J. of Supercritical Fluids 47 (2008) 309–317
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The Journal of Supercritical Fluids
journal homepage: www.elsevier.com/locate/supflu
Analyses of oxide films grown on Alloy 625 in oxidizing supercritical water
Mingcheng Sun, Xinqiang Wu ∗ , Zhaoen Zhang, En-Hou Han
State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, 62 Wencui Road, Shenyang 110016, PR China
a r t i c l e
i n f o
Article history:
Received 1 March 2008
Received in revised form 7 July 2008
Accepted 7 July 2008
Keywords:
Alloy 625
SCWO
Oxide film
XPS
TEM
a b s t r a c t
Morphologies, microstructures and chemical composition of oxide films grown on Alloy 625 at 400 ◦ C,
450 ◦ C and 500 ◦ C in oxidizing supercritical water containing 2.0% H2 O2 were investigated using weight
measurement, grazing incidence X-ray diffractometry, scanning electron microscopy, X-ray photoelectron spectroscopy and transmission electron microscopy. It was found that the mass gain of the alloy
in oxidizing supercritical water increased with exposure time. The smallest mass gain was obtained at
450 ◦ C, agreeing well with the smallest oxide film thickness at this temperature. The average thickness of
the oxide films was 1.28 ␮m, 0.72 ␮m and 1.42 ␮m at 400 ◦ C, 450 ◦ C and 500 ◦ C, respectively after 250 h
exposure. The size of oxide crystals on the alloy surface gradually grew with increasing exposure temperature. A discontinuous and thin Cr2 O3 layer was formed at 400 ◦ C in oxidizing supercritical water
as a result of leaching of Cr3+ as Cr6+ , while a continuous Cr2 O3 layer was formed at 450 ◦ C and 500 ◦ C.
Duplex oxide layer structure was observed at all three temperatures, which was identified to consist of
Ni(OH)2 /NiO/NiCr2 O4 /Cr2 O3 /Alloy 625 from outer to inner layer. The growth mechanism of oxide films on
Alloy 625 in oxidizing supercritical water seems to be similar to that in high temperature water, namely
the Ni(OH)2 /NiO outer layer growth by dissolution and precipitation mechanism and the Cr2 O3 inner layer
formation by oxygen diffusing inward and reacting with the retained Cr.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
Supercritical water is a non-polar solvent and can dissolve gases
like oxygen to complete miscibility. Supercritical water oxidation
(SCWO) is an oxidative treatment of organic wastewaters at temperatures above 374.15 ◦ C and pressures above 22.1 MPa. SCWO is
able to completely decompose and oxidize most organic materials at very short reaction times ranging from seconds to minutes
[1,2]. Heteroatoms like halogens, sulfur, or phosphorus included
in the organic wastes are transformed into their mineral acids
or salts [1,2]. However, oxidation of halogen-containing species
leads to severe corrosion of most reactor materials such as stainless steels and nickel-base alloys. Corrosion problems in SCWO
have received extensive attention [3–8]. To address corrosion that
occurs in SCWO, the influence of temperature, pressure, solution
components on the corrosion behavior and the determination of
the corrosion mechanisms under high temperature aqueous conditions should be considered. The austenitic stainless steels and
nickel-base alloys are generally considered as candidate materials for fabricating SCWO system. Especially nickel-base alloys such
as Alloy 625 and C-276 are commonly used in the SCWO reac-
∗ Corresponding author. Tel.: +86 24 23841883.
E-mail address: [email protected] (X. Wu).
0896-8446/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.supflu.2008.07.010
tors in laboratory and pilot-scale systems [9,10]. However, it has
been reported that nickel-base alloys suffer from general corrosion, pitting corrosion, intergranular corrosion and stress corrosion
cracking (SCC) in SCWO [2,5,10–14].
