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High temperature oxidation resistance of 1.25cr0.5mo wt.%
steels by zirconia coating
Y.S. Baron , A. Ruiz, G. Navas
Departamento de Ciencias de los Materiales, Universidad Simn Bolvar, Caracas, Venezuela
Received 29 April 2007; accepted in revised form 20 September 2007
Available online 4 October 2007
Abstract
A zirconia coating was applied to improve the oxidation resistance of 1.25Cr0.5 Mo wt.% steels. A 8 wt.% yttria-stabilized zirconia coating
was deposited by solgel technique. One problem with this method is that the hydrolysis of the organometallic precursors is faster than
condensation, and the formation of precipitates is favored. Ethyl acetate was used to slow the hydrolysis rate in order to obtain a more continuous
layer. The air oxidation behavior of the coating was studied at 600 C and 700 C by the continuous measurement of the weight gain. The
microstructural characterization was performed by optical and scanning electron microscopy, and the composition was determined by energy
dispersive spectroscopy (EDS). The weight gain of the 1.25Cr0.5 Mo wt.% was diminished by about 70% compared to uncoated samples.
2007 Elsevier B.V. All rights reserved.
Keywords: Oxidation; Solgel coating; Partially stabilized zirconia (PSZ)
1. Introduction
Zirconia coatings improve the high temperature oxidation
behavior of steels. Because of their low thermal conductivity
(0.05 cal/C s cm) and their thermal expansion coefficient
(similar to most metals) they can be use as a thermal barrier;
however, they have poor thermal shock properties [1]. 69%
yttria stabilized zirconia, improves the properties of the coating,
exhibiting high fracture strength and fracture toughness due to a
stress-induced phase transformation of the tetragonal phase to
the monoclinic form [25].
An easy way to obtain yttria-stabilized zirconia coatings is
the sol
gel process [1,6
8]. Organometallic compounds areusually the precursors of the sol. The gel is a rigid network built
through the polymerization of the sol. One problem with these
kinds of precursors (alkoxides) is that the hydrolysis is much
faster than condensation, and therefore the formation of
particles is favored. If the hydrolysis rate were diminished, acontinuous coating could be obtained [1].
H. Li [9] used zirconium n-propoxide as the precursor of the
solgel coating on a mild steel substrate (AISI 1008). He
obtained a single-layer coating composed of zirconia particles
that reduced the oxidation of the steel. The thickness of the
coating is a critical parameter; with the use of a six layer coating
the oxide growth is retarded [9].
To slow the hydrolysis rate of the precursors in the solgel
process, the nature of the organic group could be changed (n-
butoxides are more stable than isopropoxides). Also compounds
such as acetylacetone, ethylacetate or allylacetate, can be used
to increase the hydrolytic stability of the precursors [10
11].K. Izumi et al., [12] used various zirconium compounds as
precursors in order to observe the influences of their chemical
properties on the zirconia coatings. Among the alkoxides eva-
luated, they found that the stability of zirconium tetra-n-butoxide
was better than that of zirconium tetraisopropoxide. Their results
indicated that the stability of the solution depended upon the
hydrolysis rate of the precursors. However, the film of ZrO2obtained was discontinuous and had weak adhesive properties
[12].
M. Shane and M. Mecartney [1] used zirconium tetrabut-
oxide and yttrium acetate as the precursors for the solgel, and
Available online at www.sciencedirect.com
Surface & Coatings Technology 202 (2008) 2616 2622
www.elsevier.com/locate/surfcoat
Corresponding author.
E-mail addresses: ylianabaron@usb.ve (Y.S. Baron), aruiz@usb.ve
(A. Ruiz), gnavas@usb.ve (G. Navas).
0257-8972/$ - see front matter 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.surfcoat.2007.09.038
mailto:ylianabaron@usb.vemailto:aruiz@usb.vemailto:gnavas@usb.vehttp://dx.doi.org/10.1016/j.surfcoat.2007.09.038http://dx.doi.org/10.1016/j.surfcoat.2007.09.038mailto:gnavas@usb.vemailto:aruiz@usb.vemailto:ylianabaron@usb.ve7/29/2019 High temperature oxidation.pdf
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acetylacetone is used to slow the hydrolysis rate. Excellent
adhesion at the interface was obtained due to significant coating
substrate interfacial reactions. The crystallization of the films at
different temperatures were evaluated. At 750 C the X-ray
diffraction data showed that the peaks of the zirconia phase are
fairly weak and broad, corresponding to small amounts of very
tiny crystalline particles. At 950 C the zirconia peaks are onceagain present, however there are several extra peaks, FeCr2O4and (Fe0.6Cr0.4)2O3, that increase the strengthening of the
interface between the ceramic and substrate [1].
