ORI GIN AL PA PER

Water Vapour Effects on Fe–Cr Alloy Oxidation

Norinsan K. Othman • Jianqiang Zhang •

David J. Young

Received: 8 September 2009 / Revised: 13 October 2009 / Published online: 29 October 2009

� Springer Science+Business Media, LLC 2009

Abstract Isothermal oxidation at 700 �C of binary Fe–Cr alloys containing 9, 17

and 25 wt% chromium was measured using continuous thermogravimetric analysis.

All alloys developed thin, protective chromia scales in Ar–20O2 (vol%). Chromia

scale growth on the 17 and 25 Cr alloys was faster in Ar–20O2–5H2O and Ar–5O2–

20H2O. In these gases, the Fe–9Cr failed to form a chromia scale and suffered rapid

breakaway oxidation, growing iron-rich oxides instead. A low oxygen potential gas,

Ar–10H2–5H2O caused chromia scaling on Fe–17Cr and Fe–25Cr, but internal

oxidation of Fe–9Cr. Application of Wagner’s criterion for sustaining external scale

growth is shown to account satisfactorily for these observations.

Keywords Parabolic kinetics � Alloy depletion � Diffusion � Scale-alloy interface

Introduction

It has long been known that water vapour containing atmospheres cause more rapid

oxidation of chromia-forming alloys than do dry oxygen or air. Early studies on

binary Fe–Cr alloys [1, 2] showed that oxidation in Ar–H2O or Ar–H2O–H2 in the

temperature range 700–1100 �C led to rapid growth of iron-rich oxide—breakaway

oxidation—rather than the protective chromia formation expected in dry oxygen.

N. K. Othman � J. Zhang � D. J. Young (&)

School of Materials Science and Engineering, University of New South Wales, 2052 Sydney,

NSW, Australia

e-mail: d.young@unsw.edu.au

Present Address:N. K. Othman

School of Applied Physics, Faculty of Science and Technology, Universiti Kebangsaan Malaysia,

43600 Bangi, Selangor, Malaysia

123

Oxid Met (2010) 73:337–352

DOI 10.1007/s11085-009-9183-9

Even pure chromium metal has been shown [3–6] to oxidise faster in H2O, in

H2–H2O and in O2/H2O than it does in dry O2. Similarly, chromia scaling of

Ni–25Cr is faster in H2/H2O gases than in dry O2 [7].

Understanding these and many other similar results is made difficult by the

variety of interactions possible between H2O(g) and growing chromia scales. Water

vapour promotes chromium volatilisation [8–10] by forming CrO2(OH)2(g),

increases the amount of grain boundary oxygen diffusion in dense chromia scales

[7, 11], leads to penetration of the scale by hydrogen [1, 7, 11], modifies point defect

concentrations in the Cr2O3 lattice [4, 5, 12], adsorbs preferentially on external and

internal chromia surfaces [13, 14] and can accelerate gas phase oxygen transport

within pores inside the oxide [1, 2, 15]. In addition, it has recently been reported

[16, 17] that internal oxidation of alloy chromium is promoted in the presence of

water vapour.

The issue is important because so many environments contain water vapour in

practice. Examples include combustion gases, which usually contain free oxygen,

and more reducing gases found in gasification processes and fuel cells.

This paper concerns binary Fe–Cr alloys, used as simple models for ferritic steels.

An experimental temperature of 700 �C was chosen, corresponding to a likely upper

limit for the use of such materials. Short term experiments were designed to

minimise the possibility of the results being affected by volatilisation, which can be

shown [19] to be unimportant at this stage. The work is complementary to a

temperature cycling study [19] conducted in the same environment.

Experimental

Alloys of nominal composition Fe–9Cr, Fe–17Cr and Fe–25Cr (alloy compositions

in this paper are wt%) were prepared by argon arc melting high purity metals

(Fe—99.99%, Cr—99.995%), using non consumable electrodes. After annealing at

1050 �C for 100 h under Ar–5H2 (vol %) to produce large grained ferrite, the alloys

were cut into coupons which were ground to a 1200 grit finish and ultrasonically

cleaned in ethanol immediately before use.

