Protonic defects in oxides and their possible role in high
temperature oxidation
Truls Norby
To cite this version:
Truls Norby. Protonic defects in oxides and their possible role in high temperature oxidation.
Journal de Physique IV Colloque, 1993, 03 (C9), pp.C9-99-C9-106. <10.1051/jp4:1993907>.
<jpa-00252337>
HAL Id: jpa-00252337
https://hal.archives-ouvertes.fr/jpa-00252337
Submitted on 1 Jan 1993
HAL is a multi-disciplinary open access
archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d’enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.
JOURNAL DE PHYSIQUE IV
Colloque C9, supplement au Journal de Physique 111, Volume 3, decembre 1993
Protonic defects in oxides and their possible role in high
temperature oxidation (*)
Truls Norby
Center for Materials Research, Department of Chemistry, University of Oslo, Gaustadalleen 21,
N-037 1 Oslo, Norway
Abstract. - Protons from hydrogen or water vapour may dissolve in oxides and other corrosion
products, and may affect, and in some cases, dominate their defect structures. Defect-dependent
properties like scale growth and plastic deformation then become dependent on the ambient
hydrogen or water vapour pressure. Small concentrations of protons may enhance the transport
of oxygen in the form of hydroxide ion diffusion. Furthermore, the transport of protons through
a scale (dissolving as hydrogen in the metal), can replace the outward transport of electrons
during scale growth. Recent literature results indicate that protons can be dominant defects in
Y2O3,A1203and Cr203under certain conditions.
1. Introduction.
Effects of hydrogen and water vapour on high temperature oxidation have been known for
a long time. Of the many examples listed by Kubaschewski and Hopkins [I], some can be
attributed to stress and brittleness in the metal. The presence of water vapour may furthermore change the corrosion products from one oxide to another, or lead to the formation of
hydroxide scales or volatile hydroxides and oxyhydroxides. Other effects are interpreted in
terms of changes in interface properties of the scales, including surface-gas exchange, scale
adherence, grain texture, and grain boundary sliding. Kofstad [2] concluded that an understanding of many of the observed effects is still lacking.
Our interest in effects of hydrogen species accompanies a growing interest for effects of
other light elements (mainly C, N, and S) on oxide properties and the oxidation of metals.
While these other elements have low solubilities and diffusivities in bulk phases and mainly
affect surfaces and interfaces, protons easily diffuse in oxides and other materials. Some of
the observed effects of water vapour or hydrogen may therefore result from effects on the
bulk properties of oxides and oxide scales. The present paper outlines the formalism for
describing hydrogen defects and their possible effects on bulk transport in oxide scales. The
treatment is readily adapted to sulphide scales.
2. Hydrogen defects in oxides and metals.
2.1 INTERSTITIAL PROTONS IN OXIDES. - Until around 1980, hydrogen, water vapour, and
hydrogen defects were generally not considered to affect bulk properties of metal oxides at
(*) Invited paper.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:1993907
100
JOURNAL DE PHYSIQUE IV
high temperatures. However, hydrogen defects were relatively early described for ZnO [3]
and Si02 [4], and sought for, but not found, in NiO, COO,and C u 2 0 [5].A few papers followed which described significant levels of hydrogen defects in oxides, e.g. Zr02 [6],Ti02
[7], T h o 2 [S], and BaTi03 [9]. Interest in the subject accelerated when high proton conductivities were found in acceptor-doped perovskite oxides like Yb-doped SrCe03 [lo, 111.
Later, other systems have been characterized with respect to protons, comprising M 2 o 3[12,
131, KTa03 [14] and rare earth oxides 115-171. From these studies, hydrogen dissolves in
oxides as protons bonded to oxygen ions to form substitutional hydroxide, (OH);. These
defects may equivalently and more simply be described as interstitial protons, Hi [IS, 191.
The concentration generally increases with increasing p ( H 2 0 )(at constant p ( 0 2 ) ) and decreasing temperature. Nevertheless, protons may dissolve in dominant concentrations even
at high temperatures and in relatively dry atmospheres in oxides with low native defect concentrations. For instance, Y2O3is dominated by protons in wet atmospheres up to 1500 OC
[201.
The dissolution of interstitial protons into an oxide can be written as:
or, through the gas phase equilibrium between Ha, H 2 0 , and
02,
as
At sufficiently low water vapour or hydrogen activities, protons are minority defects. The
concentrations of native defects are then independent of p (Hz) and p (H20) (at constant
p (0211, while the concentration ofprotons, from equations (1) and (2), increases with p ( ~ 2 ) " ~
and ( ~ ~ 0 ) ' "At. higher hydrogen and water vapour pressures protons may become the
dominant defect, compensated by defect electrons, metal vacancies, oxygen interstitials, or
acceptor dopants.
