Development of Chemical Decontamination Process Based on Ozone for System Surfaces with Chromium Containing Oxides

Chromite or chromium containing oxides are formed as a protective oxide film on the stainless steel surface of heat transport systems. The chemical dissolution of these passive oxide films forms an important step in decontamination formulation development for water-cooled nuclear reactor systems. Dissolved ozone as a reagent was tested for effective chemical dissolution of Fe³⁺ substituted in nickel chromite and individual component oxides. The study showed the importance of the solution pH and temperature on the dissolution kinetics of Cr₂O₃, NiO, and NiFe_xCr_2-xO₄. Neutral water pH or 0.04 mM OH⁻ were better for achieving a high dissolution rate for chromium containing oxides compared to acidic (2.5 mM H⁺) or alkaline conditions. In an acidic condition, the release of nickel from NiO or nickel chromite was more in the ozone medium compared to a high pH condition. Substitution of Fe³⁺ in nickel chromite affected the dissolution behavior in the ozone medium. The dissolution of Fe³⁺ substituted in nickel chromite showed a small increase in the dissolution rate constant with up to composition x = 0.4, and further increase in the Fe³⁺ composition in the oxide lattice decreased the dissolution rate constant.