In this study partially stabilized zirconia (PSZ) coating on
1.25Cr0.5 Mo wt.% steel was prepared by solgel processing,
where ethylacetate is used instead of acetylacetone as an
alternative to control the hydrolysis rate. The protective effect of
the PSZ coating during oxidation was investigated.
2. Experimental procedure
2.1. Substrate preparation
Substrates of 1x1.5x0.3 cm3 of 1.25Cr0.5 Mo steel were
used in this study. The substrates were grinded using 180 to
600 grit silicon carbide paper and polished with 1 and 0.3 m
colloidal alumina.
2.2. Coating preparation
Solutions of zirconium tetrabutoxide and yttrium acetate
were prepared as procedure described by M. Shane and M.
Mecartney [1]. Ethylacetate was used instead of acetylacetate to
slow the hydrolysis rate.
2.3. Heat treatment
The dip-coated samples were heated at 500 C for 15 min,
because the decomposition of the organometallic compounds
occurs at around 450 C. For multi-layer coating, the samples
were heated after each dip. Additionally, multi-layer coatings
were treated at 800 C for 30 min, to increase the coating density.
2.4. Oxidation test
Isothermal weight gain measurements were obtained with a
Cahn 100 thermobalance with an automatic data recorder(Iotech dakbook 216). The tests were carried out at 500 C,
600 C and 700C in air, for a period of 96 h.
2.5. Characterization
The microstructural characterization was performed in a
Philips XL 30 scanning electron microscope (SEM) operated at
25 kV. The composition was determined by energy dispersive
spectroscopy (EDS) with a Philips spectrometer joint to the SEM.
3. Results and discussions
3.1. Uncoated samples
Fig. 1 presents the SEM cross-section of one sample
oxidized at 500 C. It shows the formation of two layers. The
inner one is formed by iron and chromium spinel oxide as seen
by EDS (Table 1). The outer layer is formed by hematite
(Fe2O3). At temperatures below 570 C, the iron would be
expected to form a two-layered scale of magnetite (Fe3O4) and
Fig. 1. SEM cross-section of 1.25Cr0.5 Mo wt.% steel oxidized at 500 C. It
shows the formation of two layers of iron oxides. The presence of chromiumoxides indicates the original superficies.
Table 1
EDS over the cross section of 1.25Cr0.5 Mo wt.% steel after oxidation at
different temperatures
Elements (atomic %)
(Detection level N0.5 at.%)
Fe O Cr
Fig. 1 Inner layer 59.72 35.19 1.90
Outer layer 62.66 37.34
Fig. 2 Inner layer 46.17 51.91 1.92
Intermediate layer 1 43.42 56.10 0.48
Intermediate layer 2 45.88 54.12
Outer layer 38.79 61.21
Fig. 3 Inner layer 55.07 43.06 1.51
Intermediate layer 69.89 28.99 0.23
Outer layer 64.70 34.03 0.18
Fig. 2. SEM cross-section of 1.25Cr0.5 Mo wt.% after oxidation at 600 C. It
shows the formation of different layers. The two inner layers are formed of ironand chromium oxides, the two outer layers of iron oxides.
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Fe2O3, where the Fe3O4 is next to the metal [1318]. The
presence of chromium in the inner layer is due to an internaloxidation of this element that does not form a continuous layer
because of its low concentration in the metal.
Fig. 2 presents the SEM cross-section of the sample after
oxidation at 600 C. As in Fig. 1, it shows layers of different
morphology. The inner layers are formed by iron and chromium
spinel oxide (Table 1). At both temperatures, this spinel is the
product of the reaction in solid state between the chromium
oxide (Cr2O3), formed due to the internal oxidation of this
element, and the iron oxide, formed during the process of
thickening of the scale [18]. The outer layers are magnetite and
hematite oxides with no significant amounts of chromium.
These layers are formed by outward diffusion of iron cations,
while the chromium was left behind and internally oxidized.Therefore, this border between oxides with chromium and the
other without chromium, could be taken as the original metal/
gas interface.
