Abstract
The amount of bound water in the passive film formed on AISI type 304 and 316 stainless steels under potentiostatic conditions was determined by a coulometric titration method, in which an analysis train specially designed for precise determination of very small content of water was used. From the measurements it was found that the passive films on AISI type 304 stainless steel passivated at +0.20 and +0.60V (SCE) had the bound water of 0.102 and 0.076μg/cm2 respectively. This passivation-potential dependence of the amount of bound water was in agreement with that already obtained by using a tritium tracer technique. In the case of the film passivated at elevated temperatures, the amount of bound water decreased with the rise of the passivation temperature. Furthermore, it was found that the dehydration process at 120° and 350°C was markedly different between the specimens passivated at +0.20 and +0.60V; the dehydration of the film on the +0.60V passivated specimen proceeded with intermittent release of the water, while such an intermittent dehydration behavior was not observed for the +0.20V passivated one. In the case of AISI type 316 stainless steel the amount of bound water in the passive film was in the range from 0.04 to 0.06μg/cm2 independent of the passivation potentials (+0.20, +0.40, and +0.60V). The dehydration of the passive films on 316 stainless steel was easily completed in a short time and the process was scarcely affected by the passivation potentials.
