High-Temperature Corrosion Behavior in Different Regions of 9 Cr-1 Mo Steel Weldment in SO2 + O2 Atmosphere

Abstract

This paper investigates the high-temperature corrosion behavior of microstructurally different regions of the weldment of 9 Cr-1 Mo steel used in thermal power plant boiler in SO2 + O2 environment. The weldment is produced by tungsten inert gas welding method, and the different regions of the weldment (weld metal, heat-affected zone, and base metal) are exposed in SO2 + O2 (ratio 2:1) environment at 973 K for 120 h. The reaction kinetics and corrosion growth rate of different regions of weldment in isothermal condition are evaluated. The post corroded scales of the different specimens are studied in SEM, EDS, and XRD. The results indicate that the weld metal shows higher corrosion rate followed by HAZ and base metal. The higher rate of corrosion of weldmetal is mainly attributed to the least protective inner scale of Cr2O3 with minimum Cr Content. This is due to the formation of delta ferrite, which leads to the precipitation of the Cr-based secondary phases and depletes the free Cr from the matrix. The thermal cycles during welding at high temperature are favorable for the formation of delta ferrite. On the other hand, in absence of delta ferrite, the base metal and HAZ regions of the weldment show lower corrosion rate than weld metal. The difference in corrosion rate in the three regions of the weldment is supplemented by post-corroded scale characterizations.