ORIGI NAL PAPER

Chromium-oxide Growth on Fe–Cr–Ni Alloy Studiedwith Grazing-emission X-ray Fluorescence

I. K. Koshelev Æ A. P. Paulikas Æ M. Beno ÆG. Jennings Æ J. Linton Æ M. Grimsditch ÆS. Uran Æ B. W. Veal

Received: 28 October 2005 / Revised: 17 February 2006 / Published online: 4 May 2007

� Springer Science+Business Media, LLC 2007

Abstract Using grazing-emission X-ray fluorescence (GEXRF), isothermal

oxidation of the alloys 55Fe–25Cr–20Ni and 55Fe–25Cr–20Ni(+0.3Y) (wt.%) were

studied as a function of oxidation time at 750 8C in O2. In addition, the effect of

thermal cycling was studied. Using GEXRF, oxide thickness, the Cr-depletion zone

in the substrate, and Fe and Ni concentrations in the oxide were monitored as a

function of oxidation time. Scanning-electron microscopy was used to indepen-

dently measure the Cr-depletion zone. Raman spectroscopy was used to measure the

concentration of Fe2O3 appearing in the oxides in early oxidation (less than 2 h).

Both GEXRF and Raman measurements show that the thermally-grown chromium

oxide purifies with extended oxidation; initially abundant Fe2O3 became unde-

tectable after 2 h of oxidation. However, the total Fe concentration was still *3%

after 2 h but systematically decreased with further oxidation. Thermal cycling had

no effect on these results.

Keywords Grazing emission X-ray fluorescence � Chromia scale �Scale growth

PACS 68.55.-a � 61.10,Ht

Introduction

Chromium-oxide scales which form naturally on a variety of alloys in high-

temperature environments provide essential corrosion protection for those alloys.

I. K. Koshelev � A. P. Paulikas � M. Beno � G. Jennings � J. Linton � M. Grimsditch �S. Uran � B. W. Veal (&)

Materials Science Division, Argonne National Laboratory, 9700 S. Cass Av.,

Argonne, IL 60439, USA

e-mail: veal@anl.gov

123

Oxid Met (2007) 68:37–51

DOI 10.1007/s11085-007-9053-2

For in-service use, it is important that these economically vital protective oxides be

highly adhesive, slow growing and self healing [1–4].

The oxide scale forms in a complex process involving the outward diffusion of

chromium atoms and the inward diffusion of oxygen atoms. Since Cr atoms are

extracted from the substrate as the oxide grows, a chromium-depletion region

necessarily develops at the oxide–metal interface. In early-stage growth, transient

phases appear in the scale and eventually disappear as the scale matures. For Fe–Cr–

Ni alloys, transient Fe- and Ni-rich phases appear in early oxidation, but eventually

disappear after long-term oxidation [5–7]. Apparently, after the formation of a

continuous chromia scale, Fe and Ni uptake decline; i.e., the chromia scale increases

in purity. In this study we measure, as a function of scale growth time, the

concentrations of Fe and Ni in the scale. In relatively early-stage growth, the Ni

concentration falls to a very low level (<1 at.%). While the concentration of Fe in

the scale diminishes somewhat more slowly, we observe that the total amount of Fe

in a given area of scale declines as oxidation proceeds. Thus, what is occurring is

not a simple dilution process resulting from the addition of new scale. The observed

behavior is explained using simple thermodynamic arguments.

Scales, grown isothermally on 55Fe–25Cr–20Ni(Y) (wt.%) alloys, were studied

as a function of oxidation time, at 750 8C in O2, using grazing-emission X-ray

fluorescence (GEXRF). The GEXRF measurements were complemented by Raman

spectroscopy, SEM and optical reflectivity to obtain independent determinations of

scale-characterization parameters.

In the GEXRF measurement, the intensity I(h) of X-ray fluorescence spectra for

Fe, Cr and Ni was measured vs. emission angle h. By simultaneously fitting

calculated spectral functions [7, 8] to all of the measured data [I(h) emission spectra

for Cr, Fe and Ni], and requiring an internally consistent set of materials parameters,

the scale thickness, composition, and the substrate Cr-depletion zone were

quantitatively measured. Raman spectroscopy was used to measure the ratio of

Fe2O3 to Cr2O3 in the early-stage oxidation of the evolving scales. Evolution of the

measured parameters was followed as oxidation proceeded.

