On-line monitoring of oxide formation and dissolution on alloy 800H in supercritical water

• Detailed data on elemental release from Alloy 800H to supercritical water are presented.
• Oxide film formation on Alloy 800H can be divided into five distinct stages.
• Formation of an equilibrium film requires several hundred hours.

Concentrations of dissolved metals, oxygen and hydrogen at the exit of an Alloy 800H tube exposed to oxygenated supercritical water were monitored to study the initial stages of oxidation. Initially, both water and oxygen are oxidants, generating H2, releasing low concentrations of Fe, Al, Ni and Mn. All added oxygen is consumed during this period. No Cr release is observed during these initial stages, attributed to Mn incorporation in the oxide. Rapid reappearance of oxygen in the water is attributed to thickening of the surface oxide. Manganese dissolution results in Cr release and formation of an outer iron oxide layer.