Volume 1 Paper 18
Influence of Aluminium Additions on the Rate of Oxidation of Iron-Chromium Alloys
S.E. Sadique, M.A.H. Mollah, M.M. Ali, M.M. Haque, S. Basri, M. M. H. M. Ahmad and S.M. Sapuan
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JCSE Volume 1 Paper 18
Submitted 7th September 1999, published for public review April 2000
Influence of Aluminium Additions on the Rate of Oxidation of Iron-Chromium
S.E. Sadique(previously 1, now 2), M.A.H. Mollah(2),
M.M. Ali(1), M.M. Haque(1), S. Basri(2), M. M.
H. M. Ahmad(2) and S.M. Sapuan(2)
1Department of Materials and Metallurgical Engineering,
Bangladesh University of Engineering and Technology (BUET), Dhaka : 1000,
2Faculty of Engineering, University Putra Malaysia, 43400UPM,
Serdang, Selangor D. E.
The oxidation behavior of Fe-Cr-Al alloys containing 10% chromium and
aluminum in the range of 2 - 8% by weight in pure oxygen at 1 atmosphere
pressure at higher temperatures under cyclic conditions (3 hour cycles) has been
studied. In metallographic investigation, healing layers of Cr2O3
and of Cr2O3/a -Al2O3
could be observed after breakaway of the initial protective alloy in case
of the 2%Al and 4%Al alloys but the 6%Al and 8%Al alloys reformed external a
-Al2O3 scales at all the temperatures and develops a
convoluted configuration. It is postulated that lateral growth results from the
formation of oxide within the existing oxide layer by reaction between oxygen
diffusing inward down the oxide grain boundaries and aluminium diffusing outward
through the bulk oxide. The scales developed on the four alloys maintain more or
less good contact with the alloy and thicken more slowly in case of 2%Al and
4%Al alloys. These effects can be associated with the reduction in the rate of
transport of chromium across the scale in case of 4%Al and 6%Al alloys and with
formation of intergranular and internal oxides in the underlying alloy
§2 Key Words: Oxidation Behavior, Fe-Cr-Al alloys, Cyclic Oxidation,
Oxidation Morphology, Breakaway, Oxidation, Healing Layer, Metallography.
The metallurgical problems associated with the operation of high temperature
machines nearly arise from the very high temperatures used for the working
fluid, usually gases. Those parts of the machine in contact with the hot gases
must, therefore, be made of some suitable materials which will maintain adequate
strength at its working temperatures, will not oxidize or corrode appreciably at
that temperature, will not become brittle and not be seriously subject to the
effect of creep .
§4 Many heat-resisting alloys are based on Fe-Cr and rely on the development of
Cr2O3 scales for resistance to oxidation at high
temperatures. However, such scales are often susceptible to spallation,
particularly under thermal-cycling conditions. Gardiner et al reported that
the oxide film formed on iron-chromium alloys less than 5 wt%Cr below 873K
consisted of 2 layers of magnetite and hematite. It is well known that Fe-Cr
alloys in oxygen at higher temperature formed spinel (FeCr2O4)
and Cr2O3 on the inside and Fe2O3 on
the outside of the scale. Thermodynamically findings shows that the scales of
the lowest oxygen pressure lie closest to the metal and the scale nearest the
gas phase must be of the highest dissociation pressure.
§5 Small addition of reactive metals such as Th, Al, Y, Ce, Ca, Mg, Ti, V etc
can improve oxidation resistance of high temperature alloys. It also can improve
the casting quality by reducing the formation of voids, gas cavities etc and
secondly, the resultant metal possessed higher strength and better
heat-resisting properties as a result of its inherent fine-grained structure
. Fe-Cr-Al type ferritic alloys show a great resistance to high temperature
oxidation. Their high aluminium content (5%) allows the formation of an alumina
refractory layer, which protects them. The increase of oxidation rate, which
follows this spalling, decreases the in-service lifetime of component .
Previous studies have been shown that additions of other reactive metals to such
alloys have a considerable, beneficial effect on the oxidation performance.
There is a reduction in the concentration of Cr in the alloy necessary to form a
continuous, protective Cr2O3 layer, the growth rate of
that layer, once established, is reduced, most importantly, the resistance of
the scale to spallation is improved significantly .
