Failure analysis of leaking firefighting pipeline used in nuclear power plant: Corrosion development process and key influence factors

A penetrating hole has been found in the inner wall of the firefighting pipeline (A106 steel) used for 20 years in a nuclear power plant of China, and the pipeline is filled with almost stagnant tap water before failure. The failed pipeline is taken from a vertically placed water outlet pipeline. The corrosion process and key influence factors of the failed firefighting pipeline were studied using optical microscopy (OM), scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results show that there exist two typical corrosion characteristics on the inner wall of the failed pipeline besides leakage hole, that is, corrosion nodulation and corrosion bulge. The corrosion nodulation, mainly consisting of a mixture of calcium and iron oxides, is the initiation position of perforation corrosion, and the iron-oxidizing bacteria (IOB) play a key role in the formation of corrosion nodulation. Then the sulfate-reducing bacteria (SRB) is involved in the propagation of corrosion nodulation when the dissolved oxygen concentration is low. The corrosion nodulation gradually become loose and form large bulge with extended service time. The corrosion pits under the corrosion bulge also develop gradually. Eventually, the inner wall of the pipeline is thinned and perforated.