Materials and Corrosion 2005, 56, No. 11
Oxidation of FeCrAl alloys at 500 – 900 8C in dry O2
801
Oxidation of FeCrAl alloys at 500 – 900 8C in dry O2
H. Josefsson*, F. Liu, J.-E. Svensson,
M. Halvarsson and L.-G. Johansson
This paper reports on the oxidation of a commercial FeCrAl alloy, Kanthal AF, in the temperature range 500 – 900 8C. The samples
are exposed isothermally in dry oxygen for up to 72 h using a thermo-balance. In addition, 168 h exposures are carried out in a tube
furnace. The exposed samples are investigated by grazing angle Xray diffraction (XRD), scanning electron microscopy/energy dispersive X-ray analysis (SEM/EDX), and auger electron spectroscopy (AES).
The rate of oxidation increases with temperature, the kinetics
being parabolic in the range 700 – 900 8C. At all exposure temperatures, most of the sample surface is covered by a thin smooth base
oxide. In addition, RE-rich particles, with a typical size of 1 – 3 lm
form. At 800 and 900 8C patches of thick oxide appear, featuring
needle-formed crystallites situated on top of the base oxide. The
thick oxide usually forms around Y-rich oxide particles. The concentration of iron and chromium in the oxide decreases with increasing temperature. XRD proves the formation of a-Al2O3 already at 700 8C. The low temperature of formation of a-Al2O3 is
attributed to the presence of chromium in the initial oxide. It is proposed that corundum nucleation is facilitated on a surface consisting of the isostructural escolaite, (Cr2O3). After exposure at 900 8C
AES shows large amounts of Mg in the outer part of the oxide,
MgAl2O4 being detected by XRD together with c- and a-Al2O3.
1 Introduction
cently, a life-prediction model was presented for FeCrAl alloys in the temperature range 700 – 1050 8C [14]. Publications
on FeCrAl oxidation at even lower temperature (< 800 8C) are
very scarce indeed.
The present paper reports on the oxidation of a commercial
FeCrAl alloy, Kanthal AF, in the temperature range 500 –
900 8C. The samples are exposed isothermally in dry oxygen.
Focus is on the oxidation kinetics, and on the composition and
morphology of the oxide.
At high temperature, alumina forming alloys tend to form
protective a-alumina (corundum) scales. a-Al2O3 exhibits
small defect concentrations [1] and provides unique oxidation
resistance to alloys at high temperatures. However, when alumina forming alloys are used at lower temperatures, metastable “transient” aluminas (c, h, d) tend to form rather
than the stable a-Al2O3 [1 – 3]. This is because the nucleation
of a-Al2O3 is slow at low temperature. Metastable alumina
scales generally exhibit high defect concentrations and
have relatively poor protective properties [4]. The transient
aluminas can convert to a-alumina with time.
Kofstad [1] is often cited regarding the growth of alumina
scales; “below 900 – 950 8C the alumina is generally formed as
c-Al2O3 and at higher temperatures as a-Al2O3”. In the case of
the FeCrAl alloys, most oxidation papers concern temperatures above 1000 8C and there is limited information available
concerning the low temperature oxidation of FeCrAl supporting this statement.
Several authors have investigated the role of transient aluminas in the oxidation of FeCrAl alloys and the transition
from transient alumina to a-Al2O3 formation < 1200 8C
[4 – 10]. Efforts have been made to suppress the formation
of metastable aluminas on FeCrAl alloys. For example, it
has been suggested that the formation of a-Al2O3 can be promoted by adding Y to the alloy [7, 11, 12]. Some papers investigate FeCrAl oxidation at 800 – 1000 8C [3 – 6, 13]. Re-
* H. Josefsson, F. Liu, J.-E. Svensson, M. Halvarsson,
L.-G. Johansson
High Temperature Corrosion Centre & Department of Environmental Inorganic Chemistry & Experimental Physics at Chalmers University of Technology,
S-412 96 Gothenburg (Sweden),
E-Mail: [email protected]
F 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
2 Experimental procedure
2.1 Material
The alumina forming alloy chosen for this study was
Kanthal AF, with a sample thickness of 2 mm. The chemical
composition is 73wt.% Fe, 21wt.% Cr and 5%wt. Al. The material contains small amounts of the reactive elements Zr and
Y, typically 580 ppm and 340 ppm respectively [15]. The material was cut into 10*8 mm samples, polished using 1 lm diamond spray and cleaned in ethanol using ultrasonic agitation.
