Volume 7 Preprint 4
The Effect of Nickel and Aluminum Addition on Oxidation Behavior of Austenitic Heat Resisting Steels
M. Karaminezhaad, E. Kordzadeh and M. R. Bateni
Keywords: Heat Resisting Steel, Internal oxidation, Corrosion resistance, Chromia and Alumina
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Volume 7 Paper 4
The Effect of Nickel and Aluminum Addition on
Oxidation Behavior of Austenitic Heat Resisting
M. Karaminezhaad, E. Kordzadeh, M. R. Bateni
Material Science and Engineering Dept., Kerman, Iran
The influence of nickel and aluminum addition on the oxidation
behavior of series of austenitic heat resistant Fe-Ni-Cr-Al alloys with
different nickel content has been investigated. The specimens were
annealed in argon at 1250oC for 8 hrs. Oxidation tests were carried out
at 1100oC and 25, 50, 50,100 hours in the air atmosphere. Weight
changes were measured using an automatic thermo gravimetric analyzer
and corrosion products were analyzed by a series of analytical
microscopy (SEM), energy dispersive X-ray spectrometry (EDAX) and
optical microscopy. The results showed that oxidation resistance
increased with increasing nickel content. During oxidation, the alloys
develops chromia or alumina layer on the surface, which enhances the
oxidation resistance. Al has a favorable effect as it forms alumina layer
on the surface that is more protective than chromia. Nickel also has a
good effect on the oxidation behavior of the alloys as it enters the
structure of the protective layer and decreases the defects.
resistance, Chromia and Alumina
This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and
Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at
http://www.umist.ac.uk/corrosion/jcse in due course. Until such time as it has been fully published it should
not normally be referenced in published work. © UMIST 2004.
Heat resisting steels are used in power plant equipments, gas turbines,
metallurgical furnaces, oil refinery furnaces, petrochemical furnaces,
manufacturing of glass , synthetic rubber etc. [1, 2]. Available heat
resisting alloys can be classified in two broad families, depending on
whether their crystal structure is ferritic or austenitic. Ferritic steels are
mechanically weak, with poor creep strength. The more ductile and
more creep resistant austenitic steels owe their ability to withstand high
temperature oxidation chiefly to the development of adherent chromia
based oxide scales, but these become rapidly less effective beyond 1100
to 1150oC, due to intrinsically higher ionic diffusion rates, together with
the tendency to form volatile species such as CrO3 . In Fe-Ni-Cr-Al
alloys, which contain chromium and aluminum, continuous Cr2O3 or
Al2O3 layer is formed. These oxides constitute efficient barriers to
corrosion in oxygen atmospheres . It is well known that alumina
scales provide much greater protection than do Cr2O3 based oxides.
Aluminum oxide is a non-spalling layer and is more protective than
chromium oxide film . Transport through α-Al2O3 scales is extremely
slow, relative to other oxides [6, 7]. These characteristics make these
alloys one of the most suitable one at temperatures higher than 950 oC.
Nickel content is sufficient to ensure that the alloy structure is either
austenitic or austeno-ferritic and the creep resistance is good .
The aim of this study is to investigate the effect of nickel and aluminum
content on the high temperature oxidation resistance of high aluminum
2. Experimental Techniques
composition of these alloys is shown in table 1. The dimensions of the
test specimens were 15x10x5 mm.
argon for 8 hours at
The specimens were annealed in
and then cooled in the air. At annealing
temperature only the major phases such as austenitic (γ) and ferrite (α),
are stable and grains are relatively coarse.
After annealing, the test
specimens were mechanically polished up to 1200 grit paper. Before
oxidation test, the specimens were cleaned ultrasonically in alcohol,
weighed to within 0.01 mg and then were exposed to atmosphere at
1100oC. The specimens were hung freely in the furnace using a thin
alumina rod inserted through a small hole in each specimen. To
characterize the structure and composition of the corrosion products, a
series of analytical techniques, including X-ray diffraction (XRD),
spectrometry (EDAX) and optical microscopy were used.
Table 1. Chemical composition of the alloys
(Wt . %)
20.5 21.8 7.14 0.45 0.69
The relationship between weight changes and exposure time shows in
figure 1. All alloys exhibit rapid and slow weight gain rates during
After a transient rapid rate, the growth rate of the scales
follows a parabolic law, with a lower rate. By increasing nickel content
and addition of aluminum to the alloys, the oxidation resistance
increases. (Fig. 1).
w ithout Al
Figure1. Weight gain curves for oxidation of the alloys
XRD analysis of alloy No. 5 shows that the chromia scale forms on the
surface (Fig. 2). Fe3O4 forms on the surface of the alloy due to Fe ions
diffusion through Cr2O3.
Figure 2. XRD analysis of alloy No. 5 with 100 hr exposure time
The micrograph of the alloys No. 1 and 2 shows that nickel decrease
internal oxidation of the alloys (Fig 3 and 4). Aluminum oxidized
internally in the alloys.
Fig. 3. Micrograph of Alloy No. 1 with 100 hr exposure time
Fig.4. Microscopic micrograph of Alloy No. 2 with 100 hr exposure time
XRD analysis of alloy No. 3 shows that Fe2O3 covers the surface. Also
Cr2O3 forms on the surface but it doesn’t form a continuous film on the
surface so that spinels amount such as FeCr2O4 increases and Cr2O3
amount decreases (Fig. 5).
