MIC failure of cupronickel condenser tube in fresh water application

The failure of 90/10 cupronickel heat exchanger tubes in hydrogen coolers were seen after few years in use. The tubes had river water flowing at a velocity of 2.5 m/s. The shell side was exposed to hydrogen at a pressure of 3.5 kg/cm2. The inlet and outlet temperature of the cooling water of the cooler are 37 °C and 47 °C, respectively. The failures were pin hole type with a thick layer of muck deposit on the inner side of the tube. The metallographic studies carried out for the tube showed a well annealed microstructure. The region near the pit showed attack along the grain boundaries with dislodging of the grains. The electron probe micro analysis (EPMA) studies revealed the denickelification near the pits. The energy dispersive spectroscopy (EDS) analysis of the pit showed the presence of copper chloride and also the variation in the copper and nickel concentration with respect to the unaffected regions of the tube. Microbial presence was detected in the water as well as the muck deposits in the tubes. The pin hole failure of the tube was a result of the combined action of the chloride as well as the deposits in the tube which gave a conducive environment for the microbial activity to flourish which has caused the propagation of the pit through intergranular corrosion.