The composition of Fe(III) oxy-hydroxide film anodically deposited in borate solution

Corrosion Science, Vol. 36, No. 7, pp. 1257-1266, 1994

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Copyright (~) 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0010-938X/94 $7.00+ 0.00

Pergamon

0010-938X(94)E0026--2

THE COMPOSITION OF Fe(III) OXY-HYDROXIDE FILM ANODICALLY DEPOSITED IN BORATE SOLUTION T. OHTSUKA,J.-C. Ju,* S. ITO and H. EINAGA Department of Applied Chemistry, Nagoya Institute of Technology Gokiso-cho, Showa-ku, Nagoya, 466 Japan

Abstract--The anodic formation of Fe(III) oxy-hydroxide film from Fe(II) ion has been investigated in neutral borate solutions by electrochemical quartz microbalance (EQCM), ellipsometry, and Raman spectroscopy. The comparison between the film weight from the in-situ EQCM and the film thickness from the in-situ ellipsometry provides the density of the film. The density obtained is consistent with that of partially hydrated FeOOH. A ratio of the film weight to the electricity passed during the film formation also indicates the formation of the hydrated FeOOH. The Raman spectra measured under the ex-situ condition shows that the film in pH 7.50 and 8.48 borate solutions is an amorphous FeOOH, while the film in pH 6.45 borate solution includes ),-FeOOH. INTRODUCTION

OXIDATION and deposition from ferrous ion, Fe(II), in aqueous solution to ferric oxy-hydroxide, FeOOH, are a main reaction of rust formation on iron and steel. Different kinds of rusts composed of Fe(III) oxides or oxy-hydroxides are formed and the composition greatly depends on the solution condition, i.e. pH, anionic composition and oxygen concentration of the solution. It has also been pointed out that the anodic deposition of ferrous ion plays an important role in the passivation of iron. l'e The deposition reaction has been investigated by various authors with electrochemical techniques on platinum and gold electrodes. 3-6 Markovac and Cohen 3 and Leibenguth and Cohen, 4 respectively, investigated the anodic deposition of ferric compounds on platinum electrodes from borate and sulphate solutions containing Fe(II) ions. The latter authors identified ~-FeOOH for the deposition film by reflection electron diffraction. Hashimoto and co-workers 5'6 measured the deposition films formed from ferrous perchlorate solutions containing acetate, chloride, sulphate or phosphate ions. They also found formation of a ~,-FeOOH layer by reflection electron diffraction except for film from the solution containing phosphate ions in which they estimated the formation of an amorphous ferric phosphate. They further discussed the kinetics and reaction species for the deposition process. Schultze e t al. presented a property of the deposited film from the view point of the electronic transfer reaction through the thin semiconductive layer. 7 A study using AES was presented by Ardizzone et al. 8 and Seo e t al. 9 The former authors detected a chloride ion in the deposited film formed from the ferrous chloride solution. The latter authors found a co-deposition of borate or phosphate into the film from the electrolytic solution. They further measured the film growth by *Present address: Chemistry Department, Harbin University of Science and Technology, 22 Xuefu Road, Harbin, China. Manuscript received 27 August 1993. 1257

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the in-situ gravimetry of electrochemical quartz crystal microbalance (EQCM) and discussed the film composition and the growth mechanism. In this presentation, two in-situ techniques of ellipsometry and EQCM are applied in order to confirm the composition of Fe(III) oxy-hydroxide film deposited from borate solutions. Raman spectroscopy is also utilized for the composition estimation. It could be expected to be useful for identification of film composition under the in-situ condition; however, the sensitivity is insufficient to detect a thin film covering an electrode solution. The technique was applied to the deposition film under the ex-situ condition. Raman spectra can be measured in air at room temperature, so that conversion of the deposited film due to the removal from the electrolyte may not be large, compared to other techniques such as X-ray and electron or ion beam spectroscopy used for the composition estimation. In this work, the weight change from EQCM and thickness change from ellipsometry under the in-situ condition during the growth of the anodic deposition film of Fe(III) oxy-hydroxide from neutral borate solutions containing Fe(II) ion are presented. The composition of the film from results obtained by the above techniques and from Raman spectra are discussed. EXPERIMENTAL