2.2 Oxidation tests
Isothermal oxidation was carried out in Setaram TAG 24S
16 thermo-balance at 500 – 900 8C for up to 72 h in flowing
O2. The flow velocity was 10 ml/min. An alumina reference
sample with the same geometry was used to diminish buoyancy effects. The samples were weighed before and after oxidation on a six-decimal microbalance in order to calibrate the
recorded mass gains. Several identical exposures were performed at each temperature.
Additionally, samples were exposed to dry oxygen for
168 h using a horizontal tube furnace. The flow velocity
was 1000 ml/min. The samples were exposed three at a
time and positioned parallel to the direction of the flow. After
exposure the samples were allowed to cool in dry air. The
DOI: 10.1002/maco.200503882
802
Josefsson, Liu, Svensson, Halvarsson and Johansson
Materials and Corrosion 2005, 56, No. 11
mass changes were recorded using a six-decimal microbalance.
2.3 SEM/EDX analysis
A Leo ULTRA 55 scanning electron microscope with a
field emission electron gun was used for investigating the topography of the oxidized sample surface. An accelerating
voltage of 8 kV was chosen for imaging. EDX analysis
was conducted on an Oxford INCA energy dispersive Xray spectrometry system installed on the ULTRA 55. An accelerating voltage of 20 kV was used in order to obtain information on heavy elements.
2.4 XRD analysis
The phase composition of the oxide layer was analysed by
Grazing-Incidence X-ray diffraction using a Siemens D5000
powder diffractometer equipped with a Göbel mirror. Cu-Ka
radiation was used. Measurements were performed between
20 < 2h < 60 (0.58 incidence angle, step size 0.038). PDF
numbers for reference data were 46-1212, 29-0063 and 211152 for a-Al2O3, c-Al2O3 and MgAl2O4 respectively. The
background was subtracted from the diffraction measurements.
Fig. 2. Mass gain2 plotted as a function of exposure time for
Kanthal AF at 700 – 900 8C
2.5 AES analysis
A PHI660 instrument was used for depth-profiling. Calculations of the sensitivity factors for Al and O was made by
survey of Al2O3 powder pressed on a Cu specimen. The electron beam voltage was 10 kV and the beam current was about
100 nA.
3 Results and discussion
Fig. 1 shows mass gain curves for Kanthal AF in dry O2 at
500 – 900 8C. Two independent exposures are presented for
each temperature, showing the reproducibility of the expo-
Fig. 1. Mass gain curves for Kanthal AF at 500 – 900 8C. Two identical exposures were performed at each temperature, showing the
reproducibility of the results
Fig. 3. Calculated values on the parabolic rate constant for Kanthal
AF oxidized in dry O2
sures. The oxidation rate is rapid during the first hours and
slows down due to the formation of protective oxide scales.
The oxidation kinetics is nearly parabolic (see Fig. 2 for 700,
800 and 900 8C). As expected, the parabolic rate constant, kp,
between 500 – 900 8C increases with temperature. Calculated
kp values are plotted as a function of temperature (see Fig. 3).
Cueff et al. report a kp value for Kanthal AF at 900 8C which is
in the same range [7, 13]. Values for kp below 900 8C are
scarce in the literature.
At all temperatures, most of the sample surface is covered
by a thin smooth oxide (“base oxide”). In addition, there are
RE-rich oxide particles, “pegs”, with a typical size of 1 – 3 lm
which are distributed within the base oxide. This is illustrated
by the FEG SEM image of the oxide after exposure at 500 8C
in Fig. 4a. The composition of the pegs varies. Some are dominated by Zr, while others are rich in Y or Ti. The presence of
pegs are often considered to improve oxide adherence by anchoring the oxide to the metal. At 800 and 900 8C there appears a third oxide feature, namely areas with a thick uneven
oxide, see Fig. 4b, showing a sample exposed at 900 8C. The
patches of thick oxide have needle-formed crystallites and are
situated on top of the base oxide, usually around the Y-rich
oxide pegs (see Fig. 4). A detailed description of the oxide
Materials and Corrosion 2005, 56, No. 11
Oxidation of FeCrAl alloys at 500 – 900 8C in dry O2
803
Fig. 4. FEG-SEM images showing an overview of the morphology
at 500 (a) and 900 8C (b)
morphology on Kanthal AF at 900 8C was presented recently
[16].
Fig. 5 shows SEM images of the base oxide after 72 h at
500, 700 and 900 8C. At 500 8C the typical crystallite size
is < 50 nm. At 700 8C the crystallites have grown somewhat
in size. At 900 8C we observe two types of oxide crystallites,
with faceted equiaxed and plate-like habits, respectively, the
typical crystallite size being 100 – 300 nm.
