A leakage was observed from 90/10-cupronickel tube bundles containing seawater as coolant, in the condenser unit of a heat exchanger. The failure occurred after two years service. The seawater was chlorinated to suppress microbial activities and also dosed with ferrous sulfate (FeSO4) to counteract possible sulfide and chlorine-accelerated corrosion of the cupronickel alloy. Failure analysis of the failed tubes revealed severe pitting corrosion underneath deposits (consisting mainly of CaCO3 and Cu2(OH)3Cl. The primary cause of the pitting corrosion is the continuous entrapment of aggressive sulfide species under deposits on the tube surfaces. This led to concentration effect, during the system stagnation and attack on the cupronickel alloy. Under this situation, the FeSO4 treatment program is ineffective, signifying the critical nature and limitations of the FeSO4 treatment. The deposition of FeSO4 may even be a potential corrosion initiator in the system, causing differential local cells - when trapped in the pores of the scale. Remedial measures that may be undertaken to prevent future occurrence of the problem are proposed.
Keywords: cupronickel, microbial corrosion, pitting corrosion, seawater, heat exchanger, localizedcorrosion, scale, underdeposit corrosion