The effect of 180 °C-annealing on the electrochemical behavior of nano-thick zinc-electroplated copper in aqueous solutions with different pH

The anodic polarization and a.c. impedance measurements of 4 nm- and 25 nm-zinc-electroplated copper specimens were conducted in aqueous solutions with four different pH-values. The nano-thick zinc-electroplated copper was tested after electroplating and annealing at 180 °C for 1, 2 and 3 h, respectively. The results showed that the open-circuit potential (OCP) of 25 nm-zinc electroplated copper shifted toward noble potential after annealing. A more noble OCP of 25 nm zinc-electroplated copper could be achieved when zinc plated copper was annealed at 180 °C up to 1 h. Similar change was found for 4 nm-zinc electroplated copper only in neutral and alkaline solutions. The a.c. impedance response of all nano-thick zinc-electroplated copper corresponded to two Randle's circuits in series, in which the circuit measured in a high frequency region of their Nyquist diagrams revealed the electrochemical behavior of nano-thick zinc deposits. The charge transfer resistance of the nano-thick zinc electroplated copper was significantly raised after annealing at 180 °C when tested in pH 9.5 solution. Microstructures of the aforementioned nano-thick zinc electroplated copper were examined with cross-sectional TEM specimens. A distinct phase interface between zinc and copper was observed for as plated specimens, while alloying of zinc and copper at the interface was detected after annealing at 180 °C for 1 h. Electroplated zinc diffused into the copper foil during the 180 °C-annealing and the corrosion potential of the anodic polarization curve indicated the condition of the alloy surface. The annealing effect of 4 nm- and 25 nm-zinc electroplated copper specimens could be related to the results of electrochemical measurement.