Alloy samples were reacted at 700 �C with flowing gas mixtures of Ar–20O2,

Ar–20O2–5H2O, Ar–5O2–20H2O and Ar–10H2–5H2O (all in vol %) at a total

pressure of approximately 1 atm. Water vapour levels were controlled by saturating

the gases with surplus water, then stripping out the excess in a condenser maintained

at the equilibrium temperature for the desired pH2O value.

Isothermal reactions were monitored over 48 h by continuous thermogravimetric

analysis, using a Cahn D2000 microbalance. The linear flow rate of the gas in the

tubular reactor hot zone was 0.7 cm s-1.

Reacted samples were characterised using X-ray diffraction (XRD) to identify

oxide phases, optical metallography and scanning electron micrography (SEM) to

examine product morphologies, and energy dispersive analysis of X-rays (EDAX)

for microanalysis. In one case, a protective chromia scale was examined using

transmission electron microscopy (TEM).

338 Oxid Met (2010) 73:337–352

123

Results

Weight uptake kinetics in the various gases shown in Figs. 1, 2, and 3 demonstrate

that reaction was invariably accelerated by the addition of water vapour to oxygen.

As seen in the Figures, Fe–25Cr reacts according to approximately parabolic

kinetics in all gases, whereas oxidation of Fe–17Cr is parabolic in dry O2 and 20O2/

5H2O, approximately parabolic in H2/H2O and irregular in 5O2/20H2O. The 9Cr

alloy displays parabolic kinetics in dry O2 and H2/H2O, eventually parabolic

kinetics after a fast initial reaction in 20O2/5H2O, and fast linear kinetics in 5O2/

20H2O. The onset of breakaway in both O2–H2O gas mixtures was very early.

Values of the parabolic rate constant, kw, were found from regression on the rate

equation

DW=Að Þ2¼ 2kwt þ C ð1Þ

0

2

4

6

8

10

12

0 50 100 150 200

(ΔW

/A)2

[mg

2 cm

-4]

103t (s)

Ar-5O2-20H2O

Ar-20O2-5H2O

Ar-10H2-5H2O

Ar-20O2

(b)

0

0.5

1

1.5

2

2.5

3

0 10 20 30 40 50

Time (Hour)

We

igh

t G

ain

(m

gc

m-2

)

Ar-5%O2-20%H2O Ar-20%O2-5%H2O

Ar-10%H2-5%H2O

Ar-20%O2

(a)

Fig. 1 Oxidation kinetics for Fe–9Cr: a linear plot, b parabolic plot

Oxid Met (2010) 73:337–352 339

123

where DW/A is the weight gain per unit area developed in time, t. Results are

summarised in Table 1. In the cases where kinetics were irregular, kw values were

estimated from the final stage of reaction observed.

Analysis by XRD of the reaction product surfaces revealed the phases listed in

Table 2. Cross-sections of reacted alloys are shown in Figs. 4, 5, and 6. Alloys

Fe–17Cr and Fe–25Cr formed single-phase Cr2O3 scales in all gases. Neither Fe2O3

nor (Fe,Cr)2O3 were detected by XRD. In contrast, the reaction products formed by

Fe–9Cr varied considerably with gas composition. A protective scale of Cr2O3 grew

in dry O2, whereas an external scale of Fe2O3 plus internal oxidation developed

0

0.02

0.04

0.06

0.08

0.1

0.12

50 100 150 200

(∆W

/A)2

[mg

2 cm

-4]

103 t/(s)

Ar-5%O2-20%H2O

Ar-10%H2-5%H2O

Ar-20%O2-5%H2O

Ar-20%O2

Fig. 2 Oxidation kinetics for Fe–17Cr (parabolic plot)

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 50 100 150 200

(∆W

/A)2

[mg

2 cm

-4]

