- Email: [email protected]
Transformation of γ-FeOOH to α-FeOOH in acidic solutions containing metal ions
Colloids and Surfaces A: Physicochem. Eng. Aspects 266 (2005) 155–159
Transformation of ␥-FeOOH to ␣-FeOOH in acidic solutions containing metal ions T. Ishikawa a,∗ , K. Takeuchi a , K. Kandori a , T. Nakayama b b
a School of Chemistry, Osaka University of Education, 4-698-1 Asahigaoka, Kashiwara, Osaka 582-8582, Japan Materials Research Laboratory, Kobe Steel, LTD., 5-5 Takatsukadai 1-chome, Nishi-ku, Kobe, Hyogo 615-2271, Japan
Received 21 February 2005; received in revised form 4 June 2005; accepted 10 June 2005
Abstract The transformation reaction of ␥-FeOOH to ␣-FeOOH in FeSO4 solutions dissolving Ti(IV), Cr(III), Cu(II) and Ni(II) at pH < 3 was examined by FT-IR. The TEM observation demonstrated that the transformation proceeds by dissolution of ␥-FeOOH particles and recrystallization of ␥-FeOOH particles. The transformation occurred in FeSO4 solutions but not in both FeCl2 and Na2 SO4 solutions, indicating that the transformation needs the coexistence of Fe(II) and SO4 2− . This can be explained by a strong adsorption of SO4 2− promoting the reductive dissolution of ␥-FeOOH particles by Fe(II). All the metal ions added in FeSO4 solutions before the reaction markedly interfered with the transformation in the order of Ti(IV), Cu(II) > Cr(III) > Ni(II), though the difference was not pronounced. The incorporation of the metal ions into the formed ␣-FeOOH particles was in the order of Ti(IV) > Cr(III) > Cu(II) > Ni(II) well according to the order of their hydrolysis constants. Ti(IV) and Cr(III) with a high hydrolysis constant lowered the solution pH after the reaction from 2.7 to 1.7, that is unfavorable to ␣-FeOOH formation. Taking every factor into consideration, the marked inhibitory effect of the metal ions is ascribed mainly to interruption of the reductive dissolution of ␥-FeOOH particles. © 2005 Elsevier B.V. All rights reserved. Keywords: ␥-FeOOH; ␣-FeOOH; Phase transformation; Steel rust; Steel corrosion
1. Introduction Ferric oxyhydroxides of ␣- and ␥-FeOOH are contained in steel rusts formed by atmospheric corrosion. Further, ␥FeOOH are frequently found in soils particularly in hydromorphic soils containing Fe(II) ions, and the transformation of ␥-FeOOH to ␣-FeOOH concerns with soil processes such as an Fe oxide concentration around rice roots or a rhizoconcretion. The transformation can be thermodynamically interpreted form the formation free energy of ␣-FeOOH (496 kJ/mol) [1] and ␥-FeOOH (471 kJ/mol) [2]. However, to our best knowledge, the transformation mechanism is not sufficiently explicated despite several investigations in corrosion and soil sciences. Schwertmann and Tayer investigated the transformation in KOH solutions and the influence ∗
Corresponding author. Tel.: +81 729 78 3394; fax: +81 729 3394. E-mail address: [email protected] (T. Ishikawa).
0927-7757/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2005.06.024
of silicate ions on the transformation, and they concluded that the transformation occurs via dissolution of ␥-FeOOH and precipitation of ␣-FeOOH and that silicate ions blocks dissolution of ␥-FeOOH and nucleation and/or growth of ␣FeOOH [3,4]. Oosterhout found that the transformation in solutions of KOH and FeSO4 takes place only in the presence of ␣-FeOOH nuclei at higher than 70 ◦ C and proposed the dissolution–recrystallization mechanism in which Fe(II) accelerates dissolution of ␥-FeOOH particles [5]. On the other hand, Hiller reported that the transformation mechanism is dissolution–recrystallization in alkali media but a topotactic process in FeSO4 solutions [6]. Thus, there is some inconsistency of the proposed transformation mechanisms and the detailed mechanism still remains unclear especially in acidic solutions. The transformation of ␥-FeOOH to ␣-FeOOH is an important corrosion process of steels, because the initial corrosion product of ␥-FeOOH gradually transforms to more stable
156
T. Ishikawa et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 266 (2005) 155–159
␣-FeOOH. The corrosion resistance of weathering steels used without painting or plating is reinforced by alloying with a small quantity of metal elements such as Cu, Ni, Cr, and so on. However, the role of these alloying metals is still unsolved at present. We have been systematically studying the enhancement mechanism of corrosion resistance of weathering steels by using artificially synthesized rusts of steels. The effects of various metal ions on the formation and structure of ␣, -, and ␥-FeOOH, Fe3 O4 , and poorly crystallized iron oxides were investigated [7–11]. Furthermore, the conversion of ␣-, -, and ␥-FeOOH to Fe3 O4 was examined in FeCl2 solutions and it was found out that the conversion of ␣-FeOOH is extremely low compared to - and ␥-FeOOH as presumed from the solubility of FeOOHs [12]. More recently, we investigated the transformation of ␥-FeOOH to ␣-FeOOH in FeSO4 solutions containing various metal ions at 50 ◦ C [13]. However, although it was revealed that the coexisting metal ions interfere with the conversion in different manner, the transformation mechanisms and the inhibitory effect of metal ions were not fully clarified. In the present study, the transformation of ␥-FeOOH to ␣-FeOOH was further investigated by changing the reaction conditions such as temperature, period, and metal salts. The transformation mechanism was discussed based on the acquired information. This communication must be helpful in investigating the corrosion process of steels and the role of alloying elements in corrosion resistance of weathering steels.
