ISSN 2070-2051, Protection of Metals and Physical Chemistry of Surfaces, 2009, Vol. 45, No. 3, pp. 277–282. © Pleiades Publishing, Ltd., 2009. Original Russian Text © O.A. Kozaderov, A.V. Vvedenskii, O.V. Koroleva, 2009, published in Fizikokhimiya Poverkhnosti i Zashchita Materialov, 2009, Vol. 45, No. 3, pp. 259−264.
PHYSICOCHEMICAL PROCESSES AT THE INTERFACES
Kinetics of Phase Transformations in a Binary Alloy Surface Layer at the Selective Dissolution. II. Ag–Au|Ag+* System O. A. Kozaderov, A. V. Vvedenskii, and O. V. Koroleva Voronezh State University, Universitetskaya pl. 1, Voronezh, 394006 Russia E-mail:
[email protected] Received April 12, 2008
Abstract—Based on the theoretical investigation [1], the conditions of phase transformation of gold on Ag–Au alloy surfaces, which are selectively dissolved in a nitrate environment at the overcritical anodic polarization, are experimentally found. Kinetic regularities of the formation of Au° individual gold phase on the alloy surface depending on the alloy composition and overpotential, as well as on the presence of surface active organic substances in the solutions and the passivating alloying additives in the alloy are determined. PACS numbers: 82.45.Qr DOI: 10.1134/S2070205109030022
INTRODUCTION In [1], we have proposed and theoretically founded the procedure of determining the formation rate of the individual phase of an electrochemically positive component B at the selective anodic dissolution or corrosion of an A–B alloy at potentials, which exceed the critical potential of the surface development of morphological instability. In this paper, which continues [1] and is based on the results of that theoretical investigation, we experimentally find the conditions of the phase transformation of gold on the surface of Ag–Au alloys, which are selectively dissolved in a nitrate environment at the overcritical anodic polarization. EXPERIMENTAL Investigations are carried out on polycrystalline Ag– Au alloys (ïAg = 0.65 to 0.95), as well as on Ag15Au0.5Ni, Ag15Au0.5Ti, and Ag15Au0.5Si alloys at 298 K.1 Solid solutions were obtained by directly alloying the metals in evacuated quartz ampoules, annealing for two hours at 1213 K, and quenching in water. According to the constitution diagram [2], these conditions of heat treatment at the selected chemical composition provide the formation of statistically disordered solid metallic solutions. The absence of phase inclusions in all the cases was confirmed by metallographic analysis. To make planar electrodes, the specimens were cut, polished, and armored in polymerizable epoxy resin. 1
Here and below, numbers in alloy notations show the atomic parts (%) of the components added to silver.
Conventional treatment of the electrode surfaces before experiments involved cleaning with sandpaper of the minimal grain size, washing in bidistilled water, polishing to mirror luster by chamois wetted with magnesium oxide suspension; repeated washing in bidistilled water, degreasing the electrode surface with ethanol, and final washing in bidistilled water. The roughness factor of the electrode surface was determined before experiments with a combined adsorption–electrochemical technique [3]. The fr values were 1.77, 1.82, 1.87, 1.88, and 1.91 for Ag–Au alloys containing silver in the amount XAg = 0.95, 0.90, 0.80, 0.70, and 0.65 at. %, respectively. 0.1 M NaNO3 + 0.001 M HNO3 + x MAgNO3 solutions (ı = 10–4 to 10–2) were prepared from analytically pure reagents, acid titrant, and bidistilled water. Investigations were carried out in unstirred solutions, which were deaerated with chemically pure argon directly in the electrochemical cell with a common cathodic and anodic space for accelerated transient measurements. An auxiliary electrode was platinum. A silver-chloride reference electrode was placed– in a separate vessel and connected to the cell via an electrolytic bridge filled with saturated äNO3 solution. Current density was calculated per unit original true electrode surface. In order to study the role of surface active organic substances (SAOS) in the phase-formation kinetics, we valerian took benzoic (ë6ç5ëééç), (ëç3(ëç2)3ëééç), and caproic (ëç3(ëç2)4ëééç) acids. It was found that these acids do not undergo electrochemical oxidation in the studied anodic potential range, and adding them to the solution does not affect the currentless potential Ö(0), but substantially increases the critical potential Ecr of the alloy surface development.
