The oxidation behavior of Fe-2.25Cr-1Mo in presence of low amounts of ZnCl2 at 300-400°C

Abstract

To reduce emissions of carbon dioxide, alternative sources for energy production have been adopted in high extent, e.g. biomass. However, since biomass contains higher concentrations of alkali metals and heavy metals compared to fossil fuels [1], corrosion problems e.g. on the waterwalls in biomass fired boilers occur in larger extent. In this work, the corrosion behavior of the low alloyed steel Fe-2.25Cr-1Mo, used in waterwalls due to its economical advantage and mechanical properties, in presence of low amounts of ZnCl2 at 300 – 400°C is investigated. Laboratory experiments were carried out in a tube furnace in wet atmosphere containing 5% O2, 40% H2O in N2, with exposure times up to 168 h. Prior exposures, sample coupons were cleaned, polished, dried and deposited with 0.1 mg ZnCl2/cm2. After exposure, the coupons were analyzed with scanning electron microscopy (SEM) with energy dispersive X-ray (EDX) and X-ray diffraction (XRD). In absence of ZnCl2 at 300°C, only a very small mass gain was measured by gravimetry. In the presence of ZnCl2, a local corrosion attack concentrated to the deposits was observed matching the composition of ZnO, the spinel Fe2ZnO4 and iron oxides. 1 and 24 h exposure yield a mass loss, partly explained by evaporation of solvent and chlorine. After 168 h, a mass gain was observed. In absence and presence of ZnCl2 at 400°C, the mass gain was considerably higher than at 300°C. In presence, no mass loss was observed, indicating faster oxidation rate.