The oxide films that grow on nickel-base alloys in high temperature and high-pressure water are difficult to characterize due to
their small film thickness. The morphologies, microstructures and
chemical composition of the oxide films may give us significant
clues for understanding the corrosion process and mechanisms
and finding paths to prevent corrosion in high temperature and
supercritical water. The oxide films on the surface of stainless
steels and nickel-base alloys in the high temperature water sometimes play a key role in the process of stress corrosion cracking
[14–20]. Different mechanisms are proposed for explaining general corrosion, pitting corrosion, intergranular corrosion and stress
corrosion cracking while the properties of the oxide films always
play an important role for all proposed mechanisms. Previous studies mainly focused on the characterization of oxide films formed in
high temperature (T < 370 ◦ C) and high-pressure (P < 20 MPa) water
[15,18]. The duplex oxide layer structure is normal for nickel-base
alloys. Generally, the outer oxide layer contains large crystals and
is rich in Ni, while the inner oxide layer is fine and rich in Cr
[15,18,21,22].
Several mechanisms have been proposed to explain the growth
of the oxide films on stainless steels and nickel-base alloys in
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M. Sun et al. / J. of Supercritical Fluids 47 (2008) 309–317
high temperature water [15,20,23,24]. A mechanism of oxide layers
growth on stainless steels in high temperature water has been proposed by Stellwag based on the concepts of passivity of metals [23].
Cline et al. [6] observed dealloying of Ni and Fe, intergranular corrosion and stress corrosion cracking in hydrothermal HCl solutions
on alloy N10276. Panter et al. [15] proposed that the Cr-rich layer
was formed by diffusion and selective oxidation of Cr in simulated
pressurized water at 360 ◦ C on Alloy 600. Ren et al. [4] considered
that the outer oxide layer grew by the outward diffusion of Ni and
its reaction with the dissolved oxygen, while the oxygen diffusing
inward reacted with Ni and Cr to form an inner oxide layer.
However, little work has focused on the characterization of oxide
films grown on nickel-base alloys in supercritical water compared
those with high temperature water studies. Much detailed work
should be done to promote the development of SCWO technology and its application in industrial fields. In the present work,
oxide films grown on Alloy 625 in oxidizing supercritical water
were investigated in detail using surface and microscopic analysis technologies. The morphologies, microstructures and chemical
composition of oxide films grown on Alloy 625 at 400 ◦ C, 450 ◦ C
and 500 ◦ C in oxidizing supercritical water were analyzed. The possible mechanism of oxide films grown on Alloy 625 in oxidizing
supercritical water is also discussed.
2. Experimental
2.1. Apparatus
Exposure experiments were performed with a continuous flowing SCWO system. Fig. 1 shows a schematic diagram of the SCWO
testing system consisting of an HPLC pump (Eldex Inc., BBB-4-2), a
preheater, an Alloy 625 autoclave with a volume of 850 ml, a heat
exchanger and a back-pressure regulator. The internal pressure was
measured by a pressure sensor placed at the inlet of the reactor and
controlled by an HPLC pump and a back-pressure regulator during
tests. A computer with Labview 6.0 software was used to control the
internal pressure and also the autoclave and preheater temperature
which was monitored by two thermocouples.
2.2. Specimen
The material Alloy 625 used in this study was vacuummelted and delivered in a solution-annealed condition. Coupons
Fig. 1. Schematic diagram of supercritical water oxidation experimental apparatus.
Table 1
Chemical composition of Alloy 625 (wt%)
C
Cr
Mo
Fe
Nb
Ti
Al
Ni
0.05
21.5
9.0
2.5
3.6
0.2
0.2
Bal.
(10 mm × 15 mm × 2.5 mm) were mechanically polished to 1.5 ␮m
diamond grit paste. Table 1 is the chemical composition of the
alloy. During the tests, the specimens were mounted on a rack
and put into the autoclave with the pressure maintained at 24 MPa
and exposure time from 50 h to 250 h. The 2.0% H2 O2 feed solution provided by an HPLC pump at the flow rate of 2.5 ml/min was
preheated at 350 ◦ C before entering the autoclave where the solution was heated to the experimental temperature. The effluent was
cooled down to room temperature by the heat exchanger and then
drained out of the system.
2.3. Methods
After exposure, the specimens were cleaned and dried. The mass
of all specimens before and after exposure was measured using Sartorius BP211D microbalance, working with a resolution of 10−5 g.
