Modeling the Cathodic Region in Crevice Corrosion under a Thin Electrolyte Film Including Particulates

Abstract

Crevice corrosion may be limited by the capacity of the external cathodic region to support anodic dissolution currents within the crevice. The analysis here focuses on behavior of metal surfaces covered by a thin (~microns) layer of the electrolyte film including particulates. The particulates can affect the cathode current capacity (I total ) by increasing the solution resistance ("volume effect") and by decreasing the electrode area ("surface effect"). In addition, there can be particulate effects on oxygen reduction kinetics and oxygen transport. This work simulates and characterizes the effect of a uniform particulate monolayer on the cathode current capacity for steady state conditions in the presence of a thin electrolyte film. Particulate configurations with varying particle size, shape, arrangement, volume fraction, and electrode area coverage were numerically modeled as a function of the properties of the system. It is observed that the effects of particles can be fully accounted for in terms of two corrections: the volume blockage effect on the electrolyte resistivity can be correlated using Bruggeman's equation, and the electrode coverage effect can be modeled in terms of a simple area correction to the kinetics expression. For the range of parameters analyzed, applying these two correction factors, cathodes covered with thin electrolyte films that contain particles can be represented in terms of equivalent homogenous electrolytes that can then be analyzed using simpler approaches. Continuing work will examine the effects of greater volume fractions of particles and multiple particle layers.

Current Distribution on a Cathode Covered by a Thin Film Electrolyte in Crevice Corrosion - An Analytical Model

“…The repassivation potential corresponds to the maximum allowable cathodic polarization at the crevice opening, and therefore the primary anode potential is set equal to -E rp . A more detailed discussion of the repassivation potential and its relationship to the primary anode potential in a decoupled cathode model is available elsewhere (12).…”

Modeling the Effects of Crevice Former, Particulates, and the Evolving Surface Profile in Crevice Corrosion

“…Accounting for Particles in terms of an Effective Electrolyte Conductivity (κ eff ): The presence of particles in the electrolyte along the critical distance between the crevice mouth and the actively corroding site would increase the resistance by blocking the volume of the electrolyte which otherwise would have been available for the current. Based on our earlier work of modeling cathode currents in presence of particulates (10), a suitable correction for accounting for this volume blockage effect can be made on the basis of Bruggeman's equation for the equivalent conductivity (11). For a uniform distribution of particulates in the electrolyte, Bruggeman's equation, [5], can be used to obtain an effective conductivity, κ eff , once the volume fraction occupied by the particles (Φ particles ) is known.…”

“…The crevice mouth was held at 0.05 V NHE , which is close to the repassivation potential (E rp ) of Alloy C-22 (13). The methodology for the application of the boundary conditions (potential/kinetics at the electrodes) was similar to that used earlier for the modeling of the cathode region (10). The average atomic weight of the corroding metal was taken as 58.5 and it was assumed that all the current generated at the anode led to metal dissolution (100% current efficiency).…”

Modeling the Ohmic Effects Due to Particulates and Surface Roughness Within Thin Electrolyte Films in Crevice Corrosion

“…2. In our previous presentations, 1,2 we have shown that the ohmic effects due to surface irregularities and the presence of particles can be accounted for by specific treatments based on the system properties. Here, we provide a more detailed analysis on applying these corrections to crevices with dimensions that are expected in practical systems.…”