2. Experimental methods 2.1. Materials ␥-FeOOH particles were prepared by oxidation of FeCl2 solution [14]. The 1 dm3 of 0.1 mol/dm3 FeCl2 solution prepared using O2 -freed water was oxidized by bubbling CO2 freed air at a flow rate of 4 dm3 /min at 35 ◦ C for 150 min. Through out the oxidation the solution pH was adjusted to 4.5 by injecting 2 mol/dm3 butylamine with a pH regulator. The formed ␥-FeOOH particles were filtered off, washed, and dried in an air oven at 70 ◦ C.
repeating centrifugation, and dried under the same condition as the preparation of the original ␥-FeOOH particles. 2.3. Characterization The original ␥-FeOOH particles and the products of the transformation reaction were assayed by XRD using a Rigaku diffractometer with a Cu K␣ radiation at 50 kV and 120 mA. The transformation of ␥-FeOOH to ␣-FeOOH was traced by using a Nicolet FT-IR spectrometer in a KBr method at a sample concentration of 1/100 mg KBr. The particle morphology of the products was observed by using a JEOL transmission electron microscope. The metal contents were determined by a Seiko ICP-AES spectrometer. The samples were dissolved by a concentrated HCl solution and diluted to a desired concentration by water.
3. Results and discussion 3.1. Transformation in FeSO4 , FeCl2 , and Na2 SO4 solutions Fig. 1 shows IR spectra of the products of the reaction in 0.3 mol/dm3 FeSO4 solutions at 30 ◦ C for different periods. The spectrum of the original sample shows three strong bands at 1162, 1020, 746 cm−1 due to ␥-FeOOH [15] and two very weak bands at 890 and 793 cm−1 due to ␣-FeOOH [16]. This indicates that the original product contains a small quantity of ␣-FeOOH while no ␣-FeOOH peak appeared in the XRD pattern owing to a low sensitivity of XRD compared to FTIR. The increase of reaction period intensifies the ␣-FeOOH band and weakens the ␥-FeOOH one, evidently showing that ␥-FeOOH transforms to ␣-FeOOH. The extent of transformation is expressed as Xα = lα/(lα + lγ) where lα is the area intensity of 890 cm−1 band of ␣-FeOOH and lγ is that of 1020 cm−1 band of ␥-FeOOH. Fig. 2 shows Xα values of the reaction products in 0.3 mol/dm3 FeSO4 , FeCl2 and Na2 SO4 solutions at 30 and 50 ◦ C as a function of the reaction time.
2.2. Transformation reaction The 500 mg ␥-FeOOH particles were dispersed in 25 cm3 of 0.3 mol/dm3 FeSO4 , FeCl2 , and Na2 SO4 solutions prepared using O2 -freed water and the formed suspensions were kept in a capped 50 cm3 vial at 30 and 50 ◦ C for different periods up to 120 h. To examine the influence of coexisting metal ions, the transformation reaction of ␥-FeOOH was carried out at 30 ◦ C in 0.3 mol/dm3 FeSO4 solutions dissolving Ti(SO4 )4 , Cr(NO3 )3 , CuSO4 , and NiSO4 at varied metal/Fe atomic ratios of 0–0.1. Immediately after the reaction, the particles were separated from the solutions, washed with 2 dm3 of de-ionized, distilled, and O2 -freed water by
Fig. 1. IR spectra of the products of reactions in 0.3 mol/dm3 FeSO4 solutions at 30 ◦ C for different periods.
T. Ishikawa et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 266 (2005) 155–159
Fig. 2. Plots of Xα of the products of reactions in 0.3 mol/dm3 solutions of FeSO4 , FeCl2 , and Na2 SO4 at 30 and 50 ◦ C vs. the reaction time. () FeSO4 -30 ◦ C; (䊉) FeSO4 -50 ◦ C; () FeCl2 -30 ◦ C; () FeCl2 -50 ◦ C; () Na2 SO4 -30 ◦ C.
The Xα values of the products in FeSO4 solutions shown by the circle marks increase with increasing the reaction period and the reaction rate steeply increases with raising the reaction temperature from 30 to 50 ◦ C. The activation energy of the transformation estimated by the Arrhenius equation from the temperature dependence of rate constants obtained by the half-life time method was 110 kJ/mol, which is smaller than the activation energy of phase transformation in solid at a low temperature bellow 50 ◦ C. This result does not support the phase transformation in particles such as the topotactic mechanism proposed by Hiller [6]. On the other hand,
157
the Xα values of the products in 0.3 mol/dm3 FeCl2 solutions at 30 and 50 ◦ C are constant over the whole reaction period, definitely revealing that no transformation occurs in FeCl2 solution quite distinct from FeSO4 solution. This fact contradicts the Oosterhaut’s argument that the transformation is caused by reductive dissolution of ␥-FeOOH particles due to electron transfer from Fe(II) in solution to Fe(III) in ␥-FeOOH particle. Therefore, the transformation cannot be explained only by the reductive dissolution and SO4 2− may function as a key role in the transformation. To confirm this, the reaction was examined in a 0.3 mol/dm3 Na2 SO4 solution at 30 ◦ C. As a result, no transformation took place similarly to FeCl2 solutions, which demonstrates that SO4 2− does not contribute to the transformation independently of Fe(II). Consequently, the coexistence of Fe(II) and SO4 2− is necessary for the transformation. To verify the dissolution–recrystallization mechanism, the change of particle morphology by the transformation was observed by TEM. Fig. 3 displays the TEM images of the original ␥-FeOOH particles and the particles formed in FeSO4 and FeCl2 solutions for 120 h. The product in FeSO4 solution at 50 ◦ C, which shows Xα = 1 and completely converts to ␣-FeOOH, is needle-like particles, and the product at 30 ◦ C with Xα = 0.629 is a mixture of platelike ␥-FeOOH particles and needle-like ␣-FeOOH particles. The particle morphology changes from plate to needle as ␥FeOOH converts to ␣-FeOOH, which is a firm evidence for the dissolution–recrystallization mechanism. The particles formed in FeCl2 solution at 50 ◦ C, which did not transform to ␣-FeOOH, are plate-shaped and slightly larger than the original particles. It should be noted that the fine particles observed
Fig. 3. TEM pictures of the products of reactions in 0.3 mol/dm3 FeSO4 and FeCl2 at 30 and 50 ◦ C for 120 h.
