Corrosion Evaluation of Buried Cast Iron Pipes Exposed to Fire Water System for 30 years

摘要

The corrosion evaluation of buried cast iron pipe of fire hydrant system in a nuclear power station exposed to raw water for 30 years was carried out. The samples from cast iron pipe section were characterised by inductively coupled plasma atomic emission spectroscopy, optical microscopy, scanning electron microscopy, energy-dispersive spectroscopy, X-ray diffraction and laser Raman spectroscopy. Metallographic examination revealed typical flake graphite microstructure with uniform distribution, randomly oriented graphite structure, rosette grouping, for pipes of different diameters. The presence of microbes in water side of the pipes and in contact with soil was seen. SEM examination revealed loose spherical deposits, rust layers comprising of iron oxide, silica and spherical nodules covered with network structures. EDS analysis of rust layers revealed the presence of Si, Ca, S and P apart from Fe and O. XRD analysis revealed that the rust on the surfaces comprised of SiO2 and Fe2O3. The results of the study indicated that the external corrosion of cast iron pipes was due to sand and gravel type of soil in contact with pipes. EDS elemental X-ray maps revealed regions rich in Si indicating localised graphitic corrosion. Detailed corrosion characterisation studies revealed that despite signatures of graphitic corrosion, microbes and rust formation, there was no substantial reduction in the pipe thickness, and hence, the service life of buried cast iron pipes can be further extended.