One-Dimensional Pit Experiments and Modeling to Determine Critical Factors for Pit Stability and Repassivation

Abstract

Experiments on stainless steel artificial pit electrodes in sodium chloride were used to inform a diffusion model developed based on the mass transport behavior within a one-dimensional corroding pit. Measurable estimates of the dissolution flux as well as the potential describing the conditions of interest were obtained from experiment as the one-dimensional pit stability product under a salt film (i · x) saltfilm and the repassivation potential E rp , respectively. These parameter estimates were acquired as a function of pit depth and were related to the concentration of the metal cation at the corroding surface at each depth via a one-dimensional mass transport model. These results allowed for the construction of a quantitative framework relating the various electrochemical parameters describing the transition from pit stability to repassivation. Such an analysis permitted the straightforward estimate of the critical surface concentration associated with this transition, which resulted in a single conservative lower bound of 50% of the saturation concentration for the minimum aggressive chemistry to sustain stable pitting and prevent repassivation. Along with published data, these results were utilized to advance the idea that the critical pit solution chemistry is independent of bulk chloride concentration up to 4 M, a range frequently encountered in atmospheric conditions. © The Author Many authors have separately reported the critical electrochemical conditions necessary for stable pitting and repassivation, focusing on dissolution flux, 1-8 pit solution chemistry, 9-16 or potential. 17-31The existence of these critical parameters is predicated upon the steady-state relationship that emerges between two competing processes. These processes are i) metal dissolution and hydrolysis that results in a local aggressive chemistry inside the pit, and ii) the dilution of this chemistry by diffusion out of the pit that contributes to repassivation. [32][33][34] The mathematical description of this relationship for a one-dimensional pit was framed by Galvele 1 in terms of the product of the current density and the pit depth, (i · x), which was termed the pit stability product in later studies. [35][36][37][38] Once this product decreased below a critical value, (i · x) crit , the conditions in the pit would no longer be able to sustain the local aggressive chemistry necessary for active dissolution. Galvele's formulation, originally intended to describe the conditions leading to pit initiation, has also been successfully extended to pit propagation. 4,7,[35][36][37] Experimental assessment of the Galvele pit model is typically performed using the artificial pit or lead-in-pencil electrode, 4,7,[39][40][41][42] which consists of a metal wire embedded in epoxy. The lead-in-pencil electrode is particularly useful because it closely represents Galvele's pit model configuration, i.e. a single activated surface and inert walls. 7,8 Additionally, the precipitation of a salt film 40,[43][44][45][46][47][48] at high anodic po...

On the Critical Factors for Estimating the Pit Stability Product under a Salt Film

“…1,4,5 In the past several decades, the critical pit stability product and critical solution chemistry during stable pit propagation under a salt film have been the focus of many researchers. [7][8][9][10][11][12][13][14][15] In this regard, numerous experimental and mass transport modeling works focused on one-dimensional (1D) pit propagation to estimate the pit stability product. 8,9,[16][17][18][19][20][21] The most common mass transport method is based on 1D Fick's law of diffusion.…”

“…[7][8][9][10][11][12][13][14][15] In this regard, numerous experimental and mass transport modeling works focused on one-dimensional (1D) pit propagation to estimate the pit stability product. 8,9,[16][17][18][19][20][21] The most common mass transport method is based on 1D Fick's law of diffusion. For example, Tester et al used 1D Fick's law of diffusion to calculate the time evolution of the dissolution current density during potentiostatic polarization using a 1D pit setup.…”

“…Jun et al, and Srinivasan et al have demonstrated the importance of solution viscosity on diffusivity, indicating that the diffusion coefficient would decrease with an increase in pit depth 8,19,31. Thus, it was necessary to consider a variable diffusion coefficient due to the concentration gradient inside the pit.…”

Numerical Investigation of Critical Electrochemical Factors for Localized Corrosion using a Multi-species Reactive Transport Model

“…The simplest of mechanistic models consider mass transport of solute species (e.g., metal or hydrogen ions) in the solution environment based on the Fick's law of diffusion [36]. Therefore, the 1-D Fickian diffusion model has been widely used to simulate the evolution of the chemistry inside the 1-D pit [37][38][39][40][41][42]. Srinivasan et al used this model to evaluate the effect of the external hemispherical boundary layer on the cation flux inside the pit [40] and to demonstrate the dilution of cations near the corroding surface when repassivation happens [41,42].…”

“…Therefore, the 1-D Fickian diffusion model has been widely used to simulate the evolution of the chemistry inside the 1-D pit [37][38][39][40][41][42]. Srinivasan et al used this model to evaluate the effect of the external hemispherical boundary layer on the cation flux inside the pit [40] and to demonstrate the dilution of cations near the corroding surface when repassivation happens [41,42]. Jun et al applied the similar model to analyze the effects of chloride concentration and temperature on 1-D pit growth [39].…”