158
T. Ishikawa et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 266 (2005) 155–159
in the original material diminish after the reactions in FeSO4 and FeCl2 solutions, indicating that the small particles were more easily dissolved and consumed for the formation of ␣-FeOOH in FeSO4 solution and the particle growth of ␥FeOOH in FeCl2 solution as confirmed by XRD. 3.2. Effect of metal ion on transformation The effect of metal ions on the transformation of ␥-FeOOH to ␣-FeOOH must be investigated to explore the mechanism of corrosion resistance of weathering steels as mentioned in the introduction. We previously investigated the influence of metal ions on the transformation at 50 ◦ C [13]. This reaction temperature was not suitable for investigating the corrosion of steels and it should be near a normal temperature, so that the transformation was examined at 30 ◦ C in the present study. Xα values of the reactions in 0.3 mol/dm3 FeSO4 solutions dissolving Ti(IV), Cr(III), Ni(II), and Cu(II) at different metal/Fe atomic ratios were determined. Fig. 4 plots the Xα–Xαo values against the metal/Fe ratios of starting solutions, where Xαo is 0.043 corresponding to the ␣-FeOOH content in the original sample. A small quantity of coexisting metal ions drastically reduce Xα, effectively blocking the transformation. This inhibitory effect at 30 ◦ C is remarkable compared to that at 50 ◦ C previously reported [13]. Since the difference of influences between the metal ions are not much, the magnified figure is attached in Fig. 4. The depression of Xα is in the order of Ti(IV), Cu(II) > Cr(III) > Ni(II) which well accords to the order of hydrolysis constants of Ti(IV) (−1.8) > Cr(III) (−3.7) > Ni(II) (−10.5) expect for Cu(II) (−8.0) [17]. As described later the reaction pH is lower than 3 where the hydrolysis of these metal ions except for Ti(IV) is difficult. Therefore, the influence of hydrolysis on the transformation is not prominent. It has been reported that the crystallization and particle growth of ␣-FeOOH is influenced by the coexisting metal ions [18]. So, the incorporation of the metal ions into the
Fig. 4. Plots of Xα–Xαo vs. metal/Fe ratios in the starting solutions. The added metal ions were Ti(IV)(), Cr(III)(), Cu(II)(), and Ni(II)(䊉).
Fig. 5. Plots of metal/Fe ratios of the formed particles vs. those of the starting solutions. The added metal ions were Ti(IV)(), Cr(III)(), Cu(II)(), and Ni(II)(䊉).
formed particles was examined by ICP-AES. The metal contents in the products are shown as a function of metal/Fe ratios in the starting solutions in Fig. 5. The metal contents in the particles are much less than those in the starting solutions, that is, the incorporation of the metal ions into the particles is extremely low and most of the metal ions added in the starting solutions remain in the solutions. The incorporated metal ions is contained in the formed ␣-FeOOH particles, because the products were thoroughly washed by water to remove the adsorbed metal ions as described in the experimental section. As the matter of course, unreactive ␥-FeOOH particles do not contain the added metal ions. Interestingly, the incorporation of metal ions into ␣-FeOOH particles is clearly in the order of Ti(IV) > Cr(III) > Cu(II) > Ni(II) that well corresponds to the order of hydrolysis constant despite a low solution pH less than 3. Inouye explained the inhibitory effect of Cu(II) on crystallization of ␣-FeOOH by the Jahn–Teller effect of Cu(II) [19]. The strong inhibitory effect of Cu(II) on the transformation would be due to this function of Cu(II). The incorporation of the metal ions other than Cu(II) may impede the nucleation and crystallization of ␣-FeOOH. However, the TEM pictures showed that the particles formed with the metal ions contained few irregular or fine particles besides plate-like ␥-FeOOH particle and needle-like ␣-FeOOH ones, suggesting that the disturbance of nucleation and crystallization by the incorporated metal ions is not a main factor of the inhibitory effect. The hydrolysis of metal ions reduces the solution pH by release of protons. Fig. 6 plots the solution pH after 120 h of the reaction against metal/Fe ratio of the starting solution. The solution pH is essentially unchanged by the addition of Ni(II) and Cu(II) but decreases on adding Cr(III) and Ti(IV). The decrease in solution pH is unfavorable for the formation of ␣-FeOOH and most noticeable in the addition of Ti(IV). Actually, the addition of Ti(IV) completely blocked the transformation as is seen in Fig. 4. On the other hand, the addition of Ni(II) and Cu(II) inhibiting the transformation did not
T. Ishikawa et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 266 (2005) 155–159
159
4. Conclusion This study revealed that the transformation of ␥-FeOOH to ␣-FeOOH in acidic solutions needs the coexistence of Fe(II) and SO4 2− . The addition of a small amount of metal ions effectively interferes with the transformation. The marked inhibitory effect of the metal ions is ascribed to the following three processes; interruption of electron transfer by adsorption of the metal ions, drop of solution pH by hydrolysis of the metal ions, and obstruction of nucleation and crystallization of ␣-FeOOH particles by incorporation of the metal ions. Out of these processes, the interruption of electron transfer most dominantly contributes to the inhibitory effect of the metal ions on the transformation of ␥-FeOOH to ␣-FeOOH. Fig. 6. Plots of the solution pH vs. metal/Fe ratios in the starting solutions. The added metal ions were Ti(IV)(), Cr(III)(), Cu(II)(), and Ni(II)(䊉).
