Pit Initiation and Repassivation of Stainless Steels Exposed to Cyclic Relative Humidity Changes

Abstract

Pit initiation and repassivation of stainless steels (SS) SS430 and SS304 were investigated under a thin solution film of MgCl2. The potential difference between the SS plate and the thin silver wire set 100 μm above the SS surface was measured during the wet-dry cycles. The wet-dry cycles were carried out by changing the relative humidity (RH) between 95% and 45% at a constant rate of 5%/h. A rapid potential drop due to initiation of a growth-type pit was observed when the RH was decreased from 95%, while a rapid potential rise owing to repassivation was observed when the RH was increased from 45%. The RHpit and RHrep at the onset of pitting corrosion and repassivation were determined at various temperatures. At 298 K, the RHpit was obtained to be 68%–48% for SS430 and 58%–47% for SS304. The RHrep ranged from 67% to 73% for SS430 and 56%–70% for SS304. The RHpit and RHrep increased significantly as the temperature was increased from 283 K to 313 K.