estimated as 37 ␮F cm−2 from the relationship: 1/Cfilm = ((1/22) − (1/55)). Assuming continuous coverage of the surface and a flat condenser behaviour, 37 ␮F cm−2 corresponds to 0.5 nm film thickness if 20 is taken as dielectric constant for the iron oxide [25]. This result suggests a partial coverage of the surface, in good agreement with the AFM data above reported. The capacitance C2 , associated with the redox processes (Eqs. (5) and (6)) taking place in the passive range is related with the total amount of charge, q, stored in the oxide layer, according to Eq. (7), where represents the transformation degree achieved at a given electrode potential, E. Thus, C2 is a measure of the total amount of transformable charge, according to Eq. (7) [26,27]. C2 = q

d dE

(7)

Fig. 14B depicts slightly higher C2 values for the film formed at −0.2 V comparatively to that formed at +0.2 V. Although the analytical data shows more Fe3+ in the film at +0.2 V, not all that species are transformable into magnetite (because of the 3D growing), which limits the effect of this parameter in Eq. (7). Moreover, the transformation degree, , is closer to 0.5 at −0.2 V than at +0.2 V, which makes d /dE higher in the former case [28], which explains the values depicted in Fig. 14B. EIS measurements were carried out to assess the behaviour of those anodically formed films under cathodic protection conditions. The samples were polarized for 1 h at the selected potentials (−0.2 V and +0.2 V) and later, the cathodic protection was applied by potentiostatic polarization at −0.6 V and −0.8 V. This last potential was selected because of it revealed the highest Fe2+ /Fe3+ ratio in the XPS study, and also because it is closer to that commonly employed in cathodic protection systems [29]. The evolution of the impedance with time is registered in Figs. 15 and 16. In the first case (Fig. 15A and B) the impedance obtained at −0.8 V for the mild steel previously passivated at +0.2 V, seems to be dominated by diffusion effects, as suggested by the quasi-straight lines at 45◦ to the abscissa axis in the Nyquist plot [23,30]. Nevertheless, the parameter values obtained from the fitting procedure reveal that some redox activity is also present, because the dispersion exponent, ˛2 , is higher than 0.5, as it can be seen in Table 1A. Fig. 1 shows that at −0.8 V the recorded cathodic current is about 100 ␮A cm−2 . As the oxygen limiting current in stagnant condition is about 20–30 ␮A cm−2 [31], it can be said that most of the recorded cathodic current corresponds to redox activity. At −0.6 V the diffusion feature vanishes (Fig. 16) and only one time constant can be differentiated after the first hour (Table 1B). Fig. 1 shows that at this potential the cathodic current is smaller so that oxygen reduction will represent the main contribution to the impedance which shows charge transfer control. The high C1 values reported in Table 1B can be attributed to roughness of the surface. The behaviour of the film formed at −0.2 V is depicted in Figs. 17 and 18. At the −0.8 V cathodic potential the diffusion-like feature present in Fig. 15 disappears in Fig. 17, probably because

Table 1A Best fitting parameters obtained for the fitting of data in Fig. 14 to Eq. (2). +0.2 to −0.8

R1 (k cm2 )

C1 (mF cm−2 )

˛1

R2 (k cm2 )

C2 (mF cm−2 )

˛2

t=0 t=1h t=3h t=4h

2.7 3.3 3.2 3.1

1.3 0.9 0.9 0.9

0.8 0.8 0.8 0.8

6.8 11.0 12.0 11.4

2.4 4.3 4.7 4.6

0.8 0.7 0.7 0.7

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Table 1B Best fitting parameters obtained for the fitting of data in Fig. 15 to Eq. (2). +0.2 to −0.6

R1 (k cm2 )

C1 (␮F cm−2 )

˛1

R2 (k cm2 )

C2 (mF cm−2 )

˛2

t=0 t=1h t=3h t=4h

5.3 10.9 10.2 10.2

265 267 253 253

0.8 0.8 0.8 0.8

2.0 – – –

2.6 – – –

0.7 – – –

Only the R1 C1 time constant has been considered after1 h testing time.

Fig. 17. Nyquist (A) and Bode (B) plots for a sample obtained at −0.8 V for a carbon steel plate passivated for 60 min in alkaline solution at −0.2 V.

Fig. 18. Nyquist (A) and Bode (B) plots for a sample obtained at −0.6 V for a carbon steel plate passivated for 60 min in alkaline solution at −0.2 V.

the contribution of the oxygen reduction reaction to the overall cathodic current diminishes, as revealed by the higher charge transfer resistance values (R1 ), and higher R1 /(R1 + R2 ) ratio, in Table 2A compared to those in Table 1A. Better blockage of the interface seems to be reached for films formed at −0.2 V. The behaviour at −0.6 V (Fig. 18, Table 2B) is similar to that reported for the film formed at +0.2 V (Fig. 16, Table 1B), although R1 values are higher in Table 2B, which again recalls the better blockage of the interface.