lower the solution pH. Taking account of these results, it seem most likely that the pH drop does not mainly contribute to interruption of the transformation particularly in the cases of Ni(II) and Cu(II). 3.3. Mechanism of transformation
Acknowledgement The authors are grateful to Mr. M. Fukusumi of Osaka Municipal Technical Research Institute for help with the TEM observations. This study was supported in part by the Grant-in-Aid for Science Research Fund (B) from the Ministry of Education, Science, Sports and Culture, Japanese Government.
As aforementioned, the transformation of ␥-FeOOH to ␣-FeOOH requires the coexistence of Fe(II) and SO4 2− . Further, it has been stated that Fe(II) causes the reductive dissolution of ␥-FeOOH particles by the electron transfer from Fe(II) in solution to Fe(III) of ␥-FeOOH particles which brings about distortion of ␥-FeOOH crystals due to the difference between ionic radii of Fe(II) (0.064 nm) and Fe(III) (0.078 nm) [20]. However, the surface of ␥-FeOOH particle is positively charged at low pH less than 3 from the isoelectric point of pH 8.1 to repel Fe(II) [21]. So, we must consider other cause such as adsorption of SO4 2− on the positive surface. It is well known that the surface complexation of SO4 2− on iron oxide particles takes place as follows [22]:
[10]
2 FeOH2 + + SO4 2− =
[11]
FeOH2 + + SO4 2− = FeOH + SO4 2− =
Fe2 SO4 + 2H2 O (bridging) FeSO4 − + H2 O
FeOHSO4 2−
This surface reaction alters the surface charge of ␥-FeOOH particles from positive to negative to promote the adsorption of Fe(II). However, as no transformation was observed in FeCl2 solution, Cl− does not relate to the transformation. This difference between SO4 2− and Cl− is convinced from the lower stability constant of Fe(III)–Cl− complex (0.76) than that of Fe(III)–SO4 2− complex (3.0) [23]. The inhibitory effect of the added metal ions is due to the interruption of reductive dissolution of ␥-FeOOH particles caused by a competitive adsorption of Fe(II) and the added metal ions on the ␥-FeOOH surface which would interfere with the electron transfer from Fe(II) to Fe(III).
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
[12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
K. Hashimoto, T. Misawa, Corrosion Sci. 13 (1973) 229. T. Misawa, Corrosion Sci. 13 (1973) 659. U. Schwertmann, R.M. Taylor, Clays Clay Min. 20 (1972) 151. U. Schwertmann, R.M. Taylor, Clays Clay Min. 20 (1972) 159. G.W. van Oosterhout, J. Inorg. Nucl. Chem. 29 (1967) 1235. J.E. Hiller, Werkstoffe Korrosion 17 (1966) 934. T. Ishikawa, H. Yamashita, A. Yasukawa, K. Kandori, T. Nakayama, F. Yuse, J. Mater. Chem. 10 (2000) 543. T. Ishikawa, T. Motoki, R. Katoh, A. Yasukawa, K. Kandori, T. Nakayama, F. Yuse, J. Colloid Interface Sci. 250 (2002) 74. T. Ishikawa, M. Kumagai, A. Yasukawa, K. Kandori, T. Nakayama, F. Yuse, Corrosion Sci. 44 (2002) 1073. T. Ishikawa, H. Nakazaki, A. Yasukawa, K. Kandori, M. Seto, Corrosion Sci. 41 (1999) 1665. T. Ishikawa, T. Ueno, A. Yasukawa, K. Kandori, T. Nakayama, T. Tsubota, J. Mater. Chem. 12 (2002) 2416. T. Ishikawa, Y. Kondo, A. Yasukawa, K. Kandori, Corrosion Sci. 40 (1998) 1239. T. Ishikawa, M. Minamigawa, K. Kandori, T. Nakayama, T. Tsubota, J. Electrochem. Soc. 151 (2004) 512. T. Ishikawa, H. Nishimori, A. Yasukawa, K. Kandori, J. Mater. Sci. Lett. 12 (1993) 1359. D.G. Lewis, V.C. Farmer, Clay Min. 21 (1986) 93. P. Cambier, Clay Min. 21 (1986) 191. C.F. Baes, R. Mesmer, The Hydrolysis of Cations, Wiley, New York, 1976. T. Ishikawa, T. Nakayama, Corrosion Eng. 52 (2003) 177. K. Inouye, J. Colloid Interface Sci. 27 (1968) 171. E.B. Borghi, A.E. Regazzoni, A.J.G. Maroto, M.A. Blesa, J. Colloid Interface Sci. 130 (1989) 299. Y. Zhang, L. Charlt, P.W. Schindler, Colloids Surf. 63 (1992) 259. D.A. Dzombak, F.M.M. Morel, Surface Complexation Modeling, Wiley, New York, 1990. L.G. Sillen, A.E. Martell, Stability Constants of Metal–Ion Complexes, The Chemical Society, London, 1964.
