- Email: [email protected]
Electrochemical behavior of surface films formed on Fe in chromate solutions
Corrosion Science 45 (2003) 1405–1419 www.elsevier.com/locate/corsci
Electrochemical behavior of surface films formed on Fe in chromate solutions S. Virtanen a
a,*
, M. B€ uchler
b
Institute for Materials Chemistry and Corrosion and Department of Materials, Swiss Federal Institute of Technology, ETH-H€onggerberg, 8093 Z€urich, Switzerland b Swiss Society for Corrosion Protection (SGK), Technoparkstrasse 1, 8005 Z€urich, Switzerland Received 26 November 2001; accepted 2 December 2002
Abstract The stability of passive films formed on Fe in K2 Cr2 O7 solutions during exposure at opencircuit potential or by potential cycling is studied in a chromate-free solution. The electrochemical behavior of chromate-passivated Fe is investigated with cyclic voltammetry combined with LASER light reflectance measurements which allow an in situ determination of the thickness of the iron oxide film. The electrochemical behavior of chromate-passivated Fe in chromate-free solutions strongly depends on passivation treatment. Passivation of iron by immersion at open-circuit in chromate solution leads to a passive film, in which both Fe and Cr species dissolve almost independently of the presence of the other one: Fe oxide by reductive dissolution and Cr oxide by oxidative dissolution in the corresponding potential regions. Passivation of iron by potential cycling in chromate solutions leads to much less loss of the otherwise soluble oxidized chromate and reduced ferrous species in subsequent electrochemical experiments (trapping in a protective film). Concerning the dissolution behavior, the film formed on iron by cycling in chromate solution behaves similarly as the passive film on Fe–17Cr alloy. However, the remnant passive film after reductive or oxidative dissolution on the Fe–Cr alloy is of truely protective nature as compared to the films formed on iron in chromate solutions, which show only a small contribution to the potential drop. Ó 2003 Elsevier Science Ltd. All rights reserved. Keywords: A. Iron; Iron–chromium alloys; Chromate; B. Cyclic voltammetry; Light reflectance; Inhibition; C. Passivity
*
Corresponding author. Tel.: +41-1-633-2790; fax: +41-1-633-1147. E-mail address: [email protected] (S. Virtanen).
0010-938X/03/$ - see front matter Ó 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0010-938X(02)00242-1
1406
S. Virtanen, M. B€uchler / Corrosion Science 45 (2003) 1405–1419
1. Introduction It is well-known since decades [1] that chromate ions are efficient inhibitors for Fe. Apart from the action of chromates as a strong oxidizer [2], it is a well-established fact that chromates in the solution lead to a modification of the passive film on iron, more specifically to the incorporation of significant amounts of Cr species in the surface oxide film (see e.g. [3–10]). An ellipsometric study indicated that a surface film consisting of a mixture of iron and chromium hydroxides with iron and chromium oxide is formed during open-circuit exposure of Fe in chromate solutions [7]. Further, an XPS study revealed that chromium is present in the film formed on Fe in chromate solutions mainly as Cr(3þ), iron as Fe(3þ), and oxygen both in M–O and M–OH bonds [9]. The Cr/Fe-ratio in the film depends on the passivation procedure [9]. On the other hand, the passive films on Fe–Cr alloys or stainless steels are enriched with oxidized chromium species (see e.g. [11–14]); the valency of the oxidized Cr and Fe present in the passive film as well as the Crox /Feox -ratio depends on the potential [11,15–17]. Oxygen has been identified to be present in passive films on Fe– Cr alloys in three different metal–oxygen bonds: as an oxide M–O, a hydroxide M– OH and an oxyhydride M–OOH [14–16]. Therefore, the composition of the passive film on iron formed in chromate-containing solutions shows some similarities to that of Fe–Cr alloys, even though the exact nature of the passive films in both cases clearly depends on many experimental parameters. Nevertheless, the question arises, if similarities between the electrochemical behavior of chromate-passivated iron and Fe–Cr alloys exist. However, little work has been carried out to study the stability of iron passivated in chromate solutions in electrolytes not containing the inhibiting chromate species. Therefore, in the present work the electrochemical behavior of chromate-passivated iron is investigated with cyclic voltammetry (CV) combined with LASER light reflectance measurements [18] which allow an in situ determination of the thickness of the iron oxide film.
2. Experimental Chemical passivation of Fe (99.5%) was carried out by exposing the samples to 0.0025 M K2 Cr2 O7 , pH 4.5 (open to air) at the open-circuit potential for various times (1 h to 3 days). Prior to the passivation treatment, the surfaces were mechanically ground to a 1000 grit finish for electrochemical measurements or to a 1/4 lm diamond finish for surface analytical studies. After polishing, the samples were rinsed in acetone and ethanol and dried in N2 . Impedance spectra were measured as a function of time at the open-circuit potential in the chromate solution as well as in borate buffer, pH 8.4 and in 0.1 M Na2 SO4 , pH 4.5. Apart from open-circuit passivation, Fe was treated by potential cycling in the chromate solution as will be described below. After removal from the chromate solution, the samples were thoroughly rinsed with distilled water and dried in nitrogen.
S. Virtanen, M. B€uchler / Corrosion Science 45 (2003) 1405–1419
1407
Electrochemical characterization of the treated samples was performed in borate buffer with a pH value of 8.4 (0.075 M Na2 B4 O7 10H2 O, 0.3 M H3 BO3 ). The solutions were prepared from reagent grade chemicals and distilled water. During the experiments the solutions were deaerated with N2 . A platinum gauze served as a counter electrode and a saturated calomel electrode (SCE) was used as a reference electrode. CV was carried out with a sweep rate of 10 mV/s. In order to ensure stationary conditions the sample was kept at )1 V before every subsequent cycle. For the light reflection measurements the samples were polished down to 1/4 lm diamond paste. For the light absorption measurements, the opposite wall of the electrochemical cell consisted of a quartz window (Suprasil II) to allow an absorption free penetration of UV light. As light absorption can be sensitive to ion enrichment in the electrolyte, a flow cell was used in the present work in order to exclude any influence by changes in electrolyte composition. The flow rate of the electrolyte was controlled by a flow meter and was 1 mm/s in all experiments reported in this work. The beam of a HeCdlaser which provides a continuous 325 nm line and an output of 2 mW was directed perpendicular to the sample surface. The reflected light intensity was measured with a Newport 835 Optical Power Meter. A partially transparent mirror allowed the splitting of incoming and reflected light beam. The quartz window was tilted to the light beam by 0.5° in order to separate the contribution of the light reflected on the glass from that one of the sample. Intensities ðIÞ were normalised with the reflected intensity (I0 ) at a cathodic polarisation of )1 V SCE. In the case of iron an oxide-free state is obtained at this potential. In the present work a removing of the passive film is not possible in all experiments. Therefore the highest light intensity obtained during the measurement was chosen as I0 . Due to this possible error resulting from residual oxide films and the possibly complicated effect of mixed iron–chromium oxides, the absorption data were only evaluated qualitatively and a calculation of film thickness was not performed. Details of the technique and its calibration are given elsewhere [18].
