Study of the corrosion behavior of a Cu-Sn-Ni alloy by anodic stripping voltammetry coupled with spectroscopic techniques W. Rubin 1 , I.A. Carlos 1 1 Department of Chemistry - UFSCar, São Carlos - SP - Brazil ABSTRACT The electrodeposition process is an interesting option for the production of Cu-Sn-Ni alloy since, commonly, they are produced by casting. The literature shows that the anodic stripping voltammetry (ASV) is very sensitive to study the type of alloy electroplated. Thus, the ASV of the Cu-Sn-Ni deposits was performed in order to investigate its dissolution process in HCl solution, i.e., a very corrosive environment. For this study, the anodic voltammetric curves of the electrodeposits were performed in different HCl concentrations and also at various potential sweep rates in order to assess if the corrosion products of the alloy are influenced by the electrolyte concentration and / or the sweep rate, which simulate the time action. The remaining deposits after each dissolution peak were investigated by SEM, EDX and XRD. Keywords: Cu-Sn-Ni, electrodeposition, anodic stripping voltammetry, aspartic acid, XRD. 1- INTRODUCTION The Cu-Sn-Ni alloy is an alternative to the Cu-Sn-Pb alloy, which is widely used in high-load bearings in many applications due to its high resistance and therefore used as an alternative to bronze to protect parts steel [1]. The electroplating process is an interesting way for the formation of these alloys, since generally its production is done by casting. The electrodeposition of metal alloys can produce different structures in the also the electrodeposit, not always corresponding to the equilibrium phase diagrams. Anodic stripping voltammetry (ASV) is very sensitive to the type of alloy electrodeposited, so, the study of the dissolution process of the Cu-Sn-Ni alloy by ASV becomes important due to its applications. The aim of this study was to use the ASV technique for the investigation of the dissolution process of Cu- Sn-Ni electrodeposits produced without or with aspartic acid as an additive in the plating bath in order to stydy the effect of this additive on the properties of the electrodeposits. For this study, the anodic voltammetric curves of the deposits were performed in different HCl concentrations, i.e. a very corrosive environment, and also at various sweep rates in order to assess whether the corrosion products of the alloy are influenced by the electrolyte concentration and also the sweep rate (simulating the action of time). Thus, the ASV with X-ray diffraction spectroscopy (XRD) was used to investigation of the dissolution process of Cu-Sn-Ni electrodeposits. 2- EXPERIMENTAL Encontro e Exposição Brasileira de tratamento de superficie III INTERFINISH Latino Americano 193
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Study of the corrosion behavior of a Cu-Sn-Ni alloy by anodic stripping voltammetry coupled with spectroscopic techniques
W. Rubin1, I.A. Carlos1
1 Department of Chemistry - UFSCar, São Carlos - SP - Brazil
ABSTRACT
The electrodeposition process is an interesting option for the production of Cu-Sn-Ni alloy since, commonly,
they are produced by casting. The literature shows that the anodic stripping voltammetry (ASV) is very sensitive
to study the type of alloy electroplated. Thus, the ASV of the Cu-Sn-Ni deposits was performed in order to
investigate its dissolution process in HCl solution, i.e., a very corrosive environment. For this study, the anodic
voltammetric curves of the electrodeposits were performed in different HCl concentrations and also at various
potential sweep rates in order to assess if the corrosion products of the alloy are influenced by the electrolyte
concentration and / or the sweep rate, which simulate the time action. The remaining deposits after each
dissolution peak were investigated by SEM, EDX and XRD.
Firstly, the influence of HCl concentration to be used in the dissolution studies of the electrodeposits
(produced with or without aspartic acid, as an additive) was investigated. The micrographs of the deposits
showed that they were without cracks or dendrites and also recovered totally the Pt substrate. EDS analysis of
the electr od eposi ts showed that t he deposit was compos ed of 4.1 wt % O, 3. 5 wt% Ni , 24.3 wt % Cu, 68.1 wt %
Sn. Also, DRX analysis showed that both deposits were formed of a mixture of Sn, Cu6Sn5, Ni3Sn2 and CuNi2Sn.
