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50 DECEMBER 2016 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 55, NO. 12 FIGURE 3 Water-swept pits on Tube A showing classic horseshoes. Finally, Franci s showed that chlorin e/ hypochlorite greatly increases the suscep- tibility of Al brass to impingement attack in seawater. 2 This is particularly evident when ferrous sulfate (FeSO 4 ) dosing is not carried out at regular inter vals (usually daily) to maintain a protective FeO OH film. 2 Regular FeSO 4 dosing was not carried out at the Swedish plant. It was observed, however, that other heat exchangers tubed with Al brass, in service for a number of years, had well-developed films and were not suffering from impingement attack. Francis presented data showing that well- developed films on Al brass could provide good resistance to corrosion under excur- sions with more aggressive conditions. 5 When seawater and similar brines are dosed with hypochlorite, the concentration drops quickly as it reacts with organic ma- terial in the seawater. The concentration of ClO 2 dissipates much more slowly ; and at the concentrations described previously (0.6 to 1.1 mg/L), the impingement corro- sion of the Al brass is consistent with previ- ous findings. 2 The ClO 2 concentration was originally checked with a simple colorimetric test, which showed low concentrations in the water entering the heat exchangers. This test, however, was not sensitive to chlorite, and ClO 2 changes to chlorite fairly rapidly, which produces another powerful oxidizer. Hence, the concentrations quoted were cal- culated from the volume of ClO 2 being in- jected by the chemical dosing pumps. Conclusion The corrosion of the new Al brass heat exchanger tubes was due to impingement corrosion caused by high ClO 2 /chlorite concentrations in the water entering the heat exchangers. Remedial Action The condition of the internal surfaces of the four heat exchangers was evaluated by eddy current testing. This test showed that the first two units had experienced sig- nificant corrosion , and a decision was made to retube these immediately. Due to the long delivery time and a redesign re- quirement for installing Ti heat exchang- ers, these two heat exchangers were imme- diately retubed in Al brass to avoid further leakages. New replacement Ti heat ex- changers were then scheduled to be in- stalled before summer 2016, when it was necessar y to increase the ClO 2 injection rate to control fouling. The corrosion attack in the other two heat exchangers was less severe and it was concluded that th ese h eat exchangers could have a useful life with the existing tubes if protective films could be produced. The tubes in these units were cleaned to re- move deleterious films, and FeSO 4 dosing was carried out for one hour per day, using the parameters described by Shone and Grimm, 7 to help form protective films. It is necessary to turn off the hypochlorite or ClO 2 dosing while the FeSO 4 is being intro- duced. Acid cleaning would have been pre- ferred but there was insufficient down time available to permit this. The ClO 2 levels are being kept as low as possible (~0.3 mg/L) to prevent any further corrosion of the Al brass tubes. During the winter this is not a problem, but during summer, this concentration is not suffi- cient to control fouling. To avoid further problems, Borealis decided to also replace the remaining two bundles to titanium. References 1 P.D. Goodman, Br. Corros. J. 22, 1 (1987): p. 56. 2 R. Francis, "Effects of Chlorine Additions to Cooling Water on Corrosion of Copper Alloy Condenser Tubes," MP 21, 8 (1982): p. 44. 3 G.C. White, Handbook of Chlorination and Alternative Disinfectants, 4th ed. (Hoboken, NJ: John Wiley & Sons, 1999). 4 L e n n t e c h , " D i s i n f e c t i o n ," h tt p : / / w w w. l e n n t e c h . c o m / p r o c e s s e s / d i s i n f e c t i o n / disinfection.htm (Oct. 21, 2016). 5 R. Francis, The Corrosion of Copper and Cop- per Alloys: A Practical Guide for Engineers (Houston, TX: NACE International, 2010). 6 F. Feigl , Spot Tests in Inorganic Analysis (Amsterdam , The Netherlands: Elsevier, 1958). 7 E.B. Shone, G.C. Grimm, "25 Years Experi- ence with Seawater Cooled Heat Transfer Equipment in the Shell Fleets," Trans. Inst. Mar. Eng. 98 (1985). ROGER FRANCIS, FNACE, is the director of RFMaterials, 1 The Lodge, Hadfield, Glossop, Derbyshire, SK13 1QN, United Kingdom, e-mail: rfrancis@rfmaterials.com. He has been a corrosion engineer for more than 40 years with extensive experience in marine corrosion, desalination, oil and gas, chemical and process, power, and mineral processing corrosion. He has worked with most corro- sion-resistant alloys in these environments and has published books on the corrosion of copper alloys and stainless steels. He has a Ph.D. and is a Fellow of NACE International, ICorr, and MIMMM. A NACE member for 24 years, Francis received the NACE T.J. Hull Award for his work in publishing. CLAES HEDVALL is a chief engineer at Borealis AB, 44486, Stenungsund, Sweden. He has an M.S. degree in chemical engineer- ing from Chalmers University, Gothenburg, Sweden. He has 30 years of experience working on petrochemical projects, start-up operations, and troubleshooting systems in Europe and the Middle East. MATERIALS SELECTION & DESIGN