Pitting corrosion mechanism of stainless steels under wet-dry exposure in chloride-containing environments

Two types of stainless steel were exposed to the alternate conditions of Ih immersion in chloride-containing solution and 7 h drying at 30 degrees C and 67% RH. The corrosion rates and corrosion potentials were monitored simultaneously and continuously during exposure by using combined AC impedance and corrosion potential monitoring techniques in order to study the mechanism of pitting corrosion of stainless steels under a marine atmospheric environment. The results show that pitting is initiated preferentially during the dry cycle after a critical chloride concentration is reached. Pit initiation is followed by a shift of the corrosion potential to about -400 mV (SSE) as growth commences. Afterwards, as the specimen surface dries up, repassivation commences, and it may continue towards the next wet cycle if perfect drying within the active pit is not permitted. Furthermore, it was found that the presence of thin solution layers in atmospheric corrosion contributes to the enhancement of pit initiation and growth. A mechanism was proposed to explain the pitting corrosion of stainless steel in a marine atmospheric environment. (C) 1997 Elsevier Science Ltd.