Conference-ALC 07-Influence of Anion Coexistence on Crystal Structure of Iron Oxides Deposited from Steel Surfaces ∗

The EXAFS analysis of ferric oxide particles precipitated from ferric aqueous solutions including chloride ion and with/without additional anions such as sulfate or silicate. In the solution reaction process, an akaganeite (β-FeOOH)-like oxyhydroxide with low crystallinity is precipitated and subsequently transforms to hematite (αFe2O3). Coexistence of anion species of sulfate or silicate is effective to reduce the rate of structural transformation and influence morphology of final iron oxide products. The radial distribution functions was obtained by Fourier transformation from EXAFS spectra measured at Fe K absorption edge, and Debye-Waller factor related to Fe-Fe correlation was determined. The results indicated that the linkages with face-sharing and double-corner-sharing were influenced by adding anions in corrosion reaction solutions. [DOI: 10.1380/ejssnt.2008.60]


I. INTRODUCTION
Various ferric or ferrous hydroxides, oxyhydroxides and oxides are formed from the ferric or ferrous aqueous solutions and their final structure depends on the environmental conditions.Some anions coexisting in the reaction solution can force the oxide or the oxyhydroxide to form a specific structure, without even being present in the final solid.Corrosion reaction process on steel surfaces is a case like this.Ferric or ferrous ions dissolved from the surface of steels are precipitated and formed the final products by oxidation, hydrolysis or thermal transformation via various intermediates.In case of coexistence of chloride ions in the solution containing ferric ions, akaganeite β-FeOOH is known to be formed by hydrolysis of ferric species and the precipitates transformed finally into hematite α-Fe 2 O 3 by thermohydrolysis [1,2].Recently, effects of cations or anions on the formation of akaganeite and its conversion to hematite have been focused and investigated [3][4][5][6].The results indicated that some elements strongly influence the reaction rate, formation and morphology of final products in aqueous solution systems.Therefore, to understand the relationship between conditions of coexistent ions and resultant local structure is important in order to make it possible to control the structure and morphology of iron oxyhydroxides or oxides as corrosion products.
Silicon is one of the most important elements of steel and often used as an alloying element so as to improve their corrosion resistance.The silicon contained in steel is oxidized to silicate ion during atmospheric corrosion.On the other hand, sulfate ions are contaminated from at-mospheric exposure.In this study, powder X-ray diffractometry (XRD) and X-ray absorption spectrometry were used for determining the crystalline structural parameters and the local atomic structure around iron in the final product of iron oxide precipitated from ferric aqueous solutions containing silicate or sulfate ions, and the results were used for discussion of the influences of coexisting anions on the local structure of the iron oxides in atomic revel.

II. EXPERIMENTAL A. Preparation of powder samples
Reagent-grade ferric chloride hexahydrate FeCl 3 •6H 2 O, 10 molL −1 sodium hydroxide NaOH aqueous solution, and sodium orthosilicate n-hydrate Na 4 SiO 4 • nH 2 O (n = 3.4 determined by ICP measurement) or anhydrous sodium sulfate Na 2 SO 4 as sources of silicate or sulfate anion species were used as received.The method to prepare the corrosion product particle samples was fundamentally identical to the previous one [5,6].5.4 molL −1 of sodium hydrate solution was added to the same volume of 2.0 molL −1 ferric chloride solution containing 0.05 molL −1 of silicate ion or 0.30 molL −1 of sulfate ion, and the reaction solution was agitated for more than 10 min.at 0 • C. The highly viscous gel containing the ferric hydroxides immediately precipitated in the solution was aged at 100 • C for different period till 14 days.After a given aging time, the suspension was cooled down to room temperature using running water.The solid particles separated by centrifugation of 18000 rpm for 30 min.and washed with doubly distilled water, and then freeze-dried.The crystalline phase identification and determination of structural parameters were performed by the Rigaku RINT2200 X-ray diffractometer using graphite monochromized Cu Kα radiation operated under 40 kV and 40 mA of respectively tube voltage and current.To determine lattice constant the whole powder pattern decomposition (WPPD) by Pawley method [7] using the analysis program TOPAS version 3.0 (Bulker AXS Co.) was carried out.The analysis of local atomic structure around iron in the powder samples was carried out based on the EXAFS spectra measured at Fe K absorption edge by using the Rigaku R-XAS Looper X-ray absorption spectrometer [8].White X-ray emitted from the source of the demountable X-ray tube with the target of molybdenum and the filament of LaB 6 under 14 kV of tube voltage and 100 mA of tube current is monochromized by the Johansson-type Ge(220) bent single crystal and is irradiated to the sample pelletized with boron nitride powder as binder and dilution agent.Incident beam intensity was monitored by using Ar gas sealed proportional counter and transmitted beam by sample was detected by scintillation counter.In the EXAFS analysis, the parameters of backscattering amplitude and phase shift were calculated by using FEFF 8.20 code [9] and parameter fitting calculation was carried out by using the REX2000 program (Rigaku co.).