Experimental Procedures

The oxidation study was performed on noncommercial bulk samples of composition

55Fe–25Cr–20Ni and 55Fe–25Cr–20Ni–0.3Y (wt.%). Starting materials (at least

99.99% pure) were repeatedly arc melted (usually 6 melts) in an Ar/He atmosphere,

with the samples cleaned, if needed, and rotated between melts. After the final melt,

samples were rolled to 1 mm thickness. Samples were cut from the rolled sheet,

ground flat and polished with 1 l alumina abrasive. Spectroscopic analyses showed

approximately 50 ppm levels of O, H, N and C. For the GEXRF studies, a total of

eight coupons was polished; four contained 0.3 wt.% of Y, a ‘‘reactive element’’

(RE) [1–4] and four were RE-free. Pairs of samples (with and without RE) were

concurrently treated isothermally at 750 8C in flowing O2 for 2, 4, 8 and 16 h. A

second set of eight samples (four pairs with and without RE) were given cyclic-

oxidation treatments. Sample pairs were heated in one-hour intervals at 750 8C and

38 Oxid Met (2007) 68:37–51

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were then cycled to room temperature. These sample pairs were given 2, 4, 8, and

16 heating cycles. For the Raman studies, pairs of polished samples were

isothermally oxidized for periods of 1/4, 1/2, 1 and 2 h.

For the heat treatments, samples were placed in an alumina boat which was then

inserted into a (horizontal) quartz tube furnace through which oxygen was flowing.

The boat was inserted upstream, i.e., into the flowing gas. After the heat treatment,

the samples and boat were cooled by quenching into air.

GEXRF measurements were used to monitor the evolution of oxide thickness, Fe

and Ni concentrations, and the Cr-depletion zone as a function of isothermal-

oxidation time, as the scale developed at 750 8C in O2. The GEXRF measurements

were performed using X-ray synchrotron radiation generated by a bending magnet

at the Advanced Photon Source, Argonne National Laboratory (beamline 12BM).

Monochromatized X-rays of *10 eV bandwidth struck the sample at near-normal

incidence illuminating an area of about 1 mm · 0.5 mm. Fluorescent emission

from the sample was simultaneously monitored, at emission angles near grazing,

from Fe, Ni and Cr atoms using an energy-dispersing Ge detector. Slits placed in

front of the detector were adjusted to provide a 3 milliradian acceptance aperture.

The intensities I(h) were recorded as a function of emission angle h, measured

relative to the sample surface. Although Fe, Cr and Ni fluorescence spectra were

simultaneously recorded while the sample was irradiated, an I(h) scan for each of

the three elements was also acquired separately, using different excitation energies,

in order to avoid complications resulting from secondary fluorescence. For each

fluorescing element, the impinging excitation was tuned to an energy just above the

K-edge energy of that element. Thus, 6.8 keV excitation energy was used to study

Cr, 8 keV to study Fe, and 9 keV to study Ni atom fluorescence. Measured

fluorescence spectra were always normalized to the X-ray intensity incident on the

sample.

I(h) spectra were fit using a calculated function ([7], Eq. 15). A standard sample

(the preoxidized bulk alloy with known composition) was first measured to

determine an overall scale factor for each fluorescent element under investigation

(accounting, for example, for unknown transition probabilities). These scale factors

were then used for analyzing data from the oxidized sample. Scale thickness and

elemental concentrations are obtained as fitting parameters. Independent determi-

nations of scale thickness were obtained from measurements of Ka/Kb intensity

ratios, and from optical-interference measurements [9, 10].

Raman-spectroscopy measurements were performed using the facility and

procedures described in [5]. Fe2O3 has a clear Raman signature, quite distinct

from Cr2O3 or spinels, and no competing signal is obtained from the substrate.

Further, Fe2O3 appears distinctly in early oxidation of the Fe–Cr–Ni alloy but is

replaced by a Cr2O3 signal as oxidation proceeds [5]. The Fe2O3 signal is measured

as a fraction of the total of Fe2O3 and Cr2O3. Measured intensity ratios for Fe2O3

and Cr2O3 are calibrated against standard samples consisting of known mixtures of

Fe2O3 and Cr2O3.