The XRD results are illustrated in Fig. 6. In contrast to the
commonly held view that corundum-type oxides only form
above 900 8C [1, 2], there is clear evidence for the formation
of a-Al2O3 after 168 h even at 700 8C. It is usually considered
that transient oxides form in this temperature range. While
transient oxides may very well be present in our case, we
only detect diffraction from corundum. After exposure for
168 h at 800 – 900 8C, MgAl2O4/c-Al2O3 was present together
with a-Al2O3. No crystalline phases were detected after exposure at 500 and 600 8C. This may be due to the small amount
of oxide (thickness < 50 nm) or because the oxide is amorphous.
AES profiling shows that the thickness and chemical composition of the base oxide after 168 h does not vary over the
surface. As expected, the thickness of the base oxide increases
with temperature (see Fig. 7). It may be noted that there is a
marked decrease in the concentration of iron and chromium in
the base oxide with temperature. At 500 8C the amounts of
iron and aluminium are comparable with relatively high concentrations of chromium also present. The aluminium concentration grows as we approach the oxide/metal interface. At
600 8C the entire oxide is dominated by aluminium. However,
appreciable amounts of Cr and Fe are still present (< 10 at.%).
At 700 8C the concentration of iron and chromium only
reaches a few percent.
At 900 8C (see Fig. 8) the concentration of iron and chromium is close to the limit of detection (< 1%). Instead, relatively large amount of Mg appear in the outer part of the oxide.
Fig. 5. FEG-SEM images showing the base oxide at 500, 700 and
900 8C oxidized 72 h in dry O2
Because of the low magnesium content in the alloy
( 100 ppm), the appearance of large amounts of magnesium
in the oxide implies that the diffusivity of magnesium is very
rapid in the alloy as well as in the oxide. EDX analysis after
exposure at 800 8C (see Fig. 9) shows that magnesium is en-
Fig. 6. GI-XRD results on Kanthal AF oxidized at 700, 800 and
900 8C during 168 h. The symbols indicate: a-Al2O3 (~), cAl2O3 ( *), MgAl2O4 (&) and substrate (S)
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Josefsson, Liu, Svensson, Halvarsson and Johansson
Materials and Corrosion 2005, 56, No. 11
Fig. 9. SEM/EDX on Kanthal AF oxidized 168 h at 800 8C in dry
O2
Fig. 7. AES profiling, showing the oxide formed on Kanthal AF
oxidized at 500 (a), 600 (b) and 700 8C (c) after 168 h
riched in the oxide close to the alloy grain boundaries. This
implies that magnesium transport in the alloy is dominated by
grain boundary diffusion. Close to the alloy grain boundaries,
Mg-rich particles are seen. These particles are considered to
correspond to the magnesium aluminium spinel (MgAl2O4)
found by XRD (Fig. 6). At 900 8C, spinel oxide nodules
form all over the surface (see Fig. 4b). SEM imaging showed
that the characteristic nodules do not form < 800 8C, while
MgAl2O4 is not detected by XRD. Enrichment of magnesium
in the upper part of the oxide on a FeCrAl alloy above 1000 8C
has been reported by Mayer et al. [17].
The presence of appreciable amounts of Al, Fe, Cr in the
oxide at 500 and 600 8C, shows that a solid solution and/or
a mixture of oxides is present. It may be noted that the integrated amount of iron and chromium in the oxide is roughly
the same at 500 and 600 8C. Moreover, the Fe/Cr ratio in the
oxide is the same as in the alloy. This suggests that iron and
chromium have entered the oxide during the initial stages of
oxidation. This is in accordance with the generally accepted
view on early stages of alloy (FeCrAl) oxidation [1]. Because
a-Al2O3, a-Fe2O3 and Cr2O3 are mutually soluble, it is suggested that at least part of the iron and chromium found in
the oxide form a solid solution with a-Al2O3, whenever the
latter phase is present. Moreover, it is suggested that the formation of a-Al2O3 at 700 8C on FeCrAl is connected to the
presence of chromium in the initial oxide. It is proposed
that corundum nucleation is greatly facilitated on a surface
consisting of the isostructural escolaite, (Cr2O3).
4 Conclusions
Fig. 8. AES profiling, showing the oxide formed on Kanthal AF
oxidized at 900 8C after 168 h
The rate of oxidation of Kanthal AF increases with temperature in the range studied (500 – 900 8C). The kinetics is
parabolic between 700-900 8C. At all temperatures, most of
the sample surface is covered by a thin smooth base oxide.