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log iAg [iAg, mA/cm2] –0.4
30 mV 20 mV 10 mV 0
–0.6 –0.8 –1.0 –1.2 –1.4 –1.6 –1.8 0.0
0.5
1.0
1.5
2.0 2.5 log t [t, s]
Fig. 1. Chronoammograms of Ag5Au alloy in 0.1 MNaNO3 + 0.001 M HNO3 + 0.001 M AgNO3 recorded at various excesses over the critical potential ∆Öcr.
Kinetics of the phase transformation was studied with the use of chronoammetry with an IPC-Compact computerized potentiostatic set. A series of I,t potentiostatic curves were recorded at different potentials E = Ecr + ∆Ecr, where ∆Ecr is the potential excess over the critical value. As Ecr at the first step of investigation, we took the critical potential values, which were graphically determined in [4] by the tangent method from the anodic polarization curves of the selectively dissolved Ag–Au alloys. The duration of polarization was chosen so that the following condition was met: the electric charge passing through the system should be twice as ph
iAu , µA/cm2 30
large as the qcr value determined in [5]. Insofar as gold did not dissolve under particular experimental conditions, we assumed that the measured current I = IAg. RESULTS AND DISCUSSION On all chronoammograms constructed in logarithmic coordinates, we can distinguish long segments corresponding to the diffusion limitation of the selective dissolution (Fig. 1). Starting only from a certain time moment t ≥ tcr, the current drop becomes less pronounced and, in some cases, is even replaced with an increase. The larger the ∆Ecr value, the larger the accompanying deviations of the log i Ag , log t dependence from linearity and the smaller the tcr parameter. The Ecr values found graphically turned out to be slightly overrated compared to those determined chronoammetrically. By contrast, the charges accumulated by the moment of the phase formation onset tcr were smaller than the critical charges qcr found in [5] irrespectively of the gold content in the alloy. Therefore, Ecr values were corrected by determining the potential, at which a nonlinear segment appears for the first time in the potentiostatic current-drop curve constructed in logarithmic coordinates.2 Accordingly, ∆Öcr values were determined with respect to the corrected Ecr value. Assuming that the nonlinear character of log i Ag , log t curves is caused by the appearance and growth of the individual gold phase nuclei, which proceed concurrently with the selective dissolution, the current transient of the Au* Au° phase transformation was found according to the technique founded in [1] as the difference between the total and diffusion (Cottrel’s) currents: ph
i Au (tph) = iAg(t > tcr) – const t–k, 25
4
where tph = t – tcr. The const and k parameters were calculated from the initial (linear) segment of the experimental chronoammogram using the least squares method. The dependences of the phase-formation currents
20 3
15
(1)
ph
10 2 5 1 0
50
100
150 tph, s
Fig. 2. The time dependences of the gold-phase transformation currents on Ag5Au alloy in 0.1 M NaNO3 + 0.001 M HNO3 + 0.001 M AgNO3 at ∆Öcr, mV: (1) 10; (2) 20; (3) 30; and (4) 40.
i Au on the time interval tph from the beginning of the phase transformation, which were obtained at various excesses over the critical potential (Fig. 2) on different alloys (Fig. 3), as well as in the presence of a SAOS (Fig. 4) and a passivating additive (Fig. 5), have a shape ph
quite typical of the nucleation processes, namely, i Au ph
values increase with an increase in tph. In the i Au –tph 2 In
a similar way, the critical potential of Ag–Au alloys was determined in perchlorate environments in [6].
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KINETICS OF PHASE TRANSFORMATIONS IN A BINARY ALLOY SUPERFICIAL LAYER ph
279
ph
iAu , µA/cm 2 20
iAu , µA/cm 2 7 1 6
6
18 16
5
14
5
12
4
10
3
8
1
6
4 2 3
4 2
3 1 4 0
0
20
40
60
80 tph, s
Fig. 3. Current transients of the phase transformation on Ag–Au alloys containing XAu, at. %: (1) 5; (2) 10; (3) 15; (4) 20; (5) 25; and (6) 30 recorded in 0.1 M NaNO3 + 0.001 M HNO3 + 0.001 M AgNO3 at ∆Öcr = 20 mV.