X-ray diffractometry (XRD) (X’Pert PRO PANalytical) was used to
identify the phases in the oxide films. Since the oxide film was thin,
grazing incidence X-ray diffractometry (GIXRD) was used to highlight the signals from surface oxide film other than alloy matrix
and identify the oxide phases. The measurements were carried
out using copper radiation ( = 1.542 Å) for 2 between 10◦ and
100◦ .
The chemical composition of the oxide films was analyzed by
XPS (PHI-5300 ESCA). For XPS characterization, Ni 2p, Cr 2p, Fe 2p,
O 1s, C 1s, Mo 3d and Nb 3d core level spectra have been recorded
with Mg K␣ radiation (h = 1253.6 eV), at a pass energy of 50 eV. The
spectrometer was calibrated against the binding energy (BE) of the
surface carbon contamination C 1s at 284.5 eV. In the XPS spectrometer, the base pressure of the analysis chamber is 1 × 10−7 mbar.
The argon ions sputtering speed used to remove the oxide film was
about 12 nm/min with ion energy of 3 keV. To analyze the individual contributions of the Ni 2p3/2 , Cr 2p3/2 , Fe 2p3/2 , O1s, C 1s, Mo
3d and Nb 3d core levels, peak decomposition was carried out with
XPSpeak4.1 (a computer program) using Gaussian/Lorentzian peak
shapes, and a Shirley background.
Prior to cross-section SEM observations, a thin nickel film
was deposited on the corroded sample surfaces to ensure the
intactness of the oxide films. The surface and cross-section morphologies and chemical composition of oxide films were performed
using XL30 scanning electron microscopy (SEM) equipped with
an energy-dispersive X-ray spectroscopy (EDS) system. Plan-view
and cross-section structures of the oxide films were observed
using TEM. The preparation of electron transparent foils for planview and cross-section observation was demonstrated in Fig. 2.
The alloy side was mechanically removed and the oxide film was
remained in the process of plan-view sample preparation. The
Ø3 mm disc was dimpled and thinned using an ion beam miller
at alloy side. To prepare cross-section TEM samples, the Alloy
625 strips (2 mm × 1 mm × 0.5 mm) with oxide films were glued
together. The joined sample was stuck to a copper ring (Ø3 mm) to
enable its subsequent handling and then mechanically thinned to
an 80 ␮m thickness disc. The disc was dimpled and thinned using
an ion beam miller [25]. TEM observations were performed using a
M. Sun et al. / J. of Supercritical Fluids 47 (2008) 309–317
311
Fig. 2. Schematic presentation of TEM sample preparation: (a) plan-view and (b)
cross-section.
Fig. 4. GIXRD patterns from Alloy 625 exposed in oxidizing supercritical water for
250 h at 400 ◦ C, 450 ◦ C and 500 ◦ C.
200 kV Tecnai G2 F20 TEM (FEI, Eindhoven, Netherlands), equipped
with an EDS system.
3. Results
3.1. Mass change
Fig. 3. Mass gain as a function of exposure time for Alloy 625 in oxidizing supercritical water at 400 ◦ C, 450 ◦ C and 500 ◦ C.
Fig. 3 shows the time dependent mass gain of Alloy 625 exposed
to oxidizing supercritical water. The mass gain increased with
increasing exposure time. A rapid increase in mass gain was
observed in the early stage and this increase tended to become saturated with increasing exposure time. The largest mass gain took
place at 400 ◦ C while the smallest one was at 450 ◦ C.
Fig. 5. XPS of oxide films grown on Alloy 625 in oxidizing supercritical water for 250 h at 400 ◦ C, 450 ◦ C and 500 ◦ C. (a) Ni 2p3/2 core level spectra and their decomposition
after 0 min sputtering, (b) Ni 2p3/2 core level spectra and their decomposition after 5 min sputtering, (c) Cr 2p3/2 core level spectra and their decomposition and (d) Mo 3d5/2
core level spectra and their decomposition.
