Anisotropic step etching and etch instabilities during cathodic corrosion on platinum

The process of cathodic corrosion on platinum electrodes in aqueous environments is inevitably obscured by concurrent hydrogen evolution. Insights into the corrosion mechanism and progression are therefore most straightforwardly obtained through post-corrosion characterization. In this work we use ex-situ microscopy to evaluate the progression of etch pits on (111) and (100) facets on single crystal platinum electrodes. While the corrosion process has previously been shown to be anisotropic, we observe that etching mainly occurs at undercoordinated step edges, rather than into the plane. Such a dependence on the local energetics of the platinum surface atoms is also evident by a pit nucleation barrier on the (100) facet, where nucleation occurs preferentially on dislocations in the crystal lattice. On the (100) facet we also observe an etching instability which results in both rapid downward etching and material redeposition. The observed nucleation-and-growth nature of cathodic corrosion implies the possibility of directing the process using methods known from electrodeposition.