Energy dispersive X-ray measurements were conducted on Hitachi SEM

equipment. Concentrations of Cr, Fe and Ni were measured in the substrate, as a

function of distance from the scale/metal interface, on cross sections of Fe–Cr–Ni

Oxid Met (2007) 68:37–51 39

123

samples, that had been isothermally oxidized for 4 h and for 16 h. The energy of the

electron beam was 15 kV, with spatial resolution of approximately 1 micron.

Results and Discussion

Scale characterization using GEXRF

Characterization parameters for the scales were extracted from fits to measured I(h)

normalized by Io , the incident intensity. Data were fitted to a specialized form of the

general model function describing angle-dependent fluorescent emission [7, 8]. For

multiple-layer films over a substrate, the emission intensity is given by

IðhÞ ¼X

j

IðhÞj ð1Þ

where I(h)j , the contribution from layer j, is

IðhÞj ¼Dx � Io � cj � as � Ee

lilj

sin aþlemj

sin h

� 1� exp �lilj

sin aþ

lemj

sin h

� �n ozj

h i�

expXj�1

i¼1

� lili

sin aþ lemi

sin h

� �zi

( ) ð2Þ

Here, zj is the thickness of the jth layer (numbered from the topmost layer which

is designated j = 1), Dx is the solid angle of the detector which is subtended by the

illuminated area of the specimen, Io is the flux of illuminating radiation, cj is the

concentration of fluorescing atoms in layer j (atoms/volume), as is the absorption

cross section of the fluorescing atoms for the illuminating radiation (area/atom), Ee

is the probability for the emission of an X-ray of interest after an absorption of an

illuminating photon has occurred, a is the angle of incidence of the illuminating

radiation, h the angle of emission of fluorescing radiation, lilj and lemj are

absorption coefficients for illuminating and fluorescent radiation in layer j. We

assume illumination by a monoenergetic beam of X-rays whose energy is above the

absorption edge of the element of interest.

To analyze the oxidized sample, we consider a homogeneous film over a

substrate containing a Cr-depleted zone located at the scale–metal interface. Thus,

for data fitting, contributions to the intensity from the film, from the depletion-zone

layers and from the (undepleted) substrate are included. Non-redundant information

is obtained by simultaneously fitting spectra of all of the major constituents of the

sample (i.e., Cr, Fe and Ni). Simultaneously fitting I(h)/I0 spectra from multiple

elements substantially reduces errors in the extracted parameters.

A standard sample (the preoxidized bulk alloy with known composition) was first

measured to determine an overall scale factor

Const ¼ Dx � cj � as � Ee ð3Þ

40 Oxid Met (2007) 68:37–51

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for each fluorescent element under investigation. The solid line fits to the Fe, Cr and

Ni data acquired from the alloy prior to oxidation appear in Fig. 1, panel (a). The

fitting functions, each with a single adjustable parameter (the scale factor), very

adequately describes the I(h) spectra for Fe, Cr and Ni in the bulk alloy.

Figure 1 also shows I(h) spectra after the sample has been oxidized for 2 h (panel

b) and for 16 h (panel c). Panels (b) and (c) show that the Fe and Ni signals become

progressively weaker, relative to Cr, as oxidation proceeds. This occurs because the

Fe and Ni signals come predominately from the substrate, which becomes

increasingly buried as scale growth advances. However, the Cr signal increases

since the Cr concentration in the scale exceeds that in the substrate. Note also, in

Fig. 1, that the shape of I(h) for Fe and Ni spectra near the critical angle changes

dramatically as the scale forms. This characteristic change in the shape of the

leading edge occurs when fluorescing atoms on the top surface become buried by an

absorbing layer.

Fig. 1 (a) Simultaneously acquired I(h) measurements of Cr, Fe and Ni for the unoxidized 55Fe–25Cr–20Ni alloy. Note that simulated spectra (solid lines) provide excellent fits to the measured I(h) afteradjustment of a single parameter (overall scale factor). (b) Simultaneously acquired I(h) measurements ofCr, Fe and Ni for the 55Fe–25Cr–20Ni alloy after 2 h oxidation and (c) after 16 h oxidation. In (b) and(c), solid lines are data simulations which provide measurements of scale parameters (thickness, depletionzone, Fe and Ni concentrations)

Oxid Met (2007) 68:37–51 41

123

With scale factors determined, we proceed to analyze I(h) spectra for the

oxidized sample. Scale thickness, Cr depletion in the substrate, as well as Fe and Ni

concentrations in the scale are treated as fitting parameters.