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M. Sun et al. / J. of Supercritical Fluids 47 (2008) 309–317
Fig. 6. Surface morphologies of the oxide films grown on Alloy 625 in oxidizing supercritical water for 250 h at: (a) 400 ◦ C, (b) 450 ◦ C and (c) 500 ◦ C.
3.2. GIXRD measurement
GIXRD patterns obtained for ˚ (grazing incidence angle) = 5◦
for three samples after 250 h exposure are shown in Fig. 4. Four
types of oxide products were identified on the Alloy 625 sam-
ples that were exposed to oxidizing SCW at different temperatures.
The major phase was Ni(OH)2 (according to powder diffraction file
(PDF) card 03-0177), NiO (according to PDF card 44-1159), NiCr2 O4
(according to PDF card 75-1728) and Cr2 O3 (according to PDF card
38-1479). The peaks corresponding to Ni(OH)2 , NiO, NiCr2 O4 and
Fig. 7. Back-scattering electron cross-section of oxide films grown on Alloy 625 in oxidizing supercritical water for 250 h at: (a) 400 ◦ C, (b) 450 ◦ C and (c) 500 ◦ C. 1: Ni-plating;
2: oxide; 3: Alloy 625.
M. Sun et al. / J. of Supercritical Fluids 47 (2008) 309–317
313
Cr2 O3 appeared at 400 ◦ C, 450 ◦ C and 500 ◦ C, indicating similar
oxides grown at the different testing temperatures.
3.3. XPS analysis
Fig. 5(c) shows the Cr 2p3/2 core level spectra for Alloy 625 after
exposure in oxidizing supercritical water. The Cr 2p3/2 peaks were
systematically decomposed into two components: one located at a
binding energy (BE) of 576.7 ± 0.6 eV, and another located at BE
of 586.2 ± 0.5 eV. The signal at the BE of 576.7 eV was assigned
to Cr3+ in Cr2 O3 , the one at 586.2 eV to Cr3+ in NiCr2 O4 [26].
Fig. 5(a and b) shows the Ni 2p3/2 core level spectra of the oxide
films grown on Alloy 625 in oxidizing supercritical water. Considering Ni 2p3/2 peak decomposition, only one main component
and an associated satellite were detected in this spectra: a signal at a BE of 856.3 ± 0.2 eV and a satellite at 862.1 ± 0.3 eV. The
BE signal of 856.3 eV was attributed to Ni(OH)2 on the surface.
After about 5 min sputtering, a signal at a BE of 854.4 ± 0.2 eV
and a satellite at 862.2 ± 0.4 eV appeared. The BE signal of 854.4
was definitely attributed to NiO and its satellite at 862.2 eV was
characteristic of a mixed chromate NiCr2 O4 rather than a pure
oxide NiO [27,28]. XPS showed small mounts of Mo oxides in
Fig. 5(d). The Mo 3d5/2 peaks were decomposed into two components. The signal located at BE of 228.2 ± 0.2 eV was assigned to
MoO2 and the signal at BE of 232.4 ± 0.3 eV was assigned to MoO3
[29].
