Revealing the superior oxidation resistance of alloy 690 in deaerated supercritical water at 600 °C through advanced characterization.

The corrosion behavior of a well-polished Alloy 690 in deaerated supercritical water at 600 °C was examined. The study found that the weight gain follows a near-cubic rate law. A key observation was that the direct external oxidation and the rapid transition from internal to external oxidation within the initial 24-h exposure are responsible for its superior oxidation resistance. As exposure time increases, the transformation of Cr-rich spinel oxides and Ni-rich networks in the internal oxidation zone into Cr 2 O 3 leads to the expansion of the protective chromia layer, effectively slowing down the oxidation process. Conversely, the Cr 2 O 3 layer at the grain boundaries (GBs) was less effective at preventing oxidation, resulting in a significantly faster oxidation rate in these areas compared to the bulk grains. The extensive GB oxidation exhibits little consequential correlation with carbides. • The oxidation kinetics of Alloy 690's in deaerated supercritical water (SCW) at 600 °C were investigated. • The rapid transition to external oxidation and the expansion of compact chromia layer are responsible for the superior oxidation resistance. • The severe GB oxidation proceeds about six times faster than the bulk matrix due to its faster elemental diffusivity along the migrated GB. • GB carbides does not exhibit any consequential relationship with the depth of Diffusion-induced grain migration or the extent of GB oxidation. [ABSTRACT FROM AUTHOR]