103 t/(s)

Ar-5O2-20H2O

Ar-20O2-5H2OAr-10H2-5H2O

Ar-20O2

Fig. 3 Oxidation kinetics for Fe–25Cr (parabolic plot)

340 Oxid Met (2010) 73:337–352

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in 20O2/5H2O. A thick, non-uniform scale consisting of an outer Fe2O3 layer and an

inner heterophase layer of iron and chromium-rich oxides, together with an internal

oxidation zone formed in Ar–5O2–20H2O. In Ar–10H2–5H2O, this alloy grew

Table 1 Parabolic rate constants kw (mg2 cm-4 s-1) for oxidation

Alloy Ar–20O2 Ar–20O2–5H2O Ar–5O2–20H2O Ar–10H2–5H2O

Fe–9Cr 2.5 9 10-6 3 9 10-5a Linear 3 9 10-6

Fe–17Cr 1.5 9 10-8 2 9 10-7 3.5 9 10-7a 2.5 9 10-7

Fe–25Cr 5 9 10-8 1 9 10-7 1 9 10-6 5 9 10-8a

a Estimated from last stage of reaction observed

Table 2 Scale surface phases identified by XRD

Alloy Ar–20O2 Ar–20O2–5H2O Ar–5O2–20H2O Ar–10H2–5H2O

Fe–9Cr Cr2O3 Fe2O3, (Fe,Cr)3O4 Fe2O3, Fe3O4, FeO (Fe,Cr)3O4, FeO

Fe–17Cr Cr2O3 Cr2O3 Cr2O3 Cr2O3

Fe–25Cr Cr2O3 Cr2O3 Cr2O3 Cr2O3

Fig. 4 Cross-sections of Fe–9Cr reacted for 48 h in a Ar–20O2, b Ar–20O2–5H2O, c Ar–5O2–20H2O,and d Ar–10H2–5H2O

Oxid Met (2010) 73:337–352 341

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a very thin surface layer of spinel plus FeO, and a relatively deep zone of

chromium-rich oxide precipitation. A higher magnification view of the near surface

region is shown in Fig. 7.

An additional feature observed in the reaction of high chromium alloys with Ar–

10H2–5H2O was the growth of oxide whiskers at the scale surface. Scale surfaces

developed on Fe–25Cr in the different gases are compared in Fig. 8. Some

coarsening in grain size is seen to accompany increases in the H2O/O2 ratio, with

little other change in appearance. In the H2/H2O gases, the surface oxide is much

finer grained, as well as being decorated with whiskers. Similar changes with gas

atmosphere were evident on the surface of chromia scales grown on Fe–17Cr.

The protective Cr2O3 scale was analysed in one case. Figure 9 shows an EDAX

spectrum obtained in the TEM for the chromia scale of Fig. 5a. Neglecting the

copper signal (which originates from the sample support grid), it is seen that the iron

content of the oxide is small compared with that of chromium.

Fig. 5 Cross-sections of Fe–17Cr reacted for 48 h in a Ar–20O2 (TEM bright field), b Ar–20O2–5H2O,c Ar–5O2–20H2O, and d Ar–10H2–5H2O

342 Oxid Met (2010) 73:337–352

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Discussion

The addition of water vapour to oxygen bearing atmospheres has two effects on the

oxidation of Fe–Cr alloys: acceleration of chromia scaling rates on high chromium

content alloys, and a change from protective to breakaway corrosion in the case of

Fe–9Cr. Reactions in the H2/H2O mixture led to extensive internal oxidation of Fe–

9Cr, somewhat faster chromia scaling on Fe–17Cr than in dry oxygen, and more

complex kinetics for Fe–25Cr. Patterns of behaviour in high and low oxygen

potential gases are discussed separately. First of all, however, possible loss of

chromium from the scale by formation of CrO2(OH)2(g) is considered.

