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概要

Pipe wall thinning is one of the important degradation phenomena taking place in the nuclear power plant. Major mechanism of the mainly pipe wall thinning is Flow Accelerated Corrosion (FAC), where chemical process plays the primary role. But mechanism of FAC is not yet full y understood. Especially characteristics of oxide film formed in FAC is not well known although oxide film property would be strongly related to thinning rate in FAC. The main objective of this work is finding characteristics of oxide film and examining a relationship betwee n film characteristics and FAC rate. FAC experiments were conducted for carbon steels for high pressure service (JIS G 3455) which have different Cr contents and pH. For investigation of microscopic defect structure, it was found that Transmission Electron Microscope (TEM) o bservation using Fresnel contrast is useful. As a result of observation, the oxide layer which formed on the specimens was consisted of porous and compact oxides in nanometer-scale and it was indicated that thinning rate had close relation to the thickness of compact oxides.

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