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Plate perforation occurred in the cylindrical section and walls of the inlet foot (2.38 mm thick Incoloy 825 plate welded using INCO welding rod 135) of an inert gas fire prevention system in an oil tanker. Cross-sectional microprobe analysis showed the corrosion product to contain sulfur, mainly from the flue gas, and calcium and chlorine, mainly from the sea water. The gray corrosion product was interspersed with rust and a black carbonaceous deposit. Corrosion pitting and poor weld penetration, with carbide precipitation and heavy etching at grain boundaries, indicated sensitization and susceptibility to aqueous intergranular corrosion. Chemical analysis showed the predominant acid radical to be sulfate (6.20% in the carbonaceous deposit and 0.60% in the corrosion product), suggesting that oxidation of SO2 in the flue gas caused the corrosion. Moisture condensation, the carbon acting as a cathode, and alloy susceptibility to intergranular corrosion contributed to the corrosion.

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