Critical Potential for Growth of Localized Corrosion of Stainless Steel in Chloride Media

778

467

A pit model was developed on the assumption that the metal ions hydrolyze inside the pits and that the corrosion products are transported by diffusion. Concentrations of Me e+, Me(OH)+, and H + ions, as a function of pit depth and current density, for Zn, Fe, Ni, Co, AI, and Cr were calculated. The main reason for passivity breakdown at the initial stages of pit growth, was found to be the localized acidification due to metal ions hydrolysis. Assuming a critical pH value for pit initiation, the following experimental facts could be explained: (i) the effect of the external pH on the pitting potential of Fe and stainless steel; (it) the effect of sodium borate concentration on the pitting potential of Zn; (it{) the effect of weak acid salts on the pitting potential of AI; (iv) the oscillations of the electrode potential of stainless steel and nickel in solutions of Cl-+ SO4 = ions; (v) the existence of a pitting inhibition potential; and (vi) the existence of a pitting protection potential. Through analysis of the transport processes inside a pit it was also concluded that the pitting potential of a metal should change with the CI-ion concentration according to the equation E~ = E/ --B 9 log [CI-] B = 0.059V being the slope of the curve at room temperature.

Section: F Fmentioning

confidence: 89%

Transport Processes and the Mechanism of Pitting of Metals

Galvele

1976

778

467

“…Figure 15a shows that even when the surface concentration at the pit base is diluted to lower fractions of the concentration at saturation, the numerically simulated flux continues to converge towards the analytically determined flux at pit depths which are approximately eight to ten times the pit diameter. Suzuki and Kitamura 47 reported that for 316L in chloride solution, activated pits that were about ten times as deep as their diameter registered a critical potential for repassivation similar to activated crevices. In their study, the dissolution current density was monitored versus time at progressively lower applied potentials.…”

Section: Bulk [Clmentioning

confidence: 99%

Geometric Evolution of Flux from a Corroding One-Dimensional Pit and Its Implications on the Evaluation of Kinetic Parameters for Pit Stability

Srinivasan

Liu

Kelly

2016

The flux from a one-dimensional (1-D) artificial pit electrode corroding under a salt film was examined using experimental and modeling techniques. Finite element simulations showed that the flux at shallow depths was consistently lower than analytically determined calculations for 1-D Fickian diffusion. This deviation was due to a substantial contribution of the external hemispherical boundary layer to the overall diffusion length. Increasing the pit diameter resulted in a larger boundary layer, which in turn affected the flux characteristics to greater depths. Data from experiments and simulation converged with the theoretical 1-D calculations only when pit depths approached nearly ten times the pit diameter. The experimental data from this artificial pit study as well as data from related published work were observed to span the range bounded by the numerically simulated and the analytically determined flux predictions. Comparison with published pit stability phenomenology showed that only deep pits provided kinetic data based on the cation concentration gradient unadulterated by bulk chloride effects. Finally, this work also provided insight into the origin of the dependence of the measured repassivation potential with pit depth, contributing towards a quantitative framework relating the various critical factors governing pitting. The stable growth of corrosion pits requires the presence of an aggressive chemistry at the corroding surface as characterized by a high metal chloride concentration and low pH.1,2 The conditions leading to the maintenance of such chemistry were mathematically considered by Galvele 3 through an investigation of the steady state relationship between metal dissolution and mass transport 4,5 out of a one-dimensional (1-D) pit. For the case of a 1-D pit, it was theoretically demonstrated that a minimum critical value of cation flux -expressed as the product of the current density and the pit depth, (i·x) -was necessary for the pit to maintain a critical chemistry and thus stably corrode. As such, should the product of the current density and the pit depth fall below this critical value, repassivation would set in due to the loss of the aggressive chemistry. Subsequent studies on stainless steel pitting referred to this parameter and equivalent relationships for other pit geometries as the pit stability product. 6,7 This critical pit stability product (denoted (i·x) crit ) has been employed as the anodic stability parameter to determine the maximum pit size that can be attained on a particular metal surface in a given corrosive environment. 8-11Experimental results obtained using the artificial pit or lead-inpencil electrode 12-17 can be directly used to quantify 1-D dissolution kinetics because the construction of this electrode results in inert walls surrounding a single active surface 13,14,16 corroding under a precipitated salt film 18-24 upon the application of high anodic potential in corrosive solution. The presence of the salt film results in diffusionlimited dissolution conditi...

“…Initial perspectives on a critical potential below which pitting would not be stable suggested either the open circuit potential in the pit solution or the pitting potential as possible candidates. 1,17,18,20,23,[56][57][58] The selection of the open circuit potential in the pit solution as a critical potential was based on the understanding that a net cathodic reaction would be expected to ensue should the pit electrochemistry cause the potential to decrease below this value. …”

mentioning

confidence: 99%

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

Srinivasan

Kelly

2016

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...