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Materials
at High Temperatures (2005), 22(3/4)
Phase
identification of oxide scale on low carbon
steel
S.
Birosca and R. L. Higginson
IPTME:
University,
Loughborough,
ABSTRACT
The
classical model of iron oxide scale layers; wu¨stite, magnetite and hematite,
is more complicated
in reality and its properties change with the
factors that affect their development, such as time,
temperature, alloying elements in the steel and
oxidation environment. An understanding of the oxide
scale formation and its properties can only be
achieved by careful examination of the scale
microstructure. Throughout this study, electron
back-scatter diffraction (EBSD) has proved to be a
powerful technique for verifying the individual phases
in the oxide scale accurately. The results show
that EBSD can be used to give a complete
characterisation of the oxide scale formed on low carbon
steel surface. Microstructural features such as grain
size, shape and grain boundary characteristics
have been successfully determined and analysed.
Keywords:
steel, oxidation, phase identification, electron back-scatter
diffraction
In-situ
scanning electron microscopy and electron
backscatter diffraction investigation on the
oxidation of pure iron
B.
Poter,
Max-Planck-Institute
for Iron Research Ltd.,
ABSTRACT
a-iron samples, with a 4 to 6 nm oxide layer on top of
the surface due to preparation, were oxidised in
situ within a scanning electron
microscopy equipped with electron back-scatter diffraction. At 773 and
973
K, two different growth mechanisms during the initial stage of iron oxide
formation could be
observed, layer-by-layer growth at 773K and island
growth at 973 K. The growth mechanism at
973K
is correlated to the formation of a thin layer of Fe1_XO (wu¨stite)
prior to Fe3O4 (magnetite)
crystallisation whereas at 773K only
magnetite was found as newly grown oxide. Magnetite showed
a strong epitaxial relationship to a-iron
with its {111} plane growing preferentially on the {110}
plane of iron.
Keywords:
oxidation of pure iron, Fourier electron-electron back-scatter
diffraction
Effect
of water vapour on growth and adherence
of chromia scales formed on Cr in high and low
pO2-environments at
1000 and 1050_C
M.
Michalik, M. Ha¨ nsel, J. Zurek, L. Singheiser and W.J. Quadakkers
Forschungszentrum
Julich GmbH, IWV-2, PB 1913, 52425
ABSTRACT
The
oxidation behaviour of pure Cr at 1000 and 1050_C
was studied in Ar–O2 and Ar –H2–H2O
mixtures. It was found that in the low-pO2 gases
the oxide scales exhibited higher growth rates than
in the high-pO2 gases.
The scales formed in the low-pO2 gases showed substantially
better adherence
during cooling, than scales formed in the high-pO2 gases.
These differences in growth rate and
adherence can be correlated with differences in
size and location of the in-scale voids formed during
the isothermal exposure. Exposures in Ar-O2-H2O
mixtures revealed that the differences in scale
growth rates as well as in scale void formation and
growth are not primarily related to differences in
the oxygen partial pressure of the atmosphere but
to the presence of water vapour in the test gas. At
sufficiently high H2OyO2-ratios,
water vapour promotes oxide formation at the scaleymetal
interface
thereby suppressing excessive growth of existing voids,
and also as a consequence improved scale
adherence. Whether the enhancement of inward
scale growth is related to transport of H2O-
or H2-
molecules or due to OH_ ions,
cannot be derived with certainty from the present results.
Keywords:
water vapour, growth and adherence of chromia scales, Cr, p(O2)-environments
Influence
of the oxidation environment on scale
morphology and oxidation rate of Fe–22Cr
Anette
N. Hansson and Marcel A.J. Somers
Department
of Manufacturing Engineering and Management,
ABSTRACT
The
oxidation behaviour of a commercial Fe –Cr alloy with 22 wt% Cr was
investigated at 1173K in
Ar –
9 H2 with 1% H2O (pO2 ¼ 9:8610_19),
in air with 1% H2O (pO2 ¼ 0:208),
and in a
combination of the two atmospheres. The oxide
morphology was investigated with X-ray diffraction
and scanning electron microscopy. The oxide layer
consisted of MnCr2O4 on
top of Cr2O3.
Small
oxide whiskers were present at the surface after oxidation in Ar – 9 H2 with
1% H2O but not
after oxidation in air with 1% H2O.
For samples initially oxidised in air with 1% H2O,
the
oxideyalloy
interface was wrinkled and covered with a SiO2 layer.
SiO2 particles had developed at
a rather flat oxideyalloy
interface for samples initially oxidised in Ar – 9% H2 with
1% H2O. The
results obtained can be explained assuming that oxide
growth occurs by cation diffusion only in Ar – 9
H2 with
1% H2O, whereas both cation and anion diffusion
contribute to the growth in airyH2O.
