Abstract
Evaluating the water chemistry in the primary circuit and the effect of chemical econtamination
 of the heat exchanger tubes performed by the AP-CITROX (AP: alkaline permanganate; CITROX: citric and oxalic acid) procedure at Paks NPP (Hungary), a project dealing with the comprehensive investigation of the general corrosion state of the steam generators (SGs) has been initiated. Owing to the fact that there is no investigation method available for the in-situ monitoring of the inner surfaces of heat exchanger tubes, a research program based on sampling as well as on ex-situ electrochemical (voltammetric) and surface analytical measurements (SEM-EDX, CEMS, XRD, XPS) was developed and elaborated. In the time period of 2000-2008 - within the frame of the above project - 45 stainless steel specimens, cut out from various locations of the steam generators of the Paks NPP were investigated. Based on the measured corrosion characteristics (corrosion rate, thickness and chemical composition of the protective oxide-layer) it was found that these parameters are strongly dependent on the decontamination history of steam generators. The present work gives a brief overview on the general corrosion state of the heat exchanger tubes of SGs, concerning the long-term effects of the AP-CITROX procedure on the chemical composition and structure of the protective oxide-layer.
Highlights
Some years ago, a systematic study of the primary and secondary circuit water chemistry data, and the corrosion effects of the chemical decontamination procedures performed at Paks NPP made it clear that an overall estimation of the corrosion state of the steam generators, i.e. the preparation of a so-called ‘corrosion map’ is unavoidable
The primary aim of this paper is to report some new findings obtained by electrochemical and scanning electron microscopy (SEM)-energy dispersive X-ray microanalyzer (EDX) methods in order to reveal the beneficial changes in the corrosion properties, morphology and chemical composition of the inner surfaces of the decontaminated heat exchanger tubes after 4-7 years under normal operation conditions
The electrochemical data imply that no unfavourable trend in the passivity of the inner surfaces of the heat exchanger tubes can be detected since the last steam generators (SGs) decontaminations performed in 2000 and 2001
Summary
A systematic study of the primary and secondary circuit water chemistry data, and the corrosion effects of the chemical decontamination procedures performed at Paks NPP made it clear that an overall estimation of the corrosion state of the steam generators, i.e. the preparation of a so-called ‘corrosion map’ is unavoidable This ‘corrosion map’ takes a survey of the corrosion features of the heat exchanger tubes made from austenitic stainless steel in the SGs. Owing to the fact that there are no investigation methods available for the in-situ monitoring of the inner and outer surfaces of heat exchanger tubes, a research project based on sampling as well as on ex-situ electrochemical (voltammetric) and surface analytical measurements (SEM-EDX, CEMS, XRD, XPS) was launched in the year 2000. The applied version of the AP-CITROX procedure is an eight-step process, including an oxidizing pre-treatment of the surface with alkali potassium permanganate followed by a concentrated mixture of citric and oxalic acids to remove the contaminated surface layer (VARGA, 2004; RADO et al, 2006)
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