Computational modeling of cathodic limitations on localized corrosion of wetted SS 316L at room temperature

The ability of a SS 316L surface wetted with a thin electrolyte layer to serve as an effective cathode for an active localized corrosion site was studied computationally. The dependence of the total net cathodic current, I _net , supplied at the repassivation potential E _rp (of the anodic crevice) on relevant physical parameters including water layer thickness (WL), chloride concentration ([Cl ⁻ ]) and length of cathode (Lc) were investigated using a three-level, full factorial design. The effects of kinetic parameters including the exchange current density ( i _o,c) and Tafel slope ( β _c ) of oxygen reduction, the anodic passive current density ( i _p) (on the cathodic surface), and E _rp were studied as well using three-level full factorial designs of [Cl ⁻] and Lc with a fixed WL of 25 μm. The study found that all the three parameters WL, [Cl ⁻] and Lc as well as the interactions of Lc × WL and Lc × [Cl ⁻] had significant impact on I _net . A five-factor regression equation was obtained which fits the computation results reasonably well, but demonstrated that interactions are more complicated than can be explained with a simple linear model. Significant effects on I _net were found upon varying either i _o,c, β _c, or E _rp, whereas i _p in the studied range was found to have little impact. It was observed that I _net asymptotically approached maximum values ( I _max) when Lc increased to critical minimum values. I _max can be used to determine the stability of coupled localized corrosion and the critical Lc provides important information for experimental design and corrosion protection.