Effect of Syngas Composition on Metal Dusting of Alloy 800H in Simulated Reforming Gas Atmospheres

The reactivity of various gas mixtures of carbon monoxide (CO), H₂, carbon dioxide (CO₂), and H₂O was investigated in laboratory experiments to study metal dusting in simulated syngas environments of reforming plants. Quartz and Alloy 800H (UNS N08810) samples were exposed to eight gas mixtures of different CO, H₂, CO₂, and H₂O composition at 650°C for 200 h. The amount of coke deposited from the gas mixture and the extent of corrosion were determined. Carbon deposition from the gas mixture onto the quartz and metal specimens was not understandable from the carbon activity, assuming thermal equilibrium of the entire gas mixture, but it was well interpreted according to the carbon activity derived from the gas reaction involving CO, H₂, and H₂O. Similarly, oxide scale thickness formed on the Alloy 800H specimen depended not on the oxygen potential by the thermal equilibrium, but on the partial equilibrium reaction of H₂O and H₂. This suggests that the gas mixture is not in an equilibrium condition, and partial equilibrium is dominating for reactions of carbon deposition and oxide scale formation, at least for the present experimental conditions. For Alloy 800H, carburization resulted for the gas mixtures with carbon activity more than 0.5. For the gas mixtures where the carbon activity was more than 10, significant coking and carburization resulted. Metal dusting occurred for Alloy 800H for conditions where carbon deposition and oxide formation competed, i.e., where the carbon activity and oxygen potential were both high. Metal dusting did not take place for the gas mixture where the carbon activity was high and the oxygen potential was low, or where the carbon activity was low and the oxygen potential was high. Metal dusting is considered to be initiated by some localized imperfect sites such as flaws and defects of the otherwise “protective” oxide scale.