METHOD

The electrode for the m e a s u r e m e n t s of ellipsometry and R a m a n spectroscopy was a platinum sheet of 1 x 1 × 0.1 cm 3 with a small tab for the electric connection. For the m e a s u r e m e n t of E Q C M (Electrochemical Quartz Crystal Microbalance), an Au-coated AT-cut quartz crystal was used. The resonance frequency is 5 MHz, which corresponds to a mass sensitivity of 1.77 x 10-8 g c m -2 Hz -1 , according to Sauerbrey's equation. 10 The E Q C M apparatus used was a Maxtek, Inc. Model PM-500, with an oscillation circuit of TPS-500. The sensor head, including the quartz crystal and the oscillation circuit, is immersed in an electrochemical cell with 200 cm 2 volume. The E Q C M m e a s u r e m e n t was made in an electric-magnetic shield box (Faraday cage). The ellipsometer and the cell for its in-situ m e a s u r e m e n t has been described previously. 11 It is a rotating-analyser type of automatic apparatus in which the ellipsometric parameters of xlt and A, and the reflectivity, R, are monitored simultaneously. From the three parameters, the three unknowns of the refractive and extinction indices, and the thickness of surface films on electrodes are determined without any assumptions. The m e a s u r e m e n t was carried out at an angle of incidence of 60 ° (:r/3 radians) with a light of 632.8 n m wavelength from a H e - N e laser. A R a m a n spectrometer (JASCO R-800T) was used for measurement of the composition of the Fe(III) oxy-hydroxide under the ex-situ condition. The excitation was made by a laser light with 514.5 n m wavelength from an A r ion laser. The electrolyte solutions prepared by pure water and analytical grade reagents were purged by nitrogen gas for 3 h or longer before the experiments. Ferrous ion was thereafter inserted in the solution as Mohr's salt of FeSO4(NHg)2SO46H20, u n d e r a nitrogen atmosphere. The pure water was obtained by de-ionization and subsequent distillation in a quartz vessel. A n A g - A g C l electrode in saturated KCl solution was used for a reference electrode which was connected to the cell via a Luggin capillary. The platinum plate was used for a counter electrode.

EXPERIMENTAL

RESULTS

E Q C M measurement during potential sweep Figure 1 shows the current-potential and film weight-potential relations during potential sweep at a rate of 5 x 10-3V s -1 in borate solutions containing Fe(II) ions. In the pH 6.45 and 7.40 solutions Fe(II) ions were added at a concentration of 1.0 x 10 -3 mol dm -3. In the pH 8.4 solution the Fe(II) ion concentration decreased to 0.5 x 10 -3 mol dm -3, because of the low solubility of the Fe(II) ions at that pH. It is seen in Fig. 1A that the anodic current starts to flow from E = - 0 . 2 V (Ag-AgCI) at a

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Fe(II) ion concentration of 1.0 x 10 -3 mol dm -3. The current then reveals a small peak about 0.05 V higher than this potential. The current increase at about 0.8 V corresponds to the anodic formation of an Au oxide layer underneath the Fe(III) oxy-hydroxide film. In Fig. 1B it is seen that the weight gain begins at the start potential of the current flow and the film formation hence takes place from this potential. From the dependence of the potential on pH, the following reaction is expected: F e ( O H ) ÷ + H 2 0 ~ F e O O H + 2H ÷ + e. The amount of surface film increases with potential during the anodic sweep and further increases at the reverse scan to about - 0 . 2 V. The reduction of the film takes place at the more negative potentials at which the cathodic current is observed. The amount of the surface film is proportional to the amount of electricity passed during the sweep, as will be discussed later. The formation of an Au oxide layer observed in

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the current-potential relation at the high potential region is not distinguishable in the weight-potential relation.

Ellipsometric measurements during potential sweep Figure 2 shows an example of the ellipsometric results in which the film growth is monitored during the potential sweep of a rate of 5 x 10 -3 V s -1 from - 0 . 3 5 to 0.90 V in the pH 6.45 borate solution containing Fe(II) ions of 10 -3 mol dm -3 concentration. In Fig. 2, AR/R (= [R(d)/R(O)] - 1) is a reflectivity change of the oxide-covered surface, R(d), referring to the oxide-free surface, R(0). The change in and A are seen at potentials more positive than 0.0 V in the anodic direction of the sweep, while AR/R does not change significantly.

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The ellipsometric results should be converted into the thickness of the oxyhydroxide films. For the estimation of thickness, the complex refractive index must be determined. The calculation was made from the optimization of the ellipsometric parameters, ~ , A, and AR/R with the assumption of a homogeneous layer. For the calculation, Fresnel's and Drude's equations were solved numerically. A comparison between experimental and calculated data was made on the plots of ~ vs A and AR/R vs A. The most suitable lines calculated as the parameter of thickness is shown in Fig. 3, together with the experimental loci. In Fig. 3, the values of W and A at zero thickness correspond to the complex refractive index of n = 2.065 - 3.904i for the film-free surface of the platinum electrode. No theoretical lines were found which were completely satisfied with the experimental loci. The optimal complex refractive index is n = 1.7 - 0.06i, as shown in Fig. 3, with which the A - ~ relation for the growing film is almost consistent with the experimental loci. A small deviation

1264

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between the relation and the experimental loci of AR/R vs A is probably due to a change of surface roughness during the growth. In the other experiments the plots of vs A and AR/R vs A show almost the same loci. It may be concluded that the oxyhydroxide film of Fe(III), anodically formed in the borate solution, has the complex refractive index of n = 1.7 - 0.06i.

