The corrosion characteristics of Alloy 625 (UNS 00625, Ni - 22 Cr - 10 Mo) in oxygenated ammoniacal sulphate environments are determined at room temperature and pressure and up to high temperatures and pressures (673 K, 250 bar) commensurate with the process of supercritical water oxidation (SCWO). Electrochemical methods such as linear polarization, potentiodynamic polarization and impedance spectroscopy are used.

It is found that the electrochemical and morphological response is dictated by the alloying element Cr and the formation of a Cr(III) oxide. Mo and, to a lesser extent Ni, are found to dissolve readily. Thermodynamic analysis of the Ni-NH₃-H₂O system, including new Pourbaix diagrams at temperatures as high as 653 K, has shown that Ni - ammine formation is possible at moderate temperatures but that the stability of these complexes decreases substantially with temperature. According to one of the models investigated, which is based on the only available high temperature equilibrium constant data, Ni-ammines become unstable above approximately 473 K.

Impedance spectroscopy has shown that transpassive dissolution of the alloy's ptype, cation conducting, Cr(III) oxide occurs at temperatures as low as 373 K and total pressure (oxygen saturated) as low as 40 bar. As temperature and pressure are increased the corrosion process is increasingly diffusion controlled. Transpassive dissolution results in the thinning and eventual total removal of the alloy's protective semiconductor barrier layer. Cation ejection from the barrier layer into the solution and porous outer layer phase results in precipitation of a Cr(III) scale (oxide or hydroxyl-oxide) at the alloy surface which acts as a diffusion barrier. It is hypothesized that the outer layer is either physically removed at supercritical conditions due to rapid dissolution and grain boundary attack of the alloy or chemically removed by solution acidification due to the formation of sulphuric acid at high density supercritical conditions.

Alloys 625, 316 L, Ni - 20 Cr and pure Nb are tested at SCWO conditions and it is found that the corrosion resistance increases with Cr content and Nb is found to perform well in sulphate containing SCWO solutions at oxygen concentrations up to 4 m. It is also confirmed in this work that maximum material loss occurs in the high-density supercritical region of the reactor.

New Pourbaix diagrams for Nb at elevated temperatures (348 and 368 K) are calculated and compared to electrochemical and weight loss measurements performed in concentrated acids. Through electrochemical experiments in concentrated sulphuric and hydrochloric acids, Nb is found to be an ideal candidate for the high-density supercritical sections of SCWO reactors.