Modeling of I(h) Spectra from GEXRF

To fit the measured I(h) spectra, we calculate I(h) for Cr, Fe and Ni when the set of

parameters (scale thickness, Fe and Ni concentrations, and depletion-zone depth) is

varied to include all possible combinations, within a physically reasonable range of

values. For a given set of parameter values, the deviations of each datum point from

the calculated I(h) curves are calculated and the rms of the deviations, obtained for

the three spectral fits, is recorded. In [8] plots of calculated rms deviations are

shown vs. fitting parameter values (for scale thickness, depletion-zone depth, Fe and

Ni concentrations) for the sample, included in this study, that was oxidized for 4 h.

The deviation plots display minimum values where best fits to the four parameters

are obtained. Thus we examine the whole four-dimensional parameter space of

physically reasonable values of scale thickness, depletion-zone depth, Fe concen-

tration, and Ni concentration to find those that yield the minimum root-mean-

squared differences between measured and calculated intensities, I(h).

The fitting procedure requires simultaneous fits of I(h) for Cr, Fe and Ni. This is a

very demanding constraint leading to well-defined fitting parameters, even though

four parameters are used. Further, in all of the oxidation treatments, the Ni

concentration in the scale was found to be very low, a circumstance which

effectively removes one of those fit parameters.

Cr depletion must occur in the substrate, in the vicinity of the scale–metal

interface, since the Cr which makes up the scale is extracted, by chemical diffusion,

from the substrate. Prior measurements of the Cr-depletion zone [11, 12] in related

chromia-forming alloys suggest that, for oxidation at 750 8C in O2, a Cr-depletion

zone will originate at the scale–metal interface and will extend for several microns.

For modeling I(h) obtained from GEXRF, the depletion zone is approximated by

three homogeneous layers with Cr concentrations of neighboring layers differing by

a constant increment [8]. That is, the depletion zone is represented by a histogram of

three rectangles of equal width w (in microns), and systematically changing Cr

concentrations [8]. The Cr concentration gradient is thus approximated by a

(stepped) linear function. This is, of course, a rather crude approximation to the

actual gradient, but arguably all that is justified by the multiparameter fitting

procedure.

The total number of Cr atoms removed from the zone, by oxidation, is

determined by the scale thickness since the quantity of Cr removed from the

depletion zone is equal to the quantity of Cr in the scale, i.e., the total amount of Cr

is conserved. Thus the depletion zone is represented by a histogram which

approximates a triangle (linear) function of varying width and depth depending on

scale thickness (Fig. 2, [8]). The histogram is determined by specifying a single

parameter, e.g., the histogram parameter w. The Cr concentration at the oxide–metal

interface is determined from the depletion range (3w) and the total amount of Cr

removed by the scale.

42 Oxid Met (2007) 68:37–51

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Figure 1, panels (b) and (c), shows fits to I(h) for Cr, Fe and Ni for the Y-free

alloy oxidized for 2 h and for 16 h, respectively. We observe that the data are very

well described by the model function.

Although differences between measured and calculated spectra are very subtle,

those differences might be attributable to additional effects not considered in the

fitting, such as Fe concentration gradient in the scale, or inadequacy of the

approximation for the depletion zone [8]. Note, also, that both metal and oxide

spectra (from substrate and scale) are monitored in I(h) with a different weighting of

metal and oxide signals as emission angle is changed. With monochromatic

excitation radiation (as used in these experiments), EXAFS oscillations will cause

relative absorption coefficients for metal and oxide to vary with energy. Any

spectral lineshape variations that could be associated with these oscillations were

not considered. Small unidentified systematic errors in the measurements might also

contribute to the discrepancies.

Roughness at the oxide/air and oxide/metal interfaces can change substantially

during the course of oxidation. However, the GEXRF technique is normally quite

insensitive to roughness at these interfaces. A weak sensitivity to roughness occurs

at very low emission angles for films with smooth interfaces (rms roughness on

order of X-ray wavelength k; for Fe emission, k * 2 A). However, samples in this

study have substantially thicker oxide scales. Here, the behavior of the fluorescence

signal is determined by the absorption length of X-rays which pass though the film

and the penetrated substrate. For Fe emission through chromia scales on Fe–Cr–Ni

substrates, the absorption length is *5 microns, a length scale where roughness

effects could again become discernible. Thus, the influence of interface roughness

on the GEXRF measurements, reported in this study, is expected to be negligible.

