Co-firing biomass with fossil fuels is increasingly relevant to thermal power plant operators due to new, stricter sanctions relating to carbon footprints. It has been found that the use of biomass results in an altered ash composition, which leads to increased corrosion of the superheater tube walls. This paper presents results for corrosion depth, from initial testing on P91 samples exposed to synthetic salts representative of ash compositions obtained from operational plants. SEM images and EDX element maps are obtained and presented in order to gain an understanding of the complex corrosion mechanism which occurs. A finite element methodology is presented for combined corrosion with creep damage to assess the effect of corrosion tube wall loss on creep rupture life for pressurized tubes.