3.4. SEM observation
Fig. 6 shows the surface morphologies of the oxide films grown
on Alloy 625 in oxidizing supercritical water. Significant changes
of the morphologies of the oxide films were observed at different
exposure temperatures. The size of oxide crystals gradually grew
with increasing exposure temperature. At 400 ◦ C, there were interspaces between the small and equiaxed crystals. At 450 ◦ C, some
oxide crystals developed into rods and platelets. The equiaxed and
rod-like oxide crystals fully covered the surface of Alloy 625. At
500 ◦ C, the oxide crystals grew larger due to the relatively high
temperature. Fig. 7 illustrates the back-scattering electron (BSE)
cross-sections of oxide films grown on Alloy 625 in oxidizing supercritical water at 400 ◦ C, 450 ◦ C and 500 ◦ C. BSE cross-section images
exhibited a different contrast between the outer Ni oxide and inner
Cr oxide that may be characteristic of the duplex oxide layer structure for the oxide films at all three temperatures. BSE cross-section
images show that the average thickness of the oxide films was
1.28 ␮m, 0.72 ␮m and 1.42 ␮m at 400 ◦ C, 450 ◦ C and 500 ◦ C, respectively. The inner Cr oxides were discontinuous, 0.12 ␮m, 0.29 ␮m
at 400 ◦ C, 450 ◦ C and 500 ◦ C, respectively. Fig. 8 shows the EDS line
scan profiles of the oxide films grown on Alloy 625 in oxidizing
supercritical water at 400 ◦ C, 450 ◦ C and 500 ◦ C for 250 h. There
was an obvious Cr-rich inner layer at the interface between the
oxide film and alloy at all three temperatures. Fig. 9 shows the
EDS mapping results of the oxide films grown at 400 ◦ C, 450 ◦ C
and 500 ◦ C, respectively. It was found that Mo was depleted in
the oxide films and no Fe oxide was detected in the oxide films,
as shown in Fig. 9. The outer layer of the oxide films mainly
consisted of Ni oxide and the inner layer was Cr oxide. Based
on the GIXRD result, the Cr-rich inner oxide was identified as
Cr2 O3 . The oxide films at 450 ◦ C (0.72 ␮m) were thinner than
those at 400 ◦ C (1.28 ␮m) and 500 ◦ C (1.42 ␮m). The Cr2 O3 layer
formed at 400 ◦ C was thin and discontinuous, however, a uniform and continuous Cr2 O3 layer was formed at 450 ◦ C (0.12 ␮m)
and became thicker with further increasing temperature to 500 ◦ C
(0.29 ␮m).
Fig. 8. EDS line scan profiles of oxide films grown on Alloy 625 at 400 ◦ C, 450 ◦ C and
500 ◦ C in oxidizing supercritical water for 250 h. 1:Ni-plating; 2: oxide; 3: Alloy 625.
3.5. TEM analysis
Fig. 10(a) shows the plan-view TEM morphologies of oxide film
grown on Alloy 625 at 400 ◦ C in oxidizing supercritical water. The
equiaxed oxide crystals with an average size about 300 nm existed
in the outer layer which was in good agreement with the previous SEM observations of surface morphologies. Fig. 10(b and c)
shows bright field images of the oxide films grown on Alloy 625 at
450 ◦ C in oxidizing supercritical water for 250 h. There were two
families of outer oxide films according to their size and shape.
One family was equiaxed with an average size about 500 nm;
another was rod-like with the rod length about 2.8 ␮m. The outer
large oxides were Ni(OH)2 /NiO while the subsurface oxides were
NiCr2 O4 spinel and the inner-layer oxides were Cr2 O3 based on the
GIXRD result and selected area electron diffraction (SAED) analysis
(Fig. 10(d)).
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M. Sun et al. / J. of Supercritical Fluids 47 (2008) 309–317
Fig. 9. SEM image of the oxide film and EDS mapping of the oxide film grown on Alloy 625 in oxidizing supercritical water for 250 h at: (a) 400 ◦ C (b) 450 ◦ C and (c) 500 ◦ C.
(1) BSE image; (2) O mapping; (3) Ni mapping; (4) Cr mapping; (5) Mo mapping; (6) Fe mapping.
4. Discussion
4.1. Duplex oxide layer
In the present work, it was found that the oxide films grown
on Alloy 625 in oxidizing supercritical water were duplex structure, consisting of an outer layer of loose and large grains and an
inner layer of fine-grained oxides (Fig. 10) which could be distinguished from their microstructures. The inner layer consisted of
small equiaxed grains owing to its confined growing space; the
outer layer consisted of less compact and large grains due to its
outward growth without space confinement [30]. The outer and
inner layers could also be distinguished by the chemical composition of the oxides. The outer layer was Ni-rich oxides and the inner
one retained Cr-rich oxides (Fig. 9). The presence of Cr2 O3 layer
at the interface between the oxide film and alloy was consistent
with many previous reports [4,15,18]. The uniform and continuous
Cr-rich oxide film can efficiently protect the Alloy 625 substrate
and account for the excellent high temperature aqueous oxidation
resistance [15].
