Abstract Partially melted ash particles cause increased ash fouling, agglomeration and sintering behaviors during biomass combustion. In addition, ash fouling facilitates the corrosion behavior of stainless steel. In this work, the agglomeration/sintering and corrosion behaviors of untreated herbaceous biomass ash and that pre-treated by an alkali metal leaching reaction were investigated under combustion conditions. Although the ash agglomeration tendency increased with increasing temperature, the tendency was reduced by alkali metal leaching pre-treatment. During comparison of sintering behavior, the degree of sintering (x/r) increased as a function of time but was substantially alleviated by alkali metal leaching pre-treatment. In comparison to the typical Frenkel sintering model, a modified Frenkel sintering model was shown to fit well with the experimental sintering behavior. Alkali metal leaching pre-treatment inhibited ash particle adhesion and made high melting temperature minerals, which was verified by scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX), X-ray diffraction (XRD) analysis, the use of ternary mineral phase diagrams and adoption of a new chemical classification system. Furthermore, from SEM-EDX, alkali metal leaching pre-treatment was seen to reduce the corrosion by ash fouling on stainless steel (SUS) 316. Resultingly, the alkali metal leaching pre-treatment significantly alleviated the ash adhesion and corrosion behaviors.