Atomic Structure of Metastable Pits Formed on Nickel

Scanning tunneling microscopy (STM) measurements of the atomic structure of metastable pits formed on a Ni(111) single-crystal surface prepassivated in a chloride-free sulfuric acid solution and subsequently exposed to chloride above the pitting potential are reported. The STM measurements revealed pits with a lateral size of a depth of and a density of not observable by normal current transient measurements. The atomic structure observed inside the pits was crystalline, with the same symmetry and parameters as the crystalline structure of the passivated surface prior to exposure to chloride. This structure was also observed in the nonpitted areas exposed to chloride. The location of the pits at the grain boundaries of the crystalline structure of the passive film and their preferential elongation in the direction of the substrate terraces show the deterministic role of the passive film structure and substrate structure in pit initiation.