Quick Detection of Formation of Protective Scales on Heat-resistant Steels
in High-temperature Oxidation
Toshio MARUYAMA
Professor, Department of Metallurgy and Ceramics Science
Tokyo Institute of Technology
1. Introduction
Operation at the higher temperature in plants for power generation offers the higher
efficiency and the reduction of CO2 emission.

The target temperature is 700 㷄.

Toachieve this, materials with high creep strength are required so that researches are conductedon feritic Fe-Cr steels, austenitic alloys and nickel-based superalloys.

On the other hand, thehigh-temperature operation may give problems of the material degradation by
high-temperature oxidation/corrosion which determines the life of plants. When the oxide
scales is iron oxides, the oxidation rate is too high at 700 㷄.
Cr2O3 scale is required.

The formation of the external

The present authors have reported the effect of water vapor on theinternal/external oxidation in Fe-Cr alloys by microstructure observation1,2).
The decrease of oxidation rate and suppression of spalling are important issues,
which depend on the protectiveness of scales.

In chromium-bearing heat-resisting steels, theCr contents affect the microstructure of scales.

The development of a technique is expected

to evaluate quickly the protectiveness of the scale and feedback the results to the alloy design
and the proper operation.

Fig. 1 Schematic diagram of potential distribution of oxygen in oxide scale.

㧙㧙As shown in Fig.1 (a), the formation of protective Cr2O3 scale provides the oxygen
potential at the surface of the scale to be closed to that in the atmosphere because diffusion
through the scale is the rate-determining step.

On the other hand, the formation of lessprotective scales with high ionic diffusivity such as FeO gives lower oxygen potential at thesurface than that in the atmosphere because the supply of oxidant in the gaseous boundary
layer is rate determining (Fig.1 (b)). The measurement of oxygen potential at the surface
offers the technique to evaluate the protectiveness of the scales.
The principle and the experimental set-up have been reported elsewhere3).

In thepaper, the technique was applied to high temperature oxidation of nickel, cobalt and iron at1373 K and was confirmed the validity.

In the present paper, the usefulness of thistechnique was demonstrated in the detection of formation of protective scales onheat-resistant steels in high-temperature oxidation.
2. Principle3)
Calcia-stabilized ZrO2 (CSZ) is known as an oxide ion conductor, and is often used
as an electrolyte of an potentiometric oxygen sensor.

When CSZ is exposed to a differenceof oxygen chemical potential, an electromotive force is generated.

In this study, the oxygen

concentration cell shown in Fig. 2 is proposed in order to measure the difference of theoxygen chemical potential between at the oxide surface and in the atmosphere during thehigh temperature oxidation of a metal.

A metal sheet welded a platinum lead is placed on aCSZ bar attached with a platinum lead on the other side as shown in Fig. 2(a).
When the cell is exposed to the oxidizing atmosphere at elevatedtemperatures, the oxide scale is formed on the metal.
in the scale.

In the case that the scale

On the other hand, when growth proceeds outward as shown in Fig.

2(b), the CSZ bar is partly embedded the scale growth proceeds inward (Fig. 2(c)),
the CSZ bar is always attached on the scale surface.

Fig. 2 Oxygen concentration cell for the measurement of surface oxygen potential.

㧙㧙When the oxide scale is an electronic conductor, the oxide scale itself behaves as anelectrode of the cell. The redox reaction of oxygen mostly occurs at the triple phaseboundary of CSZ, electrode of the scale and gas phase.

The cell is expressed as(-) Pt | O*ad (POSur*), oxide scale | CSZ | Pt, gas(POAtm) (+)
ad2
where POadSur*Atmand ǴO2(1)denote the oxygen chemical potentials of the adsorbed oxygen

atom at the oxide surface and oxygen molecule in the atmosphere, respectively.

Theelectrochemical reaction at the electrode in the atmosphere isO 2  4e  o 2O 2 (2)2O 2  o 2O*ad  4e (3)and that at the oxide surface is

The electromotive force of this cell, Ecell is expressed as,EcellAtm
(2ǴSur*Oad ǴO2 ) / 4 F

where F is the Faraday constant.

( RT / 4 F ) ln( POAtm/ POSur)22(4)SurPOAtm
and PO2 denote the oxygen partial pressure in the2atmosphere and that at the oxide surface, which relates to the oxygen chemical potentials ofthe adsorbed oxygen atom in the following manner.

2ǴSur*
Ǵ$O2  RT ln( POSur*)Oad2(5)
AtmThe eq. (4) gives the oxygen chemical potential at the oxide surface from Ecell and ǴO2 .
In the case of Fig. 2 (b), the redox reaction of oxygen occurs at embedded part ofCSZ and at the triple phase boundary at the surface.
oxide scale (electrode) is a mixed potential.

Therefore, the electric potential of theThe mixed potential of the oxide scale can beobtained as that which the anodic current and the cathodic current are identical.