3. Results and discussion 3.1. Electrochemical behavior of chromate-passivated iron Fig. 1 shows the impedance value at 10 mHz at the open-circuit potential as a function of time determined from impedance spectra for iron in 0.0025 M K2 Cr2 O7 , pH 4.5 and in borate buffer, pH 8.4. (Z-values at 10 mHz are used as an approximation for the polarization resistance.) In both cases, the gradual increase of the polarization resistance indicates passivation. As a comparison, the polarization resistance was measured in 0.1 M Na2 SO4 , pH 4.5. In this solution active corrosion of iron takes place, and hence the polarization resistance remains low, <1 kX. Clearly, chromate ions strongly inhibit corrosion of iron, even at relatively low pH values. In Fig. 2, the cyclic voltammograms of iron measured in borate buffer after an open-circuit exposure for 1 h either in the chromate or in borate buffer solution are
1408
S. Virtanen, M. B€uchler / Corrosion Science 45 (2003) 1405–1419
Fig. 1. Impedance at 10 mHz (approximation for the polarization resistance) as a function of time determined from impedance spectra measured at open-circuit potential in solutions open to air.
compared. In Fig. 2(a), the anodic reversal potential is below the transpassive dissolution region of Cr oxide (Emax ¼ þ200 mV SCE). In this case, a significantly different behavior is found for the chromate- and borate-passivated samples: the anodic current in the region of active/passive-transition is much lower for the chromate-passivated sample after the cathodic sweep. In Fig. 2(b) the CVs are shown with the anodic reversal potential above the Cr transpassive dissolution range. In the first anodic sweep of the chromate-passivated iron, high anodic current densities are measured below the oxygen evolution region resulting from the Cr3þ ! Cr6þ oxidation reaction. In the following cathodic and anodic sweeps, the behavior of the chromate-passivated sample is almost identical with the sample passivated in borate buffer. Thus, the Cr3þ oxidation reaction leads to a complete dissolution of the chromate species, and the stability region of chromate-passivated iron is therefore limited by the potential of onset of transpassive dissolution of chromium oxides. A similar behavior was found for all samples passivated at open-circuit potential in the chromate solution, independent of the passivation time (1 h to 3 days), temperature of the solution (RT to 80 °C), as well as chromate concentration and pH of the solution. In all these cases, the protective effect of the surface layer was completely lost after polarization in the region of oxidation of Cr(3þ). In literature, AC passivation or generally passivation by potential cycling has been shown to improve the stability of the passive film on stainless steel [19–22]. Hence, an attempt was made to change the oxide layer properties by cyclic passivation of Fe in the chromate solution. Such a cycling could lead to a thickening of the film and possibly change the Cr/Fe-ratio and/or-distribution in the film. Fig. 3 shows the cyclic voltammogram of iron measured in the chromate solution. Testing different potential windows for the treatment showed that the best treatment was obtained in the potential region as shown in Fig. 3, including the region of the Cr(3þ) oxidation and Fe(3þ) reduction. Samples, which were treated by potential
S. Virtanen, M. B€uchler / Corrosion Science 45 (2003) 1405–1419
1409
Fig. 2. Cyclic voltammogram of Fe passivated for 1 h at open-circuit potential in borate buffer, pH 8.4 or in 0.0025 M K2 Cr2 O7 , pH 4.5 (sweep rate 10 mV/s): (a) 0:3 ! þ0:2 ! 1:2 ! þ0:2 V SCE, (b) 0:3 ! þ1:0 ! 1:2 ! þ1:0 V SCE.
cycling in a potential region not exceeding these limits, showed in a subsequent electrochemical testing in borate buffer a basically identical behavior with samples passivated at the open-circuit potential, i.e. a loss of the protective quality of the oxide layer formed in the chromate solution after polarization in Crð3þÞ ! Crð6þÞ region after the treatment. In Fig. 4, the cyclic voltammograms of Fe passivated at open-circuit potential (a) or with potential cycling (b) in the chromate solution are compared. Shown are the first and the third cycle (which is identical to all subsequent cycles) in borate buffer. Fig. 4(c) shows a comparison of the third cycle of the differently treated samples. In the case of the sample passivated at open-circuit potential, the sample shows an active/passive-transition in the anodic sweep following polarization up to the Cr oxidation region. Following the oxidation peak of chromium, in the subsequent
1410
S. Virtanen, M. B€uchler / Corrosion Science 45 (2003) 1405–1419
Fig. 3. Cyclic voltammogram of Fe in 0.0025 M K2 Cr2 O7 , pH 4.5 (sweep rate 10 mV/s).
cathodic sweep, no reduction peak of chromates is seen. Since the experiment was carried out in a flow cell, only Cr(6þ) species in the oxide film would lead to a chromate reduction peak, and not species dissolved in the solution. Hence, the absence of the chromate reduction peak indicates that the chromates are completely dissolved. The corresponding experiment for CV-passivated iron shows that in this case all cycles are identical, and no activation of the sample takes place. In the cathodic sweep a reduction peak for chromates can be seen, thus suggesting that some chromate species are incorporated in the oxide layer. The different behavior of the differently treated samples as concerning the Crð6þÞ ! Crð3þÞ reduction peak in the cyclic voltammograms can be well seen in Fig. 4(c). Hence, passivation by potential cycling in the chromate solution leads to a formation of an oxide film, which can not be dissolved by polarization to high anodic potentials, since the oxidation of Cr(3þ) to Cr(6þ) does not lead to a complete dissolution of the chromate species. 3.2. Light absorption measurement on chromate-passivated iron The light absorption technique was employed to study the behavior of the iron oxide species in the surface layers during CV. Fig. 5 shows a CV of Fe in borate buffer and a simultaneously recorded light absorption data. Starting at the cathodic potential of )1 V, the surface of iron can be considered to be oxide-free, as it is wellknown that the passive film on iron as well as iron oxides reductively dissolve in this potential region in borate buffer (see e.g. [23,24]). As the potential passes through the active/passive-transition, the absorption increases indicating oxide film formation. In the passive region, a linear increase of the absorption as a function of potential is observed. Reversing the sweep direction, the absorption remains constant until the potential reaches the reduction peak (FeIII ! FeII), here the absorption intensity decreases indicating thinning of the passive film. Staying a short time at )1 V, the absorption intensity returns to itÕs original low value. It has been shown earlier by
S. Virtanen, M. B€uchler / Corrosion Science 45 (2003) 1405–1419
1411
Fig. 4. Cyclic voltammograms of Fe passivated for 1 h at open-circuit potential (a) or with potential cycling (b) in 0.0025 M K2 Cr2 O7 , pH 4.5. Shown are the first and the third cycle in borate buffer, pH 8.4, (c) shows a comparison of the third cycles of (a) and (b).
B€ uchler et al. that in the case of iron in borate buffer, anodic film formation and cathodic film dissolution are reversible––i.e. repeated anodic/cathodic cycling leads to conditions identical to the initial state, which––as mentioned above––is an oxide film-free surface [18]. Fig. 6 shows the cyclic voltammogram (a) and the corresponding light absorption data (b) for iron passivated for 1 h at open-circuit in the chromate solution. In the
1412
S. Virtanen, M. B€uchler / Corrosion Science 45 (2003) 1405–1419
Fig. 5. Cyclic voltammogram and the corresponding light absorption data for Fe in borate buffer, pH 8.4.
first anodic sweep, the anodic reversal potential was þ200 mV SCE, i.e. below the oxidation of Cr(3þ). As seen in the CV, in the following anodic sweep no active/ passive-transition is observed. The absorption data taken simultaneously with the CV, nevertheless, shows that cathodic polarization leads to an absorption close to zero indicating a strong removal of the oxide film. The small remaining absorption indicates the presence of a remnant oxide film which presumably consists of a chromium rich oxide. Hence, the lower current densities in the subsequent anodic sweep most probably result from the coverage by this remnant oxide. After cathodic polarization at )1 V, the increase of absorption in a subsequent anodic sweep is linear with potential, indicating an adaptation of film thickness to constant field strength conditions. It is striking that the slope in the second cycle is smaller than in the third, which again can be explained with the presence of the protecting chromium rich oxide on the surface that takes part of the potential drop. This conclusion is supported by the increase in slope as soon as the anodic dissolution of the chromium sets in in the second cycle. In all sweeps following polarization up to þ0.9 V (i.e. above the Cr(3þ) oxidation potential), the absorption behavior of the sample becomes almost identical to that of non-treated iron. A similar behavior was found for all samples passivated at open-circuit in the chromate solution independent of passivation time. In Fig. 7, the cyclic voltammogram in borate buffer (a) together with the absorption data (b) are shown for iron passivated by potential cycling in the chromate solution. Both, the current and absorption behavior of the second and the third cycle are very similar and the characteristic oxidation reaction of Cr(3þ) is much less pronounced. Even after several cycles the same current and absorption behavior is obtained which is very different to that of pure iron. This result indicates that the chromium deposited on the sample surface during the previous cyclic treatment in the chromate solution cannot be removed by anodic polarization. However, despite this residual oxide layer the current density in the passive range is not significantly decreased compared to that one of the sample passivated by a simple immersion
S. Virtanen, M. B€uchler / Corrosion Science 45 (2003) 1405–1419
1413
Fig. 6. Cyclic voltammogram (a) and the corresponding light absorption data (b) in borate buffer, pH 8.4 for iron passivated for 1 h at open-circuit in 0.0025 M K2 Cr2 O7 , pH 4.5.