Transformation of ␥-FeOOH to ␣-FeOOH in acidic solutions containing metal ions T. Ishikawa a,∗ , K. Takeuchi a , K. Kandori a , T. Nakayama b b
a School of Chemistry, Osaka University of Education, 4-698-1 Asahigaoka, Kashiwara, Osaka 582-8582, Japan Materials Research Laboratory, Kobe Steel, LTD., 5-5 Takatsukadai 1-chome, Nishi-ku, Kobe, Hyogo 615-2271, Japan
Received 21 February 2005; received in revised form 4 June 2005; accepted 10 June 2005
Abstract The transformation reaction of ␥-FeOOH to ␣-FeOOH in FeSO4 solutions dissolving Ti(IV), Cr(III), Cu(II) and Ni(II) at pH < 3 was examined by FT-IR. The TEM observation demonstrated that the transformation proceeds by dissolution of ␥-FeOOH particles and recrystallization of ␥-FeOOH particles. The transformation occurred in FeSO4 solutions but not in both FeCl2 and Na2 SO4 solutions, indicating that the transformation needs the coexistence of Fe(II) and SO4 2− . This can be explained by a strong adsorption of SO4 2− promoting the reductive dissolution of ␥-FeOOH particles by Fe(II). All the metal ions added in FeSO4 solutions before the reaction markedly interfered with the transformation in the order of Ti(IV), Cu(II) > Cr(III) > Ni(II), though the difference was not pronounced. The incorporation of the metal ions into the formed ␣-FeOOH particles was in the order of Ti(IV) > Cr(III) > Cu(II) > Ni(II) well according to the order of their hydrolysis constants. Ti(IV) and Cr(III) with a high hydrolysis constant lowered the solution pH after the reaction from 2.7 to 1.7, that is unfavorable to ␣-FeOOH formation. Taking every factor into consideration, the marked inhibitory effect of the metal ions is ascribed mainly to interruption of the reductive dissolution of ␥-FeOOH particles. © 2005 Elsevier B.V. All rights reserved. Keywords: ␥-FeOOH; ␣-FeOOH; Phase transformation; Steel rust; Steel corrosion
1. Introduction Ferric oxyhydroxides of ␣- and ␥-FeOOH are contained in steel rusts formed by atmospheric corrosion. Further, ␥FeOOH are frequently found in soils particularly in hydromorphic soils containing Fe(II) ions, and the transformation of ␥-FeOOH to ␣-FeOOH concerns with soil processes such as an Fe oxide concentration around rice roots or a rhizoconcretion. The transformation can be thermodynamically interpreted form the formation free energy of ␣-FeOOH (496 kJ/mol) [1] and ␥-FeOOH (471 kJ/mol) [2]. However, to our best knowledge, the transformation mechanism is not sufficiently explicated despite several investigations in corrosion and soil sciences. Schwertmann and Tayer investigated the transformation in KOH solutions and the influence ∗
Corresponding author. Tel.: +81 729 78 3394; fax: +81 729 3394. E-mail address: [email protected] (T. Ishikawa).
0927-7757/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2005.06.024
of silicate ions on the transformation, and they concluded that the transformation occurs via dissolution of ␥-FeOOH and precipitation of ␣-FeOOH and that silicate ions blocks dissolution of ␥-FeOOH and nucleation and/or growth of ␣FeOOH [3,4]. Oosterhout found that the transformation in solutions of KOH and FeSO4 takes place only in the presence of ␣-FeOOH nuclei at higher than 70 ◦ C and proposed the dissolution–recrystallization mechanism in which Fe(II) accelerates dissolution of ␥-FeOOH particles [5]. On the other hand, Hiller reported that the transformation mechanism is dissolution–recrystallization in alkali media but a topotactic process in FeSO4 solutions [6]. Thus, there is some inconsistency of the proposed transformation mechanisms and the detailed mechanism still remains unclear especially in acidic solutions. The transformation of ␥-FeOOH to ␣-FeOOH is an important corrosion process of steels, because the initial corrosion product of ␥-FeOOH gradually transforms to more stable
156
T. Ishikawa et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 266 (2005) 155–159
␣-FeOOH. The corrosion resistance of weathering steels used without painting or plating is reinforced by alloying with a small quantity of metal elements such as Cu, Ni, Cr, and so on. However, the role of these alloying metals is still unsolved at present. We have been systematically studying the enhancement mechanism of corrosion resistance of weathering steels by using artificially synthesized rusts of steels. The effects of various metal ions on the formation and structure of ␣, -, and ␥-FeOOH, Fe3 O4 , and poorly crystallized iron oxides were investigated [7–11]. Furthermore, the conversion of ␣-, -, and ␥-FeOOH to Fe3 O4 was examined in FeCl2 solutions and it was found out that the conversion of ␣-FeOOH is extremely low compared to - and ␥-FeOOH as presumed from the solubility of FeOOHs [12]. More recently, we investigated the transformation of ␥-FeOOH to ␣-FeOOH in FeSO4 solutions containing various metal ions at 50 ◦ C [13]. However, although it was revealed that the coexisting metal ions interfere with the conversion in different manner, the transformation mechanisms and the inhibitory effect of metal ions were not fully clarified. In the present study, the transformation of ␥-FeOOH to ␣-FeOOH was further investigated by changing the reaction conditions such as temperature, period, and metal salts. The transformation mechanism was discussed based on the acquired information. This communication must be helpful in investigating the corrosion process of steels and the role of alloying elements in corrosion resistance of weathering steels.
