Volume 6 Preprint 106
Potentiodynamic polarization studies of aluminium alloys with enriching alloying elements
S. Garcia-Vergara, M. A. Arenas, P. Skeldon, G. E. Thompson, K. Shimizu and H. Habazaki
Keywords: aluminium, alloys, copper, gold corrosion potentials, MEIS, RBS, TEM
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Volume 6 Paper C157
Potentiodynamic polarization studies of
aluminium alloys with enriching alloying
S. Garcia-Vergara1, M. A. Arenas1, P. Skeldon1*, G. E. Thompson1, K.
Shimizu2 and H. Habazaki3
and Protection Centre, UMIST, P.O. Box 88, Manchester
M60 1QD, UK
University Chemical Laboratory, Keio University, 4-1-1 Hiyoshi,
Yokohama 223, Japan
Graduate Engineering School, Hokkaido University, N13 W8, Kita-ku,
Sapporo 060-8628, Japan
* Corresponding author: firstname.lastname@example.org
The present work uses potentiodynamic polarization in naturally
aerated 0.1 M ammonium pentaborate electrolyte to explore the
effects of enrichment of alloying elements on corrosion potentials of
aluminium alloys. The selected alloys were Al-1.1at.%Au and Al-
0.95at.%Cu alloys deposited by magnetron sputtering, with enriching
of the alloying element being achieved by controlled anodic etching in
sodium hydroxide solution. Such etching results in large positive
shifts, of several hundred millivolts, in the corrosion potential in
ammonium pentaborate electrolyte, while the alloying element remains
in the enriching alloy layer, of thickness about 2 nm, just beneath the
surface oxide film. The results provide evidence of effects of
enrichment on the anodic reaction, i.e. oxidation of aluminium, and on
the cathodic reaction, i.e. reduction of oxygen. The former may arise
due to the relatively high concentrations of alloying elements in the
enriched alloy layer, while that later may relate to increased electron
tunnelling through the oxide film. However, there may also be effects
of flaws, which cannot be eliminated in the present experiments,
although there was no direct evidence of their role.
Keywords: aluminium, alloys, copper, gold corrosion potentials, MEIS,
The corrosion potentials of matrix and second phase regions of
aluminium alloys are important in determining the corrosion behaviour
of the alloy. Galvanic coupling and alkaline corrosion around cathodes
gives rise to localized corrosion processes [#ref1]. Various authors
have therefore determined the corrosion potentials of second phase
materials in particular environments in order to assess their function
in alloy corrosion [#ref1-4]. Copper-containing second phases are
often considered to function as cathodes, due to their relatively high
potentials, with nanoparticles of copper, formed by relocation of
copper species released by prior corrosion of the particle or by de-
alloying, assisting the cathodic reaction [#ref5]. Further, the addition
of copper to aluminium is known to increase the corrosion potential
significantly [#ref6]. Recently the present authors have examined a
possible mechanism for achieving large rises in potential for relatively
modest additions of copper to the alloy [#ref7-9]. The explanation
rests on the established mechanism for anodic oxidation of solidsolution aluminium alloys, which involves initial oxidation of only
aluminium atoms, while copper atoms accumulate in a thin layer of
alloy immediately beneath the oxide film [#ref10]. The oxidation can
take place by various processes that occur in the presence of an
amorphous, alumina-based film, such as in electropolishing, chemical
polishing, acid pickling, alkaline etching, conversion coating and
anodizing. With continued oxidation, the copper concentration builds
up in an alloy layer of thickness about 2 nm [#ref10]. Eventually, the
copper is sufficiently enriched for its oxidation to commence and,
thereafter, the oxide film contains aluminium and copper species while
the enriched alloy layer is maintained at the critical level of copper
enrichment [#ref10-11]. The level corresponds to an average
concentration of about 40 at.% Cu for an Al-1at.%Cu alloy. The
enrichment increases slightly for higher concentration of copper in the
bulk alloy, and falls off more steeply for reduced copper contents.
Thus, for relatively dilute alloys, the pre-treatments commonly used
for surface preparation, pre-condition the alloy with a high, nearsurface concentration of copper, both prior to and following the
commencement of oxidation of the copper. During the enriching
stage, it has been shown that large positive shifts in the corrosion
potential take place [#ref7-9]. For example, enrichment of an Al-
1at.%Cu alloy moves the corrosion potential by about 400 mV in a
naturally aerated borate buffer solution. Enrichment is also expected
for other alloying elements for which the Gibbs free energy per
equivalent for formation of the alloying element oxide is more positive
than that of alumina [#ref12]. Thus, gold enriches similarly to copper,
and results in broadly similar shifts in the corrosion potential [#ref9].
