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

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 (E_app) 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 (i_lim) of anodic dissolution decreased with increasing NaCl concentration. A characteristic potential, below which the current density started to decay from steady i_lim, 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 (E_sfd). E_sfdalso decreased with increasing NaCl concentration.

To explain the dependence of i_lim and E_sfd on NaCl concentration, the diffusion coefficient of metal ions (D) and saturation concentration of FeCl₂ (C_sat), which are directly associated with the product of pit depth and i_lim, were calculated using model concentration profiles in a pit and a thermodynamic database software. Both D and C_sat 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 C_sat values was used to predict i_lim for certain pit depths, and the predicted i_lim values were in the range of experimentally-measured i_lim in different NaCl concentrations, which validated the model.