The anodic

reaction occurs much more at the triple phase boundary of CSZ, where the oxygen transport
is extremely higher than the scale interior.

Therefore, the mixed potential is just slightly

lower than the oxygen potential at the triple phase boundary.
considered to be negligebly small.

㧙㧙

However, the difference is3. Experimental
3. 1 Experimental set-up
Figure 3 shows a schematic illustration of experimental set-up and the oxygen
concentration cell.

In order not only to support the metal coupon but also to ensure the freegas flow, the CSZ tube was cut into the shape as shown in Fig. 3(b), which has three bars.
The platinum electrode detecting the oxygen potential in the atmosphere is attached at theposition of 20 mm away from the sample to make the position to be outside of the boundarylayer.

A CSZ oxygen sensor is located beneath the cell in order to monitor the oxygenAtmchemical potential in the atmosphere, ǴO2 .

The reference electrode of this sensor is air.

(a)

(b)

Fig. 3 Experimental set-up for the measurement of surface oxygen potential.
(a) whole system and (b) cell.

㧙㧙

The electromotive force of the oxygen sensor, Esensor is expressed as,Esensor
Air

where ǴO2

(ǴOAtmǴOAir2 ) / 4 F
2( RT / 4 F ) ln( POAir/ POAtm)
22



(6)

Airand PO2 denote the oxygen chemical potential and the oxygen partialpressure in air.

Both the cell and the CSZ oxygen sensor should be located in theisothermal zone of the furnace to minimize errors caused by thermo-electromotive force ofCSZ and temperature dependence of gas phase equilibrium.

The temperatures at the metal

coupon and at the oxygen sensor were monitored during oxidation experiments bythermocouples (T. C. ) of type -R that were placed as shown in Fig. 3(a).
3.2 Experimental procedure
3.2.1 Sample
The plates (10x10x1 mm3 in size) of samples of Fe-5~25 mass% Cr were used after
annealing at 1473 K for 43.2 ks (12h) in vacuum.
3.2.2 High Temperature Oxidation
Dry oxidation was carried out in the stream of Ar-21% O2 gas mixture up to 20 ks.
The oxygen partial pressure of the Ar–21%O2 gas mixture was checked by a CSZ oxygen
sensor before the test furnace and was about 2.1x104 Pa at 973 K.
Steam oxidation was carried out at 973 K in the stream of Ar–30%H2O gas mixture(the dew point of 351 K) up to 20 ks. The atmosphere of Ar–30%H2O gas mixture wasobtained by bubbling purified Ar gas through the evaporator and the saturator bothcontaining distilled water. The evaporator was held at 355 K by a heating mantle and thesaturator was maintained at 351 ± 1 K by a water bath. The gas flow was controlled to theflow rate of 100 cm3 min-1. The oxygen partial pressure of the Ar–30%H2O gas mixture waschecked by a CSZ oxygen sensor before the test furnace and was about 6x10-9 Pa at 973 K.
3.2.3 Analysis
After the oxidation, the furnace was cooled down to room temperature. For
observation of cross section, the samples were mounted in epoxy resin, ground by SiC
abrasive paper up to #2000 grits, and polished by 4 ȝm diamond slurry. The cross sections
were observed by scanning electron microscope (SEM) and EPMA. Phases in the oxide wereidentified by X-ray diffraction (XRD).

㧙㧙

4.

Results

4.1 Oxidation in dry condition (Ar-21%O2)
Figure 4 shows the oxygen chemical potentials at the surface of growing scale withthat in the atmosphere calculated from the emfs of the cell and sensor during oxidation in
Ar-21%O2 mixed gas. The oxygen chemical potential at coexistence of oxides ofFe3O4/Fe2O3 equilibrium calculated from thermodynamic data of Barin4) are also shown in
the figure. The oxygen chemical potential on the surface of Fe-5Cr is lower by 3~4 orders ofmagnitudes than that in the atmosphere and the difference decreases gradually with oxidationtime.

In the case of Fe-9Cr, the surface oxygen potential is lower by 2 orders ofmagnitudes than that in the atmosphere.

On the other hand, the surface oxygen potentials

of Fe-12Cr and 15Cr are close to oxygen potential in the atmosphere.

Fig. 4 Oxygen chemical potentials at the surface of growing scale㧙㧙

4.2 Oxidation in humid condition (Ar-30%H2O)
Figure 5 shows the oxygen chemical potentials at the surface of growing scale in
Ar-30%H2O mixed gas.

The oxygen chemical potentials at coexistence of oxides of

Cr/Cr2O3, Fe/FeCr2O4/Cr2O3, Fe/FeO, FeO/Fe3O4 and Fe3O4/Fe2O3 calculated fromthermodynamic data of Barin4) are also shown.

