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49 NACE INTERNATIONAL: VOL. 55, NO. 12 MATERIALS PERFORMANCE DECEMBER 2016 action on living material (e.g., bacteria, viruses, and fungi) are different. Their oxi- dizing characteristics create a similar cor- rosion risk, as described next. Service Experience Borealis AB operates a plant on the west coast of Sweden that uses seawater for cool- ing. Four shell and tube heat exchangers are used to cool propylene gas from 45 to 39 °C. The seawater intake temperature varies from 14 to 20 °C, and the discharge temper- ature is ~32 °C. The seawater on the Swedish west coast is similar to normal seawater, but with about two thirds the concentration of chloride (salinity = 25 g/L). The seawater was formerly dosed with NaClO to control fouling, but in 2012 the NaClO was changed to ClO 2 for environmental reasons. The ClO 2 concentration in the incoming cooling water typically has been in the range of 0.6 to 0.8 mg/L, with ClO 2 excursions up to 1.1 mg/L. The heat exchanger units, which had been in service since 2000, started to expe- rience frequent leaks during 2012. The tubes were judged to be at the end of their useful life, and were retubed in Al brass (UNS C68700), the same material as the original tubes. After only eight months of service, leakage of several tubes occurred again. The heat exchangers have tube plates of Al bronze (UNS C61400) with rubber-lined carbon steel water boxes. In addition, sacri- ficial iron anodes were fitted into each water box. The heat exchanger construc- tion is the three-pass type, and the nominal water flow rate is ~1 m/s. The failures were not at the entries or discharges to the passes but along the length of the tubes. The deepest corrosion attack was at the end of the tube where the warm propylene gas enters. The depth of attack was greatest in the upper section (near the inlet), with a less severe attack in the center tubes and even less attack in the lower section. This was confirmed by eddy current testing of several tubes from each pass. The lack of corrosion at the tube ends is thought to be due to the ca- thodic protection provided by the iron anodes. FIGURE 1 Residual for different doses of chlorine and ClO 2 . 4 FIGURE 2 Inner surface of Tube A showing broad, water-swept pits. Examination of Tubes Figure 2 shows the appearance of the corrosion in one of the tubes, with broad, water-swept pits. Figure 3 shows a close-up of the tube. The classic "horseshoes" of im- pingement attack (erosion-corrosion) are clearly visible, with the horseshoes going upstream. The attack on the other tubes was similar in appearance. There are a number of possible causes of impingement attack of Al brass tubes in seawat er. 5 Th e f irst i s excessive wat er velocity, but at 1 m/s, the velocity is well within recommended guidelines. The sec- ond is the presence of sulfide, but the sodium azide (NaN 3 ) test 6 (a very sensitive and specific spot test) showed that sulfide was not present in the corrosion products.