experiment (see Fig. 4(c)). Further, the consideration of the absolute changes in absorption during the second cycle make clear that they are very similar for both passivation procedures. In neutral solutions the passive current density is mainly attributed to the passive film formation. Hence, a high passive current density indicates that a large amount of passive film has to be formed in order to compensate the potential drop. A similar result is obtained with the absorption data, as large changes in absorption indicate large changes in film thickness. Hence, the absorption and current behavior indicate that the film formed by cyclic polarization cannot be removed electrochemically but that it gives only little contribution to the protection of the metal. Nevertheless, some fundamental differences between the two procedures are observed. Contrary to the results in Fig. 6 for Fe passivated at open-circuit potential, no linear increase of the absorption is observed during anodic polarization for the cyclically passivated Fe (Fig. 7). For the cyclically passivated Fe, the significant increase in absorption coincides with the broad anodic peak in the current-potential
1414
S. Virtanen, M. B€uchler / Corrosion Science 45 (2003) 1405–1419
Fig. 7. Cyclic voltammograms (a) and the corresponding light absorption data (b) in borate buffer, pH 8.4 for iron passivated for 1 h at open-circuit or by potential cycling in 0.0025 M K2 Cr2 O7 , pH 4.5. Shown are the third cycles.
characteristic. Experiments with solutions containing iron ions have shown that absorption of Fe(2þ) is significantly lower than of Fe(3þ) [18]. Hence, the oxidation of a film consisting of mainly Fe(2þ) to a film consisting of Fe(3þ) would result in a strong increase in absorption. This effect was indeed observed for iron in 0.1 M NaOH where reduction does not lead to dissolution, but instead to a conversion of a Fe(3þ) oxide film into Fe(2þ) oxide/hydroxide film [18,25]. Therefore, the observed absorption behavior for the cyclically passivated iron could be explained with the oxidation of Fe(2þ) to Fe(3þ) within the film. Further, it is obvious that the reduction peak of Cr(6þ) to Cr(3þ) is identical in the second and the third cycle. This can only be explained with a constant amount of Cr(6þ) in the film. Hence, these results demonstrate that neither all of the reduced Fe(2þ) nor the oxidized Cr(6þ) are released into the solution. The light absorption data are shifted to higher values with increasing number of cycles. This shift is parallel over the entire potential range. This behavior is typical
S. Virtanen, M. B€uchler / Corrosion Science 45 (2003) 1405–1419
1415
for changes in reflection behavior due to changes in surface roughness or the deposition of a light scattering deposit on the surface. The findings therefore clearly indicate a significantly different behavior for the two different chromate-passivation treatments of iron: In the open-circuit passivated iron both the iron and the chromium oxide species in the oxide film are lost by dissolution during polarization in an appropriate potential region, iron oxide by reductive dissolution and chromium oxide by oxidative dissolution. This is similar to the wellknown electrochemical dissolution behavior of the passive films on pure Fe and Cr, or thin Fe2 O3 or Cr2 O3 films, respectively (see e.g. [23–26] and references therein). In the oxide film on the CV passivated iron, however, oxidation and reduction reactions do not lead to a complete dissolution of the otherwise soluble oxidized chromate and reduced Fe(3þ) species. Earlier studies on artificial Fe/Cr oxide passive films have shown that a composition range of a high chemical and electrochemical stability exists in mixed oxides where the samples show no dissolution either during reduction or oxidation [27]. The dissolution rates of the mixed oxides were determined from in situ X-ray absorption near edge spectroscopy (XANES) as a function of the Cr oxide content in the films during anodic and cathodic polarization. The XANES spectra further show that in all cases the oxide films were electroactive, i.e. reduction of Fe oxide and oxidation of Cr oxides always took place, but depending on the oxide composition, the Fe(2þ) and the Cr(6þ) species were either dissolved or trapped in the oxide film. Hence, the behavior observed with the cyclically passivated sample could be explained with the formation of a mixed iron–chromium oxide passive film that is resistant against both cathodic and anodic dissolution. Despite of the stability of the film against electrochemical dissolution, its contribution to the potential drop is small. The CVs of iron passivated with both procedures clearly show that Cr is incorporated in the passive films in both cases (presence of the oxidation peak in the Crð3þÞ ! Crð6þÞ potential region). Preliminary qualitative surface analysis indicates that for the open-circuit passivated Fe, the chromium species are present only in the outer part of the oxide layer. However, for the passive film formed by cycling, chromium seems to be present throughout the oxide film. The influence of cycling of the potential between the oxidation and reduction region (in the presence of chromate in the solution) may therefore be a continuous re-construction of the oxide film, due to dissolution of the passive film (either by oxidation of the Cr2 O3 into soluble CrO2 4 or reduction of the Fe2 O3 into soluble Fe(2þ)) and a subsequent re-formation in the reverse sweep. In this way, the oxide film may be modified until it reaches a more ‘‘stable’’ structure. This in contrast to passivation at the open-circuit potential, where less mobility of the oxide components takes place by dissolution, and hence the iron and chromium components of the oxide film may not form a real network, but are present as separate layers. In the latter case, these Cr and Fe components of the oxide film behave similar to pure Fe or Cr oxide, and hence dissolve independently of the other component in the corresponding potential regions, i.e. protection of the soluble species by the presence of the insoluble component of the oxide film is only possible for a mixed oxide film.
1416
S. Virtanen, M. B€uchler / Corrosion Science 45 (2003) 1405–1419
3.2.1. Fe–17Cr alloy To compare the behavior of chromate-passivated iron to Fe–Cr alloys, corresponding measurements were carried out on an Fe–17Cr alloy (Fig. 8). After polishing the sample was immersed into a 0.1 M Na2 SO4 (pH 4.5) solution in order to obtain comparable starting conditions. It is clear that the current densities (Fig. 8(a)) are significantly smaller than in the case of the chromate treated iron samples. This is certainly due to the fact that the passive film cannot be removed completely by cathodic polarization. This finding is supported by the fact that changes in absorption (Fig. 8(b)) are small, except in the potential region, where anodic chromium oxidation and cathodic iron reduction occur. The strong increase of the absorption at high anodic potentials can be due to the anodic dissolution of the Cr(6þ) species, which requires a replacement of the chromium in the passive film by iron ions to hold the potential drop across the film. The observed increase in absorption can then have two reasons: Either a higher film thickness is obtained or the absorption coefficient is higher for iron. As the energy gap is 1.9 eV for iron oxides while a value of 2.6 eV is reported for chromium oxides [28], a formation of an Fe-rich film could result in the
Fig. 8. Cyclic voltammogram (a) and the corresponding light absorption data (b) in borate buffer, pH 8.4 for Fe–17Cr passivated for 1 h at open-circuit in 0.1 M Na2 SO4 , pH 4.5.
