Galvanic coupling between copper and aluminium in a thin-layer cell
Introduction
In a previous study [1], a simple system consisting of a pure aluminium–pure copper couple was considered to understand the corrosion phenomena associated with copper-rich intermetallics in aluminium alloys. During immersion of the Al–Cu disk electrode in an aqueous electrolyte (10−3 M Na2SO4), aluminium is the anode and is in the passive state while copper is polarized cathodically. After 24 h of immersion, the formation of a copper deposit on the Al electrode, which is similarly observed during corrosion on commercial copper-rich aluminium alloys [2], [3], [4], [5], was clearly shown. This phenomenon was explained by the formation of an occluded zone at the Al–Cu interface, in which the local chemical reactions enable copper corrosion.
The aim of the present study is to verify that the corrosion of a copper electrode can occur even though it is polarized cathodically at the corrosion potential of the Al/Cu couple by using a thin-layer cell in order to model the occluded zone. The use of a scanning electrochemical microscope [6], [7] allowed the thin-layer cell dimension, i.e., the thickness of the thin layer, to be precisely controlled in the micrometer range. An electrochemical quartz crystal microbalance was used to quantify the copper dissolution rate.
Section snippets
SECM–EQCM set-up
All experiments were performed with a home-made SECM set-up schematized in Fig. 1A. It consisted of a 3-axis positioning system (VP-25XA, Newport) driven by a motion encoder (ESP300, Newport) allowing a spatial resolution of 100 nm in the three directions. Electrochemical measurements were performed with a home-made bi-potentiostat coupled with a low noise current-to-voltage converter (Femto DLPCA 200) and controlled by a multifunction data acquisition card (National Instrument).
The
Results and discussion
Fig. 2 shows the current response to a potential step applied at the copper electrode from −0.310 V/SSE (corresponding to the corrosion potential of a pure copper electrode) down to −0.745 V/SSE (corresponding to the corrosion potential of the Al/Cu couple measured in a preliminary experiment). This experiment was performed in a thin-layer cell formed by the ring-disk microprobe, the copper electrode, and a 10 mM Na2SO4 solution film between the electrodes. The thin-layer thickness was set at 75
Conclusions
The galvanic coupling of the Al/Cu system was studied using the combination of the SECM and the EQCM. The results obtained indisputably show the copper dissolution even if the potential of the system is negative with respect to the corrosion potential of pure copper. It was shown that the Al can also dissolve, which was observed by the presence of alumina clusters on the Cu electrode. In the absence of the Al electrode, or when the distance between the two metals was too large, no copper
References (24)
- et al.
Corros. Sci.
(1997) - et al.
Electrochim. Acta
(2007) - et al.
Appl. Surf. Sci.
(2006) - et al.
Electrochim. Acta
(2000) - et al.
Corros. Sci.
(1997) - et al.
Electrochem. Commun.
(2005) - et al.
J. Electrochem. Soc.
(2008) - et al.
J. Electrochem. Soc.
(2003) - et al.
J. Electrochem. Soc.
(2001) - et al.
J. Electrochem. Soc.
(2006)