The ASV curves of Cu-Sn-Ni-asp electrodeposits in various HCl concentrations, at v = 10 mV s-1, are
shown in Figure 1. It can be seen that they were similar for HCl concentration 0.8 mol L-1 or 1.0 mol L-1, being
that for the latter the area of anodic peak was the highest among the HCl concentrations investigated. On
contrary, for HCl concentration 2.0 mol L-1, small areas of anodic peaks were obtained during voltammetric
dissolution. Moreover, when the eletrodeposits were submitted to voltammetric dissolution in HCl 0.5 mol L-1,
they did not dissolve by complete (see by visual inspection at naked eye), even decreasing v at 5.0 mV s-1. Also,
it can be seen that the initial dissolution potential is more negative for low HCl concentration. Also, by
comparing the current efficiency (CE) of voltammetric dissolution of Cu-Sn-Ni and Cu-Sn-Ni-asp (inset in Fig.
1), it can be seen that the highest CE was obtained when the electrodeposits was dissolved in HCl 0.8 mol L-1.
Also, the smaller CE was for HCl 2.0 mol L-1, indicating, in this case, significant chemical dissolution of the
electrodeposit. In addition, it can be seen that the CE was always lower than 100%, due to the hydrogen
evolution reaction (HER) in parallel to deposition process. These results showed that the voltammetric
dissolution of the electrodeposits depends on the HCl concentration.
Figure 1 – Anodic stripping voltammetric curves of Cu-Sn-Ni-asp electrodeposits dissolution in various HCl concentrations, i.e., 0.5 mol L-
1 (__), 0.8 mol L-1 (---), 1.0 mol L-1 (…) and 2.0 mol L-1 (-.-) at v = 10 mV s-1. The insert shows the current efficiency of each HCl concentration.
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Figure 2 shows ASV curves of Cu-asp (---), Sn-asp (…), and Ni-asp (-.-) and Cu-Sn-Ni-asp (__) deposits
in HCl 0.8 mol L-1. Cu-asp and Sn-asp electrodeposits were obtained potentiostatically at Ed = -0.8 V with qd of
1.02 C cm-2 and 2.55 C cm-2, respectively, while the Ni-asp electrodeposit was obtained voltammetrically from
0.0 V until final potential of -1.7 V (to activate more Pt sites to deposition so that deposition of Ni occurs),
returning until current zero, qd = 21.0 C cm-2.
Figure 2 – ASV curves of Cu-Sn-Ni-asp (___), Cu-asp (---), Sn-
asp (...) and Ni-asp (-.-) electrodeposits in HCl 0.8 mol L-1 at v=10 mV s-1. Pt substrate.
ASV curves of Sn-asp and Ni-asp show an anodic peak at Ep= -0.56 V and at Ep= -0.09 V, respectively,
while for Cu-asp two anodic peaks were formed, i.e., Ep= -0.15 V and at Ep= +0.24 V, due to oxidation of Cu to
Cu+, forming CuCl, and after the oxidation to Cu2+.
Comparing Cu-Sn-Ni-asp dissolution voltammetric curve (__) and those of pure metals, it can be seen
that the Sn and compounds containing Sn (Cu6Sn5, Ni3Sn2 and CuNi2Sn) initiated to dissolve in the region of
anodic peak a. However, as can be seen latter, compounds containing Cu and Ni also initiated to dissolve,
already, in the region of peak a. In ord er to evaluate t he di ss ol ut ion process of Cu -Sn-Ni-as p electrod eposi ts, volt ammetric di ssol ut ions
of they in HC l 0.8 mol L-1 at 10.0 mV s-1 wer e perf or med. Th en, anal ysis of t he remaining deposits after each
voltammetric dissolution peak, i.e., anodic peaks a and b were analyzed by SEM (Fig. 2), EDS and XRD.
