Kinetics of Pit Growth for Stainless Steels under the Water Droplet Containing Chloride Ion

Kinetics of pit growth under the water droplet containing 0.87 to 5.79 mol•kg^-1 MgCl₂ were studied to elucidate the atmospheric corrosion behavior of stainless steels. Pit growth rate in this test was much higher than that obtained in an actual environment. It can be assumed that pit generation and/or repassivation process was predominant factor affecting the atmospheric corrosion resistance of stainless steels. Pit volume decreased with chloride ion concentration, with chromium, nickel, and molybdenum content, and it was independent of the size of droplet. This indicates that anodic dissolution was rate-limiting, while oxygen reduction chiefly affects initiation of pits. Current density in a pit was calculated based on the variation with time of pit volume and area of the opening. Metal dissolution rate in a pit dropped considerably with time, and the current density after 400 ks was around 100 μA.cm^-2. This was much lower than typical value required to sustain pitting (1 mA•cm^-2) in aqueous solution such as 0.5 mol• dm^-3 NaCl. Consequently, the pitting in highly concentrated solution like airborne chloride should differ from that in dilute solution.