Failure analysis of cupronickel evaporator tubes of a chilling plant

Pin-hole leakage was observed in a number of 90/10 cupronickel evaporator tubes of a chilling plant which was in service for about 6 years. Chilled water is cooled from 12 °C to 7 °C which flows inside the evaporator tubes, while the OD side has LiBr environment. The failure investigation of the failed tubes revealed greenish/blackish uniform deposits together with some spots where localized thick deposits on the ID surfaces were present. Underdeposit corrosion beneath these thick deposits was the primary reason for failure of the tubes. Intergranular corrosion was also observed underneath the thick deposits. The ID surface of the failed tubes confirmed the presence of wide variety of deposits, e.g. carbonates, silicates, phosphates, and sulphur-bearing compounds. The stagnant conditions under the thick deposits alter the chemistry inside the crevices thus formed. The altered chemistry conditions caused rapid corrosion (and intergranular attack) on the cupronickel tubes. Remedial measures that may be undertaken to prevent future occurrence of the problem are addressed.