Relations of the surface structure to the kinetics of metal electrodes

Abteilung Korrosion und Korrosionsschutz, Institut für Metallkunde und Metallphysik, Technische Universität Clausthal, D 3392 Clausthal-Zellerfeld, Germany.

Abstract

The electrochemical dissolution and deposition of metals proceeds by ion transfer at kinks in monoatomic steps according to experimental evidence from dislocation-free surfaces of silver and iron. The mean distance of kinks in steps on silver is comparable to the distance of neighbouring atoms. Due to the relatively large specific edge energy the mean equilibrium distances of kinks in steps on iron surfaces are very long. The surface concentration of kinks growing with overpotential and pH is due to a special mechanism of generating monoatomic steps and kinks. Specific edge energies are obtained by investigating twodimensional nucleation, and growth or dissolution at screw dislocations terminating at the surface. Edge dislocations and dislocation loops terminating at the surface are sites of preferential nucleation of etch pits as shown by experiments with iron single crystals.

Lattice defects influencing the rate of ion transfer can be created by electrochemical processes. The dissolution of atomic hydrogen into iron produces mechanical stresses relaxing by formation of dislocation loops. During the dissolution of alloys the faster dissolving component may also dissolve from complete steps thereby creating pairs of kinks or injecting vacancies which facilitate interdiffusion.