STRESS CORROSION CRACKING OF PURE METALS.
EFFECT OF THE EXCHANGE CURRENT DENSITY
Silvia Farina(1), Gustavo Duffó(1) (2) and José Galvele(1) (2)
(1)
Comisión Nacional de Energía Atómica, Depto. Materiales, [email protected]
Av. Gral. Paz 1499 – (1650) San Martín – Buenos Aires – Argentina
(2)
Consejo Nacional de Investigaciones Científicas y Tecnológicas (CONICET) Argentina
ABSTRACT
Based on stress corrosion cracking (SCC) studies on brass, and on Ag-Cd alloys (a model alloy used to
reproduce the behaviour of brass), it was found that both pure copper and pure silver are susceptible to SCC in
1M copper (II) nitrate and in 1M silver nitrate aqueous solutions, at the equilibrium potentials for the reactions:
Cu2+ + 2e- ↔ Cu and Ag+ + e- ↔ Ag, respectively. The results were analysed under the light of recent
developments in surface science. It was concluded that the same SCC mechanism that operates in brass and in
Ag-Cd alloys should be operating during SCC of pure copper and pure silver, under equivalent experimental
conditions.
Keywords: copper; brass; silver; stress corrosion cracking; vacancies.
In previous publications it was reported that Ag-Cd
alloys, used as model alloys to reproduce the stress
corrosion cracking (SCC) behaviour of brass [1, 2],
were susceptible to SCC when stressed in silver
nitrate and silver perchlorate aqueous solutions, at the
Ag/Ag+ equilibrium potential. It was also reported that
Cu-Zn alloys showed SCC when stressed in copper
(II) nitrate aqueous solutions, at the Cu/Cu2+
equilibrium potential [3]. It was found that, when
changing the noble metal content, in the Ag-Cd and in
the Cu-Zn alloys, from 60 up to 90 a/o, the higher the
content of the more noble metal in the alloy the lower
the crack propagation rate (CPR).
The reported CPR values [1-3] were collected in
Figure 1. If these CPR values are extrapolated to 100
a/o of the noble metal it is found that, under the same
experimental conditions, measurable CPR values
should be expected for pure copper and pure silver.
This assumption is in conflict with the frequently
reported fact that pure metals are generally considered
to be immune to SCC [4]. Nevertheless SCC of pure
copper and pure silver, in gaseous environments, was
predicted and experimentally confirmed [5,6]. On the
other hand, the conclusion based on Figure 1, that
pure copper and pure silver are susceptible to SCC is
not a trivial one. None of the SCC mechanisms
available at present could support, a priori, such a
conclusion. The studies of SCC of Ag-Cd alloys [1,2]
and Cu-Zn alloys [3] assumed that a surface mobility
SCC mechanism was operating. But the equation used
to calculate the surface self-diffusion coefficient, Ds,
is a function of the base metal content in the alloy, and
cannot be extrapolated to the pure noble metal.
Crack Propagation Rate (m/s)
1. INTRODUCTION
1x10
-5
1x10
-6
1x10
-7
1x10
-8
1x10
-9
Cu-Zn Alloy
Ag-Cd Alloy
-10
1x10
-11
1x10
60
70
80
90
100
Noble metal content (atomic percent)
Figure 1. CPR values for Cu-Zn alloys in 1M
Cu(NO3)2 solution, and Ag-Cd alloys in 1M AgNO3
solution, exposed at the Cu/Cu2+ and Ag/Ag+
equilibrium
potentials,
respectively
[1-3].
Extrapolation to 100 a/o suggests that, under
equivalent experimental conditions, pure copper and
pure silver could develop measurable CPR values.
In the SCC literature numerous SCC mechanisms start
from the assumption that surface dealloying is the
initial step for SCC. In these cases it is difficult to
rationalize that, under equal experimental conditions,
the same mechanism will be acting on the alloys and
on pure metals. According to the above mentioned
mechanisms, there is a clear possibility that, when
going from the 90 a/o alloy to the pure metal, the CPR
will not follow the extrapolation in Figure 1, but will
show a sharp drop in the CPR value, leading even to
SCC immunity.