S. Virtanen, M. B€uchler / Corrosion Science 45 (2003) 1405–1419
1417
increase of absorption without a significant change in the film thickness. The explanation with the formation of an iron rich film is supported by the fact that the increase of absorption that is observed at high anodic potentials reversibly is lost during cathodic Fe(3þ) to Fe(2þ) reduction. Further, it is clear from the data in Fig. 8 that the amount of chromium dissolution decreases with increasing numbers of cycles as the change in absorption and the peak height in iron reduction decreases with increasing number of cycles. This was investigated in up to 6 consecutive cycles. Again this result can be explained with the formation of a mixed iron–chromium oxide film that is resistant against both anodic and cathodic dissolution [27]. In the case of the passive film on Fe–Cr alloys, surface analytical studies often conclude the passive film to consist of an inner Cr-rich layer and an outer Fe-rich layer (see e.g. [13,29,30]). Even though a concentration gradient is present in the passive film, the layer responsible for stable passivity consists not only of either pure Cr or pure Fe oxides, but is a mixture of the two components. Therefore we conclude that not only the Cr/Fe ratio in the oxide film determines electrochemical stability, but also the distribution of these species can play a major role. Only in mixed oxide films––where the different film constituents can interact with each other––no complete oxidative dissolution of Cr or reductive dissolution of Fe takes place. A formation of a network of the insoluble oxide component can hence protect the otherwise soluble species from dissolution (trapping in the network). This argument is similar to the percolation model suggested to explain a critical Cr-concentration for passivity in Fe–Cr alloys [31]. Despite of the ability of the cyclically passivated iron to resist oxidative and reductive dissolution of the passive film, the results clearly demonstrate that a very different electrochemical and absorption behavior is obtained for the Fe–Cr alloy, where the remnant passive film (after reductive or oxidative dissolution) is of truly protective nature as compared to the films formed on iron in chromate solutions. This finding demonstrates that the stability of oxide films against electrochemical dissolution is only one key factor describing the behavior in the passive state. It should be pointed out that the present study only qualitatively compares the behavior of chromate-passivated Fe using two different passivation treatments with the behavior of Fe–17Cr alloy. As mentioned in the introduction, the nature of the passive film on Fe–Cr alloys depends––apart from the alloy composition––on all passivation parameters. Also for chromate-passivated iron, strong differences may arise by further changing the passivation treatment (e.g. sweep rate of cyclic passivation). As the chemical composition, thickness as well as the distribution of the different species present in the differently formed passive films can hence be manyfold, a more detailed comparison of the electrochemical behavior of the different systems is beyond the scope of this work. Further, Fe–Cr alloys are inherently much more resistant than chromate-passivated iron, since the passive film can re-form and its composition can be modified (supply of Cr from the bulk). In addition, a major drawback for practical applications in the case of chemically passivated Fe would be the lack of reactive species, if the protective surface coverage is destroyed, either mechanically (e.g. scratches), chemically or electrochemically.
1418
S. Virtanen, M. B€uchler / Corrosion Science 45 (2003) 1405–1419
4. Conclusions The electrochemical behavior of chromate-passivated Fe in chromate-free solution strongly depends on the passivation procedure. In passive films formed by immersion of iron at open-circuit in chromate solution, both the iron and chromium species dissolve almost independently of the presence of the other one: Fe oxide by reductive dissolution and Cr oxide by oxidative dissolution in the corresponding potential regions. Passivation of iron by potential cycling in chromate solutions leads to formation of oxide films, which suffer from less loss of the otherwise soluble oxidized chromate and reduced Fe(2þ) species (trapping in the film). This may be due to the formation of a network of the Fe and Cr component of the passive film (i.e. formation of a mixed Fe–Cr oxide film) by cycling. The film formed on iron by cycling in chromate solution behaves similarly as the passive film on Fe–17Cr alloy as concerns the electrochemical dissolution behavior. However, the remnant passive film after reductive or oxidative dissolution on the Fe–Cr alloy is of truely protective nature as compared to the films formed on iron in chromate solutions, which show only a small contribution to the potential drop.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
T.P. Hoar, U.R. Evans, J. Chem. Soc. (1932) 2476. M.J. Pryor, M. Cohen, J. Electrochem. Soc. 100 (1953) 203. R.A. Powers, N. Hackerman, J. Electrochem. Soc. 100 (1953) 314. M. Cohen, A.F. Beck, Z. Elektrochem. 62 (1958) 696. D.M. Brasher, A.D. Mercer, Trans. Faraday Soc. 61 (1965) 803. D.M. Brasher, J.G. Beynon, K.S. Rajagopalan, J.G.N. Thomas, Br. Corros. J. 5 (1970) 264. Z. Szklarska-Smialowska, R.W. Staehle, J. Electrochem. Soc. 121 (1974) 1146. K. Ogura, H. Wada, Electrochim. Acta 25 (1980) 913. E. McCafferty, M.K. Bernett, J.S. Murday, Corros. Sci. 28 (1988) 559. E.J. Calvo, C.D. Pallotta, S. Hild, E. Garcia, J. Electrochem. Soc. 135 (1988) 314. I. Olefjord, B. Brox, in: M. Froment (Ed.), Passivity of Metals and Semiconductors, Elsevier, Amsterdam, 1983, p. 561. K. Hashimoto, K. Asami, Corros. Sci. 19 (1919) 251. R. Kirchheim, B. Heine, H. Fischmeister, S. Hofmann, H. Knote, U. Stolz, Corros. Sci. 29 (1989) 899. C.R. Clayton, I. Olefjord, in: P. Marcus, J. Oudar (Eds.), Corrosion Mechanisms in Theory and Practice, Marcel Dekker, Inc., New York, 1995, p. 175. G. Okamoto, Corros. Sci. 13 (1973) 471. C.R. Clayton, K. Doss, J.B. Warren, in: M. Froment (Ed.), Passivity of Metals and Semiconductors, Elsevier, Amsterdam, 1983, p. 585. S. Haupt, H.H. Strehblow, Corros. Sci. 29 (1989) 163. M. B€ uchler, P. Schmuki, H. B€ ohni, J. Electrochem. Soc. 144 (1997) 2307. L. Kwiatkowski, F. Mansfeld, J. Electrochem. Soc. 140 (1993) L39. P. Schmuki, H. B€ ohni, F. Mansfeld, J. Electrochem. Soc. 140 (1993) L119. C. Duret-Thual, F. Barrau, in: P. Marcus, B. Baroux, M. Keddam (Eds.), Modifications of Passive Films, The Institute of Materials, London, 1993, p. 176. S. Fujimoto, T. Shibata, K. Wada, T. Tsutae, Corros. Sci. 35 (1993) 147. P. Schmuki, S. Virtanen, A.J. Davenport, C.M. Vitus, J. Electrochem. Soc. 143 (1996) 574.
S. Virtanen, M. B€uchler / Corrosion Science 45 (2003) 1405–1419
1419
[24] S. Virtanen, P. Schmuki, M. B€ uchler, H.S. Isaacs, J. Electrochem. Soc. 146 (1999) 4087. [25] P. Schmuki, M. B€ uchler, S. Virtanen, H.S. Isaacs, M.P. Ryan, H. B€ ohni, J. Electrochem. Soc. 146 (1999) 2097. [26] P. Schmuki, S. Virtanen, A.J. Davenport, C.M. Vitus, J. Electrochem. Soc. 143 (1996) 3997. [27] P. Schmuki, S. Virtanen, H.S. Isaacs, M.P. Ryan, A.J. Davenport, H. B€ ohni, T. Stenberg, J. Electrochem. Soc. 145 (1998) 791. [28] P. Schmuki, M. B€ uchler, S. Virtanen, H. B€ ohni, R. M€ uller, L.J. Gauckler, J. Electrochem. Soc. 142 (1995) 3336. [29] I. Olefjord, Mater. Sci. Eng. 42 (1980) 161. [30] P. Marcus, J.M. Grimal, Corros. Sci. 33 (1992) 805. [31] K. Sieradzski, R.C. Newman, J. Electrochem. Soc. 133 (1986) 1979.