Film thickness during potential sweep From the above complex refractive index, the ellipsometric parameters can be converted to the thickness of the film. Figure 4 shows the change of thickness of the Fe(III) oxy-hydroxide film calculated from Fig. 2 with n = 1.7 - 0.06i. The calculation of thickness, d, was made from the change of A by use of the next equation of the third degree. d = AA 3 + BA 2 + CA + D The coefficients, A, B, C and D, were determined from the theoretical relation between d and A, which was calculated with the complex refractive index o f n = 1.7 0.06i. It is seen in Fig. 4 that the film starts to form at a potential of E -- 0.0 V, increasing with potential to 3.0 nm at E = 0.90 V (Ag-AgC1). In the reverse direction of potential sweep the film grows further. The maximum thickness is about 4.5 nm at potential about - 0 . 2 V in the reverse scan and the reduction of the film is seen at potentials more negative than the potential. The behavior of the thickness against potential is comparable to that of the film weight shown in Fig. 1.

Constant potential oxidation The anodic formation of the Fe(III) oxy-hydroxide film from Fe(II) ions was also measured at constant potential. Figure 5 shows changes of thickness from ellipsometry, weight from E Q C M , and current from electrochemical measurement at E = 0.85 V (Ag-AgCI) in the p H 6.45 borate solution containing Fe(II) ions at 1 x 10 -3 mol dm -3 concentration for 103 s. The anodic current reveals an initial large spike which decays with time to a f e w g A after 103 s oxidation. The amount of the surface film increases with time to 4.5/zg in weight and 14.4 nm in thickness. The rate of the film formation may be limited by diffusion of Fe(II) ions in the solution 5 and/or by electron transfer through the growing film. 7 Raman spectra Attempts were made to measure the in-situ Raman spectra of the surface film. The spectra were too weak to be distinguished from the Raman spectra of the electrolyte around the electrode. H e r e the ex-situ spectra were measured in air after removal from the electrolyte solution, are reported. After the anodic oxidation, the electrode was rinsed by pure water and then dried under a stream of nitrogen gas. The ex-situ condition would change the surface film. The two spectra of a surface film were measured immediately after oxidation and after exposure in air at room temperature for 1 day. Since the two spectra observed were almost the same, the change of film after removal from the electrolyte is believed to be insignificant and the film composition was therefore examined from the ex-situ Raman spectra. Figure 6 shows Raman the spectra of the surface film formed at constant potentials for 103 s in borate solutions at various p H values. The potential for oxidation was changed with p H at a ratio of - 0 . 0 5 9 V p H -1. In Fig. 6, compensation

Compositionof Fe(III) oxy-hydroxidefilm

1265

of base lines was made for the Raman spectra in the 200-1000 cm -1 wave number region by use of a polynomial approximation. In the pH 6.5 borate solution, the spectrum of the film shows a small peak at 250 cm -1 and a small broad peak at 710 cm -1. In the higher pH solution, the small peak at 250 cm -x disappears, while a new peak at 350 cm -1 appears and the broad peak at 710 cm -1 becomes more remarkable. The two peaks at 710 and 355 cm -1 may originate from the same compound. We measured the Raman spectra of four oxy-hydroxides of a-, fl-, 7- and 6-FeOOH and three oxides of a-Fe203, y-Fe203 and Fe304 for comparison. These spectra were consistent with those previously reported by us 12and other authors. 3'14 It was not possible to find the same spectra among these compounds as those observed in Fig. 6. Only the peak at 250 cm-1 is assigned to the main peak from 7-FeOOH. The compound exhibiting the broad Raman peaks at 710 and 350 cm-1 is conceivably an amorphous type of FeOOH or Fe(OH)3. DISCUSSION The composition of the Fe(III) oxy-hydroxide anodically deposited from Fe(II) ions in the neutral borate solutions can be estimated from the Raman spectra, as shown in Fig. 6. In the pH 6.45 solution, 7-FeOOH and an amorphous type of FeOOH may be formed. In the pH 7.50 and 8.48 solutions, the deposition film may be composed of only an amorphous type of FeOOH or Fe(OH)3. Comparison of the thickness (d), weight (w) and amount of electricity (Q) passed for the film growth can provide further evidence for composition estimation. Figure 7 is a comparison between w and Q during the sweep oxidation in the three solutions. The comparison is made in the potential region below the Au oxide formation at the high potential. In the pH 7.40 and 8.48 solutions, the average ratio of w to Q is 108 g F - 1although the relation is not linear in the pH 8.48 solution. In the pH 6.45 solution it exhibits a slightly smaller value of 96 g F-1. For the composition of FeOOH and Fe(OH)3, the theoretical ratio is, respectively, 89 and 107 g F -1. The experimental ratios indicate that the film formed in the pH 6.5 solution is hydrated in a smaller degree than that in the more alkaline solutions. This is in agreement with the Raman spectroscopic identification. A comparison between w and d provides the density, p, of the film. In the pH 6.45 solution, the ratio of w to d, which is shown in Fig. 1B and Fig. 2, indicates p = 4.05 (+0.10) g cm -3. The density is comparable to that of 7-FeOOH reported in the literature.15 The film formed in the more alkaline solution may have a smaller density because of the more hydrated structure. Under constant potential, a similar comparison was made for the oxy-hydroxide film formed at 0.85 V (Ag-AgCI) in the pH 6.45 solution for 103 s. The comparisons between W and Q, and between d and W are shown in Figs 8 and 9, respectively. The relation between W and Q is not linear, because part of the electricity is consumed by Au oxide formation, which takes place together with Fe(III) oxy-hydroxide formation in the initial stage of oxidation at this potential. In the latter stage, the Au oxide layer is saturated, so that the changes of W and Q correspond to the growth of Fe(III) oxy-hydroxide. From the slope in the latter stage, a ratio of 88 g F -1 is obtained, which is consistent with the theoretical value of FeOOH. The relation between d and W in Fig. 9 is also not linear. The formation of a thin Au oxide layer in this potential region probably contributes to d and W in a small degree. The change of slope which increases with an increase of the film weight may indicate that the film changed from

