Deliquescence of NaCl and Atmospheric Corrosion of 316L Stainless Steel

Water droplets formed by the deliquescence of pre-deposited NaCl on 316L stainless steel were investigated. Different total weights of NaCl particles between 0.0005 g and 0.01 g were deposited on 316L stainless steel surface, which were exposed to an atmospheric condition at 80oC and relative humidity of 80% until NaCl droplets were stabilized. The volume of NaCl droplet was linearly proportional to the total weights of pre-deposited NaCl and consequently the chloride concentration in droplets, ranging from 3 to 6 M, did not depend on the NaCl weights. After exposed to the NaCl droplets at 80oC and relative humidity of 80% over 5, 10, and 15 days, all 316L stainless steel samples suffered from pitting. The Cl- ions in the surface films of samples were measured by ToF SIMS. The pit densities on samples were well correlated with the Cl- intensities in the surface films. These results suggest that the pit initiation under NaCl droplets by deliquescence is caused by the Cl- ions either incorporated into surface films or penetrated through them.