Electrochemical behavior of surface films formed on Fe in chromate solutions S. Virtanen a
a,*
, M. B€ uchler
b
Institute for Materials Chemistry and Corrosion and Department of Materials, Swiss Federal Institute of Technology, ETH-H€onggerberg, 8093 Z€urich, Switzerland b Swiss Society for Corrosion Protection (SGK), Technoparkstrasse 1, 8005 Z€urich, Switzerland Received 26 November 2001; accepted 2 December 2002
Abstract The stability of passive films formed on Fe in K2 Cr2 O7 solutions during exposure at opencircuit potential or by potential cycling is studied in a chromate-free solution. The electrochemical behavior of chromate-passivated Fe is investigated with cyclic voltammetry combined with LASER light reflectance measurements which allow an in situ determination of the thickness of the iron oxide film. The electrochemical behavior of chromate-passivated Fe in chromate-free solutions strongly depends on passivation treatment. Passivation of iron by immersion at open-circuit in chromate solution leads to a passive film, in which both Fe and Cr species dissolve almost independently of the presence of the other one: Fe oxide by reductive dissolution and Cr oxide by oxidative dissolution in the corresponding potential regions. Passivation of iron by potential cycling in chromate solutions leads to much less loss of the otherwise soluble oxidized chromate and reduced ferrous species in subsequent electrochemical experiments (trapping in a protective film). Concerning the dissolution behavior, the film formed on iron by cycling in chromate solution behaves similarly as the passive film on Fe–17Cr alloy. However, the remnant passive film after reductive or oxidative dissolution on the Fe–Cr alloy is of truely protective nature as compared to the films formed on iron in chromate solutions, which show only a small contribution to the potential drop. Ó 2003 Elsevier Science Ltd. All rights reserved. Keywords: A. Iron; Iron–chromium alloys; Chromate; B. Cyclic voltammetry; Light reflectance; Inhibition; C. Passivity
*
Corresponding author. Tel.: +41-1-633-2790; fax: +41-1-633-1147. E-mail address: [email protected] (S. Virtanen).
0010-938X/03/$ - see front matter Ó 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0010-938X(02)00242-1
1406
S. Virtanen, M. B€uchler / Corrosion Science 45 (2003) 1405–1419
1. Introduction It is well-known since decades [1] that chromate ions are efficient inhibitors for Fe. Apart from the action of chromates as a strong oxidizer [2], it is a well-established fact that chromates in the solution lead to a modification of the passive film on iron, more specifically to the incorporation of significant amounts of Cr species in the surface oxide film (see e.g. [3–10]). An ellipsometric study indicated that a surface film consisting of a mixture of iron and chromium hydroxides with iron and chromium oxide is formed during open-circuit exposure of Fe in chromate solutions [7]. Further, an XPS study revealed that chromium is present in the film formed on Fe in chromate solutions mainly as Cr(3þ), iron as Fe(3þ), and oxygen both in M–O and M–OH bonds [9]. The Cr/Fe-ratio in the film depends on the passivation procedure [9]. On the other hand, the passive films on Fe–Cr alloys or stainless steels are enriched with oxidized chromium species (see e.g. [11–14]); the valency of the oxidized Cr and Fe present in the passive film as well as the Crox /Feox -ratio depends on the potential [11,15–17]. Oxygen has been identified to be present in passive films on Fe– Cr alloys in three different metal–oxygen bonds: as an oxide M–O, a hydroxide M– OH and an oxyhydride M–OOH [14–16]. Therefore, the composition of the passive film on iron formed in chromate-containing solutions shows some similarities to that of Fe–Cr alloys, even though the exact nature of the passive films in both cases clearly depends on many experimental parameters. Nevertheless, the question arises, if similarities between the electrochemical behavior of chromate-passivated iron and Fe–Cr alloys exist. However, little work has been carried out to study the stability of iron passivated in chromate solutions in electrolytes not containing the inhibiting chromate species. Therefore, in the present work the electrochemical behavior of chromate-passivated iron is investigated with cyclic voltammetry (CV) combined with LASER light reflectance measurements [18] which allow an in situ determination of the thickness of the iron oxide film.
2. Experimental Chemical passivation of Fe (99.5%) was carried out by exposing the samples to 0.0025 M K2 Cr2 O7 , pH 4.5 (open to air) at the open-circuit potential for various times (1 h to 3 days). Prior to the passivation treatment, the surfaces were mechanically ground to a 1000 grit finish for electrochemical measurements or to a 1/4 lm diamond finish for surface analytical studies. After polishing, the samples were rinsed in acetone and ethanol and dried in N2 . Impedance spectra were measured as a function of time at the open-circuit potential in the chromate solution as well as in borate buffer, pH 8.4 and in 0.1 M Na2 SO4 , pH 4.5. Apart from open-circuit passivation, Fe was treated by potential cycling in the chromate solution as will be described below. After removal from the chromate solution, the samples were thoroughly rinsed with distilled water and dried in nitrogen.
S. Virtanen, M. B€uchler / Corrosion Science 45 (2003) 1405–1419
1407
Electrochemical characterization of the treated samples was performed in borate buffer with a pH value of 8.4 (0.075 M Na2 B4 O7 10H2 O, 0.3 M H3 BO3 ). The solutions were prepared from reagent grade chemicals and distilled water. During the experiments the solutions were deaerated with N2 . A platinum gauze served as a counter electrode and a saturated calomel electrode (SCE) was used as a reference electrode. CV was carried out with a sweep rate of 10 mV/s. In order to ensure stationary conditions the sample was kept at )1 V before every subsequent cycle. For the light reflection measurements the samples were polished down to 1/4 lm diamond paste. For the light absorption measurements, the opposite wall of the electrochemical cell consisted of a quartz window (Suprasil II) to allow an absorption free penetration of UV light. As light absorption can be sensitive to ion enrichment in the electrolyte, a flow cell was used in the present work in order to exclude any influence by changes in electrolyte composition. The flow rate of the electrolyte was controlled by a flow meter and was 1 mm/s in all experiments reported in this work. The beam of a HeCdlaser which provides a continuous 325 nm line and an output of 2 mW was directed perpendicular to the sample surface. The reflected light intensity was measured with a Newport 835 Optical Power Meter. A partially transparent mirror allowed the splitting of incoming and reflected light beam. The quartz window was tilted to the light beam by 0.5° in order to separate the contribution of the light reflected on the glass from that one of the sample. Intensities ðIÞ were normalised with the reflected intensity (I0 ) at a cathodic polarisation of )1 V SCE. In the case of iron an oxide-free state is obtained at this potential. In the present work a removing of the passive film is not possible in all experiments. Therefore the highest light intensity obtained during the measurement was chosen as I0 . Due to this possible error resulting from residual oxide films and the possibly complicated effect of mixed iron–chromium oxides, the absorption data were only evaluated qualitatively and a calculation of film thickness was not performed. Details of the technique and its calibration are given elsewhere [18].
