Materials Performance

NOV 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 NOVEMBER 2014 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 53, NO. 11 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 Please be advised that the items are not peer-reviewed, and opinions and suggestions are entirely those of the in- quirers and respondents.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. Cathodic protection for firefighting system Q: When buried water-handling pipelines and piping are to be externally cathodically protected, what is the maximum water conductivity for preventing internal bridging at insulation joints or f langes? A: I was involved in the design of a large gas processing plant some years ago where the buried and frewater lines were cathodically protected and isolated from aboveground deluge headers with insulating fange kits. Our cathodic protection (CP) design report suggested a minimum length of internal coating for each pipe diameter. Ty pically for 20-in (508-mm) pipe we recommended a minimum coated length of 600 mm with 2,000 Ω-cm water resistivity. Tis worked efectively but not before we had to open many fanges after the frewater system had been flled and pressurized. Te CP system died because of shorting across the isolators. Te piping engineers refused to believe that internal coating was necessary so did not require it of the contractors. However, they soon changed their minds when they saw the extensive corrosion of the aboveground fange. So, once you have determined through exper- imentation the necessary coated length for each pipe diameter, be resolute because your CP system will not function without efective electrical isolation, and corrosion of the aboveground fange face will occur. A: Internally lining the pipeline is a good solution but the internal lining should only be on the drain point (current return) side of the isolation fange and not on both sides. Te reason is obvious; any pinholes in the lining on the unprotected side can result in concen- trated corrosion. Anodes for seawater- compensated fuel tanks Q: Some time ago, some of our ships started using seawater- compensated tanks for "nav y distillate" fuel. The original plan would have involved using bladders to separate the two liquids, but that idea fell victim to the old upfront cost problem. A question has arisen concerning the need for anodes in these tanks. There is a fear that Zn anodes would not perform well as they would build up a layer of oily sludge every time the tank was pumped out. Both the fuel and the seawater are clean, so I can only assume that the oily sludge was produced by microbiological activity in the water/fuel interface. The seawater is "clean" because it is unpolluted and from the open ocean. Using Mg anodes is also strictly forbid- den for safety reasons. The tank is lined with a two-part epoxy that seems to be standing up quite well (I personally prefer to use anodes wherever possible, but there is the old adage, "If it ain't broke, don't fix it)," hence the question about the need for anodes in the first place. My questions are, will Zn perform under these circumstances? If not, what is the usual practice/anode material for seawater-compensated fuel tanks? A: Where does one obtain "clean" seawater? When closely examined, it contains sediment. Te sludge is probably from this sediment, which settles out. Under the circum- stances and the assumption that the tank is lined, you can't use Mg anodes. Because the sludge would have a relatively high resistance, can you use impressed current?

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