Keywords:
oxidation environment, scale morphology, oxidation rate, Fe– 22Cr
Microstructural
investigation of the effect of water
vapour on the oxidation of alloy 353 MA in oxygen
at 700 and 900_C
T.
Jonssona, S. Canovica,
F. Liua, H. Astemanb,
J.-E. Svenssonb, L.-G.
Johanssonb and M. Halvarssona
aDepartment of Applied Physics,
Chalmers University of Technology, SE-412 96 Go¨ teborg,
bDepartment of Environmental
Inorganic Chemistry, Chalmers University of Technology, SE-412 96 Go¨ teborg,
ABSTRACT
The
objective of this work was to study the impact of water vapour on the corrosion
behaviour of the
austenitic stainless steel 353MA at 700 and 900_C
through a detailed microstructural characterisation
of the oxide scales formed, after 168 hours, in O2 and
O2 with 40% H2O.
The oxidized samples were
investigated by scanning electron microscope/energy
dispersive X-ray, focused ion beam and
transmission electron microscope/energy dispersive
X-ray. At 700_C 353MA forms a Cr-rich
protective (Fe,Cr)2O3 oxide
scale, with some silica at the oxideymetal
interface. Breakaway oxidation
occurs in H2OyO2 mixtures
because the oxide scale is depleted in Cr due to the formation of
CrO2(OH)2(g).
However, the microstructural investigation indicated that a healing Cr-rich
oxide layer
formed beneath the Fe-rich oxide after some time. This
could be a result of the high CryFe
ratio of
353MA.
The behaviour at 900_C
was different. In spite of the loss of Cr from the oxide scale,
breakaway oxidation did not occur, i.e.
the oxide scale remained protective. The microstructural
investigation showed a thick, almost continuous
silica layer at the oxideymetal interface, which may
act as a diffusion barrier at the higher
temperature.
Keywords:
353MA, high temperature oxidation, water vapour effect, silica,
microstructure, scanning electron
microscope, focused ion beam, transmission
electron microscope, energy dispersive X-ray
Oxidation
of potential SOFC interconnect
materials, Crofer 22 APU and Avesta 353 MA, in
dry and humid air studied in situ by X-ray diffraction
Maria
Jose Garcia-Vargasa, Laurent Lelaita,
Vladislav Kolarikb, Harald Fietzekb and
Maria
aEdFyEIfER
(European Institute For Energy Research),
bFraunhofer-Institut
fu¨ r Chemische Technologie (ICT),
ABSTRACT
Several
stainless steels have been developed for the use as interconnect materials in
solid oxide fuel
cells in order to reduce costs, while maintaining the
required performance. The materials however, are
subjected to humid air, at high temperatures up
to 800_C leading to enhanced oxidation. High
temperature X-ray diffraction, combined with field
emission – scanning electron microscopy, has been
applied to study in situ the
influence of water vapour on the chromia scale formation on the ferritic
Crofer
22 APU and the austenitic Avesta 353 MA alloys in comparison to their dry air
oxidation
behaviour. Both materials form at 800_C,
during the first 100 h exposure, Cr2O3 and
MnCr2O4, the
latter mostly in the surface region of the oxide
scale. Additionally, Crofer 22 APU forms internal
Al2O3 precipitates,
while on Avesta 353 MA a SiO2 layer is found beneath the
outer oxide scale.
High
temperature XRD indicates stress formation and relaxation in the Cr2O3 scale
formed in humid
air, especially for Crofer 22 APU.
Keywords:
SOFC interconnect, oxidation in dry and humid air, in
situ X-ray diffraction
Effect
of component thickness on lifetime and
oxidation rate of chromia forming ferritic
steels in
low and high pO2 environments
P.
Huczkowski, S. Ertl, J. Piron-Abellan, N. Christiansena,
T.Ho¨ flerb, V. Shemet, L. Singheiser and W.J. Quadakkers
Forschungszentrum
Ju¨ lich, Institute for Materials and Processes in Energy Systems, D-52425 Ju¨
lich,
aHaldor Topsøe AyS,
ABSTRACT
Long
term oxidation tests were carried out with a high-Cr
ferritic steel at 800_C and 900_C
in
simulated cathode and anode gas of a solid oxide
fuel cell (air and an AryH2yH2O
mixture
respectively). It was found that with decreasing
sample thickness the life time of the steel decreases
due to breakaway phenomena. This effect is caused
by faster exhaustion of the chromium reservoir
from the bulk alloy in the case of thinner
components. During air exposure the oxidation rates
increase with decreasing specimen thickness and this has
to be taken into account in the calculation of
the Cr-reservoir exhaustion. This thickness
dependence is not found during the exposures in simulated
anode gas. Hence, especially for thin walled
components, the oxidation rates in anode gas are
substantially smaller and thus the life times are
longer than during air exposure. The differences in
oxidation behaviour in the two environments are
discussed on the basis of scale formation
mechanisms involving microcrack formation in the
surface oxide scale and depletion of major and
minor alloying additions in the bulk alloy.
