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MP BLOG Continued from page 10 In Brazil, there are many piping systems in which inhibitors cannot be used for several reasons. We have already thought of galvanic anodes, but normally the water resistivity is high. We are afraid of changing the chemical composition of the water, especially when it has to be potable. We are thinking of installing discrete impressed current anodes along the pipe. Does anybody know how we can calcu- 12 MATERIALS PERFORMANCE May 2013 A late the space of the anodes as a function You have a very challenging of the diameter? Is there any mathematiproblem. I am aware of compacal or empirical model for it? There are nies that have experimented with many different pipe diameters involved the application of CP inside (from 150 to 2,000 mm). pipelines and were unsuccessful. The The pipe has been operating for some problems I see are as follows: years. Unfortunately, we cannot coat the • It is difficult to devise an anode pipe because it supplies water for indussystem that is reliably suspended in try. If we shut off the pipe, we have to the pipe while avoiding electrical stop production. shorts to the pipe. • The anode suspension has to be such that it does not signifcantly affect the fow characteristics and reduce the throughput. • Many electrical access points through the pipe wall are needed to power the anodes because long stretches of single anodes tend not to have uniform current distribution, resulting in high current discharge close to the source and probable premature anode failure. • In systems with high or turbulent fow rates, the depolarizing effect may make protection itself diffcult to achieve, even if the other problems are solved. • The fnal problem would be developing an effective and economical method of measuring the effectiveness of the CP. This would likely involve another series of access fttings through the pipe wall to insert references. There are sacrifcial anodes that can be applied in potable water, and they are used in water heaters—activated aluminum and magnesium. These types of anodes in ribbon form may be the most practical to apply since they could be attached along the bottom or side of the inside of the pipe without causing much effect on fow. I would suggest a current requirement of ~10 mA/m2. My last thought is that one should seriously look at in situ application of a suitable internal coating. This might be less costly in the long run and usually has a bonus of lowering the fow resistance and increasing throughput. CP requires signifcant maintenance. NACE International, Vol. 52, No. 5