The present work was aimed to find out if pure silver
and pure copper in aqueous solutions, under the
experimental conditions described in Figure 1, were
susceptible to SCC. The experimental results
confirmed the predictions of Figure 1, and an analysis
of the SCC mechanism involved is made.
2. EXPERIMENTAL METHOD
only a heavily deformed surface. No cracks were
found on the strained wires. Figure 2 shows the
typical appearance of the surface of a wire strained to
rupture in air.
The samples used were 1.0 mm diameter wires of Ag
(>99.90%) and Cu (>99.90%). The specimens were
degreased with acetone, annealed for one hour in
argon at 600ºC and air-cooled in the case of Ag, and
annealed for one hour in argon at 454ºC and waterquenched, in the case of Cu. These heat treatments
were chosen in order to reproduce the experimental
conditions previously used with Ag-Cd alloys [1] and
with Cu-Zn alloys [3]. Prior to the measurements, the
surface of the samples was again degreased with
acetone and dried with hot air. The mechanical
properties of the wires measured after heat treatment
are shown in Table 1.
Table I. Mechanical properties of the material tested
(ε: elongation to rupture, σ0.2: yield strength, σUTS:
ultimate tensile strength).
σUTS
ε(%)
σ0.2 (MPa)
(MPa)
Ag
27.8±0.1
12.5±0.5
67.5±0.5
Cu
30.1±0.1
16.7±0.4
95±1
For the SCC susceptibility evaluation constant
potential slow strain rate tests (SSRT) were used. The
straining experiments were performed with a modified
Hounsfield tensometer at an initial strain rate of
4.7x10-6 s-1. The cell used in the wire straining tests
was described in a previous publication [7]. The
measurements with silver wire were made in aqueous
1M AgNO3 solution. The measurements with copper
wires were made in aqueous 1M Cu(NO3)2 solution.
The solutions were prepared with analytical grade
reagents and deionized water (resistivity = 18.2
MΩ.cm). For silver the tests were performed at the
equilibrium potential of the Ag+ + e- ↔ Ag reaction,
and for copper, at the equilibrium potential of the Cu2+
+ 2e- ↔ Cu reaction, respectively. To this purpose,
the electrode potential was maintained by short
circuiting the straining wire samples with 20 times
longer pure silver and pure copper wires, respectively,
immersed in the test solution. All tests were carried
out at room temperature. In all the tests the wires were
strained to rupture. After fracture, the specimens were
observed with a Philips SEM 500 scanning electron
microscope. Afterwards, the samples were mounted
for metallographic sectioning, and the length of the
cracks was measured. From the length of the longest
crack and the exposure time, a mean crack
propagation rate (CPR) value was calculated.
3. RESULTS
When strained to rupture in air, both, copper wires
and silver wires, showed only ductile fracture. The
observation, under high magnification with the SEM,
of the lateral surfaces of these strained wires showed
Figure 2. Lateral surface of a silver wire strained
to rupture in air. No cracks are observed.
On the other hand, when silver wires were strained in
1M AgNO3 solution, at an initial strain rate of
4.7x10-6 s-1, and at the Ag/Ag+ equilibrium potential,
abundant cracks were found on the strained metal
samples. Figure 3 shows a typical example of a
corroded silver wire. Numerous cracks are found on
the metal surface. The tests were repeated at least by
sextuplicate, and reproducible results were found.
After metallographically mounting the strained
samples, the CPR was measured.
Figure 3. Stress corrosion cracks on a silver specimen
strained to rupture in 1M AgNO3 aqueous solution.
The CPR values were measured for pure silver in 1M
AgNO3 solution and the results are shown in Figure 4.
The mean CPR found for silver strained in 1M
AgNO3 solution was (3.2±2.3)x10-10 m/s. As shown in
Figure 4, the CPR values measured for pure silver fall
closely to the extrapolation of the CPR of Ag-Cd
alloys, as the composition of the alloy reached that of
pure silver.