Volatilisation of Chromium

The reaction

1=2Cr2O3 sð Þ þ H2O gð Þ þ 3=4O2 gð Þ ¼ CrO2 OHð Þ2 gð Þ

Fig. 6 Cross-sections of Fe–25Cr reacted for 48 h in a Ar–20O2, b Ar–20O2–5H2O, c Ar–5O2–20H2O,and d Ar–10H2–5H2O

Oxid Met (2010) 73:337–352 343

123

leads to chromium loss in gases containing both oxygen and water vapour. The rates

of loss can be calculated [10] using the methods of fluid dynamics and the

thermodynamics of the solid–gas equilibrium. This approach has been shown to

Fig. 7 SEM view of near surface region of Fe–9Cr after 48 h reaction with Ar–10H2–5H2O

Fig. 8 SEM views of scale surfaces developed on Fe–25Cr in 48 h in a Ar–20O2, b Ar–20O2–5H2O,c Ar–5O2–20H2O, and d Ar–10H2–5H2O

344 Oxid Met (2010) 73:337–352

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yield close agreement with direct measurements of chromium losses from thin alloy

foils during oxidation-volatilisation [10]. Applying the same approach to the present

experimental conditions, it is calculated that the amount of chromium lost from pure

Cr2O3 in a 48 h exposure is 0.004 and 0.006 mg cm-2 in Ar–20O2–5H2O and

Ar–5O2–20H2O, respectively. These amounts are to be compared with the quantities

of chromium present in the scales.

The rate constants in Table 1 correspond to total oxygen weight uptakes by

Fe–17Cr and Fe–25Cr reacted for 48 h in the O2/H2O mixtures of order

0.2–0.6 mg cm-2. If the scales are pure Cr2O3, their chromium contents are in the

range 0.4–1.2 mg cm-2. On this basis it is concluded that chromium losses due to

volatilisation amount to about 1% of the quantity in the scales, and can be ignored.

Chromium volatilisation is not always unimportant [8–10]. The present situation

results from the use here of low pH2Op3=4O2

values and slow gas flow rates, together

with the deliberate choice of rather short reaction times [19].

Chromia Scaling Rates in O2/H2O Atmospheres

The addition of H2O to the Ar–O2 gas increases the scaling rate of Fe–17Cr by an

order of magnitude. In the case of Fe–25Cr, the increase is smaller, but still

significant, with a doubling in kw. The higher water vapour gas Ar–5O2–20H2O

increases the rate still further: by a factor of 10 for Fe–25Cr and to a varying extent

for Fe–17Cr (Fig. 2). In all cases, however, the reaction product was a single-phase

external scale of Cr2O3. No Fe2O3 or mixed (Cr1-xFex)2O3 phase was detected by

XRD. The changed rates therefore reflect changes in mass transport within the scale.

Diffusion in chromia scales, particularly at lower temperatures, is predominantly

a grain boundary phenomenon [18]. As seen in Fig. 8, oxide microstructures

developed at scale surfaces in Ar–20O2 and Ar–20O2–5H2O were closely similar,

while the grain size developed in Ar–5O2–20H2O was somewhat coarser. If the

scale surfaces are indicative of their interior, then it is apparent that changes in the

Fig. 9 EDAX results for scale shown in Fig. 5a

Oxid Met (2010) 73:337–352 345

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number density of grain boundaries do not account for the observed changes in

scaling rate.

Isotope diffusion experiments on growing chromia scales at 1050 �C using

labelled H2O [7] have shown that grain boundary diffusion of oxygen (in the form

of water molecules or hydroxyls) is accelerated in the presence of H2O, becoming

the predominant mode of mass transport. Experiments at 900 �C in mixed gases [21]

show that the Cr2O3 scale grown on pure chromium is impermeable to carbon and

nitrogen in the presence of H2O, but transmits both secondary oxidants when water

vapour is absent. These results, and the isotope diffusion experiments are interpreted

as indicating the preferential uptake by grain boundaries of H2O molecules or

species derived from them such as hydroxyls. It is therefore suggested that a similar

phenomenon is in effect at 700 �C, and accelerated scale growth results from rapid

transport of H2O derived species. The larger effect observed in the higher water

content gas mixture is on this basis attributed to competitive adsorption of O2 and

H2O on internal surfaces of the oxide scale.

