Volume 6 Preprint 104
Oxidation study of alumina-forming alloys
S. Chevalier, C. Houngninou, S. Paris, F. Bernard, E. Gaffet, Z.A. Munir, J.P. Larpin and G. Borchardt
Keywords: alumina-forming alloys, intermetallics, pack aluminisation, Al2O3 scales, reactive element, oxidation mechanisms
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Volume 6 Paper H070
Oxidation study of alumina-forming alloys
S. Chevalier1, C. Houngninou1, S. Paris1,2, F. Bernard1, E. Gaffet2, Z.A.
Munir3, J.P. Larpin1, G. Borchardt4
de Recherches sur la Réactivité des Solides, UMR 5613
CNRS, Université de Bourgogne, 9 avenue Alain Savary, BP 47870,
21078 Dijon cedex, France.
2Laboratoire Métallurgies et Cultures, UMR 5060 CNRS/UTBM, Site de
Sévenans, 90010 Belfort, France.
3Department of Chemical Engineering and Materials Sciences,
University of California, Davis CA 95616, USA.
4Institut für Metallurgie, Technische Universität Clausthal, Robert Koch
Strasse 42, 38678 Clausthal-Zellerfeld, Germany.
intermetallics have been oxidised in air under atmospheric pressure
and isothermal conditions. The high temperature efficiency of the
thermally oxide scales grown on these three materials were compared.
The kp values deduced from the parabolic plots of weight gain curves
showed that α-Al2O3 composed the oxide scale on all samples
oxidised at T>1000 °C. For lower temperatures, transient alumina
phases are observed, except for the Fe-20Cr-5Al alloys, for which only
α-Al2O3 was characterised whatever the temperature up to 800 °C. The
addition of a reactive element (Y or Y2O3) drastically improved the high
temperature oxidation resistance of the tested materials, since the
oxide growth rate decreased and the alumina scale adherence strongly
increased. The oxide morphologies and the X-ray diffraction patterns
helped to compare the high temperature behaviours under oxidant
Keywords: alumina-forming alloys, intermetallics, pack aluminisation,
Al2O3 scales, reactive element, oxidation mechanisms.
The high temperature resistance of steels is based on their capacity to
form homogeneous, adherent and dense protective oxide scales.
Alumina-forming alloys are good candidates to resist to high
temperature oxidation atmospheres because alumina scales, especially
α-Al2O3, exhibit the expected properties . Intermetallic compounds,
in particular aluminides, have been developed more recently. They
exhibit also excellent high temperature resistance because they form
Al2O3 scales acting as diffusion barriers during the oxidation process
at high temperature [2,3,4,5,6,7,8,9]. The major problem encountered
by intermetallic compounds is their brittleness, especially at room
temperature, which creates difficulties during their production [8,9].
Another possibility to elaborate aluminides consists in introducing Al
at the steel surface using an Al deposition technique. The pack
cementation process has been commonly used to cover iron-based
The aim of this work is to compare the high temperature oxidation
properties of Fe-20Cr-5Al, pack-aluminised Fe-30Cr model alloys and
FeAl intermetallics. Isothermal oxidation tests in the temperature
range from 800 to 1200°C have been performed to check their ability
to form alumina scales with the expected protective properties.
The use of two stage oxidation experiments under
observation of transmission electron microscope (TEM) cross-sections
and the use of more classical analytical techniques, as scanning
electron microscope (SEM) and X-ray diffraction (XRD), help to describe
the high temperature oxide scale growth and to understand the
differences observed between the tested samples.
A particular attention will be paid to the role played by Y or Y2O3
addition on the high temperature oxidation behaviour when added to
Fe-20Cr-5Al “model” steels were classically elaborated in laboratory
conditions by melting route. They were called “model” alloys since they
did not contain any minor elements to avoid the well known influence
of alloying elements on the high temperature corrosion behaviour
[19,20,21]. There was a low amount of sulfur (S>10ppm) in order to
susceptible to drastically influence the high temperature behaviour.
Yttrium was introduced as an alloying element in Fe-20Cr-5Al steels
to get Fe-20Cr-5Al-0.1Y alloys.
Activated Field Activated Pressure Assisted Synthesis (MAFAPAS)
technique . It consisted in applying a mechanical pressure together
with an electric field on mechanically activated powders. The complete
process was described elsewhere [28,29,30,31,32] and yielded to
obtain FeAl materials with controlled densification and nanostructure.