Fig. 2 shows micrographs of the remaining deposits after dissolution of peaks a and b. Figures 2(a) and
2(b) refers to Cu-Sn-Ni (produced without the additive) after voltammetric dissolution of deposits up to
formation of peaks a and b, respectively. Similarly, Fig. 2(c) and 2(d) refers to Cu-Sn-Ni-asp deposits after
voltammetric dissolution of deposits up to formation of peaks a and b, respectively. Comparing the micrographs
of deposits after dissolution peak a, it can be seen that they proved to be porous, dendritic and also, Pt substrate
was exposed. In turn, comparing the deposits remaining after dissolution of the peak b it can be inferred that the
morphologies of these electrodeposits are quite different, ie, Cu-Sn-Ni showed only a few isolated crystallites
distributed over the Pt surface (b), while Cu-Sn-Ni-asp (d) showed deposit with many cracks but still covering
the Pt substrate. So, the presence of aspartic acid seems to produce a deposit that possbly better protect the
substrate against corrosion.
Figure 2 – Micrographs of the remaining electrodeposits after dissolution of peak a (a, c) and b (b, d) in HCl 0.8 mol L-1. The deposits were obtained with (a and b) or without the additive (c and d). v = 10 mV s-1.
a
b
c
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The XRD analysis of Cu-Sn-Ni or Cu-Sn-Ni-asp electrodeposits, after the dissolution peaks a and b,
shows that the Sn and Cu6Sn5 phases would be dissolving in the region of peak a, as can be seen by the
suppression of the diffractogram peaks for these phases (Fig. not shown here). So, the remaining electrodeposit
was composed of SnO, result of Sn dissolution/passivation, Ni3Sn2 and CuNi2Sn. In turn, after the dissolution of
peak b, the difractogram shows that the deposit was composed only of SnO.
In addition, EDS analysis were performed on Cu-Sn-Ni and Cu-Sn-Ni-asp electrodeposits, as produced
and after the dissolution peaks a and b and it can be verified that even after dissolution process a and b, there
was still Cu, Sn and Ni in the eletrodeposit. This can be explained by the fact of the different phases formed have
different energies so different dissolution potentials too.
4- CONCLUSIONS
Comparing the results obtained from voltammetric curves and those from XRD and EDX it can be
inferred that in the region of the dissolution peak a, the dissolution/passivation of Sn together with Cu6Sn5
dissolution occurred. According to the XRD analysis, the remaining deposit was composed of SnO, Ni3Sn2,
CuNi2Sn which would be oxidizing in the region of peak b, where Ni oxidized to Ni2+ and Cu to Cu+, forming a
CuCl film. Finally, the remaining deposit after dissolution of peak b, was composed only of SnO according to
XRD but, EDX analysis showed that Cu, Sn and Ni still remains in the electrodeposit, so, in the region of peak c,
these species would be oxidizing since after that, no electrodeposit was seen at Pt substrate. Also, from XRD
analysis after each dissolution process it can be concluded that the protective character of this alloy is shown by
the formation of CuCl and SnO (peak a) and SnO2 (peak b) during the dissolution of the same in this aggressive
envi ron ment. SE M, EDX and XRD analysis aft er the dissolution of the peak b showed that as partic aci d was
advantageous in the deposition process (allowing the codeposition of metals and the formation of the ternary
alloy), since after deposit dissolution these films were more compact and homogeneous. Furthermore, EDS
results of remaining deposit after peak b, led to suggest that this film still contain CuNi2Sn alloy, indicating that
this alloy has excellent resistance to corrosive environments.
5- ACKNOWLEDGMENTS: FAPESP, CAPES e CNPq.
6- REFERENCES
1- www.mahle.com – accessed September 15th 2011
2- G. A. Finazzi, Tese de doutorado (Orientadora: I. A. Carlos), UFSCar, 2004.
3- I. A. Carlos, C. A. C. Souza, E. M. J. A. Pallone, R. H. P. Francisco, V. Cardoso, B. S. Lima-Neto. J.
Appl. Electrochem. 30: 987-994, 2000.
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Technol., 192, 2-3, 2005, 145-153.
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6- A. C. M. Moraes, J. L.P.Siqueira, L.L.Barbosa, I.A.Carlos, J. Appl. Electrochem. DOI.10.1007/s10800-
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