2. Experimental methods 2.1. Materials ␥-FeOOH particles were prepared by oxidation of FeCl2 solution [14]. The 1 dm3 of 0.1 mol/dm3 FeCl2 solution prepared using O2 -freed water was oxidized by bubbling CO2 freed air at a flow rate of 4 dm3 /min at 35 ◦ C for 150 min. Through out the oxidation the solution pH was adjusted to 4.5 by injecting 2 mol/dm3 butylamine with a pH regulator. The formed ␥-FeOOH particles were filtered off, washed, and dried in an air oven at 70 ◦ C.
repeating centrifugation, and dried under the same condition as the preparation of the original ␥-FeOOH particles. 2.3. Characterization The original ␥-FeOOH particles and the products of the transformation reaction were assayed by XRD using a Rigaku diffractometer with a Cu K␣ radiation at 50 kV and 120 mA. The transformation of ␥-FeOOH to ␣-FeOOH was traced by using a Nicolet FT-IR spectrometer in a KBr method at a sample concentration of 1/100 mg KBr. The particle morphology of the products was observed by using a JEOL transmission electron microscope. The metal contents were determined by a Seiko ICP-AES spectrometer. The samples were dissolved by a concentrated HCl solution and diluted to a desired concentration by water.
3. Results and discussion 3.1. Transformation in FeSO4 , FeCl2 , and Na2 SO4 solutions Fig. 1 shows IR spectra of the products of the reaction in 0.3 mol/dm3 FeSO4 solutions at 30 ◦ C for different periods. The spectrum of the original sample shows three strong bands at 1162, 1020, 746 cm−1 due to ␥-FeOOH [15] and two very weak bands at 890 and 793 cm−1 due to ␣-FeOOH [16]. This indicates that the original product contains a small quantity of ␣-FeOOH while no ␣-FeOOH peak appeared in the XRD pattern owing to a low sensitivity of XRD compared to FTIR. The increase of reaction period intensifies the ␣-FeOOH band and weakens the ␥-FeOOH one, evidently showing that ␥-FeOOH transforms to ␣-FeOOH. The extent of transformation is expressed as Xα = lα/(lα + lγ) where lα is the area intensity of 890 cm−1 band of ␣-FeOOH and lγ is that of 1020 cm−1 band of ␥-FeOOH. Fig. 2 shows Xα values of the reaction products in 0.3 mol/dm3 FeSO4 , FeCl2 and Na2 SO4 solutions at 30 and 50 ◦ C as a function of the reaction time.
2.2. Transformation reaction The 500 mg ␥-FeOOH particles were dispersed in 25 cm3 of 0.3 mol/dm3 FeSO4 , FeCl2 , and Na2 SO4 solutions prepared using O2 -freed water and the formed suspensions were kept in a capped 50 cm3 vial at 30 and 50 ◦ C for different periods up to 120 h. To examine the influence of coexisting metal ions, the transformation reaction of ␥-FeOOH was carried out at 30 ◦ C in 0.3 mol/dm3 FeSO4 solutions dissolving Ti(SO4 )4 , Cr(NO3 )3 , CuSO4 , and NiSO4 at varied metal/Fe atomic ratios of 0–0.1. Immediately after the reaction, the particles were separated from the solutions, washed with 2 dm3 of de-ionized, distilled, and O2 -freed water by
Fig. 1. IR spectra of the products of reactions in 0.3 mol/dm3 FeSO4 solutions at 30 ◦ C for different periods.
T. Ishikawa et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 266 (2005) 155–159
Fig. 2. Plots of Xα of the products of reactions in 0.3 mol/dm3 solutions of FeSO4 , FeCl2 , and Na2 SO4 at 30 and 50 ◦ C vs. the reaction time. () FeSO4 -30 ◦ C; (䊉) FeSO4 -50 ◦ C; () FeCl2 -30 ◦ C; () FeCl2 -50 ◦ C; () Na2 SO4 -30 ◦ C.
The Xα values of the products in FeSO4 solutions shown by the circle marks increase with increasing the reaction period and the reaction rate steeply increases with raising the reaction temperature from 30 to 50 ◦ C. The activation energy of the transformation estimated by the Arrhenius equation from the temperature dependence of rate constants obtained by the half-life time method was 110 kJ/mol, which is smaller than the activation energy of phase transformation in solid at a low temperature bellow 50 ◦ C. This result does not support the phase transformation in particles such as the topotactic mechanism proposed by Hiller [6]. On the other hand,
157
the Xα values of the products in 0.3 mol/dm3 FeCl2 solutions at 30 and 50 ◦ C are constant over the whole reaction period, definitely revealing that no transformation occurs in FeCl2 solution quite distinct from FeSO4 solution. This fact contradicts the Oosterhaut’s argument that the transformation is caused by reductive dissolution of ␥-FeOOH particles due to electron transfer from Fe(II) in solution to Fe(III) in ␥-FeOOH particle. Therefore, the transformation cannot be explained only by the reductive dissolution and SO4 2− may function as a key role in the transformation. To confirm this, the reaction was examined in a 0.3 mol/dm3 Na2 SO4 solution at 30 ◦ C. As a result, no transformation took place similarly to FeCl2 solutions, which demonstrates that SO4 2− does not contribute to the transformation independently of Fe(II). Consequently, the coexistence of Fe(II) and SO4 2− is necessary for the transformation. To verify the dissolution–recrystallization mechanism, the change of particle morphology by the transformation was observed by TEM. Fig. 3 displays the TEM images of the original ␥-FeOOH particles and the particles formed in FeSO4 and FeCl2 solutions for 120 h. The product in FeSO4 solution at 50 ◦ C, which shows Xα = 1 and completely converts to ␣-FeOOH, is needle-like particles, and the product at 30 ◦ C with Xα = 0.629 is a mixture of platelike ␥-FeOOH particles and needle-like ␣-FeOOH particles. The particle morphology changes from plate to needle as ␥FeOOH converts to ␣-FeOOH, which is a firm evidence for the dissolution–recrystallization mechanism. The particles formed in FeCl2 solution at 50 ◦ C, which did not transform to ␣-FeOOH, are plate-shaped and slightly larger than the original particles. It should be noted that the fine particles observed
Fig. 3. TEM pictures of the products of reactions in 0.3 mol/dm3 FeSO4 and FeCl2 at 30 and 50 ◦ C for 120 h.