A. Powder XRD and TEM observation
The X-ray diffraction intensity profiles of the freezedried powder samples taken after given aging time of 4, 6, 12, 24 hours and 14 days are shown in Fig. 1.The process of crystalline phase transformation for 14 days seems to be similar in all case of anion adding condition, i.e., akaganeite-like fine particles with low crystallinity were precipitated as intermediate at early stage and hematite crystalline particles were formed finally.However, the reaction rate was decelerated in case of the samples reacted in the solutions including silicate or sulfate anion.In comparing the influence of adding silicate and sulfate, the former was more effective in inhibition of crystalline structural transformation even in small amounts than the latter.On the other hand, the values of average size of crystallite calculated by using Scherrer's equation for the samples after aging for 14 days which formed singlephase hematite structure were 114, 149 and 14 nm for the samples with no additional anion, with silicate ion and with sulfate ion, respectively.These results indicate that the growth of hematite crystallites are strongly inhibited by sulfate ion while not by silicate ion.Hematite is isostructural with corundum which hcp arrays of oxygen ions where two thirds of the octahedral sites filled with ferric ions.The lattice constant a, c for the sample obtained from the solution including no additional anion determined by whole powder pattern decomposition (WPPD) using Pawley method were a = 0.50358(4) nm, c = 1.3782(1) nm.On the other hand, these for the sam- ples influenced from coexistent anions silicate and sulfate were a = 0.503243(8) nm, c = 1.38130(4) nm, and a = 0.50304(5) nm, c = 1.3804(2) nm, respectively.This result indicates that the addition of such anion like silicate or sulfate leads to expanding in c axial direction and contracting in a or b direction.Especially, in case of coexistence of sulfate ion 104 reflection peak was intense in relation to 110 reflection.It could be considered due to anisotropic crystal growth by specific adsorption of sulfate ions.
The TEM photographs of the three final products after aging for 14 days are shown in Fig. 2. The morphology of particles of final product seems to be influenced by the addition of anion.Considering this morphological observation with the results of powder XRD, however, it could be said that silicate ion influences not crystal growth but rather aggregation of crystallites and sulfate ion influences both strongly.
Figures 3 and 4 show the k 3 weighted EXAFS spectra and their Fourier transforms of the samples after aging for 6 hours and 14 days in the solution containing no additional anion species extracted from the absorbance spectra measured at Fe K absorption edge.In Fig. 4, the Fourier transformation of EXAFS spectrum gives a radial distribution function (RDF), and the first and the second peak correspond to the Fe-O and the Fe-Fe correlation distances.These data supports the result of powder XRD, i.e. the akaganeite-like intermediate structure in early stage and the hematite structure in final stage.
The RDFs of the samples obtained after aging for 14 days in the solution containing coexisting anion of silicate or sulfate are shown in Fig. 5. Fe-Fe correlation peaks in the RDFs for both of samples are reduced in amplitude compared to bulk hematite reference.Two possibilities for this decrease of amplitude can be considered as very small crystalline size and structural disorder.The former leads to reduction of coordination number and the latter leads to increase of Debye-Waller factor.However, the decrease of amplitude in the RDF reflecting the size effect generally occurs when the crystalline size is under 5 nm.Therefore, in this case the lack of long-range ordering is considered due to Debye-Waller factor effect rather than size effect.The crystal structure of hematite can be consider as combination of the four different ways of linkage between FeO 6 octahedra as structural unit.As shown in Fig. 6, Fe-Fe distances are different depending on the way of linkage between the two octahedra; about 0.290 nm between face-shared (F) A and B octahedra, about 0.297 nm between edge-shared (E) A and C octahedra, about 0.336 nm between double-corner-shared (DC) A and D octahedra, and about 0.371 nm between single-corner-shared (SC) A and E octahedra.The DC linkage corresponds to sharing one corner with each octahedron of two octahedra linked by E linkage.The parameter fitting analysis was carried out to determine the Debye-Waller factor.The parameters of interatomic distance and coordination number were fixed respectively at the value determined by Rietveld analysis using the powder XRD pattern data and the value of ideal hematite crystal structure, since the Debye-Waller factor and the coordination number are strongly correlated.The results are tabulated in Table I.It is notable that the Debye-Waller factors relating the linkages in vertical c-axis direction (F and DC) were increased by coexistence of anion of silicate of sulfate in corrosion reaction solution, though the variation of the factor for the linkage in horizontal direction of E was little.These anisotropic nature is considered due to selective adsorption of coexistent anions and/or perhaps intercalation of them in the stage forming hematite structure.The effects of coexistent anion species in corrosion reaction solutions on transforming and forming of corrosion product of iron oxide particles was investigated by using powder XRD and XAS techniques.The results indicate that adding silicate or sulfate ion into the reaction solution influenced the morphology and the local struc-ture of particles as well as inhibition of reaction rate.It was shown that the combinational use of XRD analyzing based on the information of long-range ordering and XAS determining the atomic level arrangement was effective to investigate the mechanism of forming iron oxides of corrosion product from the aspect of local structure in crystalline lattice.

FIG. 1 :
FIG.1:The powder XRD profiles the samples after aging time from 4 hours to 14 days.

FIG. 3 :FIG. 4 :
FIG. 3: The k 3 weighted Fe K EXAFS spectra of the sample obtained from reaction solution without additional anion after aging time for (a) 6 hours and (b) 14 days along with the spectra for reference samples of akaganeite (drawn by dotted line) and hematite (drawn by broken line).

FIG. 5 :
FIG. 5: The radial distribution functions for the sample obtained after aging for 14 days in the solution containing (a) 0.05 mol/L silicate and (b) 0.30 mol/L sulfate ions.Profiles drawn by gray-colored line correspond to hematite reference.

TABLE I :
Refined Debye-Waller factor σ of the Fe-Fe shells of the samples obtained after aging for 14 days.The values of coordination number N of ideal hematite crystal structure and the values of interatomic distance R determined from Rietveld analysis of powder XRD measured data were used.