The oxygen chemical potential on thesurface of oxide scales in Fe-5~15 Cr drops down more than 5 orders of magnitudes from theoxygen potential at the atmosphere, which are below the oxygen chemical potential ofFeO/Fe3O4 equilibrium. On the other hand, the oxygen chemical potentials on the surfaces ofoxide scales in Fe-20 and 25 Cr alloy are close to the oxygen potential at the atmosphere.

Fig. 5 Oxygen chemical potentials at the surface of growing scale㧙㧙

4.3 Microstructure and phases in oxide scales
Figure 6 shows cross sections of Fe-15 and -25 Cr alloys after oxidation at 973 K
for 20 ks in Ar-30% H2O.

It is clearly observed that a thick duplex scale forms on Fe-15 Cr

alloy and a thin external scale exists on Fe-25 Cr alloy.

Fig. 6 Cross sections of oxide scales formed on Ar-30%H2O mixture.

Figure 7 illustrates theXRD patterns of the sample
surfaces after oxidation for 20 ks.
In dry oxidation (Ar-21%O2), thepeaks of Į-Fe and Cr203 appear
in Fe-9~15Cr alloys. Fe2O3 peak
was detected only in the sampleof Fe-5mass%Cr.

On the otherhand, oxidation in Ar-30% H2O
gives peaks of Į-Fe, FeO andFeCr2O4 spinel in Fe-5Cr but
only Cr2O3 appears as the oxide
phase in Fe-25 Cr.

Thestronger intensity of the peak of
Į-Fe for the metal substrateindicates

thethinneroxide

scales.

Fig. 7 XRD patterns of the sample surfaces after oxidation for 20 ks㧙㧙5. Discussion
5.1 Surface oxygen potential measurement
The surface oxygen potential at growing scales is lowered by consumption ofoxygen by oxidation.

The samples contain Cr of which concentration is high enough toform a protective external Cr2O3 scale exhibit the surface oxygen potential to be close to thatin the atmosphere.

This technique is very useful in quick detection of formation ofprotective scales.
The prominent difference in surface oxygen potential between dry and humid
atmospheres is the fluctuation in potentials.

The fluctuation in humid atmosphere (Fig. 5)

is larger than that in dry atmosphere (Fig. 4).

Schutze et al.5) carried out the acousticemission (AE) measurement during high temperature oxidation of Fe-Cr steels and reported
that AE event / signal was dramatically increased in humid oxidation compared to in dryoxidation.

They proposed that the presence of water vapor increases the growth stress inthe scales and leads to the formation of crack.

The fluctuation of surface potentials may

caused by the scale cracking during oxidation.
5.2 Effect of water vapor on Cr concentration required to form an external Cr2O3 scale.
Figure 8 summarizes the mode of oxidation for Fe-Cr alloys in dry and humid
atmosphere at 973 K. Theblankandsolidsquares

denote the formation ofexternal Cr2O3 scale and theformationoxideofiron-basedscale

without

protective
respectively.

Cr2O3scale,

The left two

results were obtained atoxygen pressure of 4.3x10-17
Pa5).

The required Cr

concentration is 8~9 mass%in dry condition.

Thewater vapor in atmospheres
increases

theCrconcentration to 12 mass%

Fig. 8 Mode of oxidation for Fe-Cr alloys in dry and humid
atmosphere at 973 K.(the left half was from Asep et al.6))㧙㧙

at oxygen pressure of 4.3x10-17 Pa and water vapor pressure of 7.7x102 Pa.
4water vapor pressure to 3x10 Pa requires Cr concentration of 18 mass%.

The increase ofThe water vapor

in atmosphere affects strongly the formation of external Cr2O3 scale.
6. Conclusion
The continuous measurement of chemical potential of oxygen at the surface ofgrowing scale was applied to the investigation to clarify the effect of water vapor in hightemperature oxidation of Fe-Cr alloys.

This technique successively provided the surface

oxygen potential which is a measure of the protectiveness of scales and the indication of theformation of external Cr2O3 scale.

The water vapor in atmosphere affects strongly theformation of external Cr2O3 scale in high temperature oxidation of Fe-Cr alloys..
Acknowledgement
This study was supported in part by JFE 21st Century Foundation in 2008.
References
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2) A.R. Setiawan, M. Hanafi, M. Ueda, K. Kawamura, T. Maruyama, ISIJ Int., 50 (2),259(2010).
3) Kojiro Akiba, Mitsutoshi Ueda, Kenichi Kawamura and Toshio Maruyama, Materials
Transactions, 49(3), 629 (2008).
4) Barin, Thermochemical Data of Pure Substances, VHC Publisher, Vol. 4, (1991).
5) M. Schutze, D. Renusch and M. Schorr, Corrosion Engineering, Science and Technology,
39(2), 157 (2004).
6) A.R. Setiawan, M. Ueda, K. Kawamura, T. Maruyama, Oxidation of Metals, (to besubmitted).

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