Increased rates of corrosion during the co-firing of peat with biomass have been identified as a limiting factor on the level of biomass viable to use. The present work is motivated by the need for mechanisms-based models to simulate this process. Experimental studies on pure iron are presented, without the complexities of alloying elements, to develop a fundamental understanding of the corrosion process during co-firing of peat and biomass. A synthetic salt with chemical composition representative of in-situ plant measurements is applied to specimens in a furnace at 540 degrees C for up to 4 weeks. The corrosion layer from interrupted tests is analysed using scanning electron microscopy (SEM), optical microscopy (OM) and energy dispersive X-ray spectroscopy (EDX) to provide insight into the material degradation and structure of the corrosion layer. Two distinct types of oxide are identified from the tests. The first, a compact, uniform oxide layer which forms over the substrate initially. As corrosion proceeds, this layer degrades, leading to spallation and formation of a mixture of corrosion debris into the salt layer. Healthy iron was found to separate from the substrate and undergo oxidation from both above and below. A mechanism for this corrosion process is suggested.