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Materials at High Temperatures (2005), 22(1/2)
Theoretical
study of steam grown oxides as a
function
of temperature, pressure and p(O2)
Hugh
Davies and Alan Dinsdale
NPL
Materials Centre, National Physical Laboratory, Teddington,
Depending
on their history and composition stainless and 9/12 chrome steels can develop
two
types of protective oxide scale, comprising either
a thin layer of chromium oxide or a more
complex layer with the spinel structure.
Both these types of scale are found in environments
with low oxygen partial pressures such as in heat
exchangers for power generation plant. This
paper is concerned with the identification of the
reason for these variations and to describe how
the methods of equilibrium thermochemistry can
be applied to systems so very far from
equilibrium.
Keywords:
steam grown oxides, chemical equilibria
Solution
and transport of water in oxides
Rüdiger
Dieckmann
Department
of Materials Science and Engineering,
E–mail:
dieck@ccmr.cornell.edu
First,the dissolution of water into different types of oxides is
discussed, including the influence
of the water pressure and the oxygen activity.
Then the transport of water, i.e. of
hydrogen–
containing species, in such oxides and its
dependence on the water pressure and the oxygen
activity is analyzed. Hydrogen–containing
species of relevance in this context are water
molecules, protons and OH groups.
Crystalline and amorphous oxides are considered.
Following
these more general discussions, some specific experimental observations on the
solution and the transport of water and
hydrogen–related species in oxides are reviewed and
discussed. These observations were made by
members of different scientific communities, i.e.
by earth scientists, glass scientists, solid
state electrochemists, etc. Finally, a few possible
consequences of the observations
made and of conclusions drawn from them for the high
temperature oxidation of alloys in water–containing
atmospheres are discussed.
Keywords:
water in oxides, diffusion, oxidation of alloys
Spallation
models and their relevance to steamgrown
oxides
H.E.
Evans
Department
of Metallurgy and Materials, The
An
overview is given of existing models of oxide spallation with particular
emphasis on their
relevance to the mechanical integrity of
steam-grown oxide layers on alloys used in power
generating systems. Although the bulk of
the experimental and modelling work over the last
decade has been undertaken on
alumina-forming high-temperature alloys, the understanding
obtained can be used advantageously to
identify likely key factors affecting the mechanical
response of alloys at lower temperatures.
Various aspects are addressed, such as the mechanisms
of cracking and spallation, but a central issue
is whether creep rates in the multi-layered scale
formed on ferritic alloys can ever be
high enough to relax possible growth stresses and to permit
the Critical Strain Energy Criterion to be used
as a method for predicting oxide spallation.
Keywords:
oxide spallation, steam-grown oxides, critical strain-energy criterion
Mechanisms
of chromia scale failure during
the
course of 15–18Cr ferritic stainless steel
oxidation
in water vapour
A.
Galeriea,
aLaboratoire
de Thermodynamique et Physicochimie Métallurgiques, UMR CNRS /INPG/UJF n° 5614,
BP 75 F-38402 Saint Martin d’Hères cedex
bLaboratoire d’Electrochimie et de Physicochimie des
Matériaux et des Interfaces, UMR CNRS /INPG/UJF n° 5631,
BP 75 F-38402 Saint Martin d’Hères cedex,
cArcelor Innovation, Centre de Recherches d’Ugine, Avenue
Paul Girod F-73403 Ugine Cedex,
ABSTRACT
Breakaway
oxidation of 15–18 % Cr ferritic stainless steels occurring in water vapour is
described
in the temperature range 800–1000°C.
The failure of the protective chromia scale leads
to iron oxide(s) nodule formation with
accelerated kinetics. Characterisation of the (Fe,Cr)2O3
initial oxide scale by Raman
spectroscopy and photoelectrochemistry shows chemical evolution
with oxidation time, with increasing Cr/Fe ratio
before haematite suddenly appears at the
steel-oxide interface. The mechanisms for
such a phenomenon are discussed, first on a thermodynamic
point of view, where it is shown that chromium
(VI) volatilisation or chromia
destabilisation by stresses are not
operating. It is rather concluded that mechanical cracking or
internal interface decohesion provide
conditions for haematite stabilisation. From a kinetic
point of view, rapid haematite growth in water
vapour compared to chromia is thought to be the
result of surface acidity difference of
these two oxides.