In high temperature water, NiO as the main products was formed
on nickel-base alloys [4,16–18]. However, Machet et al. [21] and
McIntyre et al. [27] reported that Ni(OH)2 rather than NiO was
formed on Alloy 600 in high temperature water. In present study,
M. Sun et al. / J. of Supercritical Fluids 47 (2008) 309–317
315
Fig. 10. TEM results: (a) plan-view oxide film grown on Alloy 625 at 400 ◦ C in oxidizing supercritical water for 250 h, (b) cross-section observation of equiaxed crystals of
outer oxide film grown on Alloy 625 at 450 ◦ C in oxidizing supercritical water for 250 h, (c) cross-section observation of rod-like crystals of outer oxide film grown on Alloy
625 at 450 ◦ C in oxidizing supercritical water for 250 h and (d) selected area electron diffraction from inner oxide.
NiO became the major phase in the outer layer after the outer surface Ni(OH)2 (about 60 nm) was removed through 5 min argon ions
sputtering (Fig. 5).
Alloy 625 contained 9 wt% Mo and 3.6 wt% Nb, respectively
(Table 1), but no GIXRD peaks indicated the presence of Mo and
Nb oxides in the oxide film (Fig. 4). Niobium could form a stable pentavalent oxide (Nb2 O5 ). However, its concentration was too
low to form a complete oxide layer. Nb2 O5 also underwent a phase
transformation from an amorphous to a crystalline modification at
temperatures about 350 ◦ C which resulted in non-protective Nb2 O5
film for the alloy [31]. Mo was depleted in the oxide films according to the EDS mapping results (Fig. 9). Although small mounts of
MoO2 and MoO3 were detected by XPS (Fig. 5(d)), Mo is not resistant against oxidizing solutions and generally dissolves by forming
Mo6+ [31]. The addition of about 9 wt% Mo has been found to have
no significant influence on the oxidation resistance of Alloy 625
[10]. Ziemniak and Hanson [32] also reported that no Mo and Nb
oxides appeared in the corrosion layer of Alloy 625 in hydrogenated
water at 260 ◦ C. The concentration of Fe in the alloy was low and
no Fe oxide was detected in the oxide film according to the EDS
mapping results (Fig. 9).
4.2. Influence of temperature
The difference in mass gain at different exposure temperatures
(Fig. 3) can be rationalized by the characteristics of cross-sections of
the oxide films (Fig. 7). The oxide films with the smallest mass gain
at 450 ◦ C (0.72 ␮m) were thinner than those at 400 ◦ C (1.28 ␮m)
and 500 ◦ C (1.42 ␮m). At 450 ◦ C, a uniform and continuous Cr2 O3
layer (0.12 ␮m) was formed (Figs. 7 and 8), protecting the alloy as a
barrier layer of oxygen diffusion [15]. At 500 ◦ C (Fig. 7), both the continuous Cr2 O3 layer (0.29 ␮m) and whole oxide film became thicker
compared to those at 450 ◦ C due to the enhanced oxidation and corrosion at a relatively high temperature. The dissolution of metal ions
such as Ni2+ and the oxygen diffusion were enhanced at a higher
temperature. Thus, the outer Ni(OH)2 /NiO layer and the inner
Cr2 O3 layer would grow with increasing exposure temperature
even though the continuous Cr2 O3 protective layer was formed.
Zurek et al. [33] found that the oxidation rates of 9–12% Cr steels
did not steadily increase with increasing exposure temperature in a
simulated steam environment at temperatures between 550 ◦ C and
650 ◦ C. This was attributed to a very thin and protective oxide scale
developed at higher temperatures whereas less-protective oxides
formed at relatively low temperatures [33].
