Mass transfer and electrochemical kinetic interactions in localized pitting corrosion

Abstract

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...

Effects of environmental factors on key kinetic parameters relevant to pitting corrosion

“…This value of D m was seen to be comparable to values reported by Kuo and Landolt [53], Gaudet et al [50], and Ernst and Newman [57].…”

“…Finally, as bulk concentration increases, the chloride concentration gradient between the pit base and the bulk disappears and even reverses at very high chloride concentrations. ] at the pit base may be lower than that at saturation [49][50][51][52] is considered.…”

Nonequilibrium collective phenomena in the onset of pitting corrosion

“…Many authors consider stabilization to be associated with a single weakest or most favorable site that attains the conditions necessary to grow steadily (17)(18)(19). Stabilization occurs when the pit dissolution, cover, pit shape, surface roughness, and salt films all conspire to form a pit environment more aggressive than the critical chemistry needed to sustain pit growth (2,18,20,21).…”

A study of the behavior of bromide in artificial pits using in situ X-ray microprobe analysis

“…Local changes in solution composition are the dominant factors in the initiation and propagation of pits in stainless steels (10)(11)(12)(13)(14)(15). In contrast, many theories have been proposed for the breakdown of passivity but these are usually developed only up to the point where passivity is disrupted and assume that corrosion will continue because of active dissolution.…”