The selection of fuel is crucial for a Combined Heat and Power (CHP) plant in terms of cost and corrosion and the fuel will vary over time. The corrosiveness of different fuels has been studied extensively, however, not how a current corrosion attack is influenced by corrosion history (i.e. previous deposit buildup and oxide scale formation) instead of the influence of the current flue gas composition (i.e. the current fuel mix being used). This influence of the corrosion history effect can be called "corrosion memory". To investigate the corrosion memory effect on boilers, a novel setup was used. Air-cooled probe AISI304L samples were exposed in two different boilers to mimic different fuels. One boiler was a biomass-fired boiler representing a moderate corrosive fuel and the other was a waste-fired boiler representing a highly corrosive fuel. A three-step exposure program was performed in which a reference probe was exposed in the biomass-fired boiler for all three steps while another probe was exposed in the biomass-fired boiler for steps 1 + 3 and in the waste-fired boiler for the second step. All samples were investigated with material-loss measurements and SEM/EDX. The findings show that the deposit and oxide scales formed on the samples in the biomass-fired boiler had a mitigating effect on the corrosion attack. Consequently, corrosion history affects the future corrosion rate.