Figure 1 shows an example of a defect structure in the oxide M2O3as a function ofp ( H 2 0 )
at constant p (02). In the example, protons are minority defects at low p ( H 2 0 ). At higher
p ( H 2 0 ) ,protons become dominant and are compensated by defect electrons and, eventually, metal vacancies. The slopes in figure 1 can be deduced by combining the equilibria of
equations (1) and (2) with defect equilibria for the native defects [IS, 191. The defect concentrations become functions of p ( H 2 0 )when protons dominate.
Figure 1 shows that the defect structure and related properties may be very different in
dry and wet atmospheres. Similarly, large differences may be found between experiments
done in H 2 / H 2 0and CO/C02 buffers, although the oxygen activities may be identical.
For the defect structure as a function of p ( 0 2 ) , i.e., through the oxide scale, we need
to know how p (H20) and p (H2) change through the scale. Figure 2 shows schematically
the situation if p ( H 2 0 ) is assumed to be constant through the scale. This case can give high
hydrogen pressures at the metauoxide interface, high hydrogen contents in the metal, and in
some cases scale rupture. Figure 3 shows an example of defect structure vs. p (02) assuming
that p ( H 2 0 )is constant.
An example similar to this, but under the condition of constant p (H2)is shown in figure 4.
Figures 3 and 4 show distinct differences in the defect structure vs. p (02) for the two cases.
PROTONIC DEFECTS I N OXIDES
Fig. 1. - Example of defect structure vs. water vapour partial pressure at constant oxygen partial
pressure. Numbers along lines denote slopes.
Fig. 2. - Gradients in partial pressures through a scale assuming no gradient for water vapour.
Fig. 3. - Example of defect structure vs. oxygen partial pressure assuming constant water vapour
partial pressure. Numbers along lines denote slopes.
JOURNAL DE PHYSIQUE IV
p ( )~= constant
2
Fig. 4. - Example of defect structure vs. oxygen partial pressure assuming constant hydrogen partial
pressure. Numbers along lines denote slopes.
However, neither of these idealized cases may reflect the real situation for an oxide scale on
a metal.
The concentration of protons increases by acceptor doping and decreases by donor doping [18, 191. One may thus use acceptor doping to enhance and donor doping to reduce
hydrogen transport in oxides. Correspondingly, by deliberately doping a scale, one may in
principle enhance or reduce the ability of a metal to exchange hydrogen with the ambient
gas at high temperatures.
The solubility of protons in oxides ranges from below one mol-ppm to a few mol-percent
at high temperatures in wet atmospheres [21]. The activation energy for proton transport
varies in the range 0.5-2 eV [21]. In general, protons are faster than other ions, but slower
than electronic defects [19].
At high proton concentrations and moderate temperatures protons may in principle associate with interstitial oxygen to form interstitial hydroxide, OH;, or with metal vacancies
[19]. However, significant concentrations of such defects
to form, for instance, (VM(OH)~)"
at elevated temperatures have not been observed. A neutral defect formed by association
between a proton and a lower valent impurity metal, ( M l M ( O ~ ) o )has
2 , been suggested to
rationalize observed effects of hydrogen activity on the electrical properties of Cr203[22].
2.2 HYDROGEN
ATOMS AND HYDRIDE IONS. - There is spectroscopic evidence of neutral
hydrogen atoms and hydride ions in oxides at temperatures below room temperature [19]. At
high temperatures, only indirect indications of the presence of hydrogen or hydride species
have been reported, for instance in the case of a-alumina [12]. The presence of interstitial
hydride ions in MoS2 has been suggested to explain the observed dependency on p (H2) of
the parabolic sulphidation of Mo in H2+H2Smixtures [23]. However, unambiguous evidence
for these defects in oxides at high temperatures is lacking.
2.3 HYDROGEN
I N METALS. - Hydrogen normally dissolves in metals as interstitial atoms.
The solubilities are widely different for the various metals and the temperature dependencies
can be positive or negative [24].
The activation energies for diffusion of hydrogen in metals are smaller than for the diffusion of protons in oxides [24]. Thus, transport of hydrogen is always much faster in a metal
PROTONIC DEFECTS IN OXIDES
than in the oxide scale. In high temperature oxidation, a metal may therefore serve as an
efficient hydrogen source or sink which tends to equilibrate its hydrogen level with the ambient gas, the scale acting as a barrier. The resulting proton current may affect scale growth
(see below).
3. Effects on oxidation.
During normal scale growth, oxygen reacts with electrons to form oxide ions at the gadoxide
interface, while metal atoms react to form metal ions and electrons at the metauoxide interface. Oxygen ions diffuse inward while metal ions and electrons diffuse outward through the
scale. The growth is limited by the slower moving species (ions or electrons).