Cr-depletion Zone as Defined from GEXRF

In fitting the GERXF data, the depletion zone was calculated as a stepped triangle

function. For simplicity, we show in Fig. 2, smoothed triangle functions

Fig. 2 The evolution of the Cr-depletion zone in Fe–Cr–Ni at750 8C, as measured using GEXRFfor 2 (a), 4 (b), 8 (c) and 16 (d) h ofoxidation. The depletion zone isapproximated by a simple trianglefunction (from smoothed stepfunctions—see text). Horizontal linerepresents bulk concentration of Crin unoxidized alloy

Oxid Met (2007) 68:37–51 43

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representing the extracted depletion zones for the 2, 4, 8, and 16 h oxidations of the

Fe–Cr–Ni sample (Y-free). The depletion zone extends several microns into the

substrate; the total amount of depletion increases with oxidation, consistent with

scale thickening. Within experimental uncertainty, the Cr concentration at the

oxide–metal interface is independent of oxidation time for these treatments. Similar

behavior is observed for the Y-containing sample.

For Y-free samples oxidized for 4 h and for 16 h, the range and composition of

the depletion zone was independently measured using scanning-electron microscopy

(SEM). The Cr concentration in the substrate was measured, on a polished sample

cross section, using energy-dispersive X-rays (EDX), as a function of depth in the

vicinity of the scale–metal interface. SEM measurements were obtained with a

spatial resolution of 1–2 microns. Results are shown in Fig. 3 plotted with the

depletion zone obtained from GEXRF. Results obtained using the two techniques

are in reasonable agreement.

Also shown in Fig. 3 are calculated diffusion profiles obtained by considering the

diffusion of Cr from a homogeneous semi-infinite substrate to a surface sink (the

oxide layer). The concentration is given by [11]

c ¼ ci þ ðcb � ciÞ � erfx

2ðDtÞ1=2

!ð4Þ

where x is the distance into the sample from the scale–metal interface, D is the

diffusion coefficient and t is the oxidation time. cb is the concentration in the

substrate before oxidation and ci is the Cr concentration at the metal–oxide

interface, which can change as a function of time as the oxide grows. Equation (4)

assumes that the recession rate of the oxide-interface is slow.

The calculated profiles shown in Fig. 3, which reasonably represent the

experimental concentration profiles, were obtained using D = 3.5 · 10�16 m2/s in

Eq. (4). For a fixed D, the condition was imposed, by appropriately adjusting the

interface Cr concentration ci that the amount of Cr removed from the depletion zone

was equal to the amount of Cr appearing in the scale. In applying this Cr-

conservation criterion, we use scale-thickness measurements appropriate to the 2, 4,

8 and 16 h oxidation times [10]. The calculated concentrations of Cr at the scale–

metal interface as function of oxidation time are shown in Table 1.

We compare this measured value (D = 3.5 · 10�16 m2/s) to that obtained from an

analysis of the Cr-depletion profile (D = 0.8 · 10�16 m2/s) in a sample of 19Cr–

25Ni–bal Fe that was oxidized for 500 h at 800 8C in steam [12]. Cr-diffusion

measurements in alloys of approximate composition 20Cr–25Ni–bal Fe (wt.%), but

containing additions of Mn, Si and Nb, were also reported by Smith and Gibbs [13]

using tracer techniques, and by Evans and Donaldson [11] from analysis of

Cr-depletion measurements following lengthy oxidations at 900 8C. For these

alloys, with somewhat different compositions, reported values at 900 8C vary

between 4 · 10�17 and 6 · 10�16 m2/s.

The Table 1 indicates that, for the 2, 4, 8 h and 16 h oxidations, concentration of

Cr on the oxide–metal interface is nearly independent of oxidation time. Further, the

44 Oxid Met (2007) 68:37–51

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relatively high value of Cr concentration at the scale–metal interface confirms that

oxide growth is limited by diffusion through the oxide, rather than the substrate,

consistent with previous results [11].

