Abstract
Pit current densities (CDs) in small pits have been found to be potential dependent, indicating an ohmic or mixed ohmic-charge transfer controlled dissolution mechanism. In a later stage of pit growth, a diffusion-controlled dissolution has often been reported. The potential-dependent mechanism is not always restricted to only the beginning of pit development, as has been shown in galvanostatic tests with rather low CDs.
In this paper, the mechanism change from ohmic-charge transfer to diffusion-controlled pit growth was also studied in potentiostatic experiments by means of potential pulses added to the applied potential. The current response to fast potential changes differs for the two mechanisms.
The influence of a number of parameters [such as potential, position of the specimen (horizontal or vertical), agitation of the bulk electrolyte, and pit diameter] on the transition time for mechanism change could be investigated using this pulse technique.
Results also show that the rate law for pit growth can be very similar for the two types of mechanisms and different pit geometry (artificial pits of the wire type or “naturally” growing pits).
