Influence of metal ions on the transformation of γ-FeOOH into α-FeOOH

Article abstract of DOI:10.1149/1.1778167

The transformation of γ-FeOOH into α-FeOOH in FeSO4 solutions at 50°C was investigated in two different ways by dissolving Ti(IV), Cr(III), Cu(II), Ni(II), and Mn(II) in the solutions at atomic ratios of metal/Fe of 0-0.1 and adding these metal ions to th

Full text of DOI:10.1149/1.1778167

B512
Journal of The Electrochemical Society, 151 ͑9͒ B512-B518 ͑2004͒
0013-4651/2004/151͑9͒/B512/7/$7.00 © The Electrochemical Society, Inc.
Influence of Metal Ions on the Transformation of ␥-FeOOH
into ␣-FeOOH
Tatsuo Ishikawa,a,z Megumi Minamigawa,a Kazuhiko Kandori,a
Takenori Nakayama,b and Takayuki Tsubotab
aSchool of Chemistry, Osaka University of Education, Kashiwara, Osaka 582-8582, Japan
bKobe Steel, Limited, Materials Research Laboratory, Nishi-ku, Kobe, Hyogo 615-2271, Japan
The transformation of ␥-FeOOH into ␣-FeOOH in FeSO4 solutions at 50°C was investigated in two different ways by dissolving
Ti͑IV͒, Cr͑III͒, Cu͑II͒, Ni͑II͒, and Mn͑II͒ in the solutions at atomic ratios of metal/Fe of 0-0.1 and adding these metal ions to the
␥-FeOOH particles. The transition into ␣-FeOOH was examined by Fourier transform infrared spectroscopy and X-ray diffraction.
Ti͑IV͒, Cr͑III͒, and Cu͑II͒ dissolved in the solutions markedly interfered with the transition into ␣-FeOOH, whereas Ni͑II͒ and
Mn͑II͒ showed no influence. The inhibitory effect of the former three metal ions is ascribed to the protection of ␥-FeOOH particles
against dissolution in FeSO4 solution by surface coating with the hydrolysis species of metal ions, because transmission electron
microscopy and inductively coupled plasma-Auger electron spectroscopy demonstrated that the conversion proceeds by dissolu-
tion and recrystallization. The metal ions added in ␥-FeOOH particles promoted the conversion into ␣-FeOOH, which was
interpreted by the enhancement of dissolution of ␥-FeOOH particles due to the decrease in particle size by the addition of metal
ions.
© 2004 The Electrochemical Society. ͓DOI: 10.1149/1.1778167͔ All rights reserved.
Manuscript submitted July 16, 2003; revised manuscript received February 21, 2004. Available electronically August 11, 2004.
of particle size after the transformation and a promotion of transfor-
mation by adding ␣-FeOOH nuclei. Thus, the transformation pro-
ceeds by dissolution followed by crystallization, and the function of
Fe͑II͒ is to increase the dissolution rate of ␥-FeOOH in an acidic
solution.22 Bechine et al. have also reported that ␥-FeOOH trans-
forms into ␣-FeOOH and ␣-Fe2O3 in FeSO4 solutions at 95-100°C
and explained the obtained results by the dissolution and recrystal-
lization mechanism.23 The electrochemical reductive dissolution of
␥-FeOOH enhances the transformation reaction into ␣-FeOOH.24
However, as far as we know, there is no report on the effects of
coexisting metal ions on the transformation of ␥-FeOOH into
␣-FeOOH, except the report by Oosterhout that CoSO4 showed no
influence on the transformation.22
To explore the anticorrosion function of alloying metals in
weathering steels, the effect of metal ions on the structural transfor-
mation of rust components should be investigated in addition to the
influence of metal ions on the formation and structure of rusts. From
this point of view, the present study focused the transformation of
␥-FeOOH into ␣-FeOOH in the presence of various metal ions.
