Materials Transactions, Vol. 53, No. 8 (2012) pp. 1536 to 1538
© 2012 The Japan Institute of Metals
RAPID PUBLICATION
Electrochemical Phase Change of Iron Rusts
by In-Situ X-ray Diffraction Technique
Takashi Doi1, Takayuki Kamimura1 and Masugu Sato2
1
Corporate Research and Development Laboratories, Sumitomo Metal Industries, Ltd., Amagasaki 660-0891, Japan
Japan Synchrotron Radiation Research Institute, Sayo, Hyogo 679-5198, Japan
2
In order to investigate the electrochemical cathodic reduction behavior of rusts, we propose the electrochemical cell and the optical
conditions for in-situ X-ray diffraction technique. The electrochemical phase change of rust was able to be followed as a function of time. It was
observed that ¢-FeOOH was reduced, and then spinel type iron oxide was formed. [doi:10.2320/matertrans.M2012159]
(Received May 1, 2012; Accepted May 23, 2012; Published July 4, 2012)
Keywords: in-situ X-ray diffraction, electrode/electrolyte interface, phase change, rusts, atmospheric corrosion
1.
Introduction
Corrosion products (rust) formed on a steel surface during
atmospheric exposure with repetitive wet and dry environments are generally composed of iron oxy-hydroxides and
oxide such as ¡-FeOOH, ¢-FeOOH, £-FeOOH and Fe3O4.
Evans1,2) has indicated that the corrosion products strongly
influence the corrosion of a steel since the anodic dissolution
of iron is balanced by the reduction of the oxy-hydroxide
within the rust layer. The cathodic reduction behavior of
rusts, therefore, have been studied from various points of
views.3­6)
The X-ray diffraction method is a powerful tool for the
structural analysis of rusts, but it has usually been applied for
ex-situ analysis.4) On the other hand, recent in-situ XRD
measurements using synchrotron radiation has enabled us to
provide structural investigations in the electrolyte/electrode
interface.7­10) However, in-situ XRD technique is hardly
adapted for investigating the electrochemical cathodic
reduction behavior of rusts because the rust reduction
behavior can be investigated only in the presence of a large
volume of electrolyte for avoiding the large ohmic resistance.
The purpose of this study is to establish in-situ XRD
technique for investigating the rust reduction behavior. The
electrochemical cell and the optical conditions for XRD
measurements are investigated under the condition of a large
volume of electrolyte around a sample.
2.
Experiment
¢-FeOOH was prepared by hydrolysis of FeCl3,11) and
reagent grade ¡-FeOOH (Rare Metallic Co. LTD.) was used.
These were embedded in graphite powder (Wako Pure
Chemical Industries, LTD.), and pressed into a pellet which
was approximately 10 mm diameter and 0.9 mm thick. These
pellets were glued on a graphite plate (Showa Denko K. K.)
of 0.5 mm in thickness with graphite paste, and these were
used for the rust electrode.
The electrochemical cell for in-situ XRD measurements
was designed with transmission geometry. A schematic
picture of the in-situ cell is shown in Fig. 1. The length
between the two windows was adjusted to 6 mm, and this
Potentiostat
Graphite plate
Working
electrode
Reference electrode
(Ag/AgCl)
Outlet
To detector
Insident X−ray
2θ
Counter electrode(Pt)
Electrolyte Rust pellet
Inlet
Capton window
Fig. 1 Schematic diagram of experimental setup, showing the electrochemical cell with a Pt counter electrode, Ag/AgCl reference electrode
and graphite pellet with ¢-FeOOH as the working electrode.
space was filled with oxygen-free 0.03 M% NaCl (Wako Pure
Chemical) solution at room temperature. The Pt counter
electrode and saturated Ag/AgCl reference electrode (SSE)
were placed into the cell.
The technical problem for the in-situ XRD measurement
on the electrochemical reaction of the electrode in liquid
electrolyte is a low signal to noise ratio for the large
background noise from the scattering of the electrolyte, and
therefore the irradiated area in the electrolyte must be
decreased. Two technical ways can be considered; (1)
decreasing the volume of the electrolyte, and (2) restricting
the observed area instrumentally to the electrode/electrolyte
interface. The former way has some disadvantages from
an electrochemical standpoints. When the volume of the
electrolyte is decreased, the current distribution on the
working electrode surface become nonuniform, and large
ohmic resistance in the thin electrolyte gap rises due to the
geometrical arrangement around the electrode. Therefore, we
adopted the latter way in this research.
The experiments were performed with a multi-axis
diffractometer installed on the BL46XU beam line at the
SPring-8. The schematic view of the experimental layout is
Electrochemical Phase Change of Iron Rusts by In-Situ X-ray Diffraction Technique
Detector
1537
graphite (002)
Double slit
Intensity (arb. unit)
Cell
(310)
Scattering angle 2 θ
Electrode/electrolyte interface
X−ray
Electrolyte
Sample stage
Graphite
plate Rust pellet
X−ray
To detector
0.2mm
2θ
(121)
scattering from electrolyte
(220)
(110)
(400)
(240)
(200)
B
Observing area
A
0.5 0.7
0
Position, l /mm
Fig. 2 Schematic view of the multi-axis Huber diffractometer. The inset is
a cross-sectional view of the relationship between the sample volume and
the observed volume.
shown in Fig. 2. As the detector of XRD signal from the rust
electrode, a NaI scintillation counter was equipped on the
detector arm of the diffractometer. The profiles of XRD were
measured by scanning the detector position, and the scanning
plane was vertical plane parallel to the X-ray beam. The
scattering angle resolution of XRD was arranged with the
double slit equipped on the detector arm between the rust
electrode and the detector. The electrochemical cell was set
on the motorized sample stage. The sample position in the
cell was adjusted with this stage. The X-ray wave length was
0.1 nm, and the X-ray beam size was 0.1 mm (vertical) · 1 mm (horizontal).
