Corrosion behavior of NiCrMo Alloy 625 in high temperature, hydrogenated water

The corrosion behavior of NiCrMo Alloy 625 (UNS N06625) has been characterized in a 10,000 h test conducted in hydrogenated water at 260 °C. The corrosion kinetics were observed to be parabolic, the parabolic rate constant being determined by chemical descaling to be 0.074 mg dm⁻² h^−1/2. Characterizations of the corrosion oxide layer via grazing incidence X-ray diffraction and X-ray photoelectron spectroscopy in combination with argon ion milling and target factor analysis, revealed the presence of two spinel oxide phases and significant amounts of recrystallized nickel. Based on the distribution of three oxidized alloying constituents (Ni, Cr, Fe) with respect to depth and oxidation state, it was concluded that: (a) corrosion occurs in a non-selective manner, but significant amounts of nickel(II) ions are released to the water, and (b) the spinel oxides exist as a chromite-rich inner layer (Ni_0.7Fe_0.3)(Cr_0.8Fe_0.2)₂O₄ underneath a coarser, ferrite-rich outer layer (Ni_0.9Fe_0.1)(Cr_0.1Fe_0.9)₂O₄. The trivalent cation distribution in each of these phases appears to represent a solvus in the immiscible NiCr₂O₄-NiFe₂O₄ binary.