Corrosion Behavior of Alloys 625 and 718 in Supercritical Water

The corrosion behavior of two commercial, nickel-based Inconel^† alloys, 625 (UNS N06625) and 718 (UNS N07718), has been evaluated after exposure to supercritical water (SCW) at 500°C and 600°C for periods of up to 1,026 h. The 500°C tests were performed in deaerated and 2 ppm dissolved oxygen SCW, while the 600°C tests were performed only in deaerated SCW. Evaluation of corrosion has been performed using weight-change measurements, scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDS), x-ray diffraction (XRD), Auger electron spectroscopy (AES), and a surface profilometer. Surface oxidation and pitting were the principal corrosion mechanisms for both alloys at 500°C; however, intergranular corrosion was also observed in the Alloy 625 sample exposed to deaerated oxygen SCW at 600°C for prolonged durations. Oxidation was more dominant compared to pitting for the samples exposed to higher oxygen content or higher temperature SCW. The oxide thickness was lower for the higher chromium-containing Alloy 625 as compared to Alloy 718, and this can be correlated to the enrichment of chromium in the inner oxide layer for this alloy. A layered oxide structure was observed to form upon exposure to SCW with lower dissolved oxygen and/or at higher temperature and identified to be NiO/spinel/(Cr_xFe_1–x)₂O₃/(Fe,Ni).