High temperature oxidation.pdf

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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: [email protected] (Y.S. Baron), [email protected]

(A. Ruiz), [email protected] (G. Navas).

0257-8972/$ - see front matter 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.surfcoat.2007.09.038
mailto:[email protected]:[email protected]:[email protected]://dx.doi.org/10.1016/j.surfcoat.2007.09.038http://dx.doi.org/10.1016/j.surfcoat.2007.09.038mailto:[email protected]:[email protected]:[email protected]


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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.

2617Y.S. Baron et al. / Surface & Coatings Technology 202 (2008) 26162622


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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.

References

[1] M. Shane, M. Mecartney, J. of Mater. Sci. 25 (1990) 1537.

[2] K. Tsukama, J. Mater. Sci. Lett. 4 (1985) 857.

[3] T. Sato, S. Ohtaki, T. Endo, J. Am. Ceram. Soc. 68 (1985) C320.

[4] C.W. Kuo, Y.H. Lee, K.Z. Fung, M.C. Wang, J. Non-Cryst. Solids 351

(2005) 304.

[5] R. Garvie, P. Nicholson, J. Am. Ceram. Soc. 55 (1972) 303.

[6] M.J. Atik, J. Zarzycki, C. RKha, J. Mater. Sci. Lett. 13 (1994) 266.

[7] S.G. Kim, S.W. Nam, S.P. Yoon, S.H. Hyun, J. Han, T.H. Lim, S.A. Hong,

J. Mat. Sci. 39 (2004) 2683.

[8] R. Pascual, M. Sayer, C.V. Kumar, L.C. Zou, J. Appl. Phys. 70 (1991)

2348.

[9] H.B. Li, K.M. Liang, L.F. Mei, S.R. Gu, Mater. Sci. Eng. A 341 (2003) 87.

[10] C. Sanchez, J. Livage, M. Henry, F. Babonneau, J. Non-Cryst. Solids 100

(1988) 65.

[11] M. Guglielmi, G. Carturan, J. Non-Cryst. Solids 100 (1988) 16.

[12] K. Izumi, M. Murakami, T. Deguchi, A. Morita, N. Tohge, T. Minami, J. Am.

Ceram. Soc., 72 (1989) 1465.

[13] G.D. West, S. Birosca, R.L. Higginson, J. Microscopy 217 (2005) 122.

[14] J. West, Basic Corrosion and Oxidation, Ellis Horwood Limited, England,

1980.

[15] G.H. Meier, Mat. Sci. Eng. A 120 (1989) 1.

[16] O. Goncharov, Inorg. Mater. 40 (2004) 1295.

[17] P.A. Labun, J. Covington, K. Kuroda, G. Welsch, T.E. Mitchell, Metall.

Trans. A 13 (1982) 2103.

[18] A.M. Huntz, J. Mater. Sci. Lett. 18 (1999) 1981.

[19] N. Birks, G. Meier, Introduction to High Temperature Oxidation of Metals,

Edward Arnold, England, 1983.

[20] R. Chen, W. Yuen, Oxid. Met. 59 (2003) 433.[21] A. Skalli, A. Galerie, M. Caillet, Solid State Ionics 25 (1987) 27.

[22] F. Czerwinski, J. Szpunar, Thin Solid Films 289 (1996) 213.

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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