The potential–pH (E–pH) diagrams for Ni, Cr at 400–500 ◦ C in
supercritical water can be employed to estimate the chemical stability of the primary elements of Alloy 625 in oxidizing supercritical
water [34]. The oxygen concentration from almost 100% decomposition of 2.0% H2 O2 was 0.59 mol/L and the pH value of neutral
water was 7.27 at 450 ◦ C [35,36]. Based on the E-pH diagrams, the
principal chemical forms of Ni were Ni2+ and NiO, and those of Cr
were HCrO4 − and Cr2 O3 . The corrosion potential of the alloy was
closer to the transpassive condition at 400 ◦ C near the supercritical
point that would lead to the leaching of Cr3+ as Cr6+ [37]. The larger
density and ion product of water at lower temperature may make
the leaching of Cr6+ easier in the supercritical region [31,37]. As a
result, the Cr2 O3 layer formed on Alloy 625 at 400 ◦ C in oxidizing
supercritical water was discontinuous and thin.
4.3. Growth mechanism
In the present work, the outer oxide layer consisting of loose
and large grains was found to be Ni(OH)2 /NiO. The outer layer
was less protective for the alloy because of its loose nature. The
present microstructures indicated that the growth of the Ni-rich
outer layer agreed with dissolution and precipitation mechanism
[23,24]. Metal ions such as Ni2+ released from the corroding surface
can react with the dissolved oxygen in supercritical temperature
water and be precipitated on the alloy surface to form the NiO outer
layer of the oxide film. Ni(OH)2 at the initially outer surface prob-
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M. Sun et al. / J. of Supercritical Fluids 47 (2008) 309–317
References
Fig. 11. Schematic diagram showing the oxide film grown on Alloy 625 at 450 ◦ C in
oxidizing supercritical water for 250 h.
ably also precipitated in high temperature water during the cool
down [21,27].
The inner oxide layer consisting of fine grains was identified to
be Cr2 O3 . The stainless steels and nickel-base alloys might form
a continuous Cr oxide film if they satisfied the minimum 14% Cr
content [24]. Cr was oxidized much more readily than Ni even at
such comparatively low temperatures between 400 ◦ C and 500 ◦ C.
The Cr oxide layer was, however, continuous and barrier-like and
the diffusivity of Cr in the oxide film was much smaller than that
of Ni, so Cr was retained in the inner layer [30]. The enriched Cr
should react with oxygen diffusing inward through defects in the
oxide film such as micropores, grain boundaries and vacancies [4].
Therefore, a Cr2 O3 inner layer was probably formed as shown in the
present results (Figs. 7 and 10). Fig. 11 shows a schematic diagram of
the oxide film grown on Alloy 625 in oxidizing supercritical water.
5. Conclusion
The oxide films grown on Alloy 625 in oxidizing supercritical
water containing 2.0% H2 O2 at 400 ◦ C, 450 ◦ C and 500 ◦ C were analyzed using weight measurement, GIXRD, SEM equipped with an
EDS, XPS and TEM. The oxide films grown on Alloy 625 in oxidizing supercritical water were duplex structure consisting of an
outer layer of loose and large-grain oxides and an inner layer
of fine-grain oxides. The duplex oxide layer was identified to be
Ni(OH)2 /NiO/NiCr2 O4 /Cr2 O3 /Alloy 625 from outer to inner layer. It
was found that the size of oxide crystals on the alloy surface gradually grew with increasing exposure temperature. A discontinuous
and thin Cr2 O3 layer was formed at 400 ◦ C in oxidizing supercritical water as a result of leaching of Cr3+ as Cr6+ while a continuous
Cr2 O3 layer was formed at 450 ◦ C and 500 ◦ C. The smallest mass
gain and thickness of the oxide film was obtained at 450 ◦ C. The
growth mechanism of duplex oxide layers on Alloy 625 in oxidizing
supercritical water seems to be similar to that in high temperature
water. The Ni(OH)2 /NiO outer layer grew by dissolution and precipitation mechanism, while the Cr2 O3 inner layer was formed by
oxygen diffusing inward and reacting with the retained Cr.
Acknowledgements
We are grateful to the Special Funds for the Major State Basic
Research Projects (2006CB605001), the Science and Technology
Foundation of China, the Science and Technology Foundation of
Liaoning Province (20052012) and the Innovation Fund of Institute
of Metal Research (IMR), Chinese Academy of Sciences (CAS).
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