Graphitic Corrosion of a Gray Iron Water-Main Pipe Resulting in a Corroded-Through Hole

A section of cast iron water main pipe contained a hole approximately 6.4 x 3.8 cm (2.5 x 1.5 in.). The pipe was laid in clay type soil. Examination revealed severe pitting around the hole and at the opposite side of the outside diam. A macroscopic examination of a pipe section at the hole area showed that the porosity extended a considerable distance into the pipe wall. Metallographic examination revealed a graphite structure distribution expected in centrifugally cast iron with a hypoeutectic carbon equivalent. Chemical analyses of a nonporous sample had a composition typical of cast iron pipe. Chemical analyses of the porous region had a substantial increase in carbon, silicon, phosphorus, and sulfur. The porous appearance and the composition of the soft porous residue confirmed graphitic corrosion. The selective leaching of iron leaves a residue rich in carbon, silicon, and phosphorus. The high sulfur content is attributed to ferrous sulfide from a sulfate reducing bacteria frequently associated with clay soils. Reinforced coal tar protective coating was recommended.