Corrosion behavior of Alloy 625 in supercritical water environments

For a better understanding on the corrosion behavior of Alloy 625, samples fabricated from this alloy were exposed to supercritical water (SCW) environments with 8.3 ppm dissolved oxygen at 400 and 600 °C and 24.8 MPa (3600 psi) for various periods of time up to 1000 h. Pits were found on the surfaces of the samples after the corrosion tests, and the formation of these pits could be attributed to metal carbide inclusions in the as-received Alloy 625. Mass changes (w) in the samples as a function of test duration (t) could be fitted by an equation of w2.21 = 1.4 × 10−5 t, indicating that the mass change approximately followed the parabolic law in the specified SCW environment. In addition, oxides with a double-layer structure were observed on the samples. The outer layer of the oxides consisted mainly of sub-micron spinels of Ni(Cr,Fe)2O4, and the compact inner layer was mixed Ni(Cr,Fe)2O4 and Cr2O3 with a grain size of tens of nanometer.