Indeed, the density of the so-prepared FeAl samples was over 99 %
with an average crystallite size of 40-80 nm. Yttria was added in FeAl
intermetallics as dispersed particles during the mechanical activation
of powders to obtain FeAl-0.1Y2O3 (wt. %). The quantity of yttria was
determined according to the literature in order to avoid any
“overdoping” effect, generally observed when the reactive element
content is too high in metallic materials .The aluminisation of Fe30Cr “model ” alloys was carried out using the pack cementation
tecnique which involved a pack mixture made of 15 wt.% Al, 3 wt.%
NH4Cl and 82 wt.% Al2O3 powders. The metallic substrate was
embedded in the pack mixture for 5 hours at 1000 °C under argon
atmosphere, to avoid oxidation during the process. The surface of the
so-coated steels was covered with (Fe,Cr)3Al and (Fe,Cr)Al intermetallic
phases. The aluminised zone was around 100 µm thick . Yttria was
introduced as a coating elaborated via the Metal-Organic Chemical
described elsewhere . An Y2O3 film of around 200 nm thick was
applied on the aluminised surface or on the Fe-30Cr substrate, before
the pack-cementation process.
The samples were oxidised over the temperature range from 800 to
1200 °C in laboratory air under atmospheric pressure. Two stage
oxidation experiments were performed at 1000 °C. The samples were
firstly oxidised in
under 200 mbar pressure. The oxidant
atmosphere was then evacuated without cooling the samples and
(isotopic purity ≈ 91%) under the same pressure. The
oxygen isotope profiles were monitored by Secondary Neutral Mass
The corrosion products were characterised by scanning electron
microscopy (SEM) with a field emission gun (FEG) coupled with an
energy dispersive X-ray analyser (EDX). The thermally grown oxides
were identified by X-ray diffraction (XRD), using the Kα1 copper
radiation (λ=0.154056 nm).
Isothermal oxidation tests
Figure 1 exhibits the oxidation kinetic curves of the Fe-20Cr-5Al steel
in the temperature range from 800 to 1200 °C.
∆m/S (mg.cm )
Figure 1: Isothermal oxidation kinetic curves for Fe-20Cr-5Al steels in
air under atmospheric pressure.
The weight gain curves obey the parabolic rate law whatever the
studied temperature. The parabolic constants were deduced from the
slopes of the ∆m/S=f(t1/2) plots and are summarised in Table 1.
Table 1: Parabolic constants (kp) for Fe-20Cr-5Al, aluminised Fe-30Cr
and FeAl samples oxidised in air under atmospheric pressure.
Figure 2 shows the weight gain curves for the aluminised Fe-30Cr
oxidised from 800 to 1080 °C. The weight gains increase with the
temperature and the curves obey a parabolic law, as depicted in Table
∆m/s (mg.cm )
Figure 2: Isothermal oxidation kinetic curves for aluminised Fe-30Cr
steels in air under atmospheric pressure.
∆ m/S (mg.cm )
Figure 3: Isothermal oxidation kinetic curves for FeAl intermetallics in
air under atmospheric pressure.
The kinetic curves of the FeAl intermetallic compounds oxidised at
800, 1000 and 1100 °C are shown in Figure 3. The kinetic curve at 800
°C obeys a parabolic law from the beginning of the oxidation process.
At 1000 and 1100 °C, a rapid increase of the weight gain per unit area
is observed during the first hour of oxidation, followed by a classical
parabolic shape. The corresponding kp values are summarised in Table
For all tested samples, the kp values are close to each other. Note that
the parabolic rate constant is lower for Fe-20Cr-5Al at 800 °C and for
FeAl at 1000 and 1100 °C respectively.
The effect of yttrium or yttria was tested at 1000 °C. The weight gain
curves (Figure 4) clearly evidence that the addition of the reactive
element or the reactive element oxide decreases the oxidation rate and
the weight gain during 100 h oxidation, compared to the undoped
materials (Figure 1 to 3). The single exception concerns the aluminised
sample for whith the yttria coating applied after the pack cementation
process and which exhibits faster oxidation kinetics and a higher
weight gain, compared to the only aluminised sample (Figure 2).
FeCr aluminised then Y2O3-coated
Y2O3-coated FeCr then aluminised
∆m/S (mg.cm )
Figure 4: Isothermal oxidation kinetic curves for Fe-20Cr-5Al-0.1Y,
Y2O3-coated Fe-30Cr followed by aluminisation, Aluminised Fe-30Cr
followed by Y2O3 coating and FeAl-0.1 Y2O3 intermetallics in air under
atmospheric pressure at 1000 °C.
Oxide scale characterisation
The morphologies of the oxide scales formed on Fe-20Cr -5Al steels
at 1000 °C and 1100°C are different. At 1000 °C, the oxide scale is
largely convoluted with anchor points of the scale on the metallic
substrate. Many spalled areas were observed all over the surface. At
1100 °C, the oxide surface remains flat with much spallation. The
oxide scale fracture cross-section exhibits coarse grains with some
needles on the oxide surface. For all tested temperatures (from 800 to
1200 °C), α-Al2O3 was identified by XRD.
