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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38 DECEMBER 2014 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 53, NO. 12 BLOG Continued from The MP Blog, p. 13. The following items relate to cathodic & anodic protection. Please be advised that the items are not peer-reviewed, and opinions and suggestions are entirely those of the in- quirers and respondents. NACE Interna- tional does not guarantee the accuracy of the technical solutions discussed. MP welcomes additional responses to these items. They may be edited for clarity. Internal anodes for storage tanks Q: I have a question about using sacrif icial anodes for product storage tank bottoms. The tanks have mostly condensate water in the bottom. Condensate water has been shown to cause moderate corrosion of the tank bottoms. Some of the tanks have anodes, but they are not enough. I do not have enough data to support the use of the anodes or determine if the design was adequate. Any comments? A: We have designed internal cathodic protection for several product tanks. Magnesium slab anodes or zinc work equally well depending on assumed condensate resistivity. Te trick is to feld-locate the anodes in the areas where the tank foor has settled. I know of no monitoring method. One client conducted an internal inspection after f ive years. Visible deterioration of the slab anode was noted ¼ to ¾ in (6.35 to 19 mm) up the anode signif ying the water level. There was no corrosion on the tank f loor. Depleted anodes on marine pipeline Q: I have encountered an unusual situation on a marine pipeline channel crossing. Two 10-in (250-mm), well-coated 7,000-ft (2,134-m) lines have eight 11-anode strings. Each anode is 150-lb (68-kg) zinc. The pipes and groundbed are 24 years old. The pipes and most, if not all, of the anodes are silted over. Resistivity of the water is 23 Ω-cm. The pipes are well polarized to –1,125 mV to copper/copper sulfate (Cu/CuSO 4 ) electrode (CSE). However, after measur- ing current output of the eight anode strings, I found one string putting out 82 m A, four others with zero current, and three returning a total of 58 m A to the lines. I disconnected all the strings except the one with current f low in the proper direction and now achieve protection with 25 m A. I am uncertain as to the remaining life of the system and am therefore consider- ing replacing the groundbeds with alumi- num (cutting emission of zinc ion to the coastal waters is considered a plus).

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