Investigation on early formation and evolution of oxide scales on ferritic–martensitic steels in supercritical water

Early oxidation characteristics of ferritic-martensitic steel T91 in supercritical water (SCW) from 1 h to 120 h were investigated. Relative to austenitic steels, occurrence of each oxidation stage for T91 was delayed. Both inner Cr-rich layer and outer magnetite layer formed even after a 1h-exposure at 540 °C, implying their almost simultaneous formations. Several assembled-pore lines were observed in the inner layer. Abundant pores accumulated at the boundary between the inner/outer oxide layers in steam, while mainly along the inner/diffusion layer interface in SCW, due to lower difference between iron diffusion through the inner and outer layers for the later.