Table 1 Concentration of Cr at the scale–metal interface calculated from diffusion profiles and scale

thicknesses obtained from GEXRF measurements for depletion zones of Fe–Cr–Ni(Y) alloys as function

of the isothermal oxidation time at 750 8C in O2

Time (h) ci (at.%) Thickness (mm) D (m2/s*10�16)

2 0.15 0.35 3.5

4 0.18 0.49 3.5

8 0.2 0.52 3.5

16 0.198 0.97 3.5

Fig. 3 Measurements of the Cr-depletion zone in proximity to the scale–metal interface for the Y-freealloy after 4 h and 16 h oxidation treatments at 750 8C in O2. Cr concentration is plotted vs. distance intothe metal from the scale–metal interface. The step function was obtained from GEXRF, the dots wereobtained from EDX of a cross section. The dashed lines are calculated diffusion profiles (see text)

Oxid Met (2007) 68:37–51 45

123

This result is in agreement with previously measured [10] constants for parabolic

oxidation of Fe–Cr–Ni(Y). This study showed that for the oxidation at 750 8C the

parabolic constant is 1.1 · 10�18 m2/s for Fe–Cr–Ni(Y), and 1.3 · 10�18 m2/s for

Fe–Cr–Ni alloy. The ratio of kp=D is *3 · 10�3. This means that the potential rate

of supply of the metal ions from the bulk to the interface is much higher than the

rate at which these ions can be oxidized. Thus the oxidation process is limited by

diffusion within the oxide layer.

It is shown that the GEXRF measurements provide a rough determination of the

Cr-depletion zone, from which an approximate value of the Cr-diffusion rate in the

substrate can be extracted, even with relatively short oxidation times and relatively

low oxidation temperatures. More typically, the depletion zone is studied after much

longer oxidation times and higher oxidation temperatures [11, 12, 14, 15].

Fe and Ni Concentrations in Scale

The Ni concentration in the scale, obtained from GEXRF measurements, is very low

at all stages of oxidation between 2 and 16 h. For the Fe–Cr–Ni–Y sample, there is a

measurable Ni-rich residue on the oxide surface that persists throughout the

oxidation treatment, equivalent to *7 A of NiO [10]. For Fe–Cr–Ni, however, Ni is

essentially undetectable in the scale (always less than 0.5 at.%). However, the Fe

concentration in the scale is substantially higher, at all oxidation times up to 16 h.

The maximum value is about 3 at.%, after 2 h of treatment, and falls systematically

with further oxidation.

Figure 4 shows Raman spectra obtained from the Y-free alloy after 0.25 h (panel

a), 1 h (panel b) and 2 h (panel c) of oxidation at 750 8C in O2. The intensity of the

Fe2O3 peak at 230 cm�1 measured relative to the Cr2O3 peak at 550 cm�1 provides

fractional amounts of Fe2O3 and Cr2O3 in a transient scale forming on the Fe–Cr–Ni

alloy. Note that Fe2O3 is a dominant peak after 0.25 h of oxidation but that it

declines rapidly, relative to Cr2O3, with further oxidation. Structures between 600

and 700 cm�1 can be attributed to (FeCr)3O4 spinels with Fe2+/3+ valence states and/

or to (FeCr)2O3 solid solution [16]. The contribution of Fe to this high-energy-peak

structure cannot be readily obtained [5].

Figure 5 shows Fe concentrations obtained from GEXRF and Raman measure-

ments. The GEXRF results, which measure the total Fe (i.e., all valence states)

contained in the scale, shows that the Fe is declining as the thickness increases,

demonstrating that the scale purifies as growth progresses. There is no discernible

difference in Fe concentration for the Y-free and Y-containing alloy scales.

Raman spectroscopy provides an estimate of the approximate amount of Fe3+ in

the early-stage scale. The Fe2O3 measurements obtained from Raman spectroscopy

probe only one crystal structure and thus provide a lower limit on the Fe

concentration. However, in very early stages of oxidation, Fe is expected to appear

predominately as Fe2O3, consequently, Raman measurements should provide a

reasonably accurate determination of the total Fe concentration near time = 0. Thus,

the dashed line in Fig. 5, containing GEXRF and early stage Raman data, reveals the

total Fe concentration in the scale as it evolves with oxidation time. The very rapid

46 Oxid Met (2007) 68:37–51

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falloff of the Raman Fe2O3 signal suggests that Fe3+ ions are being rapidly reduced

as oxidation proceeds.