In the present work, enrichments and corrosion potentials of Al-Au
and Al-Cu alloys are compared, with further electrochemical
experiments carried out to determine the influence of the enrichments
on the anodic and cathodic processes.
Al-1.1at.%Au and Al-0.95at.%Cu alloys were deposited by magnetron
sputtering in an Atom Tech system on to electropolished high purity
aluminium substrates. The alloys were then etched at 5 mA cm-2 in 0.1
M sodium hydroxide solution at 293 K. The etching time was selected
to generate the required level of enrichment of either copper or gold in
the alloy. After etching, specimens were immersed for 30 s in 15%
nitric acid. Corrosion potential was measured in naturally aerated 0.1
M ammonium pentaborate solution (pH 8.3) at 293 K for times up to
12 ks. Further, potentiodynamic polarization curves were recorded in
the same electrolyte at a scan rate of 1 mV s-1. A three-electrode cell
was employed with a platinum counter electrode and a calomel
reference electrode (SCE).
Enrichments of alloying elements were then determined by either
Rutherford backscattering spectroscopy (RBS) or medium energy ion
scattering (MEIS). The former used 2.0 MeV He+ ions supplied by the
Van de Graaff accelerator of the University of Paris. The latter used
100 keV He+ ions supplied by the Daresbury facility. Data of RBS and
MEIS were interpreted using RUMP [#ref13] and SIMNRA [#ref14]
programs; respectively. Transmission electron microscopy (TEM) was
employed to determine the rate of alkaline etching and to examine the
morphologies of specimens at various stages of treatment. Electron
transparent specimens were first prepared by ultramicrotomy, then
examined in a JEOL FX 20000 II instrument.
The deposited alloys were columnar-grained, with thickness about
500 nm. Alkaline etching reduced the thickness of the alloy to an
extent proportional to the time of treatment. Subsequent potential
measurements in naturally aerated 0.1 M ammonium pentaborate had
no measurable effect on the alloy thickness, to an accuracy of about
20 nm. Representative micrographs, using the example of the Al0.95at.%Cu alloy, are shown in Figure 1.
Fig. 1 Transmission electron micrographs of sputtering-deposited Al0.95at.%Cu alloy. (a) Following deposition. (b) Following alkaline
etching for 75 s at 5 mA cm-2 in 0.1 M sodium hydroxide electrolyte at
293 K. (c) Following the previous alkaline etching and subsequent
measurement of the corrosion potential in naturally-aerated 0.1 M
ammonium pentaborate electrolyte for 10000 s at 293 K.
Enrichments of copper and of gold were detected readily by either RBS
or MEIS respectively (Figure 2). The enrichments increased with time of
etching as expected, with the residual oxide films remaining from the
etching treatment being free of copper species and gold nanoparticles,
as confirmed by MEIS, until enrichment to 4.5 × 1015 Cu atoms cm-2
and 5.7 × 1015 Au atoms cm-2. These enrichments are achieved after
etching for 43 and 23 s respectively.
the enriched layer
95 100 105
Fig. 2 Representative (a) RBS spectrum for the Al-0.95at.%Cu alloy and
(b) MEIS spectra for the Al-1.1at.%Au alloy; following etching for 43
and 15 s respectively, in 0.1 M NaOH at 293 K.
Following immersion in ammonium pentaborate electrolyte, the
potential underwent a period of transient behaviour, probably due to
initial modifications of the surface oxide formed in alkaline etching,
and then approached a steady value. The latter values were dependent
upon the level of enrichment of the alloy, with the potential of copper
and gold alloys increasing by 363 and 520 mV in the period where the
oxide films remained free of alloying element species (Figure 3).
Corrosion potential (VSCE )
Alloy enrichment (10 atoms cm )
Fig. 3 Dependence of the corrosion potential on copper and gold
enrichment in the alloys for Al-0.95at.%Cu and Al-1.1at.%Au alloys in
naturally-aerated 0.1 M ammonium pentaborate electrolyte at 293 K.
The alloys were first etched for up to 83 s at 5 mA cm-2 in 0.1 M
sodium hydroxide solution at 293 K.
