Selective dissolution below the critical potential and back alloying in copper-gold alloy

Below the critical potential, E sub c , selective dissolution of Cu from Cu-18 atom % Au proceeds via Cu transport through an essentially planar Au-rich product layer, which over time increases to hundreds of atom layers in thickness. Transmission electron micrographs (TEM) taken after the selective dissolution (at E < E sub c ) and subsequent (current-off) back alloying processes reveal nearly uniform Moire patterns characteristic of the superposition of two layers with similar crystal structures, i.e., the Cu-depleted (Au-rich) product layer and the parent alloy. The Moire fringe spacing decreases with the time of Cu dissolution (for approx =60 s), indicating solid-solution Au enrichment in the product layer during dissolution. The Moire patterns give no indication of discrete islands of residual Au. The decreasing Cu dissolution rate follows the parabolic law for the first 60 s with an effective diffusivity of Cu in the product layer of approx =10 exp -13 cm exp 2 s exp -1 at 23 deg C. The TEMs also show a few pits, whose diameter is in the < 10 nm range and whose number increases with time, which may account for the higher rate of Cu dissolution (than given by the parabolic law) after 1 min.