Keywords:
chromia scale, ferritic stainless steel
A
survey of test methods for scale adhesion
measurement
P.
Y. Houa and S.R.J. Saundersb
aMaterials Sciences Division, Lawrence Berkeley National
Laboratory,
bNational Physical Laboratory, Teddington,
ABSTRACT
The
spallation of oxide scales from the steam side of tubes in power generation
boilers can
severely limit component life time. The
process involves through scale cracking and crack
propagation at or parallel to the
scale/alloy interface. Many techniques have been developed in
the past two decades to assess the interfacial
fracture toughness of brittle coatings on ductile
substrates. Analyses from fracture
mechanics approaches also allow for quantitative determination
of the fracture toughness, which can be used
for life time prediction modelling. This paper
gives an overview on techniques that have been
used on thermally grown oxides, such as Ni,
Fe,
Ti, Al, and Cr oxides and spinels, and compare the results obtained on each
type of oxide
system from different techniques. In
doing so, the limitations of each technique are illustrated.
The
methods discussed are flexure, scratch, indentation, tensile and compression
tests and
those that perform direct analysis of the
spallation process. The data on steam grown oxide is
sparse, but the test methods discussed
could all be applied to these scale types.
Keywords:
spallation of oxides sales, scale adhesion measurements
Modelling
scale failure in tension (fracture and
spallation)
L.N.
McCartney
NPL
Materials Centre, National Physical Laboratory,Teddington,
The
paper addresses some important issues that relate to the prediction of
through-thickness
cracking and spallation that can occur in
oxide layers subject to local tensile stresses that arise
during cooling following periods in
service where the oxide layers form and thicken. The issues are
addressed in the context of steam
corrosion of ferrous substrates that leads to the formation of three
corrosion layers, namely, spinel,
magnetite and haematite (on the outer exposed surface). For this
system, the magnetite layer develops
tensile stresses that lead to through-thickness cracking in this
layer. The first issue concerns the failure
criteria that should be used when predicting the formation of
through-thickness cracking. A popular
approach is to assume that an oxide layer develops throughthickness
cracks when a critical tensile stress
(the oxide strength) or strain (the oxide strain to failure)
is encountered. Another approach applies
fracture mechanics principles to defects that are
assumed to exist in the oxide layer,
although there is great uncertainty regarding the relevant
defect size distributions that control
behaviour. A third lower bound (and conservative) approach,
that is discussed in some detail, is to consider
the energetics of steady state through-thickness
cracking that avoids the fracture energy
issue of needing to know the defect size that initiates
through-thickness cracking. The
applicability of the three approaches is discussed with regard
to predicting the progressive growth of
through-thickness cracking in the magnetite layer, and
the importance of residual stresses. Example
predictions are made using a proven analytical stress
transfer model that enables simulations
to be made of progressive through-thickness cracking in
the magnetite layer.
The
second issue that is discussed concerns the development of interface cracking
from throughthickness
cracks that can lead to the spallation
of oxide layers. One key factor is the influence of the
spacing of through-thickness cracks that
can determine the size of oxide fragments that might be
released during spallation. Another key
factor is the determination of conditions for steady-state
debond growth that is a critical factor
when considering conditions for spallation of oxide layers.
Issues
that are considered to require further investigation are highlighted.
Keywords:
modelling scale failure, fracture, spallation, interface cracking
The
kinetics of oxidation of diffusion coatings
in
superheated steam
Juan
Carlos Nava, Stephen Goodstine and Edward Sylva
ALSTOM
Power Inc.,
ABSTRACT
The
kinetics of oxidation of Cr-rich, Si–Cr-rich and Al-Cr-rich diffusion layers
have been studied
in superheated steam within the temperature
range 650 and 750ºC. The diffusion layers were
generated in test-pieces of alloy type
P-92 using the pack cementation approach. The thermogravimetric
analytical technique was used to determine
the kinetic in superheated steam over a
period of 100 h. The results were
benchmarked against the oxidation kinetic of the bare P92
alloy at 650ºC. The results of these short-term
experiments indicated a significant improvement
in oxidation resistance where the diffusion
layers did not experience any apparent attack. The
oxidation kinetics of a test-piece of
alloy type S304H were also determined at 750ºC. The latter
type of samples experienced oxidation with the
formation of a 1 μm thick Fe–Cr spinel type
oxide with discrete patches of break-away
oxidation. By comparison the diffusion layers
showed no apparent attack. The
diffusion layer chemistry remained stable even at 750ºC for all
coated composition. The characteristics
of the diffusion layers will be presented.