Figure 5 indicates that Fe dilution occurs as a result of a reduced rate of Fe uptake

as the scale thickens. However, it does not determine if the total amount of Fe in the

scale might be declining as growth proceeds. Since we have measurements of both

Fe concentration and scale thickness, we can determine the total amount of Fe in a

given area of scale (product of concentration times thickness). These results are

shown in Fig. 6. They indicate that, after an initial rise (lasting until the scale is

approximately 0.1–0.2 microns thick), the total amount of Fe in the scale decreases

with further oxidation. In this range, the scale purifies rapidly with increasing

thickness.

In the early stages of oxidation, we expect oxidized Fe, Ni and Cr atoms to

appear in the scale in ratios comparable to their ratios in the unoxidized alloy, i.e.,

all of the metal atoms will be oxidized without significant segregation. Since Fe

constitutes 55% of the metal atoms, it would constitute about 23% of the atoms in

the early scale (assuming a mix of Fe2O3, Cr2O3 and NiO).

Fig. 4 Raman-spectroscopy measurements from the oxide scale on Fe–Cr–Ni after 0.25, 1, and 2 h ofoxidation at 750 8C in O2. Note that, with oxidation, the Fe2O3 peak declines relative to the Cr2O3 peak.Here, star represents a peak from the illuminating laser line

Oxid Met (2007) 68:37–51 47

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The Raman measurements indicate that, after 15 min of oxidation, the ratio

Fe2O3/(Cr2O3 + Fe2O3) in the scale is about 0.35 (assuming the Ni concentration is

negligible). A lower limit for the ratio of Fe/(Fe+Cr) can thus be estimated to be

Fig. 5 Measured total Fe concentration (wt.%, circles) and Fe2O3 (wt.%, triangles) in the scales grownon the Fe–Cr–Ni(Y) alloys plotted vs. isothermal oxidation time at 750 8C in O2. With increasedoxidation, the Fe concentration in the scales decreases

Fig. 6 The number of Fe atoms per cm2 (total, squares and 3+ ions, circles) of scale for Fe–Cr–Ni(Y)alloys isothermally oxidized at 750 8C in O2. Dotted line represents model calculations of Fe quantity inthe scale. As scales thicken, the total amount of Fe in the scales decreases. Thus, as oxidation proceeds,more Fe moves out of the scale than is taken up

48 Oxid Met (2007) 68:37–51

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about 0.35. This can be compared to 0.69 for the unoxidized alloy. The Fe2O3

(Raman) signal falls rapidly, approaching the detection limit after about 1.5 h of

oxidation; here the scale is about 2,500 A thick. Thus, by 2,500 A thickness,

essentially all of the Fe3+ in the scale has been reduced (to Fe2+ and/or neutral Fe).

The dotted line in Fig. 6 shows the total amount of Fe that would be incorporated in

the scale, as the scale thickens, if the Fe, Cr and Ni ratios remained the same as in

the substrate metal. If this initial value is the upper limit on Fe concentration in the

scale, then Fig. 6 suggests that, as the scale thickens, incorporated Fe must increase

to a maximum of about 2.5 · 1017 atoms/cm2 in a scale of approximately

0.1–0.15 micron thickness, after less than 2 h of oxidation. With additional

oxidation, Fe uptake is soon halted and the total Fe loading in the scale begins a

rapid decline. This suggests that a scale of mixed oxide about 1,500 A thick is

needed to establish a PO2 sufficiently low at the scale–metal interface to reduce the

Fe solubility enough to prevent Fe uptake and to cause previously incorporated Fe to

begin migrating out of the scale.