Keywords:
high temperature oxidation, ferritic steels, chromia scales,
breakaway oxidation
Long
term steam oxidation of TP 347H FG in
power plants
A.N.
Hanssona, L. Korcakovab,
J. Halda and M. Montgomerya
aDepartment
of Manufacturing Engineering and Management, Technical University of Denmark,
Kemitorvet b. 204, DK-2800 Kgs.
bMaterials
Research Department, Risø National Laboratory, Frederiksborgvej 399, PO Box 49,
DK-4000 Roskilde, Denmark
ABSTRACT
The
long term oxidation behaviour of TP 347H FG at ultra supercritical steam
conditions was
assessed by exposing the steel in test superheater loops
in a Danish coal-fired power plant. The
steamside oxide layer was investigated with
scanning electron microscopy and energy dispersive Xray
diffraction in order to reveal the effect of
oxidation time and temperature on the microstructure.
A
double layered oxide formed during steam oxidation. The morphology of the inner
Crcontaining
layer was influenced by the oxidation temperature. At
temperatures below 585_C, it
consisted of regions of Fe –Ni – Cr spinel
surrounded by Fe –Cr oxide. At higher temperatures almost
the entire inner oxide layer was composed of Fe –Cr
oxide. Possible mechanisms for the oxide growth
are discussed and it is suggested that faster Cr
transport within the alloy at higher temperatures
explains the change in morphology.
The
thickness of the inner oxide layer appeared to be independent of oxidation time
and
temperature for exposes less than 30 000 h;
however, after 57 554 h, the thickness had increased
significantly at the lowest temperatures.
Keywords:
long term steam oxidation, TP 347H FG, power plants
Role
of nickel in the oxidation of Fe–Cr–Ni alloys
in air –water vapour atmospheres
R.F.A.
Pettersson, J. Enecker and L. Liu
Corrosion
and Metals Research Institute, Drottning Kristinas va¨g 48, 114 28
ABSTRACT
A
series of experimental austenitic alloys has been produced in which the nickel
content ranges from
14 to 43%, with constant levels of 20%Cr, 1%Mn and 0.5%Si. A
combination of isothermal,
discontinuous and cyclic oxidation testing has been
used to elucidate the performance in dry air and
in air with 10%, 45% or 62% water vapour at 700_C
and 1000_C. Evaluation was by means of
thermogravimetry, surface analysis with
glow discharge optical emission spectroscopy and scanning
electron microscopy.
Nickel
is shown to have several roles: it accelerates the kinetics of chromia
formation yet
suppresses chromia spallation at 700_C.
At 1000_C, it strongly decreases the breakaway oxidation
and
spalling associated with iron oxide formation. This
effect is particularly marked in environments
containing water vapour, where the material loss
may be decreased 10-fold by an increase in the
nickel content. Results correlate to thermodynamic and
kinetic data which show nickel to increase the
chromium activity and diffusivity in the alloy.
Keywords:
nickel, Fe– Cr–Ni alloys, air – water vapour atmospheres
High
temperature oxidation behavior of a
D.A.
Helmick, G.H. Meier and F.S. Pettit
Department
of Materials Science and Engineering,
ABSTRACT
Molybdenum
base alloys with the addition of small amounts of silicon (2 – 4.5 wt.%) and boron
(~1
wt.%) can form a passivating layer protecting the
alloy from further rapid oxidation. When such
molybdenum base alloys are exposed to oxidizing
environments at high temperatures, a borosilicate
glass layer can form that will reduce the transport
of oxygen to the alloy to limit further oxidation.
Oxidation
is then controlled by diffusion through the borosilicate glass layer. The focus
of this
research was to study the mechanisms and kinetics of
high temperature oxidation of a Mo–Si –B
alloy. The base alloy has a composition of Mo – 3Si –
1B (wt.%) and was studied in a variety of gas
environments over a range of temperatures in order
to elucidate the critical factors that allow it to
develop a protective borosilicate glass layer. The
borosilicate glass layer is protective when no
continuous channels exist in the layer extending
from the gas interface to the alloy interface. The
borosilicate layer is believed to contain channels
in the early stages of development and the
elimination of the channels is obtained by
appropriate control of the temperature and gas flow
conditions whereby MoO3 is
removed via vaporization while the borosilicate viscosity is not
increased due to loss of B2O3.
