Corrosion study of heat exchanger tubes in pressurized water cooled nuclear reactors by conversion electron Mössbauer spectroscopy

57Fe-conversion electron Mossbauer spectroscopy (CEMS) - a sensitive tool to analyze the phase composition of corrosion products on the surface of stainless steel - was applied to study real specimens from the Paks Nuclear Power Plant, Hungary. The primary circuit side of the heat exchanger tubes was studied on selected samples cut out from the steam generators during regular maintenance. Mostly Cr- and Ni-substituted magnetite, amorphous Fe-oxides/oxyhydroxides as well as the signal of bulk austenitic steel of the tubes were detected. The level of Cr- and Nisubstitution in the magnetite phase could be estimated from the Mossbauer spectra. If is suggested that Cr-Ni substitution occurs simultaneously so that the inverse spinel structure of magnetite is preserved up to a certain limit which appears to be roughly at [Fe3+]tet[Fe2+1/4Ni2+3/4Fe3+1/4Cr3+3/4]octO4. Further decrease of the iron content of this phase results in the formation of nickel chromite of regular spinel structure, with very low Fe content. This transformation may be responsible for the hybrid structure of the protective oxide layer, being substantially accelerated by previously performed, factory developed and proposed AP-CITROX decontamination cycles.