§6 As reported by Islam , observed the beneficial effects of these reactive
metal additions. The effective partial pressure of oxygen behind the less
protective Fe2O3/Fe3O4 scale is much
lower than the atmospheric and hence preferential oxidation of Al or Cr will
tend to occur in case of 2% and 4% Al alloys. The new element with higher
reactivity enables the protective element being available at the interface
region without being oxidized in the interior of the alloy.
§7 Jedilinski and Borchardt  observed that the change of the scale growth
direction from the predominant outward metal to the prevailing oxygen inward
transport occurred at the rather early stages of oxidation of Fe-Cr-Al alloys.
At these stages of reaction the development of unstable alumina is very
plausible as already demonstrated for scale on Fe-Cr-Al alloys were analyzed by
means of X-Ray.
§8 Ramanathan , Rhys-Jones et al  and Rapp and Pieraggi  summarized
various explanations to account for the beneficial effects of reactive metal
additions for the growth a chromia protective scale on pure Cr or on an alloy of
Fe-, Ni-, Co-base such as
doping of the oxide scale,
changes in defect structure of the oxide
mechanical keying through formation of oxide pages into the alloy
promotion of preferential cationic or anionic diffusion in the scale and
thus inducing the formation of oxide at one preferential interface
formation of graded oxide or interlayer containing the reactive element
reduction in accumulation of voids at the alloy/scale interface
the parabolic scaling rate constant for steady state growth is reduced by
as much as a factor of ten or more.
§9 The oxidation of Fe-14Cr-4Al and Fe-27Cr-4Al alloys proceeded by outward
diffusion of O2- ions along grain boundaries occurring simultaneously
has been reported by Golightly et al . There is a widespread technological
need for new high-temperature, oxidation-resistant alloys. Since Fe-Cr alloys
generally provide the basis of Fe-base materials used in several high
temperature applications where the formation of protective oxide scales is
required, the evaluation and mechanistic interpretation of any beneficial
effects imparted by additions of reactive elements to such alloys would be most
useful. Although studies upon the oxidation resistance of binary Fe-Cr alloys
have rather plentiful, but sufficient information of Al additions to these
alloys have not been so much available. The more economical design of machine
parts, which are, used at high temperatures such as aircraft gas turbines need
more information about the properties on the oxidation resistance at high
temperatures. The purpose of this paper is to study high temperature oxidation
seems to be rightly directed towards tackling the problem of scale adhesion and
its stabilization on the surface of the component part.
§10 Experimental Techniques
Mild steel scrap in the form of bar, Ferro-Chrome (containing 65% Cr) in the
form of lump and aluminum in the form of ingot were used as raw materials in the
production of the alloys for the investigation. The alloys were prepared in a
high frequency induction furnace. They were cast in form of rod about 2.5 cm in
diameter, 30 cm in length. The surface preparation of the specimens involved
abrading on successively finer grades of SiC papers of finess from 3 to 2/0 grit
sizes. Proper care was taken to avoid overheating of the specimen during cutting
and drilling operations. The cyclic oxidation kinetics were followed by
oxidizing the samples in a horizontal electrically heated furnace followed by
direct weight change measurements (after cooling for at least 20 minutes) in a
manual type balance.
§11 Examination Techniques
The following tests were performed on the oxidized specimens:
It was performed under ordinary light to illustrate any prominent
macroscopic feature on the specimen surface.
X Ray Diffraction :
The oxidized specimen surfaces were analyzed on a x-ray diffractometer –
JDX 8P, x-ray analytical Equipment. The different phases were identified by
the resulting X Ray patterns as separate peak using standards ASTM data.
§12 The structure and morphology of the scale in cross-section were examined
extensively under the optical microscope. The oxidized samples were mounted on
a slow heat setting solid mounting medium (plastic materials) and quick
setting solid mounting medium (Quick power) using a chemical hardener.
Specimens were positioned and supported on edge in a small cast iron mould and
liquid resin carefully pored around the specimen and allowed harden. After
that, specimens were ground down on a dry SiC paper followed by wheel
polishing with velvet cloths and finally hand polishing was completed in
velvet clothes thus minimizing "pull out" of the scale.