Steel rusts consist of ␣-, ␤-, and ␥-FeOOH, Fe3O4 , and amor-
phous iron oxides of which the composition depends on exposure
environment.1 Weathering steel intended to be free from mainte-
nance is doped with alloying elements such as Cu, Cr, Ni, and Ti to
enhance the corrosion resistance; however, the function of alloying
metals remains not fully interpreted. Yamashita et al. reported that
Cr͑III͒ is incorporated into ␣-FeOOH to form the stable protective
layer of nanosized Cr-substituted ␣-FeOOH.2 Recently, Kamimura
and Stratmann revealed that Cr inhibits the cathodic reaction in
corrosion.3 We have done a series of systematic investigations on the
influences of various metal ions on the formation and structure of
␣-FeOOH,4-6 ␤-FeOOH,7-10 ␥-FeOOH,11 Fe3O4 ,11-13 and poorly
crystallized iron oxides14,15 by using artificially synthesized rusts in
order to gain comprehensive information on the influence of metal
ions. These studies revealed that the influence of the metal ions on
the formation and structures of rusts depends on the kinds of metal
ions and rust components and suggested that alloying with more
than two kinds of metals is effective for formation of a protective
rust layer.16
The composition of rusts varies with time by structural transfor-
mation of the rust components. The most important transformation
is the conversion of ␥-FeOOH into ␣-FeOOH. ␣-FeOOH is thermo-
dynamically more stable than ␥-FeOOH, as is evidenced by the
standard free energy of formation, which is 496 J/mol for ␣-FeOOH
and 471 J/mol for ␥-FeOOH;17 thus, ␣-FeOOH is a more stable
rust component than ␥-FeOOH. For this reason, the mass ratio of
␣-FeOOH to ␥-FeOOH in rusts ͑␣/␥͒ can be used as a measure
of stability and the protective property of rusts, and the corrosion
rate of steels in atmosphere was found to be extremely low at
Ͼ
␣-FeOOH leads to fast formation of stable rusts showing a high
resistance to corrosion. However, there have been few reports on the
transformation of ␥-FeOOH into ␣-FeOOH despite voluminous
studies on the formation, structures, and properties of these
FeOOHs.19 Krause et al. have found that ␥-FeOOH transforms into
␣-FeOOH on heating at 150°C in 2 mol/dm3 KOH.20 Nitschmann
has reported that ␥-FeOOH transforms easily in an aqueous FeSO4
solution kept at 60°C and concluded that the transformation takes
place in the crystal by a topotactic and pseudomorphic process.21
After about thirty years, Oosterhout has investigated the effect of
␣-FeOOH nuclei on the transformation of ␥-FeOOH in KOH or
FeSO4 solutions at 70-150°C and detected an appreciable decrease
Experimental
The preparation of ␥-FeOOH.—The transition of ␥-FeOOH into
␣-FeOOH in rusts must be affected by the metal ions dissolved from
alloying metals into solutions and incorporated in ␥-FeOOH par-
ticles during their formation. So we prepared two series of
␥-FeOOH samples as follows.
The ␥-FeOOH particles for studying the influence of metal ions
dissolved in solution were synthesized by aerial oxidation of an
FeCl2 solution by bubbling air at a flow rate of 4 dm3/min and 50°C
for 2 h. The FeCl2 solution was prepared by adding 200 cm3 water
and 200 cm3 of a buffer solution ͑pH 7.5͒ to 300 cm3 of a 0.2
mol/dm3 FeCl2 solution. The buffer solution was prepared by adding
a 0.1 mol/dm3 NH4OH solution to a 0.1 mol/dm3 NH4Cl solution up
to pH 7.5. During oxidation the solution pH was adjusted to 4.5 by
dropping a 0.1 mol/dm3 NH4OH solution.
