Mechanism of localized corrosion issues of austenitic steels exposed to flowing lead with 10−7 wt.% oxygen at 480°C up to 16,000 h

As part of research into resistant materials for the construction of lead-cooled fast reactors, specimens of 316L and 15-15Ti stainless steel were exposed to flowing lead with 10−7 wt.% oxygen at 480 °C for 16,000 h. Post-test microscopic analyses revealed a possible mechanism of long-term susceptibility to localized solution-based attack. The corrosion of both austenitic steels was low, a thin oxide layer (80–240 nm) thin was formed, and a large-scale dissolution attack was not observed. The surface was mostly covered by a protective thin spinel film with variable Cr, Fe, and Mn ratios. With increasing exposure time, the proportion of Mn was increased replacing Fe, and the probability of localized deeper failures increased. Maintaining the spinel film depleted the subsurface layer and triggered the precipitation of new phases. The corrosion resistance was enhanced by the presence of a work-hardened surface layer formed by lathe turning. The thin film is sometimes exfoliated and then regenerated without any consequences or allows deep oxidation to start.