3.1 EFFECTSOF PROTONS ON DEFECT CONCENTRATIONS. - If the metal is oxidized in a wet
atmosphere, or if the metal contains dissolved hydrogen, protons will dissolve in the growing scale. If proton defects dominate, the concentrations of ionic and electronic defects are
changed as indicated in figures 1-4, and this, in turn, may change the oxidation rate.
The concentration of minority defects limiting the high temperature creep of scales would
also be changed, which may affect the ability of the oxide to deform plastically and alleviate
stresses, an important property for the adherence and protectiveness of a growing scale.
Silica and silicates are known to be softer under wet than dry conditions. However, the
mechanism is believed to be different than described for oxides; the softening is attributed to
loss of Si-0-Si bonds as protons hydrolyse and terminate the bond chains.
3.2 OXYGEN
TRANSPORT BY MEANS OF HYDROXYDE ION DIFFUSION. - It is generally believed
that protons diffuse faster than hydroxide ions in oxides. Nevertheless, hydroxide ions may
diffuse faster than oxide ions in oxides with slow oxide ion transport [18, 191. Thus, scale
growth and plasticity may become functions of the proton content also in cases where protons
are not necessarily dominant defects.
3.3 PROTONDIFFUSION AND GROWTH OF OXIDE SCALES. - Let us consider a possible effect
of proton transport under conditions where the metal accumulates hydrogen during the
oxidation. In this case, hydrogen (or water vapour) at the gasloxide interface dissolves as
a proton and an electron. The two move through the scale and recombine to hydrogen
at the metauoxide interface. If the scale is a predominant ionic conductor, the electronic
conductivity may limit the transport of protons.
In such an oxide, however, scale growth can possibly also take place by simultaneous inward
transport of protons and oxygen ions (or outward transport of metal ions). In these terms,
the scale growth can take place by proton diffusion and oxygen or metal diffusion without
the need for electron transport. This mechanism is illustrated in figure 5, and is valid as long
as hydrogen dissolves in the metal. In effect, systems expected to be rate-limited by electron
transport may, in the presence of hydrogen-containing gases like water vapour, be limited
instead by ionic or protonic transport, whichever is slower.
If the metal contains much hydrogen before oxidation starts, we may experience a proton
transport out through the scale. This should have the opposite effect of the previous case.
Such effects remain to be verified experimentally.
JOURNAL DE PHYSIQUE IV
Id2~'
Fig. 5. - Schematic picture of oxide scale growth accomplished by a flow of protons, dissolving as
hydrogen in the metal.
4. Example systems.
Perhaps the most classical example for the effect of water vapour on oxidation kinetics is
that of silicon, where the growth rate of the Si02 scale increases with water vapour partial
pressure. This can be explained if oxygen is transported mainly as water molecules, or it can
be attributed to the fact that protons break up (hydrolyse) the network structure, thereby
increasing the diffusion coefficient of oxygen ions [25,2].
Iron-based alloys and technical steels provide other important examples of high temperature oxidation accelerated by water vapour [Z]. In many cases these materials fail to form
protective SiOn or Cr203 scales in the presence of water vapour. There are also effects of
water vapour on the deformation, texture, and scale morphology of such materials. It is,
however, not likely that the defect structures of iron oxides are dominated by protons.
Protons are, however, found to dominate the defect structure of many rare earth oxides,
as mentioned above, and these should provide good model systems for studying the effects
of protons on the kinetics of metal oxidation. However, no such studies are known to the
author.
Hydrogen defects in a-alumina at high temperatures have been known for some years
[12]; protons are major defects in Mg-doped alumina in wet atmospheres below 1200 "C
[13]. Thus, protons are also dominant in undoped aluminas at even higher temperatures or
under drier conditions [19]. However, with the present uncertainties concerning the defect
structure and growth mechanism of alumina scales, the effect of protons on the protective
properties of alumina is difficult to predict, and a systematic study is necessary.