158
T. Ishikawa et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 266 (2005) 155–159
in the original material diminish after the reactions in FeSO4 and FeCl2 solutions, indicating that the small particles were more easily dissolved and consumed for the formation of ␣-FeOOH in FeSO4 solution and the particle growth of ␥FeOOH in FeCl2 solution as confirmed by XRD. 3.2. Effect of metal ion on transformation The effect of metal ions on the transformation of ␥-FeOOH to ␣-FeOOH must be investigated to explore the mechanism of corrosion resistance of weathering steels as mentioned in the introduction. We previously investigated the influence of metal ions on the transformation at 50 ◦ C [13]. This reaction temperature was not suitable for investigating the corrosion of steels and it should be near a normal temperature, so that the transformation was examined at 30 ◦ C in the present study. Xα values of the reactions in 0.3 mol/dm3 FeSO4 solutions dissolving Ti(IV), Cr(III), Ni(II), and Cu(II) at different metal/Fe atomic ratios were determined. Fig. 4 plots the Xα–Xαo values against the metal/Fe ratios of starting solutions, where Xαo is 0.043 corresponding to the ␣-FeOOH content in the original sample. A small quantity of coexisting metal ions drastically reduce Xα, effectively blocking the transformation. This inhibitory effect at 30 ◦ C is remarkable compared to that at 50 ◦ C previously reported [13]. Since the difference of influences between the metal ions are not much, the magnified figure is attached in Fig. 4. The depression of Xα is in the order of Ti(IV), Cu(II) > Cr(III) > Ni(II) which well accords to the order of hydrolysis constants of Ti(IV) (−1.8) > Cr(III) (−3.7) > Ni(II) (−10.5) expect for Cu(II) (−8.0) [17]. As described later the reaction pH is lower than 3 where the hydrolysis of these metal ions except for Ti(IV) is difficult. Therefore, the influence of hydrolysis on the transformation is not prominent. It has been reported that the crystallization and particle growth of ␣-FeOOH is influenced by the coexisting metal ions [18]. So, the incorporation of the metal ions into the
Fig. 4. Plots of Xα–Xαo vs. metal/Fe ratios in the starting solutions. The added metal ions were Ti(IV)(), Cr(III)(), Cu(II)(), and Ni(II)(䊉).
Fig. 5. Plots of metal/Fe ratios of the formed particles vs. those of the starting solutions. The added metal ions were Ti(IV)(), Cr(III)(), Cu(II)(), and Ni(II)(䊉).
formed particles was examined by ICP-AES. The metal contents in the products are shown as a function of metal/Fe ratios in the starting solutions in Fig. 5. The metal contents in the particles are much less than those in the starting solutions, that is, the incorporation of the metal ions into the particles is extremely low and most of the metal ions added in the starting solutions remain in the solutions. The incorporated metal ions is contained in the formed ␣-FeOOH particles, because the products were thoroughly washed by water to remove the adsorbed metal ions as described in the experimental section. As the matter of course, unreactive ␥-FeOOH particles do not contain the added metal ions. Interestingly, the incorporation of metal ions into ␣-FeOOH particles is clearly in the order of Ti(IV) > Cr(III) > Cu(II) > Ni(II) that well corresponds to the order of hydrolysis constant despite a low solution pH less than 3. Inouye explained the inhibitory effect of Cu(II) on crystallization of ␣-FeOOH by the Jahn–Teller effect of Cu(II) [19]. The strong inhibitory effect of Cu(II) on the transformation would be due to this function of Cu(II). The incorporation of the metal ions other than Cu(II) may impede the nucleation and crystallization of ␣-FeOOH. However, the TEM pictures showed that the particles formed with the metal ions contained few irregular or fine particles besides plate-like ␥-FeOOH particle and needle-like ␣-FeOOH ones, suggesting that the disturbance of nucleation and crystallization by the incorporated metal ions is not a main factor of the inhibitory effect. The hydrolysis of metal ions reduces the solution pH by release of protons. Fig. 6 plots the solution pH after 120 h of the reaction against metal/Fe ratio of the starting solution. The solution pH is essentially unchanged by the addition of Ni(II) and Cu(II) but decreases on adding Cr(III) and Ti(IV). The decrease in solution pH is unfavorable for the formation of ␣-FeOOH and most noticeable in the addition of Ti(IV). Actually, the addition of Ti(IV) completely blocked the transformation as is seen in Fig. 4. On the other hand, the addition of Ni(II) and Cu(II) inhibiting the transformation did not
T. Ishikawa et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 266 (2005) 155–159
159
4. Conclusion This study revealed that the transformation of ␥-FeOOH to ␣-FeOOH in acidic solutions needs the coexistence of Fe(II) and SO4 2− . The addition of a small amount of metal ions effectively interferes with the transformation. The marked inhibitory effect of the metal ions is ascribed to the following three processes; interruption of electron transfer by adsorption of the metal ions, drop of solution pH by hydrolysis of the metal ions, and obstruction of nucleation and crystallization of ␣-FeOOH particles by incorporation of the metal ions. Out of these processes, the interruption of electron transfer most dominantly contributes to the inhibitory effect of the metal ions on the transformation of ␥-FeOOH to ␣-FeOOH. Fig. 6. Plots of the solution pH vs. metal/Fe ratios in the starting solutions. The added metal ions were Ti(IV)(), Cr(III)(), Cu(II)(), and Ni(II)(䊉).
