Corrosion behaviour of Inconel 625 and Incoloy 800H, two of the candidate fuel cladding materials for Canadian supercritical water (SCW) reactor designs, were evaluated by exposing the metals to SCW in UNB's SCW flow loop. A series of experiments were conducted over a range of temperatures between 400oC and 600oC and the corrosion rates were evaluated as the weight change of the materials over the exposure time (typical experiments measured the weight change at intervals of 100, 250, and 500 hours, with some longer term exposures included). SEM, EDX, and TEM techniques were used to examine and quantify the oxide films formed during exposure and the corrosion mechanisms occurring on the candidate metals.
Data from in-house experiments were used to create an empirical kinetic equation for each material that was then compared to literature values of weight change. Dissolved oxygen concentrations varied between experimental sets, but for simplicity were ignored since the effect of dissolved oxygen has been demonstrated to be a minor secondary effect. Activation energies for the alloys were determined with Inconel 625 and Incoloy 800H showing a distinct difference between the low-temperature electrochemical corrosion mechanism and direct high-temperature chemical oxidation. The results were modelled using these separate effects showing dependence on the bulk density and dielectric constant of the SCW through the hydrogen ion concentration.