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...

For any particular stainless steel, a critical pitting temperature (CPT) can be measured, below which stable pits do not occur at any potential up to the onset of transpassivity. The more highly alloyed the steel, particularly with molybdenum, the higher the CPT. The CPT for 904L austenitic stainless steel with a 240 grit surface finish has been determined as 48 to 4g°C, and metastable pitting events have been analyzed in detail at lower temperatures. Below the CPT, metastable pitting activity peaks at around 300 mV (SCE), but occurs at all potentials up to the transpassive range. The morphology of these pits has been examined using SEM and a model is proposed in which precipitation of an anodic salt film within pits is the critical factor; above the CPT, a salt film is essential for stable pit growth, while below the CPT, the salt is an intermediary in oxide passivation, like that of iron in sulfuric acid. This is an outcome of a complex dynamical system and does not require the properties of the salt itself to change suddenly with temperature. ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 169.230.243.252 Downloaded on 2014-12-13 to IP ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 169.230.243.252 Downloaded on 2014-12-13 to IP

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The pitting behavior of sputter-deposited Al binary alloy thin films was studied. Pitting and repassivation potentials were determined in 0.1M NaCl for samples in freshly deposited and air-aged states. Aging for several years in laboratory air increased the pitting potential for some of the alloy systems but had no effect on others. The repassivation potentials, meaningful values for pits in thin films, were found to be very close to the pitting potentials of freshly-deposited films for many alloy systems. Stable pits initiate in these Al binary alloys at potentials just above the value at which they would repassivate, indicating that pit growth considerations control the pitting process. By determining the pit anodic current density just before passivation it is shown that alloying improves pitting resistance through a reduction in the ability of pits to maintain the critical local environment necessary for growth. The influences of alloying on the passive film chemistry and on the tendency of the metal to repassivate (depassivation pH) are secondary in nature.Sputter-deposited supersaturated Al binary alloys have been shown to exhibit remarkable pitting resistance in chloride solutions.

Corrosion pits on stainless steel often grow in such a way as to undermine the original surface, and, consequently, a thin layer of metal and oxide is left over the growing pit cavity. These pit covers develop a porous, lacelike structure, and a model for the development of this structure is described. Calculations based on this model produce pit morphologies showing the main features of real pits observed in the scanning electron microscope and also produce theoretical current transients resembling those recorded experimentally.