Potentiodynamic polarization measurements of the Al-1.1at.%Au alloy
in de-aerated 0.1 M ammonium pentaborate electrolyte were started
at the steady corrosion potential, attained by initial immersion for
10800 s, in the electrolyte. The potential was scanned firstly in the
cathodic direction, then swept from the cathodic direction to the net
positive range of currents. The initial cathodic sweep was halted just
as hydrogen evolution was beginning, as indicated by a slight increase
in the current. The findings for specimens in the as-deposited and
alkaline-etched conditions disclose net cathodic currents due to
reduction of dissolved oxygen, with magnitudes in the range 0.1 to 1
µA cm-2 following polarization by about 300 mV from the initial
corrosion potential (Figure 4). Reversal of the sweep direction led to an
increase of the net cathodic current by a factor of about 2, with the
potential at zero net current then shifting by up to 100 mV in the
positive direction relative to the initial corrosion potential, which may
be due to the increased cathodic activity. In the initial potential region
of net anodic behaviour, the current increased rapidly, in a roughly
exponential manner, which was terminated by a small peak, beyond
which the current was constant. The final plateau region corresponded
to a current density of about 5 µA cm-2. The approach to the plateau
currents was delayed for the enriched alloys by about 400 - 500 mV,
with the prior peak being broadened compared with that of the asdeposited alloy.
Potential (VSCE )
Current density (A cm )
Fig. 4 Potentiodynamic polarization curves, measured at a scan rate of
1 mV s-1, for Al-1.1at.%Au alloy in aerated 0.1 M ammonium
pentaborate electrolyte at 293 K. The curves were measured for the
as-deposited alloy, and following etching of the alloy for 15 and 23 s
at 5 mA cm-2 in 0.1 M sodium hydroxide electrolyte at 293 K.
Similar trends were also observed in potentiodynamic polarization
curves for the Al-0.95at.%Cu alloy (Figure 5). The net cathodic current
was increased for the enriched alloy, with the plateau behaviour in the
anodic region, corresponding to a current density of about 5 µA cm-2
being preceded by a small anodic peak. Plateau currents for the
enriched copper-containing alloy were achieved by about -200 mV,
but by about + 300 mV for the enriched gold-containing alloy.
Potential (VSCE )
Current density (A cm )
Fig. 5 Potentiodynamic polarization curves, measured at a scan rate of
1 mV s-1, for Al-0.95at.%Au alloy in aerated 0.1 M ammonium
pentaborate electrolyte at 293 K. The curves were measured for the
as-deposited alloy, and following etching of the alloy for 12 and 22 s
at 5 mA cm-2 in 0.1 M sodium hydroxide electrolyte at 293 K.
The increased corrosion potential of the Al-1.1at.%Au and Al0.95at.%Cu alloys following enrichment of the alloying element by
alkaline etching can be explained by increase in the cathodic reaction,
reduction in the anodic reaction, or a combination of these effects.
Considering firstly the anodic reaction, polarization curves disclose
plateau current densities of about 5 µA cm-2 for both alloys, with no
significant dependence on the level of enrichment. There is also a
common anodic peak, which represents a charge density of about 200
µC cm-2. The peak can be ascribed to the transient response of the
oxide layer at the start of oxide growth. Such transient peaks are wellknown in anodizing, when the current density changes increases from
one value to another [#ref15, 16]. Their precise origin is uncertain,
although they have been attributed to effects of adjustment of the
number of charge carriers in the film, current-driven polarization and
structural changes in the film. Thus, in the present case, the oxide
growth at the corrosion potential was relatively negligible, and
increased significantly following sufficient polarization. The growth
rate of the oxide is determined by the electric field across the oxide
which drives the ionic current through the oxide film.
A current density of about 3 µA cm-2 is expected for growth of anodic
alumina at 100 % efficiency at a sweep rate of 1 mV s-1. The higher
value of the measured current density indicates a reduced efficiency of
film growth, to about 50-60%, considering that there may be a
contribution of about up to 1 µA cm-2 from the cathodic current
density due to oxygen reduction. Thus, the results for the region of
net anodic currents following the transient peak are consistent with
growth of the oxide film at reduced efficiency. The lowered efficiency
is most probably due to loss of Al3+ ions to the electrolyte. During
oxide growth, only 1-2 nm of alloy are oxidized, which contains no
more than about 1.2 x 1014 alloying element atoms cm-2, which will
have minor effect on the total levels of enrichment that are greater
than 1 x 1015 atoms cm-2. Thus, the enrichments have negligible
influence in the growth of the alumina-based films developed in the
ammonium pentaborate electrolyte while the films are free-of alloying
element species. The increased potential required for growth of the
oxides on the enriched alloys, compared with that for growth of the
oxide on the as-deposited alloy, may indicate a thicker oxide, by
about 0.5 nm, on the former, although there is no supporting
evidence, for instance from MEIS of a thicker oxide. Alternatively, the
increased potential may relate to the reduced availability of aluminium
atoms for oxidation at the alloy/oxide interface due to influences of
the enriching alloying element.