The second series of ␥-FeOOH samples were synthesized by
aerial oxidation of an FeSO4 solution. 1 dm3 solutions dissolving
0.02 mol FeSO4 and different metal salts of Ti͑SO4)2 , Cr͑NO3)3 ,
CuSO4 , and NiSO4 at metal/Fe atomic ratios varied from 0 to 0.1
were oxidized by bubbling air at a flow rate of 8 dm3/min and 35°C
for 3 h, the solution pH being adjusted to 4.5-6.5 by dropping a 2
mol/dm3 butylamine solution throughout the oxidation. The result-
ing precipitates were separated from solution by filtration with a
␣/␥
ca. 2.18 This suggests that promotion of the conversion into
z E-mail: ishikawa@cc.osaka-kyoiku.ac.jp
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Journal of The Electrochemical Society, 151 ͑9͒ B512-B518 ͑2004͒
B513
Figure 2. Changes in X␣ of the products with sulfates of ͑᭺͒ Fe͑II͒, ͑᭝͒
Ni͑II͒, and ͑ᮀ͒ Cu͑II͒ with metal sulfate concentrations.
patterns were traced by a diffractometer ͑Rigaku, Geigerflex 2013͒
with Cu K␣ radiation ͑30 kV, 15 mA͒. The metal content of the
particles was assayed by an induction coupled plasma atomic emis-
sion spectrometer ͑ICP-AES, Seiko, SPS1200VR͒. The samples
were dissolved in a concentrated HCl, and the solutions were diluted
to a desired concentration by water. The particle morphology was
observed by a transmission electron microscope ͑TEM, JEOL,
200B͒. The specific surface area was determined by the Brunauer-
Emmett-Teller ͑BET͒ method using N2 adsorption isotherms mea-
sured by an automatic adsorption apparatus assembled in our labo-
ratory. Before the adsorption, the samples were degassed under 10Ϫ3
Torr at 100°C for 2 h.
Figure 1. IR spectra of the products by the AS method at different concen-
trations of FeSO4 .
0.22 ␮m Nuclepore, washed with distilled-deionized water, and fi-
nally dried at 70°C in air for more than 15 h.
The transition into ␣-FeOOH.—By using the ␥-FeOOH samples
thus prepared the influence of metal ions on the transformation into
␣-FeOOH was examined in different methods of addition of metal
ions in solution and ␥-FeOOH particle. Hereafter, we abbreviate the
addition in solution as the AS method and the addition in ␥-FeOOH
particle as the AP method.
The transition into ␣-FeOOH was examined by the AS method as
follows. 500 mg ␥-FeOOH powders were dispersed in 25 cm3 of 0.3
mol/dm3 FeSO4 solutions dissolving different metal salts of
Ti͑SO4)2 , Cr͑NO3)3 , CuSO4 , NiSO4 , ZnSO4 , and MnSO4 at dif-
ferent metal/Fe ratios from 0 to 0.1 and treated in a tightly closed 30
cm3 vial at 50°C for 120 h. The metal/Fe is the atomic ratio of metal
salt to Fe of FeSO4 . The reason for using Cr͑NO3)3 instead of
Cr2(SO4)3 is a low solubility of the latter. The influence of indi-
vidual metal salt was inspected by single addition of FeSO4 ,
NiSO4 , and CuSO4 at varied concentrations of 0-0.3 mol/dm3 under
the condition mentioned previously.
Results and Discussion
The influence of metal ion dissolved in solution.—Although
Oosterhout has reported that FeSO4 promotes the transition of
␥-FeOOH into ␣-FeOOH but CoSO4 shows no influence,22 there is
no report on the effect of the other metal ions. Accordingly, we
examined the influence of CuSO4 , NiSO4 , and FeSO4 by the AS
method. Figure 1 displays IR spectra of the products at different
concentrations of FeSO4 . The spectrum of the product without
FeSO4 shows the bands characteristic of ␥-FeOOH25 at 1160, 1020,
and 743 cmϪ1 and an additional weak band of ␣-FeOOH26 at 880
cmϪ1, being the same as the spectrum of the original ␥-FeOOH.
Thus, the original ␥-FeOOH contains a small amount of ␣-FeOOH.
