AC impedance monitoring of pitting corrosion of stainless steel under a wet-dry cyclic condition in chloride-containing environment

A study of the corrosion of stainless steels under a wet-dry cyclic condition has been carried out using the AC impedance method. Three types of stainless steel were exposed to the alternate conditions of 1 h immersion in chloride containing solution and 5 h drying at 25 degrees C and 60% RH. The corrosion rates were monitored continuously during exposure using an AC impedance monitoring technique. Simultaneously, the current distribution over the working electrode during impedance measurement was analysed from the phase shift Theta in an intermediate frequency. The results showed that corrosion rate monitoring using an AC impedance method is possible under the given exposure conditions even during the drying period when the metal is covered with a thin electrolyte layer. The different processes of passivation, pitting corrosion and repassivation could be distinguished from the monitored impedance data. It was found that in the wet-dry cyclic condition in the chloride-containing environment, pit generation and growth occur preferably just before the perfect drying of the metal surface, and repassivation occurs preferably when the surface dries out and is re-immersed in the solution. Copyright (C) 1996 Elsevier Science Ltd.