After 100 h at 1000 °C, the aluminised specimens are covered with
small oxide grains. At 1080 °C, the oxide scale is slightly convoluted
and still composed of small oxide grains. The oxide scale is mainly
composed of α-Al2O3 with transient alumina phases at T<1000 °C.
The FeAl intermetallic compound exhibits an “hairy” microstructure,
since its surface is covered with thin needles after 100 h at 1000 °C in
air. At 1100 °C, the oxide scale is composed of classical small
platelets. The XRD experiments clearly evidenced transient alumina
phases at 1000 °C, whereas α-Al2O3 was only detected at 1100 °C.
Two-stage oxidation experiments
The SNMS profiles of
are presented in Figure 5. The
experiments were performed at 1000 °C for the three tested materials.
The oxygen isotope profiles clearly indicate the presence of two major
for the three materials: one peak is located at the oxide
surface and the second main
peak is situated within the pre-
existing oxide scale close to the metal-oxide interface (Figure 5 a to
experiments at 1000 °C in
a) Fe-20Cr-5Al, b)
aluminised Fe-30Cr and c) FeAl intermetallics.
intermetallic compounds oxidation curves obey the parabolic rate law.
kp values deduced from the parabolic plots are comparable for
T>1000 °C. At lower temperature, the Fe-20Cr-5Al steel exhibited a kp
value two order of magnitude lower than the values determined for the
aluminised steel and the intermetallics. Grabke et al.  exposed
Arrhenius plots of kp together with the corresponding alumina phases
formed on NiAl: α-Al2O3 at T>950 °C, θ-Al2O3 at 850<T<950 °C and γAl2O3 at T<850 °C. The comparison of our results to the Grabke's
Arrhenius plots evidences that α-Al2O3 is formed on the Fe-20Cr-5Al
steels all over the temperature range (from 800 to 1200 °C), that
transient alumina phases (θ and γ) grow at low temperature for the
aluminised samples and the intermetallic FeAl compounds, whereas αAl2O3 is the main thermally grown phase for T>1000 °C on these two
These conclusions are confirmed by the observation of the oxide
morphologies and above all by the XRD analyses, which identify
transient alumina phases at low temperature for the aluminised steels
and the FeAl specimens.
The three tested materials possess better high temperature behaviour
with the presence of yttrium or yttria, verifying hence the beneficial
effects of the so-called reactive elements . Note that for the
aluminised steels, the way to introduce the yttria has a major influence
on its high temperature oxidation performance. When it is introduced
after the aluminisation process, the yttria coating does not improve
the oxidation resistance; the oxidation rate and the weight gain during
100 h oxidation tests are higher, compared to those of the undoped
aluminised samples. This detrimental phenomenon was already
observed [33,46,47] and attributed to an “overdoping” effect and bad
incorporation of the reactive element within the growing alumina
scale, inhibiting then any beneficial effect of the reactive element.
The oxidation mechanisms were clarified at 1000 °C on the Fe-20Cr-
5Al steels, the aluminised Fe-30Cr steels and the FeAl intermetallic
compounds. Both inward diffusion of oxygen and outward diffusion of
aluminium participate to the growth of the alumina scales. Two-stage
oxidation experiments at lower temperature (800 °C for example) are
actually in progress to understand the effect of the presence of
transient alumina phases on the transport mechanism of the thermally
grown alumina scales.
The oxidation behaviour of Fe-20Cr-5Al steels, pack cemented
aluminised Fe-30Cr steels and FeAl intermetallic compounds were
tested in air between 800 and 1200°C. Kinetic results and XRD
characterisation evidenced that the alumina scales were composed of
α-Al2O3 at high temperature (T>1000 °C). Transient alumina phases
(θ-Al2O3 and γ-Al2O3) were identified at lower temperature (T<1000 °C)
for the aluminised and the FeAl specimens. α-Al2O3 was the main
phase even at 800 °C for the Fe-20Cr-5Al steel.
The introduction of yttrium or yttria gave the well known reactive
element effect, which consisted in decreasing the oxidation rate and
improving the oxide scale adherence. For the aluminised samples, the
beneficial effects were observed when the yttria coating was applied
prior to the aluminisation.
Outward diffusion of aluminium and inward diffusion of oxygen
constituted the main diffusive specie paths during the growth of the
alumina scales at 1000 °C.
The comparison of the preliminary results clearly evidenced that pack
aluminisation or the use of FeAl intermetallic could replace FeCrAl
steels in order to form protective alumina scales, even if it is obviously
to early to relate the base material microstructure to the alumina scale
The authors are thankful to Dr G. Strehl (Post-doc at the University of
Burgundy) for the two-stage oxidation experiments and to S. Weber
(Ecole des Mines, Nancy, France) for the SNMS profiles. A part of this
study was performed with the financial support of a PROCOPE program
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