AC-IMPEDANCE MONITORING OF PITTING CORROSION OF STAINLESS-STEEL UNDERA WET-DRY CYCLIC CONDITION IN CHLORIDE-CONTAINING ENVIRONMENT

A study of the corrosion of stainless steels under a wet-dry cyclic co ndition has been carried out using the AC impedance method. Three type s of stainless steel were exposed to the alternate conditions of 1 h i mmersion in chloride containing solution and 5 h drying at 25 degrees C and 60% RH. The corrosion rates were monitored continuously during e xposure using an AC impedance monitoring technique. Simultaneously, th e current distribution over the working electrode during impedance mea surement was analysed from the phase shift Theta in an intermediate fr equency. 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 electr olyte 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 chlori de-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 th e solution. Copyright (C) 1996 Elsevier Science Ltd.