lower the solution pH. Taking account of these results, it seem most likely that the pH drop does not mainly contribute to interruption of the transformation particularly in the cases of Ni(II) and Cu(II). 3.3. Mechanism of transformation
Acknowledgement The authors are grateful to Mr. M. Fukusumi of Osaka Municipal Technical Research Institute for help with the TEM observations. This study was supported in part by the Grant-in-Aid for Science Research Fund (B) from the Ministry of Education, Science, Sports and Culture, Japanese Government.
As aforementioned, the transformation of ␥-FeOOH to ␣-FeOOH requires the coexistence of Fe(II) and SO4 2− . Further, it has been stated that Fe(II) causes the reductive dissolution of ␥-FeOOH particles by the electron transfer from Fe(II) in solution to Fe(III) of ␥-FeOOH particles which brings about distortion of ␥-FeOOH crystals due to the difference between ionic radii of Fe(II) (0.064 nm) and Fe(III) (0.078 nm) [20]. However, the surface of ␥-FeOOH particle is positively charged at low pH less than 3 from the isoelectric point of pH 8.1 to repel Fe(II) [21]. So, we must consider other cause such as adsorption of SO4 2− on the positive surface. It is well known that the surface complexation of SO4 2− on iron oxide particles takes place as follows [22]:
[10]
2 FeOH2 + + SO4 2− =
[11]
FeOH2 + + SO4 2− = FeOH + SO4 2− =
Fe2 SO4 + 2H2 O (bridging) FeSO4 − + H2 O
FeOHSO4 2−
This surface reaction alters the surface charge of ␥-FeOOH particles from positive to negative to promote the adsorption of Fe(II). However, as no transformation was observed in FeCl2 solution, Cl− does not relate to the transformation. This difference between SO4 2− and Cl− is convinced from the lower stability constant of Fe(III)–Cl− complex (0.76) than that of Fe(III)–SO4 2− complex (3.0) [23]. The inhibitory effect of the added metal ions is due to the interruption of reductive dissolution of ␥-FeOOH particles caused by a competitive adsorption of Fe(II) and the added metal ions on the ␥-FeOOH surface which would interfere with the electron transfer from Fe(II) to Fe(III).
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
[12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
K. Hashimoto, T. Misawa, Corrosion Sci. 13 (1973) 229. T. Misawa, Corrosion Sci. 13 (1973) 659. U. Schwertmann, R.M. Taylor, Clays Clay Min. 20 (1972) 151. U. Schwertmann, R.M. Taylor, Clays Clay Min. 20 (1972) 159. G.W. van Oosterhout, J. Inorg. Nucl. Chem. 29 (1967) 1235. J.E. Hiller, Werkstoffe Korrosion 17 (1966) 934. T. Ishikawa, H. Yamashita, A. Yasukawa, K. Kandori, T. Nakayama, F. Yuse, J. Mater. Chem. 10 (2000) 543. T. Ishikawa, T. Motoki, R. Katoh, A. Yasukawa, K. Kandori, T. Nakayama, F. Yuse, J. Colloid Interface Sci. 250 (2002) 74. T. Ishikawa, M. Kumagai, A. Yasukawa, K. Kandori, T. Nakayama, F. Yuse, Corrosion Sci. 44 (2002) 1073. T. Ishikawa, H. Nakazaki, A. Yasukawa, K. Kandori, M. Seto, Corrosion Sci. 41 (1999) 1665. T. Ishikawa, T. Ueno, A. Yasukawa, K. Kandori, T. Nakayama, T. Tsubota, J. Mater. Chem. 12 (2002) 2416. T. Ishikawa, Y. Kondo, A. Yasukawa, K. Kandori, Corrosion Sci. 40 (1998) 1239. T. Ishikawa, M. Minamigawa, K. Kandori, T. Nakayama, T. Tsubota, J. Electrochem. Soc. 151 (2004) 512. T. Ishikawa, H. Nishimori, A. Yasukawa, K. Kandori, J. Mater. Sci. Lett. 12 (1993) 1359. D.G. Lewis, V.C. Farmer, Clay Min. 21 (1986) 93. P. Cambier, Clay Min. 21 (1986) 191. C.F. Baes, R. Mesmer, The Hydrolysis of Cations, Wiley, New York, 1976. T. Ishikawa, T. Nakayama, Corrosion Eng. 52 (2003) 177. K. Inouye, J. Colloid Interface Sci. 27 (1968) 171. E.B. Borghi, A.E. Regazzoni, A.J.G. Maroto, M.A. Blesa, J. Colloid Interface Sci. 130 (1989) 299. Y. Zhang, L. Charlt, P.W. Schindler, Colloids Surf. 63 (1992) 259. D.A. Dzombak, F.M.M. Morel, Surface Complexation Modeling, Wiley, New York, 1990. L.G. Sillen, A.E. Martell, Stability Constants of Metal–Ion Complexes, The Chemical Society, London, 1964.