Effect of pH and temperature on corrosion of nickel-base alloys in high temperature and pressure aqueous solutions

The properties of water change significantly across the critical point of water and these changes affect the degradation behavior of constructional alloys. The corrosion behavior of nickel-base alloys is investigated across the critical temperature of water at a constant pressure in aqueous solutions. At subcritical temperatures, the corrosion behavior of alloys of 625 and C-276 is influenced by the pH values of the aqueous solutions. In neutral pH, a thin surface oxide film develops on the substrate. In pH 2, Ni and Fe are selectively dissolved, and Cr and Mo form stable oxides. In pH 1, Cr, Ni, and Fe are dissolved from the substrate and Mo forms a stable oxide. At supercritical temperatures, no dealloying is observed independent of pH and a thin surface oxide film develops. Changes in water properties and thermodynamics explain the dealloying behavior of elements and the highest corrosion rate at high subcritical temperature.