Corrosion analysis and diagnosis – useful tools for plant life management and long term operation programmes in nuclear power plant

Abstract

Steam Generator degradation has caused substantial losses of power generation, resulted in large repair and maintenance costs. Institute for Nuclear Research has carried out an extensive R&D program focused on the understanding of the degradation processes especially for the tubing material and on developing remedial actions in the purpose to prevent and diminish the ageing process of which evolution supposes some considerable economic costs. Because of the huge impact of corrosion, it is imperative to have a systematic approach to recognizing and diminish corrosion problems as soon as possible after they become apparent. A proper failure analysis includes collection of pertinent background data and service history, followed by visual inspection, photographic documentation, material evaluation, data review and conclusion procurement. In analyzing corrosion failures, one must recognize the wide range of common corrosion mechanisms. The features of any corrosion failure give strong clues as to the most likely cause of the corrosion. The principal steps of analysis and diagnosis of the steam generator tubes degradations consist in: visual inspection, chemical analysis, cross section examination by optical and scanning electron microscopy (SEM) and X-ray diffraction (XRD), data review, conclusions and recommendations. The paper details a proven approach to properly determining the root cause of a failure, and includes metallographic illustrations of the most common corrosion mechanisms, including general corrosion, pitting, crevice corrosion, corrosion fatigue and intergranular corrosion.