Identification and characterization of coupled diffusional and electrochemical kinetics effects was achieved under potentiostatic anodic dissolution conditions. A one-dimensional artificial pit geometry with sample wire electrodes embedded in an inert support exposed to NaCl solutions was used to study the dissolution of stainless steel and highnickel Alloy 600. Multiple steady states for both materials were determined at conditions where the diffusional transport rates balanced the electrochemical rate of dissolution at the surface of the wire electrode. A theoretical transport model was developed to quantitatively explain the observed multiple steady state phenomena.
G. T. Gaudet
SCOPECoupling of the rates of mass transfer and chemical reaction is common in systems of interest to chemical engineers. One example of this occurs in pitting corrosion. In the presence of chloride ions, pitting corrosion of alloys such as stainless steel occurs only when the electrochemical potential exceeds a minimum value called the critical pitting potential. Tester and lsaacs (1975) and Beck (1973) suggested that the corrosion rate in this range of potential is possibly controlled by the diffusion rate of metal ions out of the pit, since the true metal dissolution rate is much faster. Diffusion control was experimentally verified for potentiostatic conditions above the critical pitting potential using a one-dimensional artificial pit consisting of a metal wire mounted in an inert support with its top surface exposed to a stagnant chloride solution.In later experiments by Newman and lsaacs (1983) the potential of an artificial pit undergoing quasi-steady dissolution was suddenly lowered to below the critical value. The dissolution rate was thereby reduced, and diffusion was no longer the single rate-limiting process. The observed rapid decline in current after a short induction period at the lower potential suggested the possible existence of multiple steady states where the rates of diffusion and metal dissolution reaction were balanced.In this work, the transient coupling of diffusion and electrochemical reaction were examined, both theoretically and experimentally, to verify the existence of multiple steady states of pitting corrosion, where different current densities (metal dissolution rates or corrosion rates) occur under the same operating conditions.
CONCLUSIONS AND SIGNIFICANCEBy imposing a potential step change on a wire electrode exposed to a 1 mol/L sodium chloride solution, current-time data were collected: these data were then reduced using a diffusion model. The results show that at the higher potentials metal ion diffusion is adequately described by Fick's law with a single effective diffusivity, in which the effects of ion concentration on the diffusion coefficient and electromigration under a potential gradient are taken into account. The current-
AIChE JournalJune 1986 time data were also corrected for the effect of electrical resistance changes in the pit as the wire dissolved and the diffusion length increa...
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