The massive synthesis of pure ␥-FeOOH by aerial oxidation is dif-
ficult, because residual Fe͑II͒ facilitates the conversion into
␣-FeOOH as described previously. It is known that PO34Ϫ and eth-
ylenediaminetetraacetic acid ͑EDTA͒ inhibit the conversion into
␣-FeOOH;27,28 however, the samples synthesized with these addi-
tives is unsuitable for the purpose of the present study due to anxiety
that the additives remaining in the particles affect the conversion
into ␣-FeOOH. As seen in Fig. 1, the peaks due to ␣-FeOOH inten-
sify with the increase in FeSO4 concentration whereas those due to
␥-FeOOH weaken, indicating that the transition of ␥-FeOOH into
␣-FeOOH is promoted by the presence of FeSO4 . The transition
into ␣-FeOOH is quantified by a peak intensity ratio of I␣/(I␣
ϩ I␥) ϭ X␣, where I␣ is the area intensity of the 880 cmϪ1 band
In the AP method, 500 mg of ␥-FeOOH particles containing
different amounts of Ti͑IV͒, Cr͑III͒, Cu͑II͒, and Ni͑II͒ were dis-
persed in 25 cm3 of a 0.3 mol/dm3 FeSO4 solution and treated under
the same condition as the AS method. After the treatment in the AS
and AP methods, the samples were washed and dried in the same
manner as the preparation of ␥-FeOOH particles.
The characterization of products.—IR spectra of the products
were taken by a KBr method using a Fourier transform infrared
͑FTIR͒ spectrophotometer ͑Nicolet, Protage 460͒. The sample con-
centration was 1 mg/500 mg KBr. Powder X-ray diffraction ͑XRD͒
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B514
Journal of The Electrochemical Society, 151 ͑9͒ B512-B518 ͑2004͒
Figure 4. XRD patterns of the products by the AS method with CuSO4 at
varied Cu/Fe.
17-536͒ weaken. The pattern of the sample before the reaction
shows no peak of ␣-FeOOH different from the IR spectrum, which
is due to a lower sensitivity of XRD than FTIR.
To compare the influences of the metal ions, X␣ of the reactions
with different metal ions is plotted vs. metal/Fe in Fig. 5. Interest-
ingly, X␣ does not decrease on adding Ni͑II͒ and Mn͑II͒, while the
addition of Zn͑II͒, Cu͑II͒, Cr͑III͒, and Ti͑IV͒ decreases X␣ or im-
pedes the transition into ␣-FeOOH, markedly for the latter three
metal ions. The inhibitory effect seems to be caused by interfering
with the dissolution of ␥-FeOOH and the recrystallization of
␣-FeOOH, which are possible mechanisms of the conversion into
␣-FeOOH. However, no diminution of particle size and drop of
crystallinity was observed after the reactions with Cu͑II͒, Cr͑III͒,
and Ti͑IV͒ as confirmed by TEM and XRD. This strongly suggests
that these metal ions inhibit the dissolution of ␥-FeOOH particles,
Figure 3. IR spectra of the products by the AS method with CuSO4 at varied
Cu/Fe.
of ␣-FeOOH and I␥ is that of the 1020 cmϪ1 band of ␥-FeOOH.
Figure 2 plots X␣ of the reactions in the presence of CuSO4 ,
NiSO4 , and FeSO4 vs. concentration of the metal salts. The X␣
value of the reaction with FeSO4 steeply rises with increasing the
FeSO4 concentration, whereas those of the reactions with CuSO4
and NiSO4 are constant. From this figure it can be clearly conceived
that only FeSO4 promotes the conversion into ␣-FeOOH. This func-
tion of FeSO4 has been explained by Oosterhout, who considered
that the electron transfer from Fe͑II͒ in solution to Fe͑III͒ in
␥-FeOOH particles enhances the dissolution of ␥-FeOOH
particles.22 It goes without saying that the electron transfer from
Cu͑II͒ and Ni͑II͒ to Fe͑III͒ does not take place. Consequently, the
addition of FeSO4 is inevitable for the transition into ␣-FeOOH;
hereafter, all experiments used a 0.3 mol/dm3 FeSO4 solution.
