Pitting corrosion of Type 430 stainless steel under chloride solution droplet

To clarify the critical relative humidity (RH) to initiate pitting corrosion and the rusting mechanism in a marine atmospheric environment, pitting corrosion of Type 430 stainless steels under drops of MgCl₂ solutions were investigated. A pitting corrosion test was performed at different relative humidities under droplets with various diameters and thicknesses. The probability of pitting decreased as the diameter and thickness decreased. Pitting progressed only when the chloride concentration exceeded 4 M, which is the equilibrium concentration at 80% RH. Accordingly, pitting of Type 430 could be initiated when the RH was less than 80%. Additionally, a pitting corrosion mechanism of Type 430 stainless steel under droplets containing chloride ions is proposed.