The situation is probably more complex than this, as the scales formed on these

alloys are not pure chromia. It is known [13] that the presence of iron at low

concentrations in the oxide affects the ability of gas species to penetrate the scale.

The different concentrations of iron expected in the scales formed on alloys of

different iron:chromium ratios might therefore affect the scale susceptibility to

permeation by H2O. Present results do not provide the information required to

explore this possibility.

Breakaway Oxidation in O2/H2O

Addition of water vapour to oxygen accelerates the oxidation rate of Fe–9Cr, as the

reaction product changes from a protective, chromium-rich oxide formed in dry O2

to fast-growing, iron-rich oxides in wet gas mixtures. This finding is consistent with

earlier results [14] for the ferritic 9Cr steel, P91, at 650 �C. Accelerated attack on

Fe–10Cr in air plus 10% H2O at 650 and 800 �C has also been observed [20].

Two gas mixtures, Ar–20O2 and Ar–20O2–5H2O had the same oxygen potentials,

but produced different reaction products on Fe–9Cr. The effect is illustrated by the

diffusion paths mapped onto the Fe–Cr–O phase diagram in Fig. 10. Although

errors result from the use of a phase diagram appropriate to a much higher

temperature, the general conclusion is clear. If the system is described as a simple

ternary, then two identical diffusion couples (Fe–9Cr vs. O2(g) at pO2¼ 0:2 atm)

develop different diffusion paths, an impossibility if local equilibrium is everywhere

closely approximated. Since the reaction kinetics are parabolic (after an initial

transient period in the case of the wet gas), it seems likely that steady-state

conditions were achieved. Drawing the usual inference, therefore, that local

equilibrium was arrived at, it must be concluded that the system is not a simple

ternary, and hydrogen is recognised as being a chemical constituent of the reacting

system.

This conclusion is supported by the ample evidence that hydrogen can be present

throughout the alloy-gas reaction zone. Fujii and Meussner [1] demonstrated long

ago that hydrogen dissolves in Fe–Cr during its oxidation in water vapour. Thus the

346 Oxid Met (2010) 73:337–352

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hydrogen had passed through the oxide scale. The isotope diffusion experiments

referred to above show very clearly that inward grain boundary transport is

promoted by the presence of H2O, but not O2, implying the presence of both oxygen

and hydrogen on the internal surfaces. Unfortunately, however, the actual chemical

form of these species (water molecule, hydroxyl or more loosely associated oxygen

and hydrogen point defects) is unknown. Nonetheless, it is clear that some hydrogen

species enters the scale. It is possible to devise a phenomenological model based

simply on the knowledge that chromia scale growth is faster in the presence of water

vapour (Figs. 2, 3).

For a given oxygen potential, the nature of the external scale formed is

determined by conditions at the alloy-scale interface. If sufficient chromium is

available, then its selective oxidation to form a protective chromia scale results. If

not, then iron-rich oxide, rather than the desired Cr2O3, co-exists with the alloy. The

two situations for an Fe–9Cr alloy represent different extents of depletion (Fig. 10).

The interfacial concentration of chromium, NðiÞCr, is determined by the rates at

which the metal is consumed by oxide formation and replaced by alloy diffusion.

