Diffusion-Limited 1D Pit Growth of S13Cr in Brine at Elevated Temperature

Monday, October 12, 2015: 11:00
102-A (Phoenix Convention Center)
J. Jun (Fontana Corrosion Center, The Ohio State University), G. Frankel (Fontana Corrosion Center, The Ohio State University), and N. Sridhar (DNVGL)
Diffusion-limited pit growth of Super 13Cr stainless steel (S13Cr) was studied using one-dimensional (1D) pit electrodes in 0.1 – 3 M NaCl solutions at 85 °C. Applied potential (Eapp) was controlled in four different steps to achieve a target pit depth and measure the anodic current density for S13Cr dissolution under potentiostatic polarization and during a backward scan. For pits with similar depths, the diffusion-limited current density (ilim) of anodic dissolution decreased with increasing NaCl concentration. A characteristic potential, below which the current density started to decay from steady ilim, was found during the backward scan. This potential is associated with dissolution of a salt film at the pit bottom and was defined as the potential of salt film dissolution (Esfd). Esfdalso decreased with increasing NaCl concentration.

To explain the dependence of ilim and Esfd on NaCl concentration, the diffusion coefficient of metal ions (D) and saturation concentration of FeCl2 (Csat), which are directly associated with the product of pit depth and ilim, were calculated using model concentration profiles in a pit and a thermodynamic database software. Both D and Csat were predicted to decrease as NaCl concentration increased, indicating slower diffusion and easier precipitation of salt film, which both increase the stability of a salt film. The product of the calculated D and Csat values was used to predict ilim for certain pit depths, and the predicted ilim values were in the range of experimentally-measured ilim in different NaCl concentrations, which validated the model.