A simple thermodynamic argument suggests that the Fe concentration, and

eventually the total amount of Fe in the scale, should indeed decline at some time

after a continuous chromia scale has developed. As new scale growth slowly

proceeds, the oxygen activity (essentially the oxygen partial pressure) at the scale–

metal interface will adjust to that value where the metal and Cr2O3 are in

thermodynamic equilibrium. At 750 8C, this value is about 10�30 atm O2 [17]. The

oxygen partial pressure will rise rapidly with distance away from the scale–metal

interface, to a value of approximately 1 atm at the outer surface of the scale. At

750 8C, iron oxides will decompose to neutral Fe at a PO2 * 10�19 atm [5, 16, 17],

a substantially higher pressure than needed to reduce chromium oxides. Conse-

quently, within the scale, in the vicinity of the scale–metal interface, the oxygen

activity will fall to a value sufficiently low to decompose any iron oxides [18]. It

will not be energetically favorable for these reduced Fe atoms to remain in the

extreme low PO2 region of the scale, near the buried interface. Effectively, the iron

solubility in this portion of the scale falls to a very low value. A driving force will

exist promoting Fe diffusion out of this region of the scale. Further, the extremely

low solubility acts as a diffusion barrier, blocking the uptake of new Fe atoms into

the scale. Since Fe uptake is blocked and a driving force has been established for

inward diffusion of Fe, it follows that the total amount of Fe in the scale should

decline, as observed.

Figures 5 and 6 show that Fe purification rates in scales thermally grown on

Fe–Cr–Ni and Fe–Cr–Ni (Y) alloys are the same, within experimental uncertainty;

i.e., the average Fe concentration is the same for the two scales after a given heat

treatment.

For the Y-free alloy, the Ni concentration within the scale is sufficiently low to

escape detection with GEXRF. However, for the Y-containing alloy, a residue of

oxidized Ni is readily observed on the top surface of the scale [10]. The presence of

the Ni residue on the Y-containing alloy and its absence on the Y-free alloy is taken

as evidence that, for the Y-containing alloy, new scale forms at the scale–metal

interface (or at least internal to the scale) and the early stage oxidized Ni simply

floats on the growing scale. For the Y-free alloy, some new chromia growth occurs

Oxid Met (2007) 68:37–51 49

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at the top surface burying early-stage oxidized Ni, eventually reducing and

expelling the Ni from the scale. This shift in the location of new scale growth

resulting from the addition of a reactive element has also been reported in

independent studies [18–20]. The top-surface residue of Ni oxides contains about

4 · 1015 Ni atoms/cm2, a layer of perhaps 7 A average thickness. The residue

persists, relatively unchanged, with further oxidation [10]. These results suggest

that, very early after the formation of a continuous scale, the solubility of Ni in the

oxide becomes very low so that Ni atoms are rapidly excluded.

Thermal Cycling

As noted previously, eight samples were given cyclic-oxidation treatments. Sample

pairs (samples with and without RE) were heated in one-hour intervals at 750 8Cand were then cycled to room temperature (air quench to room temperature

followed by insertion into a hot furnace). These four sample pairs were given 2, 4,

8, and 16 heating cycles. Thus the total oxidation times for these pairs were

equivalent to the oxidation times for corresponding (noncycled) isothermally-

treated specimen pairs discussed above. We find no discernable effect in any of the

scale-characterization parameters that we can attribute to the thermal cycling. Thus,

for oxidation at 750 8C in O2, the cyclical compression of the scale that occurs

during cooldown to room temperature, apparently does not cause sufficient damage

to the scale to alter its growth properties. A substantial room temperature

compressive stress results from thermal expansion mismatch between scale and

substrate.

Conclusions

Using grazing emission X-ray fluorescence (GEXRF), isothermal oxidation of

55Fe–25Cr–20Ni (wt.%) alloys was studied as a function of oxidation time after

treatments at 750 8C in O2. Samples were treated for 2, 4, 8 and 16 h. Alloys with

and without the reactive element Y were examined. Also, the effect of thermal

cycling, after 1 h heating intervals, was studied. Scale thickness, the Cr-depletion

zone, as well as Fe and Ni concentrations in the chromia scale were monitored as a

function of oxidation time. No effect of thermal cycling was observed in the scale-

characterization parameters. To supplement the GEXRF measurements, the

concentration of trivalent Fe in the scale was measured in early oxidation using

Raman spectroscopy.

It was observed that Ni concentrations in the thermally-grown oxide were

consistently very low. Initial Fe concentrations were substantially higher but

showed a systematic decrease with oxidation time. It is argued that this scale

purification process is a consequence of the very low PO2 in the vicinity of the

scale–metal interface.

Acknowledgments This research is supported by the US Department of Energy, Basic Energy Science,Materials Science under Contract No. W-31-109-ENG-38.

50 Oxid Met (2007) 68:37–51

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