Fig. 10 Isothermal section of Fe–Cr–O phase diagram at 1200 �C showing diffusion paths at 700 �C forFe–9Cr reacted with Ar–20O2 (dashed line) and with Ar–20O2–5H2O (dotted line)

Oxid Met (2010) 73:337–352 347

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In the case where an external scale of pure chomia grows according to parabolic

kinetics, Wagner’s diffusion analysis [22] leads to

Nð0ÞCr � N

ðiÞCr ¼

VA

VCrO1:5

pkp

2 ~D

� �1=2

ð2Þ

Here Nð0ÞCr is the original alloy chromium concentration (mol fraction), VA and

VCrO1:5the molar volumes of alloy and oxide, ~D the alloy interdiffusion coefficient,

and kp is the parabolic rate constant for scale thickening

X2 ¼ 2kpt ð3Þ

with X the scale thickness formed in time, t, and movement of the scale-alloy

interface with time has been ignored.

Evaluation of NðiÞCr from Eq. 2 is made difficult by the lack of alloy diffusion data for

this temperature. Extrapolation from high temperature data [23] leads to the estimate~D ¼ 2� 10�12 cm2 s�1 at 700 �C. Calculating kp = 9 9 10-13 cm2 s-1 from the

value of kw measured for Fe–9Cr in Ar–20O2 and the density of Cr2O3 and setting

VA = 7.2 cm3 mol-1, VCrO1:5¼ 14:6 cm3 mol�1; one calculates N

ð0ÞCr � N

ðiÞCr ¼ 0:41:

In Fe–9Cr, Nð0ÞCr ¼ 0:10; this requirement cannot be met, and the alloy is predicted not

to form a protective scale.

This prediction is incorrect, as Fe–9Cr forms a thin chromia scale in dry Ar/O2.

The two sources of error are the already mentioned inaccuracy of the ~D estimate and

the assumption that the chromia scale contains no iron. The much faster rate of

Cr2O3 scale growth on Fe–9Cr compared to the rate observed for Fe–17Cr (Table 1)

indicates the latter assumption might be seriously in error. In this case, the scale

contains both iron and chromium, and the amount of chromium withdrawn from the

alloy is overestimated by any measure based on overall scale growth. If instead kp is

calculated from the Fe–17Cr value of kw, a value of Nð0ÞCr � N

ðiÞCr ¼ 0:03 is

calculated, corresponding to NðiÞCr ¼ 0:07; consistent with the stability of chromia.

Turning now to the effects of water vapour, it is evident that the values of kw

given in Table 1 for Fe–9Cr in 20O2–5H2O cannot be used to calculate kp for use in

Eq. 2. They reflect growth of iron-rich oxide and internal oxidation, not chromia

scaling. The question at issue is what value of Nð0ÞCr would be required to enable the

establishment of a Cr2O3 scale, and sustain its growth in Ar–20O2–5H2O. It is

proposed that this can be answered by using the kp value for chromia scale growth in

Fe–17Cr in this gas. The hypothesis is that during the very initial, transient period of

oxidation, both iron and chromium oxides would nucleate at the surface. For the

chromia nuclei to survive and grow, sufficient alloy chromium must be available to

support their growth at the rapid rate characteristic of this gas. Failure of the Fe–9Cr

alloy in O2/H2O gases is seen in Fig. 1a to occur very early in the reaction,

apparently in agreement with this hypothesis.

As seen earlier, water vapour accelerates chromia scale growth. Using the value

of kw in Table 1 for Cr2O3 scaling on Fe–17Cr in Ar–20O2–5H2O, it is found that

kp = 7.5 9 10-14 cm2 s-1. Applying this value to Eq. 2, it is calculated that Nð0ÞCr �

NðiÞCr ¼ 0:12: This condition cannot be met by an Fe–9Cr alloy, but is met by Fe–

17Cr and Fe–25Cr. Thus the faster chromia scale growth brought about by the

348 Oxid Met (2010) 73:337–352

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presence of water vapour accounts for the failure of the low chromium alloy. Using

the measured oxidation rate of Fe–17Cr in Ar–5O2–20H2O, it is found that

kp = 1.3 9 10-13 cm2 s-1 and therefore Nð0ÞCr � N

ðiÞCr ¼ 0:16: Again the failure of

the 9Cr alloy and success of the 17 and 25 Cr alloys in this gas are explained.

