With an expanding use of low quality bio fuels, corrosion problems on water wall tubes are increasing. In this study, the possible corrosion reducing effect when adding digested sewage sludge to the fuel in a used wood (also known as waste or recycled wood) fired furnace has been evaluated. The low alloyed steel 16Mo3 and the stainless steel 310S were exposed for 14.25 h at the furnace wall position when firing only used wood and used wood with sewage sludge additions. The exposures were performed in a bubbling fluidized bed boiler and the metal temperature of the test samples was controlled to 350 °C. Chemical analysis of the deposits and microscopic evaluation of the metallic samples showed reduced amount of alkali metals and chlorine in the deposit together with reduced initial corrosion for both materials when co-firing with digested sewage sludge. In the corrosion process, metal chlorides were formed for both materials when firing only used wood, iron chlorides for the low alloyed steel, and chromium chlorides for the stainless steel. When co-firing with sewage sludge, this behavior was suppressed.