The corrosion behavior of 9Cr ferritic/martensitic (F/M) steels with three different Si contents in oxygen saturated static lead-bismuth eutectic at 500 °C was investigated. The oxide scale on the Si-modified steels exhibits a three-layer structure, including magnetite layer, spinel layer and internal oxidation zone. In the F/M steel with the highest Si content, a continuous Cr-rich layer containing more Cr and Si elements compared to the IOZ was observed. This Cr-rich layer was located at the interface of IOZ and spinel layer after exposure for 1000 h. The critical Si content for the formation of this layer was determined to be 1.58 wt.% at 500 °C in 9Cr F/M steels based on the third-element effect theory. After 2000 h, the Cr-rich layer was transformed into FeCr 2 O 4 spinel and Fe 2 SiO 4 , while continuously formed SiO 2 and Cr 2 O 3 remained distributed along the interface. Further investigation of the formation and evolution mechanisms of the Cr-rich layer helps understanding how Si enhances the corrosion resistance of F/M steels.
Chen et al. (Sun,) studied this question.