It turned out that in all the cases, the current transients of the phase transformations on the surfaces of the studied Ag-based Ag–Au alloys can be linearized only in the 3D-nucleation criterial coordinates. There are usually two linear segments corresponding to different models of the gold nuclei growth at small and large times. At the final step, chronoammograms can be linearized at the sufficiently large correlation coeffiph
cients only in i Au –tph [1/2] coordinates that are criterial
10 20 30 40 50 60 70 80 tph, s
Fig. 4. The time dependence of the phase transformation currents of gold on (1) Ag15Au, (2) Ag15Au0.5Si; (3) Ag15Au0.5Ni; and (4) Ag15Au0.5Ti in 0.1 M NaNO3 + 0.001 M HNO3 + 0.001 M AgNO3 at ∆Öcr = 20 mV.
curves, we can sometimes see a pronounced current maximum, however, in the most of cases, current reaches a plateau. Such a shape of current transients is particularly typical of the multiple nucleation [7] when the formation of islets of a new phase proceeds at a certain delay, which results in the mergence of the peaks of nucleation and nuclei growth. The time dependences were analyzed in terms of the deterministic nucleation models [7–10]. Taking into account the peculiar phase regrouping that proceeds concurrently with the selective dissolution of the alloy, we considered the kinetic and diffusion conditions of the growth of two- (2D) and three-dimensional (3D) new-phase nuclei. Coordinates, in which the current transients are linearized, are listed in Table 1 for the instantaneous (ν = 0) and continuous (ν = 1) nucleation; ä1, ä2, ä3, and ä4 are the apparent rate constants of the corresponding processes. We assumed that the electrode by-processes, as well as the adsorption of the solution components, do not take place, but the possibility of covering the gold nuclei was taken into account.
2
2
ph
iAu , µA/cm 2 25
1
20
2
15 3
10
4 5
0
50
100
150
200 tph, s
Fig. 5. Chronoammograms of the gold-phase transformation on Ag10Au alloy at ∆Öcr = 20 mV in a 0.1 M NaNO3 + 0.001 M HNO3 + 0.001 M AgNO3 solution containing (1) no SAOS; (2–4) 0.001 M (2) valerian, (3) benzoic, and (4) caproic acids.
for the instantaneous 3D nucleation proceeding at diffusion limitations (Fig. 6).3
3 Note
that according to the 3D-nucleation models, irrespectively of ν value, all the linearized current curves should be extrapolated to the frame origin, which is not the case. This may have different reasons, namely, simplifications [7–10] used when constructing the corresponding theoretical nucleation models, inaccuracies in determining the characteristic parameter tcr, as well as the presence of an additional term in Eq. (1), which reflects the accumulated changes in the surface area.
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Table 1. Current transients of the anodic phase formation via the 2D and 3D nucleation stages Dim
Nuclei Current transient linearization growth coordinates conditions
Ref.
ph
⎛ i Au ⎞ 2+ν - = const1 – K1 t ph ln ⎜ --------1 + ν⎟ ⎝ t ph ⎠
2D Kinetic
[7]
ph
⎛ i Au⎞ 1+ν - = const2 – K2 t ph ln ⎜ -----ν ⎟ ⎝ t ph ⎠
Diffusion 3D Kinetic
ph
2+ν
ph
1 --- + ν 2 K4 t ph
[7, 8] [9, 10]
i Au = K3 t ph
Diffusion
i Au =
[7, 8]
Note: Dim is the nuclei growth geometry.
At small times of nuclei formation, the situation is less clear. Here, two variants are equally possible, namely, the instantaneous 3D nucleation at kinetic limitations and the progressive 3D nucleation at the surface-diffusion limitations (Fig. 7). In our opinion, much more probable is the change in the nuclei growth conditions (from the kinetic to the diffusion limitation) rather than the change in the way of activation of the potential nucleation sites. Taking into account the above facts, the part of diverse factors (XAu and ∆Ecr values, alloying, and the presence of SAOS) in the kinetics of the phase regrouping of gold on the surfaces of selectively dissolved Ag−Au alloys was quantitatively estimated based only on the data obtained at the heightened tph values under conditions when the phase formation mechanism did not generally ph
change. Not only the i Au currents (Figs. 2–5), but also the ph
iAu , µA/cm 2 30
K4 formal constant obtained from the slope of i Au −tph [1/2] dependence was analyzed. Note that, according to [7, 8], ph
1
K4 = constn0(Ds∆c)3/2,
25 20
where n0 is the number of active nucleation sites per unit surface, Ds is the surface diffusivity of gold adatoms, and ∆c is their concentration gradient on the electrode surface.