Figure 3 shows IR spectra of the products by the reaction in 0.3
mol/dm3 FeSO4 solutions dissolving CuSO4 at different metal/Fe.
The absorption bands of ␥-FeOOH disappears at metal/Fe ϭ 0, in-
dicating that ␥-FeOOH is completely transformed into ␣-FeOOH
without the additives except FeSO4 . However, at Cu/Fe у 0.025
the ␥-FeOOH bands reappear and intensify with the increase of
Cu/Fe, that is, the transition into ␣-FeOOH is suppressed by adding
Cu͑II͒. Figure 4 shows XRD patterns of the products with Cu͑II͒,
which demonstrate the inhibitory effect of Cu͑II͒ as well as the IR
spectra; with increasing Cu/Fe the diffraction peaks of ␥-FeOOH
͑JCPDS 8-98͒ increase in intensity and those of ␣-FeOOH ͑JCPDS
Figure 5. X␣ changes of the products by the AS method with ͑᭺͒ Ti͑IV͒,
͑᭝͒ Cr͑III͒, ͑ᮀ͒ Ni͑II͒, ͑᭹͒ Cu͑II͒, ͑᭡͒ Mn͑II͒, and ͑᭿͒ Zn͑II͒ metal salts
with metal/Fe of the starting solutions.
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Journal of The Electrochemical Society, 151 ͑9͒ B512-B518 ͑2004͒
B515
Table I. Hydrolysis constants „K… for Men¿ ¿ H2O
Ä MeOH„nÀ1…¿ ¿ H¿ and precipitation pH of metal ions at
25°C.
Metal ion
K
pH
Ͻ1
Ti͑IV͒
Fe͑III͒
Cr͑III͒
Cu͑II͒
Zn͑II͒
Fe͑II͒
Ni͑II͒
Mn͑II͒
Ϫ1.8a
Ϫ2.4
Ϫ3.7
Ϫ8.0
Ϫ9.1
Ϫ9.3
Ϫ10.5
Ϫ10.6
2.0
4.7
6.0
7.0
7.5
8.0
8.0
a Ti͑OH)3ϩ ϩ H2O ϭ Ti͑OH)22ϩ ϩ Hϩ.
despite the coexistence of Fe͑II͒ assisting the dissolution. It was
confirmed that without Fe͑II͒ all the metal ions including Ni͑II͒ and
Mn͑II͒ did not affect the morphology and crystallinity of ␥-FeOOH
particles ͑the data not shown here͒. Therefore, we can infer that
Zn͑II͒, Cu͑II͒, Cr͑III͒, and Ti͑IV͒ interrupt the dissolution of
␥-FeOOH particles by Fe͑II͒ and protect the ␥-FeOOH particles
from dissolution in the FeSO4 solution. Such stabilization of
␥-FeOOH particles may be explained by considering that the
␥-FeOOH particles are coated by the hydrolysis species of the added
metal ions. The formation of the hydrolysis species depends on the
hydrolysis constants ͑K͒ of the metal ions shown in Table I.29,30
Ti͑IV͒, Cr͑III͒, and Cu͑II͒, which inhibit the transition into
␣-FeOOH, show a larger K than Ni͑II͒ and Mn͑II͒ exhibiting no
influence. For more easily understanding this relation, X␣ at
metal/Fe ϭ 0.05 is plotted against K in Fig. 6. It is noteworthy that
Ni͑II͒ and Mn͑II͒ with K less than Ϫ9.3 of Fe͑II͒ show X␣ ϭ 1 and
do not interfere with the transition into ␣-FeOOH, whereas X␣ of
Ti͑IV͒, Cr͑III͒, and Cu͑II͒ with K more than Ϫ9.3 is less than unity
and these metal ions impede the conversion. This fact is a convinc-
ing evidence for the stabilization of ␥-FeOOH particles in FeSO4
solution by the surface coating.
