還元性硫安水溶液中でのステンレス鋼の防食

抄録

A mechanistic aspect of passivation of stainless steels in reducing ammonium sulfate solution was studied.
When a reducing agent such as sulfur dioxide, thiosulfate or sulfite was added in ammonium sulfate solution, the redox potential of Fe²⁺/Fe³⁺ and the corrosion potential of stainless steels moved to less noble direction, and then steels were severely corroded. When the ammount of such reducing additives exceeded the critical value, the rest potential of the stainless steels shifted to noble direction into passive region and corrosion was protected, while the redox potential of Fe²⁺/Fe³⁺ remained at a constant base level, and Fe³⁺ in solution were strongly reduced to Fe²⁺. The critical value corresponds to the condition that the limiting diffusion current density of reduction of HSO₃^- to S is beyond a anodic peek current density in active region. In this case, the corrosion behavior was strongly controlled by cathodic diffusion process. Investigation was also made on various factors related to the critical value.