Explanation of the effect of high chloride concentration on the critical pitting temperature of stainless steel

We correlate the effect of high chloride concentration on the critical pitting temperature (CPT) of type 316L stainless steel with its effect on the critical pit solution chemistry as determined by the artificial pit technique. It is shown that the change in CPT with bulk chloride concentration (0.5–9 mol kg−1) can be correlated with a change in the ratio of C∗/Cs, where C∗ is the critical dissolved alloy cation concentration to sustain pitting, and Cs is the solubility of FeCl2 at the pit surface. A complicating factor is that natural pits can only grow with C∗ = Cs at the lower chloride concentrations, but can grow without the salt film at very high chloride concentrations; this transition is believed to occur close to 5 or 6 m bulk chloride concentration. The dependence of Cs on bulk chloride concentration is given a new interpretation based on a common-ion effect operating within an altered local chemistry with complexation.