Experimental studies of Zn and Pb induced high temperature corrosion of two commercial boiler steels

Fuels such as demolition wood and MSW usually contain high levels of alkali chlorides and heavy metals especially Zn and Pb which may form low melting, highly aggressive deposits speeding up the corrosion reactions. In order to investigate the corrosion tendency of these specific heavy metals two common superheater materials were exposed to various Zn and Pb compounds in a laboratory tube furnace. Afterwards, the maximum and the mean oxide layer thicknesses were determined. Furthermore, the thickness distribution over the steel sample was also determined. Metallographic cross-sections of the specimens were characterized by means of SEM/EDX. The analysis of the specimens showed that the most severe corrosion attack occurred on both tested steels during the exposure to PbCl2 and already below the PbCl2 melting temperature. The poor resistance of the austenitic stainless steel (AISI 347) in this case was subscribed to PbCrO4 formation that decreased significantly the protectiveness of Cr2O3. The exposures to ZnCl2 showed no adverse effects on AISI 347 while 10CrMo9-10 suffered from an increased oxide layer growth at 350 degrees C. The laboratory experiments were supplemented with thermodynamic calculations in order to gain insight in the behavior of Zn and Pb in the corrosion mechanisms.