Extrinsic Parting Limit for Dealloying of Cu-Rh

Abstract

Electrochemical dealloying of Cu-Rh alloys was systematically studied in 1 M H2SO4 aqueous solution in order to clarify the effect of Rh oxide passivation on dealloying behavior. Active dealloying, oxide passivation and transpassive dealloying regions were consecutively identified in their anodic polarization curves. The dealloying is rapid in both active and transpassive regions. The active dealloying region vanishes as Cu content decreases to below 85 at.%. When the Cu content is further decreased to below 75 at.%, the transpassive dealloying also ceases and the material becomes “stainless”. This indicates that the parting limit for the dealloying of Cu-Rh is located at ∼75 at.%, which is much higher than that of Au-Ag and other alloys that do not involve oxide passivation in dealloying. The high parting limit for Cu-Rh is associated with a passive layer of Rh oxide formed by corrosion, which hinders dealloying and nanoporosity evolution processes. The parting limit identified here may be an extrinsic property. The intrinsic parting limit of this alloy, i.e., the threshold composition below which the dealloying is hindered by a passive layer of Rh (instead of oxide passivation), could be lower – it may be obtained by avoiding any form of oxide passivation in corrosion. Moreover, all dealloyed nanoporous Rh shows a face centered cubic crystal structure, in contrast to the hexagonal close packed structure reported in Rh nanoparticles.