Fig. 3 presents the cross-section of 1.25Cr0.5 Mo wt.% steel
oxidized at 700 C. It shows two layers. The inner layer is formed
by iron and chromium oxides (Table 1), and has similar
dimensions as the cross section of Fig. 2. The outer layer could
be hematite. We can explain the formation of these two layers asthe result of two processes. At the beginning, there is formation of
chromium oxides due to internal oxidation process, and the
formation of wustite (FeO), Fe3O4 and Fe2O3 due to the outward
cation diffusion of iron. As in the sample oxidized at 600 C, there
is reaction in solid state between the chromium oxide and the iron
oxide formed during the thickness of the scale. However the
hematite in the outer layer continues its growth process due to
cationic diffusion and destabilizes the layers beneath, forming a
layer of magnetite. As a result of the destabilization of the inner
layers, the iron can change from Fe2+ to Fe3+and create metallic
vacancies. Thus, the porous zone can be attributedto the diffusion
and coalescence of these vacancies.The oxidation kinetics for 1.25Cr0.5 Mo wt.% steel in air is
illustrated in Fig. 4. It shows a linear dependence between the
square of the weight gain, (m/S)2, versus the time of
exposition at 500 C and 600 C, indicating that the transport
of the ions across the scale is the rate controlling process [18
21]. At 700 C, there are two linear behaviors for (m/S)2 vs
time. During the firsts 30 h, there is oxidation and development
of the characteristics scales (parabolic rate constant
kp=98.64 mg2/cm4s), with predominance of FeO; at longer
times the oxidation rate decreases, as a result of the formation of
more protective scales (kp=42.96 mg2/cm4s) of Fe3O4 and
Fe2O3, and the presence of a porous zone. The porous zone
reduces the cationic diffusion of iron from the inner layer and,therefore, reduces the weight gain of the sample.
Fig. 3. SEM cross-section of 1.25Cr0.5 Mo wt.% steel oxidized at 700 C. It
shows the formation of different layers and a porous zone.
Fig. 4. Oxidation kinetics for 1.25Cr0.5 Mo wt %steel in air at different temperatures.
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3.2. Coated samples
Fig. 5 shows the SEM micrograph of the zirconia coating on
1.25Cr0.5 Mo wt.%. The single layer coating is formed of
clusters (Table 2) having an average diameter of about 5 m
(Fig. 5a). The three-layer coating is formed of bigger clusters
(30 m) that cover a larger area of the metal surface (Fig. 5b).The coating is mainly formed by tetragonal zirconia, as seen on
the XRD of Fig. 6. The presence of these clusters may be an
indication that there was hydrolysis of the precursors of the sol
gel process.
The oxidation kinetics curves at 600 C, were presented in
Fig. 7 for coated and uncoated 1.25Cr0.5 Mo wt.% steel. It
shows that all samples demonstrate a parabolic dependence
between the square of the weight gain versus the time of
exposition at 600 C. For the uncoated samples, the deviation
from the linear behavior at the first 20 h can be attributed to the
rapid formation of wustite as explained before. For coated sam-
ples, although there is no formation of a continuous layer of thezirconiacoating, the oxidation of the 1.25Cr0.5 Mo wt.%steelat
600 C is reduced due to the zirconia coating. A single layer
coating decreases the oxidation rate by about 40%, and a three-
layer coating decreases it by 80%.
The same mechanism were carried out in the experiment for
formation of the oxide scale with zirconia coating as per results
obtained by F. Czerwinski and J. Szpunar [22]. They proposed
that CeO2 coatings decrease the oxidation rate of chromia former
steels, because the Ce4+cations form pairs with cationic vacancies
in oxide grain boundaries, blocking these fast ionic paths.
As a parallel system, the same mechanism could be occu-
rring in this case. There is formation of the oxide scale with the
zirconia coating in it. The ZrO2 coating can react with the oxide
Fig. 5. SEM micrograph of the zirconia coating on 1.25Cr0.5 Mo wt.%. (a)
Single layer coating. (b) Three-layer coating. The presence of particles may be
an indicative that there was hydrolysis of the precursors of the solgel process.
Fig. 6. XRD of the surface of an oxidized and coated sample. It presents iron oxides and tetragonal zirconia. The iron oxides are due to the oxidation of the steel at700 C that promotes the spalling of the coating.
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and form pairs with vacancies Eq. (1). This pairs are formed due
to coulombian attractive forces.
3ZrO2 Y2Fe2O3
6OO 3Zr
Fe VjFe
1
Although there is formation of a new vacancy of iron, there
are three zirconia cations available to form pairs with the
vacancies of iron from the oxide. Consequently the mobilevacancy concentration in the oxide decreases due to the
formation of such pairs. Once this protective scale is formed,
the ionic diffusion is slowed, decreasing the oxidation rate.
Fig. 8 shows the surfaces of samples with one (Fig. 8a) and
three layers (Fig. 8b) of the zirconia coating after oxidation at
600 C. The formation of fine grain oxides (Table 2) is observed
that are the product of a slow grow with an increment of the
protective behavior of the oxide layer.
