Marcus P. Corrosion mechanisms in theory and practice
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Anodic Dissolution 169
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5
Thin Oxide Film Formation on Metals
Francis P. Fehlner
Corning Incorporated, Corning, New York
Michael J. Graham
National Research Council of Canada, Ottawa, Ontario, Canada
INTRODUCTION
A thin oxide film on base metals provides the protective layer required to make
metals useful. Without such a layer, they would adhere to and react with each other
as well as with other materials. The film, invisible to the naked eye, forms at low
temperatures. By low, we mean near room temperature. This contrasts with high-
temperature oxidation, which occurs at several hundred degrees Celsius and above.
The formation of an oxide film at low temperatures may be illustrated by the
machining of an aluminium block. The cutting tool removes the surface of the metal,
exposing pristine metal. In less than a millisecond, atmospheric oxygen attacks the
exposed metal atoms and a thin oxide layer begins to form. It attains a limiting
thickness after a few days. The thickness is in the range of a few nanometers (nm).
The kinetic rate laws of low-temperature oxidation are logarithmic, either
direct or inverse. At high temperatures, parabolic kinetics are usually observed.
For intermediate temperatures, logarithmic kinetics transform with time into the
parabolic type.
The mechanism by which a thin oxide film forms on a metal must explain the
transition from a two-dimensional adsorbed oxygen layer to a three-dimensional
oxide film. The process at one time appeared to be impossible at room temperature
because growth of an oxide requires that ions overcome an energy barrier to move
into and through the oxide. The thermal energy available at room temperature is
insufficient to overcome this barrier, which is approximately 1 electron volt.
Fortunately, the work of Cabrera and Mott [1] showed how tunneling electrons and
an electrochemical mechanism could explain the phenomena.
The process of oxygen adsorption on metals has been discussed in Chapter 2 of
this book. The transition to an oxide film is a gradual one in the sense that islands of
oxide nucleate from the adsorbed oxygen and then grow laterally across the surface.
Fehlner and Mott [2] proposed that islands of oxide grow on the metal by a process
of place exchange. This requires cooperative movement of both cations and anions.
The driving force is postulated to be the image force between an oxygen ion and the
171
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metal. Mitchell and Graham [3] suggested that activated processes are unnecessary
for island growth when it occurs at a step edge on a metal surface. However, they
subsequently reported [4] that misorientation of the (111) plane on a nickel
surface did not affect the rate of low-temperature oxidation.
Island growth is the initial stage of three-dimensional oxide growth and has
been emphasized for the low-temperature oxidation of Ba and Mg [5a]. During the
transition from island growth to a three-dimensional oxide film, the kinetics of the
process undergo a marked change from linear to logarithmic. This is readily
observed only at low temperatures.
MECHANISM OF THIN OXIDE GROWTH
The model of Cabrera and Mott is discussed in the book on low-temperature
oxidation by Fehlner [5b]. The basis of the model is the quantum mechanical concept
of electron tunneling. An electron can penetrate an energy barrier without the
requirement for thermal activation. As soon as a three-dimensional oxide forms on a
metal, electrons tunneling through the oxide are captured by adsorbed oxygen on the
oxide surface. The charge separation thus established between the oxide surface and
the metal sets up an electric field across the oxide. The proposed mechanism is
illustrated in Figure 1.
Ion movement into and through an oxide is required if an oxide film is to
thicken. Such movement requires that an activation energy of approximately 1
electron volt be supplied. At high temperatures, thermal energy is sufficient to
overcome the energy barrier and the ion can move from site to site within the oxide.
This is no longer true at low temperatures. However, the electric field across the oxide
lowers the activation energy for ion movement into the oxide, allowing the oxide to
continue growing thicker.
The process is self-limiting. The amount of charge is fixed so that the voltage
across the film is relatively constant. Thus, the field across the oxide decreases as
the oxide thickens. At some critical thickness, the oxide essentially stops growing
because the reduction of the activation energy by the field is insufficient to allow
172 Fehlner and Graham
Figure 1 Proposed mechanism for low-temperature oxidation. (From Ref. 1.)
