Abstract
Carbon steel exhibits passivity in highly alkaline and nitrite-containing solutions warranting its use in storing high pH radioactive waste. The boundaries of corrosion resistance of carbon steel in radioactive waste solutions stored in underground tanks are derived from the repassivation potential for localized corrosion of steel in various solutions. If the corrosion potential is higher than the repassivation potential, stable localized corrosion is likely to occur. The repassivation process can be considered in terms of a competition between stable oxide-type passive film formation versus an active metal salt-enriched layer formation, depending on the potential. This approach has worked reasonably well for stainless steels and Ni-base alloys. It is often argued that carbon steel is not a strongly passivating metal unlike stainless steels and, therefore, the stability of the oxide film on carbon steel is questionable. An alternative hypothesis is that repassivation potential is related to the corrosion potential of the bottom of the localized corrosion region. This paper evaluates this argument by conducting repeated cyclic potentiodynamic polarization (CPP) measurements in different environments for both carbon steel and Type 304L stainless steel. The effect of different environmental species, nitrite and hydroxide on repassivation/reactivation mechanism of carbon steel is examined.
