Corrosion micromorphology of noble metal alloys and depletion gilding

Abstract

Noble metal alloys commonly undergo selective dissolution during anodic corrosion whereby the less noble component is dissolved preferentially. This is particularly interesting, as it leads to a depletion of the less noble species not only at the surface of the metal but also at an appreciable depth within it, implying that a mass transport of atoms is occurring either through the bulk of the crystal lattice or across the surface to support the continuing dissolution. Experiments in which the micromorphology of silver–gold alloys is observed during such corrosive treatment have shown that surface diffusion of gold plays an important part in this process. As surface silver atoms are dissolved the residual gold atoms reform into gold-rich islands so that fresh silver atoms are continuously exposed to the environment layer by layer. These observations will be described in detail elsewhere¹. Here the mechanism of surface re-arrangement is considered, which leads to the accumulation of gold-rich surface regions, and throws a new light on the ancient craft of depletion gilding.