2
15 10
ph
3 4
5 0
0.3
5
6
7
8 9 1/2, s tph
Fig. 6. Chronoammograms of (1, 3) Ag10Au, (2) Ag15Au, and (4) Ag15Au0.5Ni alloys recorded in a nitrate solution containing (1, 2, 4) no SAOS and (3) 0.001 M valerian acid constructed in coordinates corresponding to the 3D nucleation at the diffusion limitation of the growth at the instantaneous activation.
ph
ph
2 (a) iAu , µA/cm 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 10 20 30 40 50 60 70 0 20 40
iAu , µA/cm 2 6 5 4 3 2 1 0
(2)
3/2 , s tph
Fig. 7.
(b)
The gold content in the alloy affects the Ecr, i Au , and ä4 values in a complex manner. For example, on going from Ag5Au to Ag15Au alloy, the phase transformation currents measured at ∆Öcr = 20 mV decrease (Fig. 3). However, with a further increase in the gold content to 20 and 30 at. %, the phase-transformation current density, which corresponds to the formation of the individual gold phase, substantially increases. More demonstrative are the changes in the ä parameter depending on XAu (Table 2). When the alloy potential excess over the critical value is quite small (∆Öcr = 10 mV), ä4 monotonically increases, so that the phase transformation rate also increases with an increase in the gold content in the alloy. At the higher ïAu values, ph
ä4, similarly to i Au , reaches a minimum at a gold conph
60 80 2 ,s tph
tent of 15 at.%. This character of i Au –XA dependence is probably caused by the differences in the effects produced by the alloy composition on the n0, Ds, and ∆Ò values. Note that the Ecr value itself increases sufficiently monotonically with an increase in the gold content in the alloy. Compared to the alloy composition, the role of ∆Ecr is more definite: for each studied alloy, an increase in this parameter resulted in a sharp increase in the phasetransformation current (Fig. 2a) and in an increase in ä4 parameter (Table 2). According to Eq. (2), this may be
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KINETICS OF PHASE TRANSFORMATIONS IN A BINARY ALLOY SUPERFICIAL LAYER Table 2. Dependence of the K4 parameter (µA/(cm2 s–1/2)) on the ∆Ecr value and the alloy composition in 0.1 M NaNO3 + 0.001 M HNO3 + 0.001 M AgNO3 XAu
Ecr, mV
5 10 15 20 25 30
770 825 925 1015 1085 1095
∆Ecr, mV 10
20
30
40
0.39 0.50 0.46 0.63 1.20 1.29
2.52 2.82 0.53 1.12 1.98 3.69
4.57 6.47 2.72 4.00 2.70 6.42
9.91 6.83 2.79 7.49 3.60 16.9
Table 3. The effect of the alloying additive (0.5 at. %) on the critical potential and K4 parameter (µA/(cm2 s–1/2)) of Ag15Au alloy in 0.1 M NaNO3 + 0.001 M HNO3 + 0.001 M AgNO3 Alloying component
Ecr, mV
– Ni Si Ti
925 935 945 930
∆Ecr, mV 10
30
40
0.46 – 0.13 0.01
2.72 0.78 0.61 0.49
2.79 0.79 2.37 1.12
Table 4. The effect of SAOS on the critical potential and K4 parameter (µA/(cm2 s–1/2)) of Ag10Au alloy in 0.1 M NaNO3 + 0.001 M HNO3 + 0.001 M AgNO3 SAOS – benzoic acid caproic acid valerian acid
∆Ecr, mV
Ecr, mV
10
20
30
40
825 830 835 840
0.50 0.41 0.20 0.36
2.82 0.48 0.41 0.73
6.47 0.88 0.74 1.57
6.83 5.46 1.30 –
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and, hence, in the concentration of gold ad-atoms when the critical potential is reached. As was already mentioned, introducing alloying microadditives (Ni, Si, or Ti) in the alloy, as well as adding surface active organic substances (valerian, caproic, or benzoic acid) to the solution, does not change the phase formation kinetics: in all the cases, the current-drop curves are linearized only when a 3D-nucleation model is assumed valid (Fig. 6). However, as one may expect, the current of the individual gold phase formation and the ä4 parameter noticeably decrease upon doping the alloy and in the presence of SAOS in the solution (Figs. 4–6, Tables 3 and 4). Being adsorbed on more active surface sites, organic acid molecules seemingly restrict the mobility of superficial gold atoms, which hampers their joining in an Au0 phase islets. Probably, for the same reason, the selective dissolution rate of silver from the alloy decreases, which decelerates the formation of overcritical vacancies, increases the Ecr value, and, finally, hampers the phase formation on the whole. At the silver dissolution potential, nickel, titanium, or silicon doping additives are oxidized to oxides, which