-5
1x10
-5
1x10
-6
1x10
-6
1x10
-7
1x10
-7
1x10
-8
1x10
-8
1x10
-9
1x10
-9
Crack Propagation Rate (m/s)
Crack Propagation Rate (m/s)
1x10
-10
1x10
Ag-Cd Alloy
Pure Ag
-11
Cu-Zn Alloy
Pure Cu
-10
1x10
-11
1x10
60
70
80
90
100
1x10
60
Noble metal content (atomic percent)
Figure 4. Open symbols: Crack propagation rate
values for several Ag-Cd alloys in 1M AgNO3, at the
Ag/Ag+ equilibrium potential [1, 2]. Closed symbols:
The same for pure silver.
Numerous cracks were also found for copper wires
strained in 1M Cu(NO3)2 solution, at an initial strain
rate of 4.7x10-6 s-1, and at the equilibrium potential of
the Cu2+ + 2e- ↔ Cu reaction. Figure 5 shows a
typical example of a corroded copper wire. The tests
were repeated at least by sextuplicate, and
reproducible
results
were
obtained.
After
metallographically mounting the strained samples, the
CPR was measured.
70
80
90
100
Noble metal content (atomic percent)
Figure 6. Open symbols: Crack propagation rate for
several Cu-Zn alloys in 1M Cu(NO3)2 at the Cu/Cu2+
equilibrium potential [3]. Closed symbols: The same
for pure copper.
4. DISCUSSION
The experimental results show that the CPR values
found for pure copper and pure silver fit with the
extrapolation made in Figure 1. From the above
results it seems reasonable to expect that, under the
same experimental conditions, the same SCC
mechanism operating in the Ag-Cd alloy and in the
Cu-Zn alloy, will be operating also in pure silver and
in pure copper.
The specific action of the noble metal cation, which
induced SCC in the Ag-Cd alloys [1,2] and in the CuZn alloys [3], and that recently allowed to predict the
experimental conditions for SCC of 18 carat gold [8],
seems to be responsible for the SCC of pure silver and
pure copper.
Figure 5. A typical example of the lateral surface of a
pure copper sample strained in 1M Cu(NO3)2 aqueous
solution. Abundant cracks are observed.
The mean CPR value found for pure copper was
(2.0±1.4)x10-10 m/s, and the measured values are
shown in Fig. 6. As shown in Figure 6, the CPR
values measured for pure copper fall closely to the
extrapolation of the CPR of Cu-Zn alloys, as the
composition of the alloy reached that of pure copper.
In the discussion of the previous publications on SCC
of Ag-Cd alloys [1,2] and of Cu-Zn alloys [3] it was
concluded that the results were in coincidence with
the predictions of the surface mobility SCC
mechanism [9-11]. To this purpose an equation
developed by Galvele and Duffó [12] was used to
calculate the surface self-diffusion coefficient, Ds.
This equation assumed that most of the vacancies
present on the alloy surface were produced by the
preferential dissolution of the less noble component of
the alloy. The effect of the noble metal cation in the
solution, was to provide a significant exchange
current density to induce high surface mobility of the
metal surface [1-3].
In the case of the pure metals, the noble metal cations,
present in the aggressive solution, will also induce an
exchange current density similar to that found for the
alloys. But, for the concentration of vacancies on the
metal surface the use of Galvele and Duffó's equation
is not applicable.
On a pure metal, at room temperature, the
concentration of vacancies on the surface is small. On
the other hand, up to very recently, it was believed
that the surface structure was practically immobile
[13]. Nevertheless, van Gastel et al. [14,15], using the
scanning tunneling microscope (STM) found that,
even at room temperature, the mobility of surface
atoms on a pure metal is surprisingly high. In the
cases studied in the present work, the mobility is
further increased by the exchange current density
induced by the noble metal cations in the solution.
In the present case no attempt was made to calculate
the probable Ds value for the pure metals in the
corrosive environment, because Hirai et al. [16] have
shown that the surface self-diffusion coefficient, Ds,
for a pure metal, in the presence of an electrolyte, can
be measured with the atomic force microscope
(AFM).
5. CONCLUSIONS
It was found that silver and copper undergo SCC in
solutions of their respective ions (Ag+ and Cu2+) at the
equilibrium potentials Ag/Ag+ and Cu/Cu2+,
respectively.
It can also be concluded that the SCC mechanism for
pure copper and pure silver, in the presence of the
respective cations, is the same as found for Cu-Zn
alloys and Ag-Cd alloys in equivalent experimental
conditions.
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