Primary and Secondary Dealloying of Au(Pt)-Ag: Structural and Compositional Evolutions, and Volume Shrinkage.

Systematic study of the dealloying of Au-Ag and Au(Pt)-Ag alloys shows that the dealloying occurs by two processes: a primary dealloying process that selectively dissolves Ag from the parent alloy and creates a nanoporous (np) structure, and a secondary dealloying process that occurs behind the corrosion front and further dissolves the residual Ag from the nano-ligaments. The secondary dealloying can occur during coarsening, and/or when a more anodic potential is applied. With suppressed np structure coarsening in Pt-containing samples, we found that the intrinsic np structure created by the primary dealloying contains small ligament diameter (3-7 nm) and high concentration of residual Ag (~50 at.%), irrespective of the dealloying potentials. Dilatometry experiments show that the volume shrinkages are rather small during primary dealloying and are large during secondary dealloying. The primary dealloying can be explained by the percolation dissolution mechanism. Although the mechanism of the secondary dealloying (without coarsening) remains unclear, we point out that the kinetics of the secondary dealloying is decisive to some important characters of np metals, such as the crack formation and the final residual Ag concentrations. [ABSTRACT FROM AUTHOR]