The corrosion of the nickel-base alloy 625 under hydrothermal conditions. The influence of temperature, pressure, pH value, and presence of the anions chloride, sulfate, nitrate, and phosphate on the corrosion behaviour

Reactor materials such as nickel-base alloys are subject to a strong corrosion in acidic high-temperature, high-pressure aqueous solutions. Such conditions are present e.g. during the oxidation of organic pollutants in supercritical water (SCWO). In this work, the corrosion of the nickel-base alloy 625 (NiCr22Mo9Nb) in hydrothermal solutions (T = 25-500 C; p = 24-38 MPa) of the acids HCl, H2SO4, HNO3, and H3PO4, of the salts NaCl, NaHSO4, and Na2SO4, and of NaOH was investigated. The experimental parameters were the concentrations of the acid or the salt (0.05- 1.0 mol/kg), the concentration of oxygen (0-3.6 mol/kg), the pH value of the solution, and the reaction time. In acidic solutions, corrosion was dependent of the anion at temperatures up to about 250 C. In the temperature range between 250 and 380-450 C, a transpassive dissolution of the alloy occurred in HCl, H2SO4, and HNO3 solutions. At these temperatures, a protective oxide film cannot be formed at the surface of the alloy due to the transformation of the alloying constituent chromium to chromate and a simultaneous instability of solid nickel oxides. The anion has only influence on the corrosion rate. The low corrosion found in H3PO4 solutions can be explained with an incorperation of phosphate into the oxide film. At temperatures above approx. 380-450 C, the corrosion rates correlate with the physical properties of water and are low with water densities being below about 300 kg/m3. Under these conditions, NiO protects the alloy. Solutions of H3PO4 are extremely corrosive at temperatures of about 440 C at acid concentrations above 0.1 mol/kg. To explain this behaviour, the presence of two phases under these conditions was postulated. (orig.)