Gravimetric runs on samples of various compositions were performed
within the temperature range of 9500C – 10500C. It is
apparent that most of the alloys exhibit rates, which are approximately
parabolic. Some of the runs were repeated to check the reproducibility of the
data; generally fair agreement was found. The general oxidation trend for each
alloy was almost identical. A protective scale initially forms and suffers
breakaway oxidation within a few cycles as displayed by an increase in the
oxidation rates and subsequently in fact a second protective oxide forms which
reduces the oxidation rate to a more or less steady state value.
§14 Effect of Aluminum-Content
A graph of specific weight gain value vs. percentage of aluminum is shown in
Fig. 1 for all the three temperature levels (950oC, 1000oC,
and 1050oC). It will be noticed that the oxidation rate, as measured
by specific weight gain, declines gradually reaching a minimum value with 8% Al.
This is also evident from Fig. 2; Fig. 3 and Fig. 4 represented oxidation
kinetics of the alloys at 950oC, 1000oC, and 1050oC
respectively. In Fig.3 the oxidation kinetics of the binary alloy-both
isothermal and cyclic (hypothetical-have been included along with the cyclic
data for the modified alloys at 1000oC. It will be noticed that even
the slope of the kinetics curve is much lower than that for the binary Fe-10Cr
alloy (at 1000oC) i. e. the oxidation rate of the modified alloy with
the lowest Al-content in even lower than that for the binary Fe-10Cr alloy.
Clearly, the alloys containing higher Al have oxidation rates much lower that
the 2%Al alloy. In general, the onset of initial spallation has been found to be
delayed with increasing Al-content. The individual spall particles are noticed
to be progressively finer with the increase of Al-content in the base alloy and
a gradual decrease in the amount of spall particles. They were grayish black in
color and are in the form of large thin flakes being magnetic in nature for 2%Al
and 4% Al alloys. In contrast, 6% and 8% Al containing alloys forms a very fine
powder particles being creamy brown in color and nonmagnetic in nature. It will
be noticed that the oxidation rate in terms of specific weight gain values
increase with the higher temperatures for each of the alloys. It also appears
that 2% Al alloys are found to be oxidized completely so that no parent metal
can be observed at higher temperatures.
§15 To sum up the following general observation can be made:
Under cyclic conditions, the rate of oxidation in terms of sp. Wt. gain
attains a minimum value with 8% Al in the alloy.
The effect of thermal cycling has been observed to be more severe upon the
lower Al alloys as evident from sp. wt. gain values at various temperatures
tends to increase in general as the alloy Al-content decreases.
On visual examination, the surface appearance shows the presence of black
patches running between gray to dark gray areas with the 2%Al and 4%Al
alloys, but oxidized specimens for the 6%Al and 8%Al alloys reveals creamy
brown areas with a few large to gradually small nodular eruptions. The
number of eruptions increases at higher temperatures.
§16 Metallographic Observation
A general observation of the transverse cross-sections from oxidized samples
was that spalling during cooling became more severe as the Al content of the
alloy decreased. It has been done to examine properties such as scale density,
adherence to the metal, and the nature of the metal/oxide interface. As observed
in Fig. 5 and Fig. 6, it is evident that the rapidly oxidizing specimen shows a
usual duplex scale with a continuous subscale layer of Cr2O3 and
at the scale base in 2% Al and 4% Al alloys respectively whereas a
-Al2O3 scale in the 6% Al and 8% Al alloys as shown in
Fig. 7 and Fig. 8. The substrate surface of the specimen shows convoluted
structure and the presence of void as usual.
§17 Oxide Morphology
The development of the convoluted morphology resulted in extensive detachment
of the oxide from the alloy as per cent Al increases, creating cavities beneath
the oxide ridges. The coarsening of the convoluted oxide morphology was the
result of a progressive increase in the length of the oxide comprising
individual oxide ridges. This occurred as a consequence of lateral growth of
both the oxides, which remained in contact with the alloy and that, which, as
part of an oxide ridge, had become detached.
§18 Observations and Discussion
Comparison of the weight gains for the four alloys after a given period of
oxidation must take into consideration the markedly different oxide
morphologies, which were developed. Although parabolic kinetics were generally
observed for all compositions and temperatures, it is apparent from the
structural features of the scales that other that the other factors besides
diffusive transport mechanisms must be considered in attempting to understand
the oxidation of Fe–Cr alloys. Comparison of the weight gains for the three
alloys after a given period of oxidation must take into consideration the
markedly different oxide morphologies, which were developed.
