Corrosion behavior of SUS316L in nitric acid solution containing seawater components

Sano, Yuichi   ; Ambai, Hiromu ; Takeuchi, Masayuki ; Iijima, Shizuka; Uchida, Naoki

During the Fukushima Daiichi Nuclear Power Plant accident, significant volume of seawater was injected into the storage pool for spent nuclear fuel as the coolant. In this study, we investigated the effect of seawater components on the corrosion behavior of SUS316L stainless steel in HNO solution. Electrochemical and immersion tests were carried out using a mixture of HNO and artificial seawater (ASW). In the HNO solution containing high amounts of ASW, the cathodic current densities increased and uniform corrosion progressed. This might be caused by strong oxidants, such as Cl and NOCl, generated in the reaction between HNO and Cl ions. In addition, pitting corrosion occurred simultaneously at low HNO concentrations. The corrosion rate decreased with the immersion time at low concentrations of HNO, while it increased at high concentrations. It is assumed that the cathodic reactions proceed slowly and the anodic reactions are gradually prevented by the growth of an oxide film on the surface of the coupon in low-concentration HNO. On the other hand, high-concentration HNO triggers a vigorous cathodic reaction, which disturbs the growth of the oxide film and activates the anodic reactions. This competition between the cathodic and anodic reactions causes a change in the corrosion rates with the immersion time in a mixture of HNO and ASW.