probably hamper the Au* Au0 phase transformation in the surface layer of the dissolved Ag–Au alloy by increasing the critical potential of the alloy. Such an effect of the alloying additives was earlier discussed in [11] in the case of the decelerated copper phase transformation at the selective dissolution of β-and γ-brasses. Besides that, similarly to introducing SAOS into the solution [5], adding a third component, which differs in its properties from both silver and gold, to the alloy makes it possible to decrease the concentration of overequilibrium vacancies formed in the surface layer during the selective dissolution of silver, which results in the decrease in iAg already at Ö < Öcr. CONCLUSIONS
explained by the increase in the surface diffusivity of the gold ad-atoms and by the sharper change in their concentration.4 However, anyhow, this trend, as well as the fact that the higher the overpotential, the earlier the phase formation begins, indicates the decisive role of the selective dissolution of silver from Ag–Au alloys in the formation kinetics of the individual gold phase in the overcritical potential range. In fact, it is the dissolution of silver that results in a sharp increase in the concentration of vacancies in the surface layer of the alloy 4A
change in the number of the active nucleation sites on the alloy surface with the change in ∆Ecr value seems unlikely.
1. An assumption about the declivity of the solidphase diffusion and gold-phase regrouping currents on the surface of Ag–Au alloys during their selective dissolution at overcritical potentials, which was theoretically founded earlier, enables one to estimate the rate of phase transformations from the data of anodic chronoammetry. 2. At relatively small duration, the phase transformation of gold proceeds at diffusion limitations of the nuclei growth corresponding to the instantaneous 3D-nucleation model. At the initial stage of the process, 3D nuclei of the individual gold phase also form, but their growth is seemingly limited by the embedding of ad-atoms rather than the surface diffusion. 3. The anodic potential increases with respect to the critical value of the beginning of the surface develop-
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ment, while the phase transformation rate increases. The changes in the volume composition of the alloy nonmonotonically affect the phase transformation rate: it reaches minimum at XAu = 15 at. %. 4. Introducing valerian, caproic, or benzoic acid into the acidic nitrate solution, as well as doping the Ag–Au alloy with passivating Ti, Ni, or Si additives, does not change the general kinetic scheme of the phase transformation of gold in the nonequilibrium alloy surface layer. Both alloying and adding SAOS result in a substantial decrease in the phase transformation rate and an increase in the critical potential of the surface development of Ag–Au alloys. ACKNOWLEDGMENTS The work was financially supported by the Grant Council of the Russia President (project no. MK1426.2007.3) and the Russian Foundation for Basic Research (project no. 08-03-00194).
REFERENCES 1. Kozaderov, O.A., Koroleva, O.V., and Vvedenskii, A.V., Zashch. Met., 2009, vol. 45, no.1 p. 34. 2. Hansen, M. and Anderko, K., Constitution of Binary Alloys, New York: Genium Pub Corp, 1988. 3. Shcheblykina, G.E. and Bobrinskaya, E.V., Zashch. Met., 1998, vol. 34, no. 1, p. 11. 4. Vvedenskii, A.V., Kozaderov, O.A., and Koroleva, O.V., Korroziya: Materialy, Zashchita, 2007, vol. 3, p. 7. 5. Vvedenskii, A.V., Bobrinskaya, E.V., Marshakov, I.K., et al., Zashch. Met., 1993, vol. 29, no. 4, p. 561. 6. Dursun, A., Phil. Dr. Theses, Blacksburg, Virginia (USA), 2003. 7. Gamburg, Yu.D., Elektrokhimicheskaya kristallizatsiya metallov i splavov (Electrochemical Crystallization of Metals and Alloys), Moscow: Yanus, 1997. 8. Armstrong, R.D. and Harrison, J.D., J. Electrochem. Soc., 1969, vol. 116, no. 3, p. 328. 9. Isaev, V.A. and Baraboshkin, A.N., Elektrokhimiya, 1985, vol. 21, p. 960. 10. Isaev, V.A. and Baraboshkin, A.N., Elektrokhimiya, 1994, vol. 30, p. 227. 11. Marshakov, I.K., Karavaeva, A.P., and Saryan, S.A., Zashch. Met., 1970, vol. 6, no. 2, p. 241.
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