Exposure of Stainless Steel 316 to Subcritical, Transition, and Near-Supercritical Water

This work investigates the oxidation of austenitic stainless steel 316 at temperatures close to the critical point of water: subcritical water at 330°C and 27 MPa, the transition between subcritical and supercritical water at 374°C and 27 MPa, and supercritical water (SCW) at 380°C and 27 MPa. The surfaces of the samples were characterized using x-ray diffraction, Raman spectroscopy, scanning electron microscopy, and x-ray photoelectron spectroscopy before and after exposure. Magnetite (Fe₃O₄) was found to be present on all surfaces of all samples. The samples exposed to subcritical water also showed the presence of a nickel-iron chromium mixed spinel Ni(Fe_2-xCr_x)O₄. The oxide morphology was observed to vary based on exposure temperature; an increased nucleation and growth rate, and an increased generation of Fe²⁺ was observed on samples exposed to the transition region and SCW.