It has been reported that the crystallization of ␣-FeOOH is im-
peded by Ni͑II͒, Cu͑II͒, Cr͑III͒, and Ti͑IV͒ coprecipitating with
Figure 7. Plots of metal/Fe of the particles formed by the AS method with
͑᭺͒ Ti͑IV͒, ͑᭝͒ Cr͑III͒, ͑ᮀ͒ Cu͑II͒, ͑᭹͒ Ni͑II͒, ͑᭡͒ Zn͑II͒, and ͑᭿͒ Mn͑II͒
metal salts vs. metal/Fe of the starting solutions.
Fe͑III͒.6,31 Especially, the hindrance with Cu͑II͒ is most effective,
presumably due to the Jahn Teller effect distorting the octahedron of
Cu͑II͒ coordinated by O2Ϫ and OHϪ.32 However, in the previous
study ␣-FeOOH was synthesized at a high pH, у12, so that the
added metal ions were nearly precipitated with Fe͑III͒. In the
present study the solution pH after the reaction was less than ca. 3.
Table I lists the theoretical precipitation pH of the metal ions at 0.01
mol/dm3 calculated from the solubility products of metal hydroxides
and the ion product of water.29 According to this table only Ti͑IV͒
can precipitate with Fe͑III͒. However, metal ions with different pre-
cipitation pH are frequently coprecipitated by induced precipitation
due to adsorption and occlusion of hydrolysis species of metal ions
showing a higher precipitation pH. To confirm the coprecipitation
with Fe͑III͒, we determined the metal ion content of the formed
particles by ICP-AES. Figure 7 plots metal/Fe of the formed par-
ticles against that of the starting solutions. The metal/Fe of the par-
Ͼ
Ͼ
Ͼ
Zn͑II)
ticles is on the order of Ti͑IV)
Cr͑IV)
Cu͑II)
Ͼ
Ͼ Ni͑II)
Mn͑II), being well equivalent to the order of K and
precipitation pH in Table I. Note that the particles formed with the
metal ions impeding the transition into ␣-FeOOH show a high
metal/Fe ratio, which is evidence of the surface coating. The prod-
ucts with Ti͑IV͒ show an extremely high Ti/Fe ratio due to a partial
Figure 6. Plots of X␣ of the products by the AS method at metal/Fe
ϭ 0.05 vs. the hydrolysis constant ͑K͒ of the added metal ions.
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B516
Journal of The Electrochemical Society, 151 ͑9͒ B512-B518 ͑2004͒
Figure 9. TEM images of the particles before and after reaction by the AP
method with NiSO4 at varied Ni/Fe.
Figure 8. TEM images of the particles formed by the AS method with
Ti͑IV͒, Cr͑III͒, Ni͑II͒, Cu͑II͒, Mn͑II͒, and Zn͑II͒ metal salts at metal/Fe
ϭ 0.05.
needle ␣-FeOOH particles. The similar needle ␣-FeOOH particles
are formed by the reactions with Ni͑II͒, Mn͑II͒, and Zn͑II͒. The
reactions with Ti͑IV͒, Cr͑III͒, and Cu͑II͒ do not noticeably change
the particle morphology, and the thin ␥-FeOOH particles remain
after the reaction, through a few needle ␣-FeOOH particles are vis-
ible. Similar to FTIR and XRD, TEM demonstrates that ␥-FeOOH
converts into ␣-FeOOH by dissolution and recrystallization.
dissolution of Fe͑III͒ from ␥-FeOOH particles, because the solution
pH after the reaction with Ti͑IV͒ of 1.6-1.9 depending on Ti/Fe was
lower than 2 of the precipitation pH of Fe͑III͒ but higher than that of
Ti͑IV͒. Ni͑II͒ and Mn͑II͒ not affecting the conversion into
␣-FeOOH show less metal/Fe of the formed particles and smaller K
than the metal ions impeding the conversion, and a higher precipi-
tation pH than Fe͑III͒. Consequently, the surface coating with the
hydrolysis species of Ti͑IV͒, Cr͑III͒, and Cu͑II͒ stabilizes the
␥-FeOOH particles in FeSO4 solution. Ni͑II͒ and Mn͑II͒ do not
inhibit the conversion into ␣-FeOOH due to less surface coating,
and further, they are almost uncontained in the formed ␣-FeOOH
particles because of their higher precipitation pH than Fe͑III͒. These
results can be explained by the dissolution and recrystallization
mechanism of the transition into ␣-FeOOH.
