Corrosion behavior of ni-base alloys in supercritical water

Abstract

Corrosion of nickel-base alloys (Hastelloy C-276, Inconel 625, and Inconel X-750) in 500 °C, 25 MPa supercritical water (with 10 wppb oxygen) was investigated to evaluate the suitability of these alloys for use in supercritical water reactors. Oxide scales formed on the samples were characterized by gravimetry, scanning electron microscopy/energy dispersive spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. The results indicate that, during the 1000 h exposure, a dense spinel oxide layer, mainly consisting of a fine Cr-rich inner layer (NiCr2O4)underneath a coarse Fe-rich outer layer (NiFe2O4), developed on each alloy. Besides general corrosion, nodular corrosion occurred on alloy 625 possibly resulting from local attack of γ″″ clusters in the matrix. The mass gains for all alloys were small, while alloy X-750 exhibited the highest oxidation rate, probably due to the absence of Mo.