Copyright © 2002 Marcel Dekker, Inc.
continued ion movement. The overall process is quite analogous to anodic
oxidation at constant voltage, which is discussed in Chapter 4 of this book. Thus,
the rate of oxide growth is dependent on a preexponential factor and an activation
term. The activation energy in the exponential is reduced by the electric field
across the oxide. Corrosion, too, has been discussed in terms of an electrochemical
mechanism [6,7]. See Chapter 1.
The rate of oxide growth according to the Cabrera-Mott expression can be
written as
Thin Oxide Film Formation on Metals 173
where x is the oxide thickness, t the time, N the number of potentially mobile ions,
Ω the oxide volume per mobile ion, v the atomic vibration frequency, W the
energy barrier to ion movement into the oxide, q the ionic charge Ze, a half the ion
jump distance, E the electric field V/x, k Boltzmann’s constant, and T the
temperature. Once the ion enters the oxide, it is assumed to pass easily to the other
side, where reaction takes place.
Ghez [8] has integrated Eq. (1) to give the inverse logarithmic law:
where
and
The constant τ can be neglected if it is small compared with t. The value of u is
expressed as nanometers per second and x
1
as nanometers. Fehlner [9] has utilized
these equations to fit literature data on low-temperature oxidation. Some of these
results will be included in this work.
Fehlner and Mott [2] pointed out that direct logarithmic kinetics could be
derived from Eq. (1) if two conditions were met. First, the film must grow under
constant field rather than constant voltage. This may happen when the growing field
across the oxide causes ion movement to occur through the oxide before the
maximum value of V is attained. Second, the oxide must rearrange with time so that
the value of W increases by some factor, say ξx, where ξ is a constant and x is the
oxide thickness. Thus, the activation term in Eq. (1) becomes
which integrates to a logarithmic expression.
The implication then is that the kinetics of low-temperature oxidation depend
on the structure of the oxide. This can be considered in terms of network formers
and modifiers. A network-forming oxide is one in which covalent bonds connect the
atoms in a three-dimensional structure. There is short-range order on the atomic
scale but no long-range order. For example, oxygen atoms form tetrahedra around
Copyright © 2002 Marcel Dekker, Inc.
ions such as silicon, triangles around aluminum. These continuous random
networks can be broken up by the introduction of modifiers. Oxides such as
sodium oxide have ionic bonding. When added to a network-forming oxide, they
break the covalent bonds in the network, introducing ionic bonds, which change
the properties of the mixed oxide.
It seems likely that direct logarithmic kinetics would be observed for modi-
fying oxides rather than for network-forming oxides because the former have
lower single oxide bond strengths and could rearrange more easily. See Table 1.
The network formers would be expected to follow the inverse logarithmic kinet-
ics of Eq. (2).
Kingery et al. [10a] have collected the experimental results for cation and
anion self-diffusion coefficients in various oxides. For instance, the rate of
diffusion of copper in Cu
2
O, a modifier, is six orders of magnitude larger than
that of aluminum in Al
2
O
3
, a network former when the aluminum is four coordi-
nate. This result is for a temperature of approximately 1500°C, but the relative
relationship is expected to hold at room temperature. This finding also supports
the postulate that network modifiers can recrystallize more easily than network
formers.
174 Fehlner and Graham
Table 1 Values of Single Oxide Bond Strengths
a
a
Note: Glass formers have bond strengths > 75 kcal mol
–1
, intermediates lie between 75 and ~50
kcal mol
–1
, and modifiers lie below 50 kcal mol
–1
.
Copyright © 2002 Marcel Dekker, Inc.
CONTROLLING FACTORS
Ion entry into a growing oxide occurs at the metal-oxide interface for cations and
at the oxide-gas interface for anions. Davies et al. [11] have shown that one or both
species can be mobile in an oxide undergoing anodization. The choice of mobile
ion is related to the structure of the oxide, whether it is crystalline or noncrystalline,
as discussed below. In the case of cation movement, the properties of the metal
affect the rate of oxide growth. Such characteristics as crystal orientation, defects,
and impurities are important. In the case of anion movement, gas pressure and the
presence of moisture are most significant.
