Effect of a chemical decontamination procedure on the corrosion state of the heat exchanger tubes of steam generators

In the present work, a comprehensive study of the general corrosion state of the heat exchanger tubes originating from different steam generators (SGs) of the Paks NPP (Hungary) was carried out. While the passivity of the inner surface of 25 stainless steel specimens was investigated by voltammetry, the morphology, chemical and phase compositions of the oxide-layer formed on the surface were analyzed by SEM-EDX, Conversion Electron Mössbauer spectroscopy (CEMS), and X-ray diffraction (XRD) methods. The systematic studies have revealed that corrosion characteristics (corrosion rate, thickness and chemical composition) of the surfaces observed in the long run are strongly dependent on the decontamination history of steam generators. Specifically, some adverse effects (general attack, formation of “hybrid” layer with accelerated corrosion rate and great mobility) have been detected with applying the AP-CITROX decontamination procedure. Process restrictions and modifications to minimize corrosion damages should be defined, and there is ample data available now for utilities to select a CITROX based process for particular application.