Effects of Hydrogen Peroxide on Corrosion of Stainless Steel, (III) Evaluation of Electric Resistance of Oxide Film by Equivalent Circuit Analysis for Frequency Dependent Complex Impedances

Corrosive conditions in BWRs are determined mainly by hydrogen peroxide (H2O2). Then, a high temperature, high-pressure H2O2 water loop was fabricated to identify the effects of H2O2 on corrosion and stress corrosion cracking of stainless steel. By changing concentrations of H2O2 and O2, in situ measurements of electrochemical corrosion potential (ECP) and frequency dependent complex impedance (FDCI) of test specimens were carried out and then characteristics of oxide film on the specimens were evaluated by analyzing FDCI data based on the equivalent circuit analysis. The following points were experimentally confirmed. 1. The ECP and FDCI data of the specimens exposed to 100 ppb H2O2 were not affected by co-existing O2 with the same level oxidant concentration and they were also not affected by pre-exposure to 200 ppb O2. From the viewpoint of ECP, this meant that corrosive conditions of hydrogen water chemistry were the same as those of normal water chemistry.2. The low frequency semi-circles of the FDCI data for the specimens exposed to 100 ppb H2O2 reached a saturation value which was much smaller than saturation values for specimens exposed to 200 ppb O2 and to 10 ppb H2O2.3. Smaller oxide dissolution resistance and larger electric resistance of the oxide film were obtained for the specimens exposed to 100 ppb H2O2. This caused ECP to increase by shifting the anodic polarization curve of stainless steel to the high potential side.