For further verification of the transition mechanism discussed
previously, we observed the morphology change of the particles by
the reaction. Figure 8 displays the TEM images of the particles
before and after the reaction in a 0.3 mol/dm3 FeSO4 solution dis-
solving different metal ions at metal/Fe ϭ 0 and 0.05. The original
␥-FeOOH particles before the reaction are aggregates of thin plates.
The reaction at metal/Fe ϭ 0 or only with Fe͑II͒ produces small
The influence of metal ion contained in particles.—The influence
of the metal ions contained in the ␥-FeOOH particles was studied by
the AP method. Figure 9 displays the TEM images of the ␥-FeOOH
particles containing Ni͑II͒ at Ni/Fe ϭ 0-0.1 before and after the
reaction. The particles before the reaction get smaller with the in-
crease of Ni/Fe due to the interruption of particle growth by Ni͑II͒.
The particles become smaller after the reaction with the increase of
Ni/Fe corresponding to the particles before the reaction; smaller
particles give smaller ones. As described later, ␥-FeOOH was com-
pletely transformed into ␣-FeOOH at Ni/Fe у 0.01. A glance at this
therefore indicates that ␥-FeOOH converts into ␣-FeOOH by a
solid-state transition, being different from the AS method, but this
was denied by measuring the metal content of the particles before
and after the reaction, as discussed later. The result that smaller
␣-FeOOH particles result from smaller ␥-FeOOH ones can be inter-
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Journal of The Electrochemical Society, 151 ͑9͒ B512-B518 ͑2004͒
B517
Figure 10. Plots of X␣ of the products by the AP method with ͑᭺͒ Ti͑IV͒,
͑᭝͒ Cr͑III͒, ͑ᮀ͒ Ni͑II͒, and ͑᭹͒ Cu͑II͒ metal salts vs. metal/Fe of the par-
ticles before the reaction.
Figure 11. Plots of X␣ of the products by the AP method with ͑᭺͒ Ti͑IV͒,
͑᭝͒ Cr͑III͒, ͑ᮀ͒ Ni͑II͒, and ͑᭹͒ Cu͑II͒ metal salts vs. specific surface area of
the particles before the reaction.
preted by the inhibitory effect of Ni͑II͒ on the particle growth of
␣-FeOOH.6 The similar trend was found in the addition of the other
metal ions.
incorporated again in the formed ␣-FeOOH particles, affecting the
particle growth of ␣-FeOOH.
Figure 10 plots X␣ determined in the same way as the AS
method vs. metal/Fe of the ␥-FeOOH particles before the reaction.
The product at metal/Fe ϭ 0 shows X␣ of 0.37, less than 1.0 of X␣
obtained by the AS method ͑Fig. 5͒, which is due to the difference in
particle size of these samples. From observing the TEM pictures in
Fig. 8 and 9, the original ␥-FeOOH particles in the AS method ͑Fig.
8͒ are smaller than the particles before reaction at metal/Fe ϭ 0 in
the AP method ͑Fig. 9͒, so that the former is more soluble in the
FeSO4 solution and easily transforms into ␣-FeOOH than the latter.
As seen in Fig. 10, X␣ is increased by the addition of all the metal
ions. The addition of Cr͑III͒ and Ni͑II͒ abruptly increases X␣ up to
1.0 at a low metal/Fe of 0.01 and effectively promotes the conver-
sion into ␣-FeOOH, but Ti͑IV͒ does not prominently increase X␣.
