Microstructural evolution of pre-oxidized T91 steel during LBE dissolution corrosion

Abstract The microstructural evolution of T91 steel by pre-steam oxidation during LBE dissolution corrosion was investigated. A bi-layered pre-oxide film with Fe-rich outer layer and Cr-rich inner layer was formed on T91 steel, which was similar to the oxide film of T91 after oxidation corrosion in LBE. The pre-oxide film effectively protects the matrix from LBE corrosion at 620 °C. However, the composition and microstructure of the pre-oxide film changed dramatically. Unlike the original duplex structure, the pre-oxide film exposed to LBE undergoes a process of reduction of the outer layer and oxidative growth of the inner layer and changes into five layers, the loose and easily peeling outer Fe layer, the dense and intact inner layers successively consist of Fe-Cr spinel layer, a transition layer of matrix and Cr-rich oxide, continuous Cr-rich oxide layer with tetragonal distorted spinel structure and amorphous SiO2 layer. The evolution mechanism of the pre-oxide film during LBE dissolution corrosion is discussed.