3. Results and discussion 3.1. Electrochemical behavior of chromate-passivated iron Fig. 1 shows the impedance value at 10 mHz at the open-circuit potential as a function of time determined from impedance spectra for iron in 0.0025 M K2 Cr2 O7 , pH 4.5 and in borate buffer, pH 8.4. (Z-values at 10 mHz are used as an approximation for the polarization resistance.) In both cases, the gradual increase of the polarization resistance indicates passivation. As a comparison, the polarization resistance was measured in 0.1 M Na2 SO4 , pH 4.5. In this solution active corrosion of iron takes place, and hence the polarization resistance remains low, <1 kX. Clearly, chromate ions strongly inhibit corrosion of iron, even at relatively low pH values. In Fig. 2, the cyclic voltammograms of iron measured in borate buffer after an open-circuit exposure for 1 h either in the chromate or in borate buffer solution are
1408
S. Virtanen, M. B€uchler / Corrosion Science 45 (2003) 1405–1419
Fig. 1. Impedance at 10 mHz (approximation for the polarization resistance) as a function of time determined from impedance spectra measured at open-circuit potential in solutions open to air.
compared. In Fig. 2(a), the anodic reversal potential is below the transpassive dissolution region of Cr oxide (Emax ¼ þ200 mV SCE). In this case, a significantly different behavior is found for the chromate- and borate-passivated samples: the anodic current in the region of active/passive-transition is much lower for the chromate-passivated sample after the cathodic sweep. In Fig. 2(b) the CVs are shown with the anodic reversal potential above the Cr transpassive dissolution range. In the first anodic sweep of the chromate-passivated iron, high anodic current densities are measured below the oxygen evolution region resulting from the Cr3þ ! Cr6þ oxidation reaction. In the following cathodic and anodic sweeps, the behavior of the chromate-passivated sample is almost identical with the sample passivated in borate buffer. Thus, the Cr3þ oxidation reaction leads to a complete dissolution of the chromate species, and the stability region of chromate-passivated iron is therefore limited by the potential of onset of transpassive dissolution of chromium oxides. A similar behavior was found for all samples passivated at open-circuit potential in the chromate solution, independent of the passivation time (1 h to 3 days), temperature of the solution (RT to 80 °C), as well as chromate concentration and pH of the solution. In all these cases, the protective effect of the surface layer was completely lost after polarization in the region of oxidation of Cr(3þ). In literature, AC passivation or generally passivation by potential cycling has been shown to improve the stability of the passive film on stainless steel [19–22]. Hence, an attempt was made to change the oxide layer properties by cyclic passivation of Fe in the chromate solution. Such a cycling could lead to a thickening of the film and possibly change the Cr/Fe-ratio and/or-distribution in the film. Fig. 3 shows the cyclic voltammogram of iron measured in the chromate solution. Testing different potential windows for the treatment showed that the best treatment was obtained in the potential region as shown in Fig. 3, including the region of the Cr(3þ) oxidation and Fe(3þ) reduction. Samples, which were treated by potential
S. Virtanen, M. B€uchler / Corrosion Science 45 (2003) 1405–1419
1409
Fig. 2. Cyclic voltammogram of Fe passivated for 1 h at open-circuit potential in borate buffer, pH 8.4 or in 0.0025 M K2 Cr2 O7 , pH 4.5 (sweep rate 10 mV/s): (a) 0:3 ! þ0:2 ! 1:2 ! þ0:2 V SCE, (b) 0:3 ! þ1:0 ! 1:2 ! þ1:0 V SCE.
cycling in a potential region not exceeding these limits, showed in a subsequent electrochemical testing in borate buffer a basically identical behavior with samples passivated at the open-circuit potential, i.e. a loss of the protective quality of the oxide layer formed in the chromate solution after polarization in Crð3þÞ ! Crð6þÞ region after the treatment. In Fig. 4, the cyclic voltammograms of Fe passivated at open-circuit potential (a) or with potential cycling (b) in the chromate solution are compared. Shown are the first and the third cycle (which is identical to all subsequent cycles) in borate buffer. Fig. 4(c) shows a comparison of the third cycle of the differently treated samples. In the case of the sample passivated at open-circuit potential, the sample shows an active/passive-transition in the anodic sweep following polarization up to the Cr oxidation region. Following the oxidation peak of chromium, in the subsequent
1410
S. Virtanen, M. B€uchler / Corrosion Science 45 (2003) 1405–1419
Fig. 3. Cyclic voltammogram of Fe in 0.0025 M K2 Cr2 O7 , pH 4.5 (sweep rate 10 mV/s).
cathodic sweep, no reduction peak of chromates is seen. Since the experiment was carried out in a flow cell, only Cr(6þ) species in the oxide film would lead to a chromate reduction peak, and not species dissolved in the solution. Hence, the absence of the chromate reduction peak indicates that the chromates are completely dissolved. The corresponding experiment for CV-passivated iron shows that in this case all cycles are identical, and no activation of the sample takes place. In the cathodic sweep a reduction peak for chromates can be seen, thus suggesting that some chromate species are incorporated in the oxide layer. The different behavior of the differently treated samples as concerning the Crð6þÞ ! Crð3þÞ reduction peak in the cyclic voltammograms can be well seen in Fig. 4(c). Hence, passivation by potential cycling in the chromate solution leads to a formation of an oxide film, which can not be dissolved by polarization to high anodic potentials, since the oxidation of Cr(3þ) to Cr(6þ) does not lead to a complete dissolution of the chromate species. 3.2. Light absorption measurement on chromate-passivated iron The light absorption technique was employed to study the behavior of the iron oxide species in the surface layers during CV. Fig. 5 shows a CV of Fe in borate buffer and a simultaneously recorded light absorption data. Starting at the cathodic potential of )1 V, the surface of iron can be considered to be oxide-free, as it is wellknown that the passive film on iron as well as iron oxides reductively dissolve in this potential region in borate buffer (see e.g. [23,24]). As the potential passes through the active/passive-transition, the absorption increases indicating oxide film formation. In the passive region, a linear increase of the absorption as a function of potential is observed. Reversing the sweep direction, the absorption remains constant until the potential reaches the reduction peak (FeIII ! FeII), here the absorption intensity decreases indicating thinning of the passive film. Staying a short time at )1 V, the absorption intensity returns to itÕs original low value. It has been shown earlier by
S. Virtanen, M. B€uchler / Corrosion Science 45 (2003) 1405–1419
1411
Fig. 4. Cyclic voltammograms of Fe passivated for 1 h at open-circuit potential (a) or with potential cycling (b) in 0.0025 M K2 Cr2 O7 , pH 4.5. Shown are the first and the third cycle in borate buffer, pH 8.4, (c) shows a comparison of the third cycles of (a) and (b).
B€ uchler et al. that in the case of iron in borate buffer, anodic film formation and cathodic film dissolution are reversible––i.e. repeated anodic/cathodic cycling leads to conditions identical to the initial state, which––as mentioned above––is an oxide film-free surface [18]. Fig. 6 shows the cyclic voltammogram (a) and the corresponding light absorption data (b) for iron passivated for 1 h at open-circuit in the chromate solution. In the
1412
S. Virtanen, M. B€uchler / Corrosion Science 45 (2003) 1405–1419
Fig. 5. Cyclic voltammogram and the corresponding light absorption data for Fe in borate buffer, pH 8.4.
first anodic sweep, the anodic reversal potential was þ200 mV SCE, i.e. below the oxidation of Cr(3þ). As seen in the CV, in the following anodic sweep no active/ passive-transition is observed. The absorption data taken simultaneously with the CV, nevertheless, shows that cathodic polarization leads to an absorption close to zero indicating a strong removal of the oxide film. The small remaining absorption indicates the presence of a remnant oxide film which presumably consists of a chromium rich oxide. Hence, the lower current densities in the subsequent anodic sweep most probably result from the coverage by this remnant oxide. After cathodic polarization at )1 V, the increase of absorption in a subsequent anodic sweep is linear with potential, indicating an adaptation of film thickness to constant field strength conditions. It is striking that the slope in the second cycle is smaller than in the third, which again can be explained with the presence of the protecting chromium rich oxide on the surface that takes part of the potential drop. This conclusion is supported by the increase in slope as soon as the anodic dissolution of the chromium sets in in the second cycle. In all sweeps following polarization up to þ0.9 V (i.e. above the Cr(3þ) oxidation potential), the absorption behavior of the sample becomes almost identical to that of non-treated iron. A similar behavior was found for all samples passivated at open-circuit in the chromate solution independent of passivation time. In Fig. 7, the cyclic voltammogram in borate buffer (a) together with the absorption data (b) are shown for iron passivated by potential cycling in the chromate solution. Both, the current and absorption behavior of the second and the third cycle are very similar and the characteristic oxidation reaction of Cr(3þ) is much less pronounced. Even after several cycles the same current and absorption behavior is obtained which is very different to that of pure iron. This result indicates that the chromium deposited on the sample surface during the previous cyclic treatment in the chromate solution cannot be removed by anodic polarization. However, despite this residual oxide layer the current density in the passive range is not significantly decreased compared to that one of the sample passivated by a simple immersion
S. Virtanen, M. B€uchler / Corrosion Science 45 (2003) 1405–1419
1413
Fig. 6. Cyclic voltammogram (a) and the corresponding light absorption data (b) in borate buffer, pH 8.4 for iron passivated for 1 h at open-circuit in 0.0025 M K2 Cr2 O7 , pH 4.5.