Keywords : kinetics of oxidation,
diffusion coatings, superheated steam
Japanese
experience with steam oxidation of
advanced
heat-resistant steel tubes in power boilers
N.
Nishimuraa, N. Komaia,
Y. Hirayamaa and F. Masuyamab
aMitsubishi Heavy Industries, Ltd., 5-717-1
Fukahori-machi,
bKyushu
ABSTRACT
Japanese
experience of steam oxidation behavior of advanced heat-resistant steel tubes
in
power boilers is discussed. Severe scale
separation, cracking, and exfoliation were observed in
T91 pendant reheater tubing in a Japanese utility boiler after
around 40,000 hours of operation.
Separation
occurred at the interface between the inner and outer layers of scale. A
highpressure
steam oxidation test rig in which the steam
conditions could be controlled in a similar
manner to that of an actual boiler was
developed and T91 steel samples were tested up to
10,269
hours. The scale separation behavior of T91 was reproduced in the steam
oxidation test.
The
growth rate of T91 was lower than that for conventional 9Cr-1Mo steel reported
by EPRI.
The
scale separation was related to void formation at the interface between the
inner and outer
layers of scale, as well as the
transformation of magnetite into hematite in the outer layer.
Field
exposure testing was carried out for T23 and T122 for 80,000 hours, and the
properties of
steam oxidation scale were obtained.
Keywords:
steam oxidation, advanced heat-resistant steel tubes, power boilers
The
influence of laboratory test procedures on
scale
growth kinetics and microstructure during
steam
oxidation testing
Steve
Osgerbya and Joe Quadakkersb
aNational Physical
bForschungszentrum, Juelich
Laboratory
exposures cannot reproduce all the features present in service conditions. The
experimentalist is faced with the
conflict between increasing the complexity of laboratory tests
to replicate service more closely and keeping
testing costs low by maintaining a simple procedure.
The
influence of various experimental parameters, which can be controlled in the
laboratory,
on the steam oxidation response of materials is
reviewed and recommendations for best
practice are proposed.
Keywords:
scale growth kinetics, microstructure, steam oxidation testing
Fracture
behavior of steam-grown oxide scales
formed
on 2–12%Cr steels
Nobuo
Otsuka
Sumitomo
Metal Industries, Ltd,
A
room temperature three point bent-beam test was conducted for preoxidized
coupon specimens
of
ferritic 2%Cr (T22), 9%Cr (T91), and 12%Cr (T122) steels in order to examine
the
fracture/spalling behavior of
steam-grown oxide scale. Test specimens were reacted with atmospheric
100%
steam at 550–750˚C for 1000-4800 h. Oxide scale thickness of the tested
steels
was 15–1150 ìm for the 2Cr
steel, 30–450 ìm for the 9Cr steel, and 25–60 ìm
for the 12Cr
steel. External tensile strain of up to 1.86% was
loaded to each preoxidized specimen surface
and the fracture/spalling behavior of
steam-grown oxide scale was investigated visually and by
microscopic observation. For the 2Cr steel,
scale greater than 380 ìm exfoliated without
applying any external strain. For
thin-scaled specimens of 15–20 ìm thick, a tensile
strain of
0.25% and more caused through-scale cracking perpendicular to the
scale/metal interface. For
these specimens, cracks along the scale/metal
interface also resulted, and the oxide scale
became separated from the base metal.
For the 9Cr steel, scale exfoliation due to cooling was
not prominent even for specimens with 450 ìm
thick scales. External tensile strain of 0.91%
and 1.86% caused through-scale cracking to the oxide
scale, but the scale/metal interface
remained intact and scale exfoliation did
not take place. This was the same for the 12Cr steel.
