Materials Performance

DEC 2014

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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39 NACE INTERNATIONAL: VOL. 53, NO. 12 MATERIALS PERFORMANCE DECEMBER 2014 While researching the literature on zinc/ aluminum combinations I found "Long-Term Performance of A luminum Anodes in Seawater and Marine Soil," January 1999 MP. This article mentions that aluminum in combination with zinc will cause the zinc to passivate and current output from zinc will cease quick ly. I also think that at some point these pipes were shorted to land-side impressed systems. I cannot explain why 13,200 lb (5,988 kg) of zinc, with such low current output required, is not working after this time. Articles from the University of Manchester claim that zinc maintains very high eff iciency even at low current outputs, so self corrosion of the zinc does not appear to be likely. Could the zinc anodes have become passivated? If so, can I reverse it? I disconnected four of the zero or reverse current strings and found open circuit potentials in the –1,029 to –1,112 mV range to CSE (less negative than the protected structure). I hooked these up one at a time to a temporary carbon steel structure in seawater and got zero current out of two and 215 and 180 m A (both dropping as the structure polar- ized) out of the others. I consider the two zero current strings depleted. The other two strings appear to be good, but when hooked up they return small (10 to 14 m A) currents to the structure. Is this current f low entirely because of the potential difference between open circuit anode strings and the protected structure? For testing, I used a digital multi- meter and the 0.01-Ω shunts built into the cathodic protection system. The meter was calibrated within the last six months, but even on 4 ½-digit mode, I was pushing the lower limits of the instrument's accuracy. Even if the readings were off by an order of magnitude, the current outputs are too low to consume the anodes, assuming that the current demands have remained consistent over the lifetime of the structure and system. However, I cannot make this assump- tion. I am obligated to design and install new groundbeds unless I can show that they will not be needed in at least 10 years. I suspect that there is still a lot of good zinc out in the channel, but I can't demonstrate this to be true. Of the four strings that I tested against a temporary cathode, two had almost no current output. This indicates to me that something has happened to either deplete the anodes or to passivate them. A: It sounds like you may have some high-resistance connections or maybe some broken wires in the zero output strings. A: It is clear that the string provid- ing 82 m A to the structure also provides 58 m A to the other " low electro- negative potential" strings, considering just 25 m A when you disconnect all strings except the one with 82 m A (82 – 58 = 29 m A). Te high zinc anode consump- tion may be caused by high initial current demand and buildup of calcium carbon- ate (CaCO 3 ) in time. A: A luminum anodes would be a good choice, but you will have to control the current output closely. You probably will also fnd very low current output now with the pipeline polarized to –1,125 mV vs. CSE, as you have deter- mined. I have dealt with many cases where the current demand is very low from magnesium as well as zinc ribbon, after polarization has been reached. Impedance measurements Q: Has anybody ever measured the impedance of bare metal in soil or read about it? Specif ically, I would like to know the ty pical polarization resis- tance and the capacitance of carbon steel in soil when it is cathodically protected. A: You might look at harmonic analysis for corrosion monitor- ing. Most impedance gear should be able to do the measurements. A: Tere are several papers describ- ing the application of electro- chemical impedance spectroscopy to evaluate the efectiveness of cathodic protection in soil: • "Monitoring Cathodically Protected Steel in Concrete Structures with Electrochemical Impedance Tech- niques," N.G. Tompson, et al., Corrosion 44, 8 (1988): pp. 581-588. • "Corrosion Monitoring and Life Prediction of the Bottom Plate of a Fuel Storage Tank," T. Tsuru, S. Sudoh, MP 31, 6 (1992): pp. 53-56. • "How to Monitor Internal Corrosion in Gas Lines," K.B. Burnhan, Pipe Line Ind. 65, 12 (1986): pp. 30-31.

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