Once the borosilicate layer is continuous and free of channels,
subsequent oxidation occurs by inward diffusion
of oxygen and the outward diffusion of molybdenum
through this layer with vaporization of MoO3 occurring
at the gasyborosilicate layer interface, and
MoO2 and
additional borosilicate forming at the alloyyMoO2 interface.
Keywords:
high temperature oxidation behavior, Mo – 3Si – 1B(wt.%)
alloy
The
halogen effect for improving the oxidation
resistance of TiAl-alloys
A.
Donchev, H.-E. Zschau and M. Schu¨ tze
DECHEMA
e.V Karl-Winnacker-Institut, Theodor-Heuss-Allee 25, D-60486 FrankfurtyMain,
ABSTRACT
Alloys
based on TiAl intermetallics are potential candidates for high temperature
applications in e.g.
aero engines or automotive engines because of their
low specific weight and good high temperature
strength. To improve their oxidation resistance at
temperatures up to 1000_C the halogen effect offers
an innovative and cost-effective way. The addition
of small amounts of halogens into the surface
leads to the preferential formation of gaseous
aluminium halides which are oxidised to aluminium
oxide during their outward migration forming a dense,
protective and slowly growing alumina scale
on the surface. In this paper two methods were
used to apply halogens to the surface, ion implantation
(F
and Cl) and a liquid phase process (F). Ion beam analysis with detection limits
in the ppm-range
was applied to quantify the needed amount of
halogens to achieve the halogen effect. Thermocyclic
oxidation experiments at 900_C
were performed in laboratory air and wet air. Depth concentration
profiles of fluorine were measured by PIGE within the
first 1.4 mm without destruction of the sample
before and after oxidation. Furthermore, the loss of
fluorine during heating up and oxidation was
measured characterising the stability of the effect.
Simultaneous RBS-measurements of the O-, Aland
Ti-depth
profiles prove the formation and growth of an almost pure alumina scale.
Correlation
with the fluorine profiles validates the proposed
model for the halogen effect. Furthermore,
metallographic methods, REM, EPMA, AES and the proton
micro beam (PIXE) were applied to
study cross-sections. A virtually pure alumina scale
was found after F-treatment and oxidation up to
1500
hours at 900 – 1000_C in air. The fluorine
depth profiles after ion implantation and liquid phase
treatment, respectively, show similar levels for
both methods before and after oxidation. The
development of the fluorine interfacial
concentration underneath the oxide scale as a function of
oxidation time and temperature was recorded. The
results are discussed in the light of the existing
model considerations on the halogen effect and with
regards to differences in the behaviour between
F-
and Cl-doping.
Keywords:
halogen effect, oxidation resistance, TiAl-alloys
Photoelectrochemical
and Raman imaging studies
of chemical and mechanical properties of thermally
grown titanium oxide scales
Y.
Woutersa, A. Galeriea,
P. Bouvierb, M. Mermouxb and
J.-P. Petitb
aLaboratoire de Thermodynamique
et de Physicochimie Me´ tallurgiques, UMR CNRSyINPGyUJF
5614
bLaboratoire d’Electrochimie et
de Physicochimie des Mate´ riaux et des Interfaces, UMR CNRSyINPGyUJF
5631
Institut
National Polytechnique de Grenoble – Ecole Nationale Superieure d’Electrochimie
et d’Electrome´ tallurgie de
BP
75 Domaine Universitaire, F – 38402 Saint-Martin-d’He` res Cedex
ABSTRACT
The
present work focuses on thin TiO2 scales (up to 3 mm).
The use of both micro-photoelectrochemistry
(MPEC)
and Raman imaging aims to get chemical and mechanical information at a
micron scale. In particular, the MPEC technique allows
images of the history of strain and adhesion
levels of the oxideymetal
interface to be realised. A set of photocurrent images recorded at different
applied potentials allows different images to be
constructed, Structural Quality Image (SQI) and
Interface
Cohesion Image (ICI). In addition, Raman spectroscopy images the chemical nature
of the
scale, topography of the surface and local stress
level in the scale. Combining both techniques is a
powerful tool in elucidating the mechanisms of thermal
oxidation.
Keywords:
thermal oxidation, titanium, MicroPhotoElectroChemistry (MPEC), Raman
spectroscopy
Effects
of Pt on the short-term oxidation behavior
of g-Niþ+g’-Ni3Al
alloys
Shigenari
Hayashia and Brian Gleesonb
aDivision of Materials
Science and Engineering,