§19 The influence of the Al additions is marked both in the initial and latter
stages of the oxidation processes, affecting oxide nucleation, scale adhesion
and the rate of oxide growth. During the initial stages of oxidation, Al
additions promote the rapid formation of an oxide film; i.e. oxide nucleation
processes are enhanced. In the case of Al-containing materials this can be
attributed to the rapid formation of Al oxide nuclei on the surface of the
alloys, causing a greater total number of nuclei and hence decreasing the
internuclear distances. Consequently, complete surface coverage by oxide is more
rapidly attained that in the base materials.
§20 As reported by Wood et al , binary Fe – Cr alloys containing Cr in the
range of about 14 – 25% at 900 – 11000C immediately forms a
continuous protective scale of upon the surface. When a more reactive element is
present, this Cr2O3 scale is formed even with much lower
percentages of Cr (10-13%). In case of the first two alloys, it is apparent from
the X-ray diffractometer data that the scale is of Cr2O3
since a strong peak for Cr2O3 is noticeable. This external
Cr2O3 scale suffers breakaway as it proceeds Fe begins to
oxidize as depletion of Cr in the bulk alloy has already taken place. This is
why the oxidation rate after breakaway increases to a high values corresponding
to that for the formation of Fe2O3/Fe3O4.
In the course of time, Cr2O3 may combine with FeO to form
spinel, FeCr2O4, and also a -Al2O3
may combine with both FeO and Cr2O3 to form Fe-Cr-Al
spinel, Fe(Cr, Al) 2O4. These internal oxide particles will
ultimately coalesce together to form a more or less continuous subscale layer
through which the cationic diffusion is relatively slow and the oxidation rate,
therefore, settles down to a more or less steady state value. The
photomicrographs of 2% Al and 4% Al alloys as shown in Fig.5 and Fig. 6 support
the X-ray data in which a spinel layer in the outer region and subscale
formation at the alloy/oxide interface is the principal rate-determining factor
for further oxidation. A paradoxical behavior as observed in 2% Al alloy at 10500C
reveals that the specimens are completely oxidized which has been proved under
the optical microscopic examination, probably due to gradual loss of the Cr2O3
by volatilization and it is found to consist of Fe3O4 upon
magnetic test and a dark blackish color.
§21 Golightly et al  and R A Rapp and B Pieraggi  indicate that binary
Fe-Cr alloys containing Cr of about 14-28% along with a more reactive element in
the range of 4-5% initially forms an a -Al2O3
scale. The oxidation kinetics of 6% Al and 8% Al alloys represent that an
external protective scale forms initially which suffers breakaway within a short
period. Subsequently, a second or a third protective scale has been observed
which level down the rates of oxidation to a more or less steady state. It is
apparent from X-ray diffractometer data that a number of strong peaks for a
-Al2O3 have been observed. Also the spall particles
released from the specimens are in the form of a fine powder, non-magnetic in
nature, creamy brown in color and small in quantity. Microstructures of the
scales at 1050oC for the above alloys as shown in Fig. 7 and Fig. 8
show that the scale surfaces bear marks of convoluted growth with void formation
at the interface region and cracked here and there. The convolutions are
believed to be the result of the development of compressive stresses due to
lateral growth of the oxide scale  and also due to the differential
contraction of the oxide scale and metal during thermal cycling. These two
factors are supposed primarily to be responsible for the formation of cracks in
the scale through which contacts between the hot alloy surface and the oxidizing
§22 The phenomena of multiple breakaway and repeated protective scale formation,
as observed at higher temperature in the oxidation of the above alloys is due to
the failure of the initial external protective scale definitely produces
depletion of the alloy-content in the interface region which mat even reach the
minimum level of the alloying element necessary for the reformation of the
protective scale, but eventually replenished to a higher level of alloy content
to enable the formation of a second protective scale. This particular behavior
is widely known as healing.
§23 It has been postulated that the generation of a complete protective scale of
alumina on the alloy surface requires the aluminium-content in the alloy to be
above a certain critical values, which is a function of temperature. For
example, an Fe – 4.9% Al alloy forms iron oxides and spinel below 570�C and
alumina above 570�C. In the present study, it is observed that the 6%Al alloy
did form such a protective scale whereas this was not possible with the 4%Al
alloy. This finding confirms the same fact that the minimum level of Al
necessary for the formation and maintenance of a -Al2O3
scale on the alloy surface is about 5% in the present instance as well.
