Mechanisms of hot corrosion in coal fired boiler gas T91 and EM12 steels

摘要

The corrosion of two 9 Cr-Mo steels (T91 and EM12) has been evaluated in synthetic environments related to those experienced by tubes in coal-fired boilers. The effects of water vapour,SO{sub}2, and ash deposits with and without alkali salts on the corrosion, compared to that in ambient air, have been measured in the temperature range 460°-800°C. The corrosion kinetics of both steels in air follow a cubic rate law. In the synthetic atmospheres, a maximum in the corrosion rate was observed at approximately 600°C, which is attributed to the effect of water vapour and/or SO{sub}2. This maximum is also observed for specimens oxidized with an ash deposit enriched in alkali salts. The mechanism in this case is related to the melting of alkali iron trisulfates, (Na,K) {sub}3Fe(SO{sub}4) {sub}3. The corrosion also increases to very rapid rates as the temperature approaches 800°C, which is attributed to the melting of alkali silicates.