experiment (see Fig. 4(c)). Further, the consideration of the absolute changes in absorption during the second cycle make clear that they are very similar for both passivation procedures. In neutral solutions the passive current density is mainly attributed to the passive film formation. Hence, a high passive current density indicates that a large amount of passive film has to be formed in order to compensate the potential drop. A similar result is obtained with the absorption data, as large changes in absorption indicate large changes in film thickness. Hence, the absorption and current behavior indicate that the film formed by cyclic polarization cannot be removed electrochemically but that it gives only little contribution to the protection of the metal. Nevertheless, some fundamental differences between the two procedures are observed. Contrary to the results in Fig. 6 for Fe passivated at open-circuit potential, no linear increase of the absorption is observed during anodic polarization for the cyclically passivated Fe (Fig. 7). For the cyclically passivated Fe, the significant increase in absorption coincides with the broad anodic peak in the current-potential
1414
S. Virtanen, M. B€uchler / Corrosion Science 45 (2003) 1405–1419
Fig. 7. Cyclic voltammograms (a) and the corresponding light absorption data (b) in borate buffer, pH 8.4 for iron passivated for 1 h at open-circuit or by potential cycling in 0.0025 M K2 Cr2 O7 , pH 4.5. Shown are the third cycles.
characteristic. Experiments with solutions containing iron ions have shown that absorption of Fe(2þ) is significantly lower than of Fe(3þ) [18]. Hence, the oxidation of a film consisting of mainly Fe(2þ) to a film consisting of Fe(3þ) would result in a strong increase in absorption. This effect was indeed observed for iron in 0.1 M NaOH where reduction does not lead to dissolution, but instead to a conversion of a Fe(3þ) oxide film into Fe(2þ) oxide/hydroxide film [18,25]. Therefore, the observed absorption behavior for the cyclically passivated iron could be explained with the oxidation of Fe(2þ) to Fe(3þ) within the film. Further, it is obvious that the reduction peak of Cr(6þ) to Cr(3þ) is identical in the second and the third cycle. This can only be explained with a constant amount of Cr(6þ) in the film. Hence, these results demonstrate that neither all of the reduced Fe(2þ) nor the oxidized Cr(6þ) are released into the solution. The light absorption data are shifted to higher values with increasing number of cycles. This shift is parallel over the entire potential range. This behavior is typical
S. Virtanen, M. B€uchler / Corrosion Science 45 (2003) 1405–1419
1415
for changes in reflection behavior due to changes in surface roughness or the deposition of a light scattering deposit on the surface. The findings therefore clearly indicate a significantly different behavior for the two different chromate-passivation treatments of iron: In the open-circuit passivated iron both the iron and the chromium oxide species in the oxide film are lost by dissolution during polarization in an appropriate potential region, iron oxide by reductive dissolution and chromium oxide by oxidative dissolution. This is similar to the wellknown electrochemical dissolution behavior of the passive films on pure Fe and Cr, or thin Fe2 O3 or Cr2 O3 films, respectively (see e.g. [23–26] and references therein). In the oxide film on the CV passivated iron, however, oxidation and reduction reactions do not lead to a complete dissolution of the otherwise soluble oxidized chromate and reduced Fe(3þ) species. Earlier studies on artificial Fe/Cr oxide passive films have shown that a composition range of a high chemical and electrochemical stability exists in mixed oxides where the samples show no dissolution either during reduction or oxidation [27]. The dissolution rates of the mixed oxides were determined from in situ X-ray absorption near edge spectroscopy (XANES) as a function of the Cr oxide content in the films during anodic and cathodic polarization. The XANES spectra further show that in all cases the oxide films were electroactive, i.e. reduction of Fe oxide and oxidation of Cr oxides always took place, but depending on the oxide composition, the Fe(2þ) and the Cr(6þ) species were either dissolved or trapped in the oxide film. Hence, the behavior observed with the cyclically passivated sample could be explained with the formation of a mixed iron–chromium oxide passive film that is resistant against both cathodic and anodic dissolution. Despite of the stability of the film against electrochemical dissolution, its contribution to the potential drop is small. The CVs of iron passivated with both procedures clearly show that Cr is incorporated in the passive films in both cases (presence of the oxidation peak in the Crð3þÞ ! Crð6þÞ potential region). Preliminary qualitative surface analysis indicates that for the open-circuit passivated Fe, the chromium species are present only in the outer part of the oxide layer. However, for the passive film formed by cycling, chromium seems to be present throughout the oxide film. The influence of cycling of the potential between the oxidation and reduction region (in the presence of chromate in the solution) may therefore be a continuous re-construction of the oxide film, due to dissolution of the passive film (either by oxidation of the Cr2 O3 into soluble CrO2 4 or reduction of the Fe2 O3 into soluble Fe(2þ)) and a subsequent re-formation in the reverse sweep. In this way, the oxide film may be modified until it reaches a more ‘‘stable’’ structure. This in contrast to passivation at the open-circuit potential, where less mobility of the oxide components takes place by dissolution, and hence the iron and chromium components of the oxide film may not form a real network, but are present as separate layers. In the latter case, these Cr and Fe components of the oxide film behave similar to pure Fe or Cr oxide, and hence dissolve independently of the other component in the corresponding potential regions, i.e. protection of the soluble species by the presence of the insoluble component of the oxide film is only possible for a mixed oxide film.
1416
S. Virtanen, M. B€uchler / Corrosion Science 45 (2003) 1405–1419
3.2.1. Fe–17Cr alloy To compare the behavior of chromate-passivated iron to Fe–Cr alloys, corresponding measurements were carried out on an Fe–17Cr alloy (Fig. 8). After polishing the sample was immersed into a 0.1 M Na2 SO4 (pH 4.5) solution in order to obtain comparable starting conditions. It is clear that the current densities (Fig. 8(a)) are significantly smaller than in the case of the chromate treated iron samples. This is certainly due to the fact that the passive film cannot be removed completely by cathodic polarization. This finding is supported by the fact that changes in absorption (Fig. 8(b)) are small, except in the potential region, where anodic chromium oxidation and cathodic iron reduction occur. The strong increase of the absorption at high anodic potentials can be due to the anodic dissolution of the Cr(6þ) species, which requires a replacement of the chromium in the passive film by iron ions to hold the potential drop across the film. The observed increase in absorption can then have two reasons: Either a higher film thickness is obtained or the absorption coefficient is higher for iron. As the energy gap is 1.9 eV for iron oxides while a value of 2.6 eV is reported for chromium oxides [28], a formation of an Fe-rich film could result in the
Fig. 8. Cyclic voltammogram (a) and the corresponding light absorption data (b) in borate buffer, pH 8.4 for Fe–17Cr passivated for 1 h at open-circuit in 0.1 M Na2 SO4 , pH 4.5.
