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Before the causes of passivation of impure commercial copper anodes can be addressed, an understanding of the mechanism of copper dissolution is needed. Copper appears to dissolve by a two-electron transfer (Eq. 1). However, the likelihood of this occurring is minimal based on statistical thermodynamics and a series of single electron reactions is more likely. Mattsson and Bockris were the first to indicate that the reaction occurs by the two single electron steps with one being the rate-determining step. In a sulfate medium, the reactions occur as shown in (Eq. 8) and (Eq. 9).Cu ® Cu+ + e(Eq. 8) Cu+ ® Cu2+ + e(Eq. 9)
The reaction between the cuprous and cupric ions was declared as the rate determined step based on the anodic and cathodic transfer coefficients being asymmetric, 1.64 +/- 0.25 and 0.46, respectively. Stankovic more recently verified these transfer coefficients in copper and sulfuric acid concentrations similar to those observed in commercial copper electrorefining.
Bockris and Enyo and Bockris and Kita proposed that the rate-determining step is controlled by charge transfer or surface diffusion at low current densities and only charge transfer at high current densities. Slaiman and Lorenz used a double-pulse galvanostatic method to illustrate that at low current densities the overpotential causing the slowness of (Eq. 9) contains charge transfer and surface diffusion components. The preference appeared to be related to the experimental conditions. De Agostini, Schmidt, and Lorenz indicated that the oxidation of cuprous ions may be occurring between cuprous ad-ions on the surface and cupric ions in solution. The mechanism of the reaction was investigated as a function of current density by Jardy et al. using a quartz crystal microbalance and rotating ring-disk electrode. It was shown that the dissolution valence was a function of current density and that the mechanism for copper dissolution in 0.1 M Na2SO4 acidified to pH 1.5 was:Cu ® Cu+(ads) + e(Eq. 10)
at current densities less than 10-6 A/cm2:Cu+(ads) ® Cu+(sol)(Eq. 11) Cu+(sol) ® Cu2+(sol) + e(Eq. 12)
at current densities greater than 10-2 A/cm2:Cu+(ads) ® Cu2+(sol) + e(Eq. 13)
It is readily observed that the complexity of the dissolution mechanism of pure copper leads to some inherent difficulties when discussing the passivation of impure copper anodes.
To further complicate the dissolution mechanism is the ability of copper to disproportionate. At high current densities, cuprous ions can be generated so quickly that they react with each other (Eq. 13). This provides yet another possible chemical or electrochemical route for copper dissolution.2 Cu+(sol) ® Cu2+(sol) + Cuo(Eq. 13)
Anode Passivation Main Page
Passivation of Pure Copper
Passivation of Impure Copper Anodes
Secondary Phases Within the Anode
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