In chromia, it appears that intrinsic electronic defects are dominant at high temperatures,
eventually shifted by aliovalent impurities. If these are divalent, ~ 1 ' + ,the electroneutrality
condition, including protons, becomes
p
+ [Hi] = n + [MIL,]
(3)
where n can be neglected at moderate temperatures. It has recently been demonstrated that
the p-type conductivity of chromia samples decreases with increasing hydrogen partial pressure (at constant oxygen partial pressure) at temperatures below 1000 O C [22]. The observed
p ( H ' ) - I / ~ dependence is tentatively interpreted as follows: protons, dissolved through equation (I), and localized at an oxygen ion, associate with the lower valent impurities, forming
PROTONIC DEFECTS IN OXIDES
105
neutral (Mlc,(O~)o)xdefects. At high temperatures and low p (H2), protons are minority
defects and the majority of the MI acceptors are unassociated, keeping the compensating
hole concentration constant. With lower temperatures and higher hydrogen partial pressures, however, the concentration of protons increases and the majority of the M1 acceptors
become associated; the concentration of remaining free acceptors and compensating holes
become a function of temperature and hydrogen partial pressure. This, in turn, changes the
concentration of metal and oxygen defects, but it is difficult to foresee the effect on oxidation
as the growth mechanism for chromia scales is not unambiguously known.
For many systems, including alumina and chromia, there are indications that grain boundary transport is important for scale growth [2]. In addition to the bulk aspects of protons
previously mentioned, future studies should also consider the role of protons in grain boundaries.
5. Summary.
Protons diffuse relatively easily in oxides at high temperatures. They may dissolve in significant concentrations and in some cases dominate the defect structure. The defect concentrations and defect dependent properties then become functions of the hydrogen and
water vapour partial pressures and of the hydrogen content of the metal. This may affect the
oxidation rate and the ability of the scales to deform pIasticalIy.
Another possible effect of protons is to enable oxygen to be transported as hydroxide ions.
Finally, inward diffusion of protons to a hydrogen-deficient metal may accompany inward
diffusion of oxygen ions (or outward diffusion of metal ions). This alleviates the need for
electron transport, and may enhance scale growth for ionically conducting oxides.
The presence and effects of protons have been demonstrated in several oxides, including
alumina and chromia, but more research is needed to clarify eventual effects on oxidation.
References
[l] KUBASCHEWSKI
O., HOPKINS
B.E., Oxidation of Metals and Alloys (Butterworths,
London, 1962).
F!, High Temperature Corrosion (Elsevier, London, 1988).
[2] KOFSTAD
D.G., LANDER
J.J., J. Chem. Phys. 25 (1956) 1 136.
[3]THOMAS
[4] BRUNNER
G.O., WONDRATSCHEK
H., LAVESE, 2. Elektrochem. 65 (1961) 735.
C., Ber. Bunsenges. Phys. Chem. 70 (1967) 78 1.
[5] STOTZS., WAGNER
C., Ber. Bunsenges. Phys. Chem. 72 (1968) 778.
[6]WAGNER
[7] HILLG.J., Br. J. Appl. Phys. l ( 2 ) (1968) 1151.
[8] SHORES
D.A., RAPP R.A.,J. Electrochem. Soc. 119 (1972) 300.
[9] POPEJ.M., SIMKOVICH
G., Muter. Res. Bull. 9 (1974) 11 11.
[ 1 01 TAKAHASHI
T., IWAHARA H .,Rev. Chim. Miner. 17 ( 1980) 1 17.
[ l l ] IWAHARA
H., UCHIDA
H., KONDO
J., J. Appl. Electrochem. 13 (1983) 365.
[12] EL-AIAT
M.M., KROGER
F.A., J. Appl. Phys. 53 (1982) 3658.
[13] NORBY
T., KOFSTAD
P., High Temp. High Pressures 20 (1988) 345.
[14] LEEW.-K., NOWICK
A.S., BOATNER
L.A., Solid State Ionics, 18-19 (1986) 989.
[15] NORBY
T., KOFSTAD
F!, Solid State Ionics, 20 (1986) 169.
106
JOURNAL DE PHYSIQUE IV
[16] NORBYT., DYRLIEO., KOFSTAD
P.,J. Am. Ceram. Soc. 75 (1992) 1176.
[17] NORBYT, DYRLIE O., KOFSTAD
F!, Solid State Zonics 53-56 (1992) 446.
[18] NORBYT., Adv. Ceram. 23 (1987) 107.
[19] NORBYT., Studies in Inorganic Chemistry , A.G. Andersen, @ . Johannesen Eds. 9
(Elsevier,Amsterdam, 1989) p. 101.
[20] NORBYT., KOFSTADF!, J. Am. Ceram. Soc. 69 (1986) 780.
[2 11 NORBYT., Sold State Zonics 40-41 (1990) 857.
[22] HOLTO., Thesis, Univ. of Oslo (1992).
[23] CHEUNG
W.H., YOUNGD.J., Oxzd. Met. 36 (1991) 15.
[24] PHILIBERT
J., Atom Movements, Diffusion, and Mass Transport in Solids (Les Editions d e Physique, Les Ulis, France, 1991).
[25] DEALB.E., GROVEA.S., J.Appl. Phys. 36 (1965) 3770.