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 chlor ide above the pitting potential are reported. The STM measurements revealed pits with a lateral size of 43 +/- 13 nm, a depth of 3 +/- 1 nm, and a den sity of similar to3 x 10(8) cm(-2), not observable by normal current transi ent measurements. The atomic structure observed inside the pits was crystal line, with the same symmetry and parameters as the crystalline structure of the passivated surface prior to exposure to chloride. This structure was a lso observed in the nonpitted areas exposed to chloride. The location of th e pits at the grain boundaries of the crystalline structure of the passive film and their preferential elongation in the direction of the substrate te rraces show the deterministic role of the passive film structure and substr ate structure in pit initiation. (C) 2000 The Electrochemical Society. S109 9-0062(00)07-072-3. All rights reserved.