Because the solubility of the ␥-FeOOH particles must affect the
conversion into ␣-FeOOH by dissolution, the size of the ␥-FeOOH
particles used in the AP method was estimated from the specific
surface area determined from N2 adsorption isotherms, because the
reliable mean particle size was difficult to measure from the TEM
pictures of the particles with an irregular shape. The addition of the
metal ions increased the specific surface area of the ␥-FeOOH par-
ticles, that is, it decreased the particle size. Figure 11 plots X␣ vs.
the specific surface area, which quite resembles Fig. 10 showing the
relation between X␣ and the metal/Fe of the particles before the
reaction. X␣ increases with the specific surface area, that is, smaller
␥-FeOOH particles more easily convert into ␣-FeOOH, strongly
supporting the dissolution and recrystallization mechanism.
Conclusions
The transformation of ␥-FeOOH into ␣-FeOOH in FeSO4 solu-
tion was influenced by the metal ions dissolved in solution. Cu͑II͒,
Cr͑III͒, and Ti͑IV͒ in solutions markedly interfered with the trans-
formation, but Ni͑II͒ had no affect. These metal ions added to
␥-FeOOH particles promoted the transition into ␣-FeOOH by reduc-
ing the particle size; Ni͑II͒ most effectively enhanced it. Therefore,
If the conversion into ␣-FeOOH proceeds in the particles by a
solid-state transition, the metal content of the ␥-FeOOH particles
should not vary during the conversion. To confirm this, the metal/Fe
ratios of the particles before and after the reaction are shown in Fig.
12. The products with Ti͑IV͒ and Cr͑III͒, of which the precipitation
pH is close to that of Fe͑III͒, shows the equivalent metal/Fe ratio
before and after the conversion, whereas the metal/Fe ratio of the
products with Cu͑II͒ and Ni͑II͒ after the reaction is less than that
before the reaction. It becomes more apparent that the transition
takes place by the dissolution of ␥-FeOOH and the recrystallization
of ␣-FeOOH. The metal ions once dissolved into the solutions are
Figure 12. Plots of metal/Fe of the particles formed by the AP method with
͑᭺͒ Ti͑IV͒, ͑᭝͒ Cr͑III͒, ͑ᮀ͒ Ni͑II͒, and ͑᭹͒ Cu͑II͒ metal salts vs. metal/Fe of
the particles before the reaction.
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B518
Journal of The Electrochemical Society, 151 ͑9͒ B512-B518 ͑2004͒
8. T. Ishikawa, T. Motoki, R. Katoh, A. Yasukawa, K. Kandori, T. Nakayama, and F.
Yuse, J. Colloid Interface Sci., 250, 74 ͑2002͒.
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Interface Sci., 260, 320 ͑2003͒.
10. T. Ishikawa, R. Isa, K. Kandori, T. Nakayama, and T. Tsubota, J. Electrochem. Soc.,
Submitted.
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Corros. Sci., 44, 1073 ͑2002͒.
12. T. Ishikawa, H. Nakazaki, A. Yasukawa, K. Kandori, and M. Seto, Mater. Res.
Bull., 33, 1609 ͑1998͒.
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1665 ͑1999͒.
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15. T. Ishikawa, T. Ueno, A. Yasukawa, K. Kandori, T. Nakayama, and T. Tsubota,
Corros. Sci., 45, 1037 ͑2003͒.
among the metal ions used in the present study Ni͑II͒ is thought to
be most effective in promoting the transition into ␣-FeOOH. This
finding allows us to infer that the anticorroding function of the al-
loying metal elements is not only to promote the formation of ther-
modynamically stable ␣-FeOOH rusts but also to reduce the particle
size and crystallinity of ␣-FeOOH rusts, leading to the formation of
compact and stable rust layers. The information presented in this
article provides understanding of the anticorroding effect of allow-
ing metals in weathering steels, because the transition of ␥-FeOOH
into ␣-FeOOH rusts is an important process concerning the stabili-
zation and protective nature of rusts.
Acknowledgment
The authors are grateful to Masao Fukusumi of Osaka Municipal
Technical Research Institute for help with the TEM observations.
This study was partly supported by the Grant-in-Aid for Science
Research Funds ͑B͒ from the Ministry of Education, Science, Sports
and Culture, Japan.
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Osaka University of Education assisted in meeting the publication costs
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