At potentials below the transient anodic peak, the passive oxide can
only grow at low rates. The pre-existing film developed during alkaline
etching and modified during the initial stage of immersion in
ammonium pentaborate electrolyte is of thickness about 3 nm, as
determined by MEIS. The electric field necessary to form anodic
alumina at 100 % efficiency at a constant rate corresponding to the
sweep rate is about 6.25 x 106 V cm-1. Thus, a potential drop of about
1.87 V is required across the oxide film. The equilibrium potential for
oxidation of aluminium to form alumina at pH 8.3 is –2.32 V (SCE).
Accordingly thickening of the oxide is not anticipated until a potential
of about – 450 mV, which is reasonably close to the values at which
net anodic activity commences. Low anodic currents due to oxidation
of aluminium and growth of alumina, at rates in the range 1 nA cm-2
to 1 µA cm-2, can occur in the vicinity of the corrosion potential and
the potential range just above the corrosion potential. In this range,
thinning of the alumina, or its transformation to non-protective
hydroxide may have a significant role in determining the magnitude of
The increased time of alkaline etching, and hence enriching of the
alloying element, clearly influences the cathodic reaction. Thick films
of anodic alumina are insulating over the macroscopic surface, but
local electron conductivity can occur at flaw sites. Thus, increase of
flaws through etching could explain the enhancement of cathodic
activity. However, the time of etching does not appear to increase the
thickness of the film significantly. Further, the film is thinned at a
similar rate to which it is growing, such that flaws may be removed at
a similar rate to which they are formed, particularly if they are of the
residual rather than mechanical type. Regarding the latter, roughening
of the surfaces during etching may increase the flaw population,
although there is no evidence for a major range in roughness from
examining of surface by atomic force microscopy.
If the cathodic reaction is not dominated by flaws, the increased
current with increased time of etching can be ascribed to facilitation of
electron tunnelling [#ref17]. The tunnelling current should increase if
the alumina thickness reduces with increased time of etching, which is
not evident form MEIS data, or if the height of the energy barrier for
electron transfer from the metal to adsorbed oxygen at the oxide
surface is reduced. The latter can be achieved by the altered
composition of the alloy in the enriching alloy layer. It is notable that
the reduction current densities for oxygen are about an order of
magnitude lower than measured on metals such as platinum that allow
ready electron transfer.
Assuming that the altered corrosion potential is due to macroscopic
effects of enriching of the alloying elements, the present results are
consistent with the behaviour resulting from an increased potential of
the anodic reaction combined with increased electron tunnelling
through the oxide film. Alternatively, or in association with the
previous, the enriching of the alloying elements during alkaline
etching may be accompanied increased activity due to flaws, although
there is no firm evidence from the results of increased flaw
1. Alkaline etching of metastable, solid-solution Al-1.1at.%Au and
Al-0.95at.%Cu alloys is accompanied by enriching of the alloying
element in an alloy layer of thickness a few nanometres located
just beneath the alumina-based films formed during the etching
process. The enriching of the alloying elements correlates with
an increase of the corrosion potentials of the alloys in naturallyaerated ammonium pentaborate electrolyte at 293 K.
2. At comparatively high potentials, potentiodynamic polarization
measurements for the alloys at various stages of enriching,
measured in de-aerated ammonium pentaborate electrolyte,
dependence upon the level of enrichment of the alloy, which
correspond to the growth of the oxide film at a constant rate
related to the potential sweep rate. This region is preceded by
an anodic peak, corresponding to a charge of 200 µC cm-2,
which is a transient response of the film prior to achieving the
steady-state conditions for growth. The magnitude of the
current density indicates that the oxide grows at an efficiency of
50-60%. The potential at which steady-state growth of the oxide
commences increases with increased enrichment of the alloying
3. At regions of net negative current, corresponding to domination
of the oxygen reduction reaction, currents increase with
increased time of etching, and hence enriching of the alloying
element. A possible explanation of the behaviour is related to
enhanced tunnelling of electron through the oxide film.
4. In the vicinity of the corrosion potential, the anodic current may
be due to slow growth of the film, supported by electron
tunnelling, with the enhanced cathodic kinetics and increased
anodic potential of the anodic reaction due to enriching leading
to enhanced corrosion potentials. However, it is not possibly to
eliminate some contributions from reactions from flaws, which
could not be isolated in the present experiments.
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