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...

The importance of diffusion was investigated under potentiostatic dissolution conditions with wire electrodes contained in inert supports. The artificial cavities created simulated localized corrosion conditions. Current‐time behavior at voltages in excess of the critical pitting potential [>0.5V (SCE)] was examined for nickel and stainless steel specimens in concentrated chloride solutions. The effect of changing the concentration or activity gradient of the dissolving metal cations within the artificial cavity was studied by altering the composition of the bulk solution. Solutions of FeCl2 , NiCl2 , CrCl3 , normalLiCl , normalNaCl , MgCl2 , and CeCl3 ranging from 0.5 to 10M were used. Mass transfer models were developed for the observed transient and quasi steady‐state periods of dissolution.

An existing, validated elementary reaction model for hydrogen oxidation in supercritical water, with theoretically consistent modifications for high pressure, was expanded to allow modeling of carbon monoxide oxidation. The carbon monoxide model was less successful, exhibiting a higher overall activation energy than the data and lacking an oxygen dependence. Data for fuel-rich CO oxidation, including hydrogen formation, were well predicted, but results for fuel-lean conditions were not correctly predicted. Both models, when extended to subcritical conditions, successfully reproduced the majority of the experimentally observed pressure (waterdensity) dependence. The primary effect of the high water density on oxidation kinetics is the increase in the rate of the HO2 + H20 -H202 + OH reaction, which is effectively a branching step; the dissociations of hydrogen peroxide (H202) and the hydroperoxyl radical (HO2) are also at or near their high-pressure limits.The increase in the rate of the branching reaction with increasing water density accounts for the majority of the oxidation pressure dependence. The models for hydrogen and carbon monoxide oxidation exhibited high sensitivities to the rate constant and equilibrium constant for the branching reaction, and experimental data could be reproduced only if the value of this rate constant in supercritical water is significantly lower than its most probable value based on gas-phase measurements.

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