Fig. 5. XRD analysis of alloy No. 3 with 100 hr exposure time
As it can be seen from optical microscopy of the alloy No. 4, internal
oxidation occurs in the alloy.
Fig. 6. Optical microscopy of alloy No. 4 after 75 hr exposure time
According to XRD results Al2O3 formed on the surface of alloy No. 4 has
impurities but it becomes more pure with increasing the time (Fig. 7).
Fig. 7. XRD analysis of alloy No. 4 with 100 hr exposure time
The oxidation product on the surface of alloy 6 is Al2O3 according to
XRD analysis as it can be seen in fig. 8.
Fig. 8. XRD analysis of alloy No. 6 with 100 hr exposure time
In all curves, weight gains could be separated in two regions (Fig. 1).
The first region is related to the nucleation and the formation of
continuous scale, while the second region corresponds to the growth of
protective layer. Oxidation test results have shown that heat resistant
steels with higher nickel content have higher oxidation resistance in
atmospheres. During first step, the reaction follows a linear rate law.
The linear rate law observes if the reaction rate controls by gas phase
transport or phase boundary reaction. By increasing time, the thickness
of the scale increased and diffusion through corrosion products became
rate determining. During this Stage, parabolic rate law is predominated.
For good oxidation resistance in the heat resisting steels it is necessary
that the chromia forms on the surface. In the oxidizing atmospheres
chromium oxidized preferably and forms protective layers of Cr2O3 .
During oxidation, the layer depleted from chromium and riched in the
metal/oxide interface. In this case the chromium should not be depleted
less than the necessary amounts for chromia formation .
XRD analysis of alloy No. 5 shows that the chromia scale forms on the
surface and provide the protection of the alloy (Fig. 2). This layer loses
its protective properties at temperatures more than 950oC. Fe3O4 forms
on the surface of the alloy due to Fe ions diffusion through Cr2O3. The
protective layer is riched with iron. This causes wear and spallation in
the alloy and is due to free routs for diffusing iron ions.
As chromia grows with cations diffusion outward, the cationic defects
moves inward and gathered at the interface to form pores and holes in
this region and decreases the adherence of the layer [10, 11].
With increasing nickel content in alloy No. 2 the oxidation resistance
increases (Fig 1, 3 and 4). Nickel has a good effect on the oxidation
resistance. Although the affinity of nickel with oxygen is low, but it can
mechanical prosperities of the layer. It is suggested that with only small
amount of nickel dissolved in the chromia scales, it decreases the
cationic diffusion and oxidation resistance of the alloys. It seems that
nickel delays the transformation of chromia scale to spinels and ferric
Nickel also decreases the difference between alloy and oxide thermal
coefficient and the stresses at the alloy/oxide interface during cooling
. In the heat resisting alloys containing aluminum, chromium is of
importance. As Petit showed if the chromium and aluminum content of
the alloy is low, aluminum oxidizes internally under the protective layer
. In the first stage of oxidation, the chromium oxide formation
provides time for aluminum to diffuse to the metal/oxide interface and
reaches its concentration to the content to be able to form a continuous
alumina on the surface. With increasing the time of exposure chromia
layer volatiles and underlayer alumina protects the alloy. The alumina
layer grows and as its growth rate is low, the producing stresses at the
matrix/ oxide interface become lower, so it doesn't cause the crack at
In the alloys No. 1 and 2, aluminum has been oxidized internally (Fig.3
and 4). This is due to aluminum ions don’t have sufficient time for
penetration and enrichment under the layer for alumina formation, so
aluminum oxidizes internally and as the growth rate of the oxide is high
after some times these oxides appears in the oxide layer and forms
multilayer of aluminum, chromium, iron and nickel. The alloying
elements such as chromium, silicon, aluminum etc. are more stable than
iron, nickel and cobalt in the cast heat resisting steels. When the amount
of these elements which is added to the iron is low, internal oxidation
occurs. With increasing the amount of element, it changes to external
oxidation and the protective layer protects the alloy form rapid
In alloy No. 3 the rate of oxidation decreases (Fig. 1). XRD analysis
shows that Fe2O3 covers the surface and oxygen diffuses easily through
it so that internal aluminum oxides form (Fig. 5). Also some Cr2O3 forms
on the surface but it doesn't form a continuous layer of Cr2O3 on the
surface and the amount of spinels such as FeCr2O4 increases. The
existence of some oxides increases the pores and causes oxidation and
decreasing of oxide layer to the matrix.
Optical microscopy shows that increasing the nickel content causes
internal oxidation in alloy No. 4 (Fig. 6). XRD analysis shows that an
Al2O3 layer forms on the surface has impurity but it becomes pure with
time of exposure (Fig. 7).
The oxidation product on the surface of alloy 6 is Al2O3, which protect
the alloys from internal oxidation. The amount of aluminum is sufficient
for the formation of a continuous alumina. The alumina layer is pure and
this purity increases with increasing the time (Fig. 8).
1. Aluminum addition to the alloys increases the oxidation resistance of
2. Alumina layer has better protection prosperities than chromia layer.
3. Increasing Nickel improves oxidation resistance of the alloys.
4. The formation of the spinels decreases the oxidation resistance of the
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