The inset of Fig. 2 shows the schematic view of the
vertical cross section of the observing area of the rust
electrode. The observing area can be arranged by the vertical
size of the X-ray beam and the aperture of the double slit. The
aperture of the double slit was set at 0.1 mm so as to restrict
the irradiation area within sample thickness. In order to adjust
the position of the observing area around the electrode, the
rust electrode position was scanned along the X-ray beam
monitoring the intensity of the XRD peak (002) of the
graphite from the graphite plate and the rust pellet, and then
we adjusted the position of the center of the observing area
to 0.2 mm from electrode/electrolyte interface, as shown in
Fig. 2.
Figure 3 shows the XRD profile of the rust electrode
containing ¢-FeOOH in the electrochemical cell filled with
the aqueous 0.03 M NaCl solution. In this figure, we
compared the data obtained using the double slit (Fig. 3A)
and the data obtained using Soller slit12) with the aperture
of 13 mm vertical instead of the double slit (Fig. 3B). The
intensity of these data were normalized by the intensity of the
graphite (002) peak. In the case of the optical condition using
Soller slit, large background noise from X-ray scattering of
the electrolyte disturbed the XRD profile from ¢-FeOOH. On
the other hand, in the case of the optical condition using the
double slit, the XRD profile from ¢-FeOOH was clearly
observed, because the X-ray scattering of the electrolyte was
decreased effectively.
10°
20°
Diffraction Angle, 2θ
Fig. 3 The XRD patterns of ¢-FeOOH pellet in 0.03 M NaCl solution with
changing the receiving slits height from (B) 13 mm to (A) 0.1 mm. The
intensities of these data were normalized by the intensity of the graphite
(002) peak.
graphite
Intensity (arb. unit)
−0.7
b
b
E
b+sp
sp
b sp
b
b sp
D
C
B
A
10°
20°
Diffraction Angle, 2θ
Fig. 4 In-situ XRD spectra of ¢-FeOOH pellet. (A) open circuit potential
and ¹1.2 V (SSE) after (B) 0 s; (C) 753 s; (D) 1505 s; (E) 2257 s. b: ¢FeOOH, sp: spinel iron oxide, graphite: graphite.
3.
Results and Discussion
Figure 4 shows the result of potentiostatic reduction of
¢-FeOOH. The electrochemical cell was filled with the
aqueous 0.03 M NaCl solution. The rusts were stable at the
open circuit potential in this electrolyte, and then the ¢FeOOH electrode were polarized at ¹1.2 V (SSE).
Successive scans between 2ª angles of 6 and 25 deg. were
repeated approximately every 750 s, simultaneously. While
the XRD measurements were run for over 3000 s, the
intensity of ¢-FeOOH peaks (represented by b in Fig. 4) was
decreased and the formation of spinel type iron oxides was
observed as shown in Fig. 4. Since the spinel type iron
1538
T. Doi, T. Kamimura and M. Sato
graphite
Intensity (arb.unit)
a
a
a
a
aaa
the superior ability for protection during the atmospheric
corrosion. Future work is to determine the effect of some
cations for the electrochemical stability of rusts. More
detailed experiments of the electrochemical phase change
of rusts are in progress and will be reported in the near future.
4.
E
D
C
B
A
10°
20°
Diffraction Angle, 2θ
Fig. 5 In-situ XRD spectra of ¡-FeOOH pellet. (A) open circuit potential
and ¹1.2 V (SSE) after (B) 0 s; (C) 838 s; (D) 1544 s; (E) 2318 s. a: ¡FeOOH, graphite: graphite.
oxides, Fe3O4 and £-Fe2O3, have a similar structure, which
cannot easily be distinguished by only the XRD technique,
these spinel type iron oxides, both Fe3O4 and £-Fe2O3, were
expressed as sp.
The result of a potentiostatic reduction of ¡-FeOOH
electrode at ¹1.2 V (SSE) are also shown in Fig. 5. The
intensity of ¡-FeOOH peaks (represented by a) was not
decreased while measuring. It is well known that £-FeOOH
also decreased and spinel type iron oxide increased as
reduction proceeded, while ¡-FeOOH did not change.4)
It was confirmed that ¢-FeOOH is also easily reduced
compared to ¡-FeOOH.
The decrease in the ratio of the electrolyte volume to the
observing area improved the XRD spectra of rust through an
electrolyte. The synchrotron radiation light source, which
is the high brilliance and flux of advanced X-ray source,
enables us to obtain the XRD pattern of rusts in 0.03 M
NaCl solution in a short time, even though the field of view
of XRD measurements are controlled by narrowing the
double slit. As a result, the XRD spectra of rusts could
be obtained under control of the electrochemical reductive
condition.
Many researchers13­15) have reported that the rust layer
which contains some cations, Cu, P, Cr, etc., possesses
Conclusion
The purpose of this study is to put to practical use the
in-situ XRD technique for investigating the electrochemical
cathodic reduction behavior of rusts. The X-ray diffraction
was collimated using the double slit to control the stray
scattering from an electrolyte and thus the electrochemical
phase change of rust was able to be followed as a function of
time. It was observed that ¢-FeOOH was reduced and spinel
type iron oxide was formed.
Acknowledgments
The authors would like to express their gratitude to Dr. T.
Kudo for valuable discussions. The synchrotron radiation
experiments were performed in SPring-8 with the approval of
JASRI (Proposal No. 2009A1940).
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