Clearly,
spalling resistance of the steam-grown oxide scale was significantly higher for
the 9Cr
and 12Cr steels than for the 2Cr steel.
Keywords:
fracture behavior, steam-grown oxide scales, 2–12%Cr steels
Morphological
characteristics of oxide scales
grown
on H11 steel oxidised in dry or wet air
B.
Pieraggi*, C. Rolland and P. Bruckel
CROMeP,
Research Centre on Tools, Materials and Processes, Ecole des Mines d’Albi,
81013
*E-mail:
Bernard.Pieraggi@ensiacet.fr
ABSTRACT
The
oxidation behaviour in dry and wet air of H11 steel was studied at 600 and 700°C
by
Thermo
Gravimetric Analysis (TGA) and in situ oxidation
in the specimen chamber of an
Environmental
Scanning Electron Microscope (ESEM) equipped with a hot stage specimen
holder. The oxidation kinetics of H11
steels are quite sensitive to the presence of water
vapour
and, although the final mass gains show a good
overall reproducibility, they are locally rather
irregular. The morphology and
microstructure of oxide scale are complex and often heterogeneous,
particularly at 700°C.
In situ oxidation tests permit to follow
the evolution of oxide
scales and to observe several growth
modes of oxide scales. The diversity of the observed scale
growth modes can explain the complexity
and irregularity of scale growth mechanisms. Some
additional in
situ oxidation tests were performed in wet
nitrogen. Morphology and growth
modes of oxide scale grown in wet air and wet
nitrogen differ strongly.
Keywords:
oxide scales, H11 steel, dry or wet air
Steam
oxidation of ferritic steels – laboratory
test
kinetic data
W.
J. Quadakkers, P. J. Ennis, J.Zurek and M. Michalik
E-mail:
j.quadakkers@fz-juelich.de
The
oxidation resistance of 9–12% chromium steels in steam-containing environments
simulating
the service conditions of steam power plant has
been investigated for exposure times
ranging from 1 h up to 10 000 h. In the
long-term experiments the steels were evaluated by the
mass changes determined in exposure tests, which
were interrupted every 250 h, thus providing
information concerning the scale growth and
spalling characteristics. The morphologies of the
oxide scales were examined using optical
microscopy, scanning electron microscopy, secondary
neutrals mass spectrometry and Raman
spectroscopy. Different mechanisms of oxidation
were observed for the various steels in different
temperature regimes and exposed for different
durations. For some steels, the steam
oxidation resistance increased with increasing exposure
temperature. The scale thickness at which
spalling was observed varied according to the exposure
temperature and the interval between thermal
cycles, so that a critical scale thickness for
spalling cannot be derived. The cracking
and spallation of scales was correlated with the type,
morphology and growth of pores and voids in
the scale and could be influenced by the steel
microstructure.
Keywords:
chromium steels, ferritic steels, steam oxidation
Steam
oxidation and spallation in
advanced
gas-cooled reactors
Andy
Rudge
British
Energy, Barnwood,
The
were developed on the principle of delivering
high
pressure (~170 bar) high temperature
(~540°C) steam to
the turbine, thereby benefiting from secondary
circuits
that retain many similarities with conventional
power
plants. Among these similarities,
therefore, are the risks
associated with oxidation and spallation in
high temperature
high pressure steam. However, for a nuclear plant
where the role of the boiler is to provide
essential reactor
cooling as well as delivering steam to
the turbine, sensitivity
to boiler tube failure is far greater than in
conventional
plant. Furthermore, the boilers are housed within
the concrete reactor pressure vessels, and so
inspection,
section removal and boiler replacement
are effectively
impossible. As a result of these factors, a
large amount of
work was carried out in the 1970s and early 1980s
at the
Central
Electricity Research Laboratory (CERL) in
Leatherhead
to improve understanding of steam oxidation
and spallation behaviour. The outcome of that
work
was to develop predictive models for steam
oxidation
and spallation behaviour and thereby demonstrate
that
the design lifetimes of the AGR boilers could be
supported.
Much
of this work is described in [1].
Chemical-mechanical
failure of oxide scales on
9%
Cr steels in air with H2O
M.