In addition, RE-rich oxide particles, with a typical size of
1 – 3 lm are present. At 800 and 900 8C patches of thick oxide
appear, featuring needle-formed crystallites that are situated
Materials and Corrosion 2005, 56, No. 11
on top of the base oxide, the thick oxide usually forms around
the Y-rich oxide pegs.
At 500 8C the concentrations of iron and aluminium in the
oxide are comparable with relatively high concentrations of
chromium also present. The concentration of iron and chromium in the oxide decreases with temperature. XRD proves
the formation of a-Al2O3 already at 700 8C where the concentration of iron and chromium in the oxide is in the order of a
few percent. At 900 8C the concentration of iron and chromium in the oxide is very low. Instead, large amounts of
Mg appear in the outer part of the oxide. Magnesium transport
in the alloy is dominated by grain boundary diffusion. Magnesium aluminium spinel, MgAl2O4, forms at 800 and 900 8C
together with c- and a-Al2O3. It is suggested that iron and
chromium enter the oxide during the initial stages of oxidation
and that part of the iron and chromium may form a solid solution with a-Al2O3. The formation of a-Al2O3 at low temperature (700 8C) is attributed to the presence of chromium in the
initial oxide. It is proposed that corundum nucleation is
greatly facilitated on a surface consisting of the isostructural
escolaite, (Cr2O3).
5 Acknowledgement
The authors wish to acknowledge the support from the
Swedish High Temperature Corrosion Center (HTC), Kanthal
AB, Sandvik Materials Technology AB and National Graduate School in Materials Science at Chalmers.
6 References
[1] P. Kofstad, High Temperature Corrosion, Elsevier Applied
Science 1988, 342 and 408.
Oxidation of FeCrAl alloys at 500 – 900 8C in dry O2
805
[2] F. H. Stott, G. C. Wood, J. Stringer, Oxidation of Metals 1995,
44, 113.
[3] M. Boualam, G. Beranger, M. Lambertin, Proc. 2nd Int. Conf.
Microsc. Oxid. 2 1993, 243 – 252.
[4] D. Naumenko, W. J. Quadakkers, A. Galerie, Y. Wouters, S.
Jourdain, Materials at High Temperatures 2003, 20, 287.
[5] A. Andoh, S. Taniguchi, T. Shibata, Materials Science Forum
2001, 369 – 372, 303.
[6] M. J. Bennett, R. Newton, J. R. Nicholls, H. Al-Badairy, G. J.
Tatlock, Materials Science Forum 2004, 461 – 464, 463.
[7] R. Cueff, H. Buscail, E. Caudron, C. Issartel, F. Riffard, Annales de Chimie (Paris, France) 2003, 28, S207.
[8] R. M. H. El Kadiri, Y. Bienvenu, Materials Science Forum
2004, 461 – 464, 1107.
[9] R. Molins, Materials Science Forum 2004, 462-464, 29-36.
[10] V. K. L. Rol, M. Juez-Lorenzo, H. Fietzek, N. Eisenreich, Materials Science Forum 2004, 461 – 464, 521.
[11] C. Mennicke, E. Schumann, M. Ruhle, R. J. Hussey, G. I.
Sproule, M. J. Graham, Oxidation of Metals 1998, 49, 455.
[12] B. A. Pint, A. J. Garratt-Reed, L. W. Hobbs, Materials at High
Temperatures 1995, 13, 3.
[13] E. Andrieu, A. Germidis, R. Molins, Materials Science Forum
1997, 251 – 254, 357.
[14] J. R. Nicholls, M. J. Bennett, R. Newton, Materials at High
Temperatures 2003, 20, 429.
[15] R. Newton, M. J. Bennett, J. P. Wilber, J. R. Nicholls, D. Naumenko, W. J. Quadakkers, H. Al-Badairy, G. Tatlock, G.
Strehl, G. Borchardt, A. Kolb-Telieps, B. Jonsson, A. Westerlund, V. Guttmann, M. Maier, P. Beaven, European Federation of Corrosion Publications 2001, 34, 15.
[16] H. Josefsson, F. Liu, J.-E. Svensson, M. Halvarsson, L.-G. Johansson, Electro Chemical Society 2005, accepted proceeding
article.
[17] J. Mayer, H. J. Penkalla, A. Dimyati, M. Dani, P. Untoro, D.
Naumenko, W. J. Quadakkers, Materials at High Temperatures 2003, 20, 413.
(Received: April 29, 2005)
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