On the Oxidation Resistance of Alloy 800HT Exposed in Supercritical Water (SCW)

The purpose of this study was to examine the structure and composition of the oxide scales formed on Alloy 800HT in a supercritical environment after a relatively short exposure time (circa 500 h) to provide a more complete physical description of the metallurgical factors that influence the protectiveness of the scale formed. This was achieved using transmission electron microscopy and associated techniques on site-specific cross-section samples prepared by focused ion beam milling. No physical evidence in support of oxide scale exfoliation was found. Rather, the formation of the discontinuous magnetite (Fe₃O₄) outer nodular layer is believed to be controlled to a large extent by the underlying microstructure and the presence of ε-martensite plates in particular. The formation of a thin compact corundum-type ([Cr,Fe]₂O₃) base scale is considered to be sufficiently encouraging to justify an effort to reduce or eliminate the small volume fraction of ε-martensite from the starting microstructure of Alloy 800HT as a means to optimize the corrosion resistance.