Schütze, D. Renusch and M. Schorr
Karl-Winnacker-Institut
der DECHEMA e.V, D-60061 Frankfurt am Main,
E-mail:
schuetze@dechema.de
The
oxidation behaviour of 9% Cr steels P91 and Nf616 has been investigated at 650°C
in dry
air and in air with water vapour, where
particular attention was given to breakaway failure.
Additional
emphasis was given to the quantitative characterisation of the kinetics of
chromium
depletion in the metal subsurface zone
resulting from scale growth, CrO2H4
evaporation, and
scale cracking and healing, with scale cracking
being monitored by acoustic emission measurements.
While
in dry air the steels show protective oxidation behaviour up to 10000 h,
breakaway oxidation may occur already
after 100 h in humid environments, which was correlated
with the stronger Cr-depletion and the
development of intrinsic oxide scale growth
stresses exceeding a critical value, in
the case of water vapour containing air. In the paper the
different parameters that are responsible
for breakaway oxidation were identified and discussed
with regard to the role of water vapour in the
environment. As a conclusion it turns out that
breakaway is not a consequence of
Intrinsic Chemical Failure (InCF) but of a Mechanically
Induced Chemical Failure (MICF).
Keywords:
oxidation behaviour of 9% Cr steels, water vapour
Experience
with the use of the acoustic
emission technique for monitoring
scale
failure in wet and dry environments
Daniel
Renusch and Michael Schütze
Karl-Winnacker-Institut
der DECHEMA e.V.,
A brief survey of the acoustic emission technique for
monitoring scale cracking and failure on
2.25–24% Cr steels in wet and dry environments is
given. A number of acoustic emission test
rigs are described. Some of the more
simple test rigs are used for testing small oxidation
coupons during isothermal oxidation. More sophisticated rigs
have been used for testing full
size heat exchanger tubes during thermal cycling.
Most acoustic emission measurements in a wet
environment come from testing at
temperatures below 650°C. There are examples from Alloy 800 and
thermal barrier coatings
that were tested at higher temperatures, 900°C and 1100°C,
respectively. Through the years
acoustic emission tests have been performed in dry air, dry
air+10%H2O, dry air+0.5%SO2,
and Ar+5%H2+50%H2O. Consequently, a wide variety of exposure
temperatures and
atmospheres can be investigated using acoustic emission
techniques.
Qualitative acoustic emission results can detect when
scale cracking occurs at exposure
temperature, where such cracks are produced by growth stress.
Acoustic emission signals have
been measured during sample cooling, where the signal
arises from scale cracking that is
caused by the thermal expansion mismatch stress. Measured
results have clearly shown that
scale cracking caused by both growth stress and thermal
expansion mismatch stress are
affected by water vapor in the exposure environment. Post-test
metallographic investigations
show that crack orientation and the oxide scale phases are
also affected by the gas composition
in the test rig. Additionally the sample mass gain and
scale thickness is affected by water
vapor content.
Finally, acoustic emission techniques are helpful for
understanding the phenomena of
breakaway oxidation and spallation/exfoliation.
Keywords: acoustic emission, Cr steels, water vapor,
growth stress, thermal expansion mismatch stress, breakaway
The
effect of water vapor in increasing growth
stresses
in the oxide scale on martensitic steam
plant
alloys
Alexander
Doncheva, Harald Fietzekb,
Vladislav Kolarikb, Daniel Renuscha
and
Michael
Schützea
aKarl-Winnacker-Institut der DECHEMA e.V., D-60486 Frankfurt
am Main,
bFraunhofer Institut für
Investigation
of the oxidation behavior of 9% Cr steels at 650°C
in dry air and air + 10% H2O
have shown that oxide scale growth stresses may
play a significant role in breakaway. For this
reason, preliminary studies on the
development and characteristics of growth stresses in oxides
on these materials have begun. These studies
include in situ acoustic
emission (AE) for
monitoring scale cracking, deflection
testing in monofacial oxidation (DTMO) and in situ
X–ray measurements. The measurements
were complemented by detailed post-experimental
metallographic investigations.
From
the DTMO measurements the stresses in the oxide on the specimens tested in
humid