The oxidation kinetics curves at 700 C, for coated and
uncoated 1.25Cr0.5Mo wt.% steel, are presented in Fig. 9. For
uncoated samples, the behavior is linear for both curves of (m/
S)2
versus time. The first part of the curve (tb30 h) correspondsto a transient stage characterized mainly by a rapid formation of
wustite oxide and internal oxidation of chromium, and the last
part (tN30 h) characterized by the formation of hematite in the
outer layer due to cationic diffusion [20]. For coated samples,
the square weight gain versus time curves are the same for one-
layer and three-layer zirconia coating, and the decrease of the
uncoated sample weight gain is of the same magnitude for both
coated samples. Additionally, the slope of the coated sample
curve is the same as the slope of the last part of the uncoated
sample curves, indicating that cationic diffusion through
hematite is controlling the rate of the oxidation process.
Fig. 10 shows the surfaces of samples with one (Fig. 10a)
and three layers (Fig. 10b) of zirconia coating after oxidation at
Fig. 7. Oxidation kinetics curves at 600 C, for coated and uncoated 1.25Cr0.5 Mo wt.% steel. It shows that the use of the zirconia coating reduces the oxidation rate
in 40% for one layer and 80% for a three layer coating.
Fig. 8. SEM of the surfaces of 1.25Cr0.5 Mo wt.% steel coated with the
zirconia coating after oxidation at 600 C. (a) One layer coating. (b) Three layer
coating. It can be seen the formation of fine particles that are mono-crystals ofoxide, product of a very slow grow.
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700 C. These figures don't show the evidences of clusters
(Table 2) seen previously (Fig. 5). This may indicate that, even
though the coating acts in the first 30 h, for longer times there is
spallation of the zirconia coating.
Fig. 11 presents the SEM of the cross-section of a coated
sample, oxidized at 700 C for 96 h. It shows the formation of
two layers which are named as inner and external layer (outer).The inner layer is dense, and the external layer is formed by
columnar grains separated from one another, which was
produced by diffusion of cations. Comparing Fig. 10 with the
Fig. 3, it can be seen that the thickness of the oxides layers
formed on coated and uncoated samples after oxidation is
almost same. However, there are differences between the weight
gain of coated and uncoated samples. Based on these
observations, it can be assumed that the coating makes possible
the formation of vacancieszirconia cations pairs in the first
Fig. 9. Oxidationkinetics curves at 700 C, for coated and uncoated 1.25Cr0.5 Mowt.% steel. With the use of the zirconiacoating (one layer and three layer)there is a
decrease of the oxidation rate in a 30%, but the increase of the thickness of the coating doesn't decrease the oxidation rate.
Fig. 10. SEM of the surfaces of 1.25Cr0.5 Mo wt.% steel with the zirconia
coating after oxidation at 700 C. (a) One layer coating. (b) Three layer coating.It does not show evidence of the coating clusters.
Table 2
EDS over the surfaces of 1.25Cr0.5Mo wt.% steel coated with zirconia.
Elements (atomic %)
(Detection level N0.5 at.%)
Y Zr Cr Fe O Mn
Fig. 5a Cluster 23.04 35.76 0.91 39.82
Base 0.46 0.41 1.44 96.99
Fig. 5b Cluster 15.34 16.82 30.72 36.08
Base 1.29 85.67 11.48
Fig. 8a,b Clusters 15.34 16.82 0.58 30.72 36.08 0.46
Oxide 0.33 0.22 0.22 48.73 49.84 0.66
Fig. 10a Oxide 0.32 0.36 0.19 51.04 47.78 0.31
Fig. 10b Oxide 0.27 0.20 0.18 50.86 47.89 0.61
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30 h of oxidation. But, at longer times, the growing of the oxide
layers promotes the spallation of the coating.
4. Conclusions
At 500 C, 600 C and 700 C the oxidation process of the
1.25Cr0.5 Mo wt.% steel shows a parabolic relationship
between the weight gain and time, indicating that the
transport of the ions across the scale is the rate controlling
process.
The zirconia coating is formed by clusters, indicating that
there was hydrolysis of the precursors of the solgel
process.
At 600 C, the oxidation rate of a 1.25Cr0.5 Mo wt.%
steel decreases when coated with the zirconia coating. Asingle layer coating decreases the oxidation rate by about
40%, and a three-layer of coating decreases it by 80%.
This indicates that this coating can be used to increase the
temperature of operation of these steels to 600 C.
The use of multilayered coating does not improve the
resistance at 700C. Therefore, the coating is not effective
at this temperature.
Acknowledgments
The authors wish to thank the Decanato de Estudios de
Postgrado of the Universidad Simn Bolvar for the financial
support offered to this project.
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Fig. 11. SEM cross-section of 1.25Cr0.5 Mo wt.% steel coated with zirconia
after oxidation at 700C. It shows the formation of two different layers.
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