S. Virtanen, M. B€uchler / Corrosion Science 45 (2003) 1405–1419
1417
increase of absorption without a significant change in the film thickness. The explanation with the formation of an iron rich film is supported by the fact that the increase of absorption that is observed at high anodic potentials reversibly is lost during cathodic Fe(3þ) to Fe(2þ) reduction. Further, it is clear from the data in Fig. 8 that the amount of chromium dissolution decreases with increasing numbers of cycles as the change in absorption and the peak height in iron reduction decreases with increasing number of cycles. This was investigated in up to 6 consecutive cycles. Again this result can be explained with the formation of a mixed iron–chromium oxide film that is resistant against both anodic and cathodic dissolution [27]. In the case of the passive film on Fe–Cr alloys, surface analytical studies often conclude the passive film to consist of an inner Cr-rich layer and an outer Fe-rich layer (see e.g. [13,29,30]). Even though a concentration gradient is present in the passive film, the layer responsible for stable passivity consists not only of either pure Cr or pure Fe oxides, but is a mixture of the two components. Therefore we conclude that not only the Cr/Fe ratio in the oxide film determines electrochemical stability, but also the distribution of these species can play a major role. Only in mixed oxide films––where the different film constituents can interact with each other––no complete oxidative dissolution of Cr or reductive dissolution of Fe takes place. A formation of a network of the insoluble oxide component can hence protect the otherwise soluble species from dissolution (trapping in the network). This argument is similar to the percolation model suggested to explain a critical Cr-concentration for passivity in Fe–Cr alloys [31]. Despite of the ability of the cyclically passivated iron to resist oxidative and reductive dissolution of the passive film, the results clearly demonstrate that a very different electrochemical and absorption behavior is obtained for the Fe–Cr alloy, where the remnant passive film (after reductive or oxidative dissolution) is of truly protective nature as compared to the films formed on iron in chromate solutions. This finding demonstrates that the stability of oxide films against electrochemical dissolution is only one key factor describing the behavior in the passive state. It should be pointed out that the present study only qualitatively compares the behavior of chromate-passivated Fe using two different passivation treatments with the behavior of Fe–17Cr alloy. As mentioned in the introduction, the nature of the passive film on Fe–Cr alloys depends––apart from the alloy composition––on all passivation parameters. Also for chromate-passivated iron, strong differences may arise by further changing the passivation treatment (e.g. sweep rate of cyclic passivation). As the chemical composition, thickness as well as the distribution of the different species present in the differently formed passive films can hence be manyfold, a more detailed comparison of the electrochemical behavior of the different systems is beyond the scope of this work. Further, Fe–Cr alloys are inherently much more resistant than chromate-passivated iron, since the passive film can re-form and its composition can be modified (supply of Cr from the bulk). In addition, a major drawback for practical applications in the case of chemically passivated Fe would be the lack of reactive species, if the protective surface coverage is destroyed, either mechanically (e.g. scratches), chemically or electrochemically.
1418
S. Virtanen, M. B€uchler / Corrosion Science 45 (2003) 1405–1419
4. Conclusions The electrochemical behavior of chromate-passivated Fe in chromate-free solution strongly depends on the passivation procedure. In passive films formed by immersion of iron at open-circuit in chromate solution, both the iron and chromium species dissolve almost independently of the presence of the other one: Fe oxide by reductive dissolution and Cr oxide by oxidative dissolution in the corresponding potential regions. Passivation of iron by potential cycling in chromate solutions leads to formation of oxide films, which suffer from less loss of the otherwise soluble oxidized chromate and reduced Fe(2þ) species (trapping in the film). This may be due to the formation of a network of the Fe and Cr component of the passive film (i.e. formation of a mixed Fe–Cr oxide film) by cycling. The film formed on iron by cycling in chromate solution behaves similarly as the passive film on Fe–17Cr alloy as concerns the electrochemical dissolution behavior. However, the remnant passive film after reductive or oxidative dissolution on the Fe–Cr alloy is of truely protective nature as compared to the films formed on iron in chromate solutions, which show only a small contribution to the potential drop.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
T.P. Hoar, U.R. Evans, J. Chem. Soc. (1932) 2476. M.J. Pryor, M. Cohen, J. Electrochem. Soc. 100 (1953) 203. R.A. Powers, N. Hackerman, J. Electrochem. Soc. 100 (1953) 314. M. Cohen, A.F. Beck, Z. Elektrochem. 62 (1958) 696. D.M. Brasher, A.D. Mercer, Trans. Faraday Soc. 61 (1965) 803. D.M. Brasher, J.G. Beynon, K.S. Rajagopalan, J.G.N. Thomas, Br. Corros. J. 5 (1970) 264. Z. Szklarska-Smialowska, R.W. Staehle, J. Electrochem. Soc. 121 (1974) 1146. K. Ogura, H. Wada, Electrochim. Acta 25 (1980) 913. E. McCafferty, M.K. Bernett, J.S. Murday, Corros. Sci. 28 (1988) 559. E.J. Calvo, C.D. Pallotta, S. Hild, E. Garcia, J. Electrochem. Soc. 135 (1988) 314. I. Olefjord, B. Brox, in: M. Froment (Ed.), Passivity of Metals and Semiconductors, Elsevier, Amsterdam, 1983, p. 561. K. Hashimoto, K. Asami, Corros. Sci. 19 (1919) 251. R. Kirchheim, B. Heine, H. Fischmeister, S. Hofmann, H. Knote, U. Stolz, Corros. Sci. 29 (1989) 899. C.R. Clayton, I. Olefjord, in: P. Marcus, J. Oudar (Eds.), Corrosion Mechanisms in Theory and Practice, Marcel Dekker, Inc., New York, 1995, p. 175. G. Okamoto, Corros. Sci. 13 (1973) 471. C.R. Clayton, K. Doss, J.B. Warren, in: M. Froment (Ed.), Passivity of Metals and Semiconductors, Elsevier, Amsterdam, 1983, p. 585. S. Haupt, H.H. Strehblow, Corros. Sci. 29 (1989) 163. M. B€ uchler, P. Schmuki, H. B€ ohni, J. Electrochem. Soc. 144 (1997) 2307. L. Kwiatkowski, F. Mansfeld, J. Electrochem. Soc. 140 (1993) L39. P. Schmuki, H. B€ ohni, F. Mansfeld, J. Electrochem. Soc. 140 (1993) L119. C. Duret-Thual, F. Barrau, in: P. Marcus, B. Baroux, M. Keddam (Eds.), Modifications of Passive Films, The Institute of Materials, London, 1993, p. 176. S. Fujimoto, T. Shibata, K. Wada, T. Tsutae, Corros. Sci. 35 (1993) 147. P. Schmuki, S. Virtanen, A.J. Davenport, C.M. Vitus, J. Electrochem. Soc. 143 (1996) 574.
S. Virtanen, M. B€uchler / Corrosion Science 45 (2003) 1405–1419
1419
[24] S. Virtanen, P. Schmuki, M. B€ uchler, H.S. Isaacs, J. Electrochem. Soc. 146 (1999) 4087. [25] P. Schmuki, M. B€ uchler, S. Virtanen, H.S. Isaacs, M.P. Ryan, H. B€ ohni, J. Electrochem. Soc. 146 (1999) 2097. [26] P. Schmuki, S. Virtanen, A.J. Davenport, C.M. Vitus, J. Electrochem. Soc. 143 (1996) 3997. [27] P. Schmuki, S. Virtanen, H.S. Isaacs, M.P. Ryan, A.J. Davenport, H. B€ ohni, T. Stenberg, J. Electrochem. Soc. 145 (1998) 791. [28] P. Schmuki, M. B€ uchler, S. Virtanen, H. B€ ohni, R. M€ uller, L.J. Gauckler, J. Electrochem. Soc. 142 (1995) 3336. [29] I. Olefjord, Mater. Sci. Eng. 42 (1980) 161. [30] P. Marcus, J.M. Grimal, Corros. Sci. 33 (1992) 805. [31] K. Sieradzski, R.C. Newman, J. Electrochem. Soc. 133 (1986) 1979.