Materials Performance

DEC 2016

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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48 DECEMBER 2016 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 55, NO. 12 MATERIALS SELECTION & DESIGN C This case history reviews the reactions that occur when hypochlorite or chlo- rine dioxide (ClO 2 ) is added to sea- water to control biological fouling in cooling water systems. The active concentrations of ClO 2 and its prod- ucts, such as the chlorite ion, remain in significant concentrations through- out the system. The failure of four heat exchangers, tubed with Al brass, is related to the change from hypo- chlorite to ClO 2 dosing. The problems of monitoring the concentration of chlorite are discussed. Chlorin e/hyp o chlorit e i s frequ ently added to seawater used for cooling to c ontrol biofoulin g in pipin g and h eat exchangers. It is usually added in liquid form as sodium hypo chlorit e (NaClO) s o l u t i o n s . Th e c h e m i c a l re a c t i o n s of chlorine in seawater were reviewed in detail by Goodman. 1 Chlorine is a powerful oxidizer, and in more than small concentra- tions (a few mg/L) it can cause accelerated corrosion of stainless steel and copper alloy heat exchanger tubing. 2 In recent years, there has been a move away from chlorine to chlorine dioxide (ClO 2 ) as a disinfectant in fresh waters, and ClO 2 is now starting to be used in seawater antifouling systems as well. A corrosion problem was found with Al brass heat ex- changer tubes, highlighting a potential cor- ro sion pro bl em th at can o c cur w h en switching from chlorine to ClO 2 . Chlorine Dioxide vs. Chlorine Before describing the corrosion prob- lem, it is important to highlight the chief differences between ClO 2 and chlorine. ClO 2 is superior to chlorine for removing un- wanted tastes and smells associated with phenols and decaying vegetation . 3 It is equally as effective as chlorine when used as a bactericide, and is superior as a viri- cide. ClO 2 also is more environmentally friendly because it does not produce chlori- nated byproducts such as trihalomethanes. ClO 2 also kills some pathogens that are re- sistant to chlorine, and it does not react with ammonia or amines. When chlorine reacts with species in water, it is usually reduced to the chloride ion or a chlorinated organic compound, which will react no further. In contrast, ClO 2 is present as a free radical when dis- solved in water. Like chlorine, ClO 2 is an oxidizing disinfectant; however, it reacts with less of the matter in seawater than chlorine (i.e., it is more specific in its action on living organisms). Hence, after addition, ClO 2 is consumed at a much slower rate than chlorine, as shown in Figure 1. 4 Addi- tionally, the primary reaction product with ClO 2 is chlorite, which is a less powerful oxidizer. The net result is that the oxidizing power of water with added ClO 2 remains high for some time throughout the cooling water system. In contrast, the free chlorine concentration decreases sharply very soon af t er a d dition an d th en d e cay s m ore slowly. 1 In summary, both chlorine/hypochlo- rite and ClO 2 are strongly oxidizing, al- though the detailed mechanism of their CASE HISTORY The Effect of Chlorine Dioxide on Corrosion in Seawater Roge R F R an C is, F na C e , RFMaterials, Glossop, United Kingdom Claes h edvall, Borealis AB, Stenungsund, Sweden

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