The Effect of Dissolved Chlorine on the Pitting Behavior of 304L Stainless Steel in a 0.5 N NaCl Solution Available to Purchase

Electrochemical experiments were performed on a 304L (UNS⁽¹⁾ S30403) stainless steel alloy in a 0.5 N NaCl solution as a function of chlorine content (0 to 180 mg/L) at room temperature. Experiments performed included a measurement of the corrosion potential as a function of time, the determination of both the breakdown and repassivation potentials using cyclic polarization curves, and the use of a scratching electrode technique to measure the kinetic aspects of the breakdown of passivity. The addition of chlorine to a solution of pH 5 in the concentration range of 20 to 60 ppm chlorine resulted in a significant shift in the corrosion potential in the noble direction. At higher concentrations of chlorine, the corrosion potential shifts back toward that observed without chlorine additions. Chlorine also results in a monotonic shift in the breakdown potential, suggesting that the passive film is rendered more stable against the initiation of localized corrosion. However, the repassivation potential exhibits a minimum in the chlorine concentration range where the corrosion potential exhibits a maximum. This data suggests that the rate of propagation of localized corrosion should be maximized in this chlorine regime (20 to 60 mg/L). When scratching electrode experiments are performed, the number of pits is minimized, but the size of the pits is maximized. Analysis of the kinetic pit growth results suggest that, when the pits are small, the rate of pit growth is controlled by diffusion of corrosion products through the bulk aqueous phase, but, as the pits age, the mechanism changes, and pit growth is governed by diffusion through a solid or semisolid film on the walls of the pits.