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a hydrated oxy-hydroxide to a denser oxy-hydroxide with the film growth. From the ratio of d to W the density of the film is estimated to be p = 3.97 g c m -3 in the latter stage of oxidation. The density is a slightly smaller value than the Fe(III) oxyhydroxide. CONCLUSION The Fe(III) oxy-hydroxide film which is formed from Fe(II) ions in solution is an important c o m p o u n d for formation of iron rust and passivation. In the present work, film formation has been investigated by E Q C M , ellipsometry, and R a m a n spectroscopy in borate solutions. The comparison between the film weight from E Q C M and electricity from current m e a s u r e m e n t directly provides an equivalent weight of the film, and also the comparison between the weight and thickness from ellipsometry provides a density of the film. Both values of the equivalent weight and the density indicate formation of partially hydrated F e O O H for the deposition film. From the R a m a n spectra, it was found that the film is composed of an amorphous hydrated F e O O H in p H 7.40 and 8.48 solutions; however, in p H 6.45 solution it includes 3/F e O O H with the amorphous hydrated F e O O H . The hydration degree perhaps increases with the solution pH. We have not checked for any trace of an electrolyte anion, (i.e., borate anion) in the film. On the ex-situ R a m a n spectra, there are no peaks from the borate ions, but as the R a m a n spectroscopy is not sensitive enough to detect a trace amount of chemical species, the possibility of the co-deposition of borate ions into the film cannot be excluded. Acknowledgements--Jiang-Chong Ju wishes to acknowledge the kind support of Professor Fumio Hine

(Professor Emeritus of the Nagoya Institute of Technology) for her stay at the Nagoya Institute of Technology. REFERENCES M. NAGAYAMAand M. COHEN,J. electrochem. Soc. 110,670 (1963). V. MARgOVACand M. COHEN,J. electrochem, Soc. 114, 674 (1967). V. MARKOVACand M. COHEN,J. electrochem. Soc. 114, 678 (1967). J. L. LEtaENGUTHand M. COHEN,J. electrochem. Soc. 119,987 (1972). K. HASmMOTOand M. COHEN,J. electrochem. Soc. 121, 37 (1974). M. COHEN,D. MICHELand K. HASmMOTO,J. electrochem. Soc. 126, 442 (1979). J. W. SCHULTZE,S. MOHRand M, M. LOHRENGEL,J. electroanal. Chem. 154, 57 (1983). S. ARDXZZONEand L. FORMARO,J. eleetroanal. Chem. 246, 53 (1988). M. SEO,K. YOSHIDA,H. TAKAHASHIand I. SAWAMURA,J. electrochem. Soc. 139, 3108 (1992). G. SAUERBREY,Z. Phys. 155, 206 (1959). T. OHTSUgA,Denki Kagaku 60, 1123 (1992). T. OHTSUgA,K. KuaO and N. SATO,Corrosion 42, 476 (1986). D. THIERRY,D. PERSSON,C. LEIGRAF,D. DELICHERE,S. JOIRET,C. PALLOTTAand A. HUGOT-LEGOFF, J. electrochem. Soc. 135,305 (1988). 14. J. GuI and T. M. DEWNE,Corros. Sci. 32, 1105 (1991). 15. U. SCHWERTMANNand R. M. CORNELL,in Iron Oxides in the Laboratory, VCH, Weinheim (1991). 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.