Effects of hydrogen peroxide on intergranular stress corrosion cracking of stainless steel in high temperature water, (IV) Effects of oxide film on electrochemical corrosion potential

In order to determine the effects of hydrogen peroxide. on electrochemical corrosion potential (ECP) of type 304 stainless steel (SUS304), ECPs were measured using a high temperature, high pressure water loop with polytetrafluoroethylene (PTFE) inner liner at controlled hydrogen peroxide concentration. It is observed that the ECP of SUS304 exposed to hydrogen peroxide is higher than that when exposed to oxygen at the same oxidant concentration. The ECP shows a hysteresis pattern for its concentration dependency. Those results were attributed mainly from the chemical form of oxide film on stainless steel specimens. The oxide film was affected by the corrosive circumstances. Hematite (alpha -Fe2O3) was observed for the specimens exposed to hydrogen peroxide, while Fe3O4 was a main oxide when exposed to oxygen. The difference of the anodic polarization curves between O-2 and H2O2 environments was caused by the difference of the stability between alpha -Fe2O3 and Fe3O4. Since the alpha -Fe2O3 is reduced to the Fe2+ when hydrogen is added to water, the ECP decreases with decreasing oxidant concentration without showing the hysteresis that keep the ECP higher value.