The overall pattern of behaviour observed for these alloys in all dry and wet

oxygen gas mixtures can be understood in terms of chromia scale growth

acceleration by water vapour. Faster scaling leads to more extensive chromium

depletion, increasing the initial alloy level required for protection. However, an

alternative hypothesis is available, as discussed in the next section.

Finally, it is noted that water vapour can have a variety of effects on chromia

scale growth [18], depending on reaction conditions, and the analysis provided here

will not always apply. In cases where the CrO2(OH)2 volatilisation reaction is fast as

a result of gas composition and/or its velocity [24], that process can be the dominant

effect.

Chromia Scaling in H2/H2O

The oxygen potential in the H2–H2O gas mixture controlled by the equilibrium

H2O ¼ H2 þ 1=2O2 ð4Þ

has a value of pO2¼ 7:4� 10�21atm: This is just high enough to form FeO, but not

magnetite or hematite. The observation that Cr2O3 forms on both Fe–17Cr and Fe–

25Cr is therefore expected, as is the mixture of FeO and FeCr2O4 developed at the

surface of Fe–9Cr.

The high chromium content alloys developed protective chromia scales in Ar–

10H2–5H2O. In the case of Fe–17Cr, the scaling rate was similar to those found in

O2–H2O mixtures, and much faster than in dry oxygen. The observation of faster

scaling in H2/H2O than in O2 has been reported before for Fe–Cr alloys [1, 2, 17]

and for pure chromium [6, 11, 12]. It seems likely that the same mechanism of rapid

grain boundary transport of water molecules, or species derived from them, applies.

As seen in Table 1, the 17Cr alloy is more susceptible to water vapour in the

reducing gas than is Fe–25Cr. This may be due to the higher levels of iron found in

chromia scales on dilute Fe–Cr alloys.

The failure of Fe–9Cr to form a protective chromia scale in this gas is of

particular interest. Instead, the alloy formed a very thin, two-phase surface layer of

FeCr2O4 plus FeO. A higher magnification SEM image in Fig. 7 shows that the

surface oxide is discontinuous. Extensive internal precipitation of very finely

divided chromium-rich oxides also developed. Similar observations have been

reported by Essuman et al. [17] for an Fe–10Cr alloy oxidised in Ar–4H2–7H2O at

900 and 1050 �C. It was proposed by those authors that interaction between water

vapour and the alloy leads to dissolution of both oxygen and hydrogen.

Thermodynamic and kinetic interactions between the interstitial solutes were

suggested to lead to increased oxygen permeability in the alloy, causing internal

oxidation of chromium. Precipitation of chromium in this way leads to a lowering of

NðiÞCr to the point where an external chromia scale cannot be maintained, and

breakaway oxidation results.

Oxid Met (2010) 73:337–352 349

123

Oxygen permeability can be gauged from the rate at which the internal oxidation

zone grows into the alloy. Assuming this process to be controlled by inward

diffusion of dissolved oxygen, Wagner’s diffusion analysis [25] yields

X2

ðiÞ ¼ 2kðiÞp t ð5Þ

kðiÞ

p ¼eD0N

ðsÞO

vNCr

ð6Þ

Here X(i) is the internal oxidation depth, kðiÞp the internal oxidation rate constant, D0

the self-diffusion coefficient and NðsÞO the surface concentration of dissolved oxygen,

e a tortuosity constant, and m the stoichiometric coefficient for the precipitated

compound CrOm.