Metal Structure
The effect of crystallographic orientation on the rate of low-temperature oxidation
is shown dramatically in the case of copper. Rhodin [12] published an early study of
the copper-oxygen system at temperatures from –195° to 50°C. The (100) face
oxidized approximately twice as fast as the (110) and (111) faces. Young et al. [13]
showed that impurities could affect the rate of oxidation, especially on the (110)
plane. Their results at 70°C are shown in Figure 2. Oxidation rates follow the order
(100) > (111) > (110) > (311). Film thickness as a function of time appears to follow
a linear relationship, but logarithmic kinetics have also been applied to the data.
The value of N in Eq. (1) is related to the number of sites where ion entry into
the oxide can occur. However, in the case of cation movement, it has not been
possible to predict the relative rates of oxidation according to the density of atomic
packing on the various crystal faces. Cation loss from a surface occurs preferentially
from steps and kink sites, so metal surface roughness and impurities play a major
role. All these factors need to be considered when evaluating N.
Thin Oxide Film Formation on Metals 175
Figure 2 Copper oxidation. Oxidation of single-crystal copper surfaces in an atmosphere
of oxygen at 70°C. (From Ref. 13.)
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Single-crystal studies as referenced above reveal the basic behavior of oxide
growth on metals, but most metals are found as polycrystalline bodies. The gain
boundaries that separate randomly oriented grains must accommodate the
discontinuities between misoriented lattices. The resulting region of disorder serves
as a sink for impurities and often a region of fast oxidation. As a result, a
polycrystalline body can develop a very uneven, polycrystalline oxide layer. Metal
grains with different crystallographic orientations oxidize at different rates. In
addition, grain boundaries serve as points of weakness. The advent of glassy metal
alloys has overcome some of these problems, at least at low temperatures [14, 15].
The lack of both a crystal lattice and grain boundaries eliminates orientation effects
as well as impurity concentrations.
Oxide Structure
The book by Fehlner [5] emphasizes the role that vitreous (glassy or noncrystalline)
oxide structure can play in oxide growth on metals. The structure of a vitreous
oxide can have perfection equal to that of a single crystal. This fact is not well
recongnized. At the atomic level, both structures are similar in that the short-range
order is the same. For example, the local environments of silicon atoms in cristobalite
and fused silica are very much alike. Each silicon atom is coordinated with four
oxygen atoms in a tetrahedral arrangement. The difference in the structures is seen
in the long-range order present in the crystal but absent in the fused silica glass. As
a result, physical properties that depend on the local bonding are similar in the two
structures but those dependent on long-range order differ. Both structures are
expected to be more protective of a metal surface than a polycrystalline oxide would
be. This is because the grain boundaries present in the polycrystalline oxide provide
paths for easy ion movement and hence more rapid oxide growth.
The tetrahedral arrangement of atoms in silica lends itself to the formation of
a three-dimensional network, which makes up the polymer-like structure of a glass.
Glass can be formed from a single oxide or a mixture of oxides. Silicon dioxide is
the permier glass-forming oxide either by itself or with other oxides. Additives to
silica can either enter into the silica network or modify it. Modifiers such as sodium
oxide break up the network, leading to l ower softening points (glasses have no
sharp melting points), lower chemical durability, and higher ionic conductivity.
Some oxides like zinc oxide can act as an intermediate oxide, i.e., either a network
former, where it enters the silica tetrahedral network, or a modifer, where it breaks
up the network. The choice depends on the other constituents in the glass. A basic
review of the subject can be found in Kingery et al. [10b].
Several schemes are used to classify metal oxides as network formers,
intermediates, or modifiers. A useful one is based on single oxide bond strengths.
Values for the more common metals are collected in Table 1 from Sun [16] and
Fehlner and Mott [2]. Glass formers tend to have single oxide bond strengths greater
than 75 kcal mol
–1
. The directional covalent bonds interfere with the crystallization of
an oxide while it is being formed on the surface of a metal. Intermediates lie between
75 and approximately 50 kcal mol
–1
and modifiers lie below this value. Their ionic
bonds are not directional, so it is easier for the atoms to align in crystalline order.
This division can be related to thin oxide films on metals. The metals that fall
into the network-forming or intermediate classes tend to grow protective oxides that
176 Fehlner and Graham
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