3.3.3. Galvanic current measurements Nominally identical samples (2.0 cm2 each) were galvanically coupled and cathodically polarized to −0.8 V. One of the samples was placed in an entangled area of the electrochemical cell so that IR drop makes the local potential increase to −0.6 V. Fig. 19 depicts the evolution of the cathodic current registered in such kind of galvanic coupling for specimens having oxide layers formed at +0.2 V. The record at −0.6 V reveals a nearly steady behaviour; the current remains constant and little events are

Table 2A Best fitting parameters obtained for the fitting of data in Fig. 16 to Eq. (2). +0.2 to −0.8

R1 (k cm2 )

C1 (mF cm−2 )

˛1

R2 (k cm2 )

C2 (mF cm−2 )

˛2

t=0 t=1h t=3h t=4h

5.2 16.8 7.1 8.0

2.7 2.5 2.1 2

0.8 0.8 0.8 0.8

6.8 34.0 11.9 8.8

2.6 2.5 3.0 3.0

1 1 1 1

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Table 2B Best fitting parameters obtained for the fitting of data in Fig. 17 to Eq. (2). +0.2 to −0.6

R1 (k cm2 )

C1 (␮F cm−2 )

˛1

R2 (k cm2 )

C2 (mF cm−2 )

˛2

t=0 t=1h t=3h t=4h

8.0 13.6 14.0 15.0

217 204 192 185

0.9 0.9 0.9 0.9

1.6 – – –

2.8 – – –

0.7 – – –

Only the R1 C1 time constant has been considered after1 h testing time.

Fig. 21. Idealized drawing representing passive film growth iron in alkaline media at potential values slightly anodic than that of the magnetite formation peak.

Fig. 19. Cathodic currents for a galvanic couple between two identical samples (2 cm2 active surface) passivated at +0.2 V. IR drop in the solution makes the potential to shift from the nominal −0.8 V to −0.6 V.

observed. The data recorded at −0.8 V shows increasing current densities with time and a number of current transients that can be associated to film breakdown and/or hydrogen evolution. The integrity of the passive layer seems more preserved at −0.6 V probably because of a shallow reduction of Fe3+ species, in agreement with impedance data. When the applied potential is −0.8 V more important fluctuations occur and the electrode cannot reach a steady state. The passive layer formed at −0.2 V behaves differently under cathodic polarization. The results are presented in Fig. 20. The current/time records are smoother than in Fig. 19, and a steady state

is reached at −0.8 V. Hence the lower oxidation degree of the passive film formed at −0.2 V leads to more homogeneous local current distribution, in agreement with impedance data. The data presented in Figs. 19 and 20 suggest that cathodic protection applied to pre-rusted specimens shall be operated gradually to reduce oxides stepwise. This operating procedure will reduce the possibility of local hydrogen embrittlement. In this context, the concept of cathodic prevention [32] widens its original meaning to account also for the modifications in the structure of the passive layer under mild cathodic conditions. 3.4. Electrochemical measurements The above presented physical and electrochemical data suggest the iron in alkaline media develops a passive film of duplex structure: an inner layer rich in Fe2+ oxides whose thickness depends on the potential, and an outer layer richer in Fe3+ hydroxides. Growing of the outer layer is of topotactic nature and dependent on the potential. Dehydration tends to occur as electrode potential increases. Fig. 21 depicts an idealized drawing of such film growth near the magnetite formation peak. 4. Conclusions

Fig. 20. Cathodic currents for a galvanic couple between two identical samples (2 cm2 active surface) passivated at −0.2 V. IR drop in the solution makes the potential to shift from the nominal −0.8 V to −0.6 V.

This work reveals the importance of the electrode potential in the formation of the passive films on mild steel in alkaline media. The structure of the passive film is potential dependent and affects the efficiency of the cathodic protection current. Surface analysis revealed the existence of a duplex structure in the passive layer: an inner layer rich in Fe2+ oxides whose thickness depends on the potential, and an outer layer richer in Fe3+ hydroxides. Growing of the outer layer is not homogeneous but of topotactic nature. The operative conditions for efficient cathodic protection requires oxide scales not too much oxidized in order to avoid hydrogen embrittlement. The concept of cathodic prevention can be applied to modify the structure of the passive film and make cathodic protection more efficient.

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