§24 The rate of oxidation decreases with the increase of Al-content as shown in
Fig. 1 due to rapid oxidation of base metal in the lower Al alloys. It is
evident from the observation of Mosely et al  that oxides of base metal
(iron) initially grow more rapidly – outwards from the metal surface – but
ultimately the surface becomes covered by a layer of the more stable oxides
(alumina) and oxidation of base metal ceases in higher Al alloys.
§25 The size of the individual spall particles is observed to be progressively
finer and smaller with the increase of Al-content. As the rate of oxidation
increases, oxide layers being thick and stratified and coarsen the spall
particles in lower Al alloys. Distinctive color of the spall particles and of
the oxidized specimens is likely due to the compositional changes in the oxide
layers. As per cent Al increases in the alloy, oxidation of Fe and Cr almost
ceases and Al begins to oxidize preferentially. This is why, the spall particles
released from the 6%Al and 8%Al alloys has been observed to be creamy brown in
color (most identical to the appearance of a -Al2O3
§26 Concluding Remarks
Al additions have been shown to mark effects upon the oxidation
characteristics of Fe-10Cr at all temperatures. The scale growth process results
in the generation of stresses and a convoluted scale configuration is
established. Considerable deformation of the alloy substrate and eventual
fracture of the oxide accompany lateral growth of the scale. For the modified
Fe-10Cr alloys containing 2-8 percent aluminum, a general reduction in the rate
of oxidation in terms of specific weight gain values was noticed with increasing
aluminum content. The amount of spallation decreased with increasing aluminum
content. The individual spall particles become smaller in size and progressively
finer accompanied by a change in color from blackish to creamy brown.
§27 Tendency towards the formation of a -Al2O3
scale was definite and conspicuous with the increase of aluminum content in the
alloy. Scale thickness reduced progressively with increasing aluminum content
marked by an almost complete cessation of spallation with the 8% Al alloy.
§28 The oxide nucleation processes occurring on the alloy surface is favored by
Al due to the provision extra oxide nuclei of Al oxide. A decreased internuclear
distance is achieved, thus causing time required by alloy to form a protective
film to be shortened. Al markedly enhances the adhesion of the Cr2O3. Failure of
scale was inhibited either through the immediate reformation of an external
protective scale or through the internal oxidation and subsequent formation of a
sub-scale layer. A minimum level of aluminum, depending upon temperatures,
exists for formation of a complete protective scale on the alloy surface.
The authors express thanks to the Lab. Assistants of the Department of
Metallurgical Engineering, Bangladesh University of Engineering and Technology.
Appreciation is extended to Dr M S Islam and Dr M M Haque for their
encouragement during the study.
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p. 547 (1984).
(In all cases the images are reduced in size for the page display, and a
larger version can be obtained by clicking on the image, or right-clicking to
save the file for printing).
§32 Fig.1 Effect of Al-content and temperature upon sp. wt.
gain (48-hour basis) of the modified Fe-10Cr alloys.
§33 Fig. 2 Influence Al-content on the oxidation kinetics of the modified Fe-10Cr alloys
(2-8%) at 9500C.
§34 Fig. 3 Comparison of cyclic oxidation kinetics of the modified Fe-10Cr alloys with
2-8%Al at 10000C
§35 Fig. 4 Effect of Al-content on the oxidation kinetics of the modified Fe-10Cr alloys
with 2-8%Al at 10500C
Fig.5: Optical cross-section of oxide scale
on an Fe-10Cr-2Al alloy specimen after exposure of 51 hours (17 3-hour
cycles) at 950� C, x 300.
Fig.6: Metallographic cross-section of
oxide scale on an Fe-10Cr-2Al alloy Specimen oxidized for 48 hours(17
3-hour cycles) at 1000� C, x 1200.
Fig.7: Microstructure of oxide scale on an
Fe-10Cr-6Al alloy after 51 hours exposure (119 3-hour cycles) at 1050�
C, x 1200.
Fig.8: Microstructure of oxide scale on an
Fe-10Cr-8Al alloy specimen specimen oxidized for 19 3-hour cycles at 1050�
C, x 1200.
Editor's note - the magnifications quoted above relate to an image that is
approximately 10 cm wide.