Values for D0 are available [26] and the thermodynamics of oxygen dissolution

in a-Fe have been measured [26]. Oxygen concentrations are related to activities by

Sievert’s equation

NðsÞO ¼ KpO2

1=2 ð8Þ

and the difficulty lies in specifying the oxygen activity. If the scale is continuous,

then the maximum value of pO2is set by the Fe/FeO equilibrium at 2.5 9 10-22

atm, assuming NFe & 1. However, if the oxide is discontinuous, the appropriate

value for pO2is that of the gas, 7.4 9 10-21 atm. The corresponding values of N

ðsÞO

are 2 9 10-7 and 1 9 10-6. Using Do = 2 9 10-7 cm2 s-1 and setting e = 1,

m = 1.5, one calculates kðiÞ

p from Eq. 6 as 3 9 10-13 or 1.5 9 10-12 cm2 s-1 for the

two cases. Evaluation of X(i) from Fig. 4d as 25 lm after 48 h reaction leads to the

estimate kðiÞp � 2� 10�11 cm2 s�1; significantly higher than predicted from Eq. 6.

A condition for the validity of Eq. 6 is that DoNðsÞO � DCrNCr; where DCr is the

self-diffusion coefficient of chromium in the alloy. At the very low oxygen

solubilities expected, DoNðsÞO ¼ 4� 10�14 or 2 9 10-13 cm2 s-1 whereas DCrNCr =

3 9 10-14, based on extrapolation from high temperature data for Fe–Cr diffusion

[25]. Thus the condition for the validity of Eq. 6 is met, at least in the case of the

higher pO2values, and an order of magnitude difference remains between the

predicted and observed internal oxidation rates. Unfortunately, it is not possible to

conclude with confidence whether the discrepancy is due to the data (and its

extrapolation from higher temperatures) or to an effect of dissolved hydrogen, as

proposed by Essuman et al. [16, 17].

Alternatively, the failure of Fe–9Cr to form a protective chromia scale in

Ar–10H2–5H2O can be explained in terms of the value of NðiÞCr required to sustain the

scale, in the same way as was done for the O2/H2O gases. Using the chromia scaling

rate kw = 2.5 9 10-7 mg2 cm-4 s-1 for Fe–17Cr in Ar–10H2–5H2O (Table 1), it is

found that kp = 9.2 9 10-14 cm2 s-1. Application of Eq. 2 then yields the require-

ment for chromia scale growth of Nð0ÞCr � N

ðiÞCr ¼ 0:13: This condition cannot be met

by the Fe–9Cr alloy, but is satisfied by the 17 and 25 Cr alloys. Again it is seen that

the different reaction morphologies can be understood quite simply from the

accelerating effect water vapour has on chromia scaling rates.

350 Oxid Met (2010) 73:337–352

123

Internal oxidation can be a result of chromium depletion caused by more rapid

chromia scaling, or it can be the precursor to breakaway oxidation by removing

chromium from the alloy matrix, thereby rendering passivation impossible.

Reasonable arguments can be advanced for both, and their relative contributions

cannot be determined at this time.

Summary and Conclusion

Additions of water vapour to oxygen-rich gases accelerated chromia scaling rates on

Fe–17Cr and Fe–25Cr. Reaction of these alloys in Ar–10H2–5H2O also led to fast

chromia scale growth on Fe–17Cr, but oxidised Fe–25Cr at much the same rate as

did dry oxygen. This acceleration in chromia scaling is consistent with earlier

results.

The marginal chromia former, Fe–9Cr, is strongly affected by the presence or

absence of water vapour. Whilst growing a protective chromia scale in dry oxygen,

this alloy loses the ability to passivate when water vapour is present. When the

oxygen potential is low, very little iron-rich oxide formed, and internal oxidation of

the chromium results.

The observed changes in reaction morphology with gas composition can

satisfactorily be accounted for on the basis of alloy chromium levels necessary to

sustain external chromia scale growth. Wagner’s diffusion analysis of this

requirement provides very good agreement with the observed changes, solely on

the basis of changes in the growth rate of chromia scales.

Although the effect of water vapour on scale transport properties, and the

consequences for alloy passivation are clear, it remains possible that water vapour

(or hydrogen) might also alter conditions within the alloy itself.

Acknowledgements Financial support from the Australian Research Council and the provision of a

scholarship (to N.K.Othman) by the Malaysian Ministry of Higher Education are gratefully

acknowledged.

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