Co-firing biomass with traditional fuels is becoming increasingly relevant to thermal power plant operators due to increasingly stringent regulations on greenhouse gas emissions. It has been found that when biomass is co-fired, an altered ash composition is formed, which leads to increased levels of corrosion of the superheater tube walls. Synthetic salt, which is representative of the ash formed in the co-firing of a 70\% peat and 30\% biomass mixture, has been produced and applied to samples of P91 at 540 degrees C for up to 28 days. This paper presents results for oxide layer thickness and loss of substrate from testing. Scanning electron microscopy (SEM) images and energy-dispersive X-ray spectroscopy (EDX) element maps are obtained and presented in order to gain an understanding of the complex corrosion mechanism which occurs. A finite-element (FE) methodology is presented which combines corrosion effects with creep damage in pressurized tubes. The effects of corrosion tube wall loss and creep damage on tube stresses and creep life are investigated.