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CP BLOG Continued from page 41
–1,112 mV range to a 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 mA (both dropping as the structure polarized) 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 mA) currents to the structure. 1[ \PQ[ K]ZZMV\ ÆW_ MV\QZMTa JMKI][M WN \PM potential difference between open circuit anode strings and the protected structure? For testing, I used a digital multimeter
IVL \PM ¤ [P]V\[ J]QT\ QV\W \PM KI- thodic protection system. The meter was calibrated within the last six months, but even on 4.5-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 con- sistent 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 out- put. This indicates to me that something has happened to either deplete the anodes or to passivate them.
strings, considering just 25 mA when you disconnect all strings except the one with 82 mA (82 – 58 = 29 mA). The high zinc anode consumption may be caused by high initial current demand and buildup of calcium carbonate (CaCO3
A A
) in time. NACE International, Vol. 51, No. 11
It sounds like you may have some high-resistance connections or maybe some broken wires in the zero output strings.
It is clear that the string providing 82 mA to the structure also pro- vides 58 mA to the other "low electro-negative potential"
low current output now with the pipeline polarized to –1,125 mV vs. CSE, as you have determined. I have dealt with many cases where the current demand is very low from magnesium as well as zinc rib- bon, after polarization has been reached.
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Internal anodes for storage tanks
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I have a question about using sacrificial an- odes for product stor- age 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 old enough. I do not have enough data to support the use of the anodes or determine if the design was adequate. Any comments?
We have designed internal ca- thodic protection for several product tanks. Magnesium slab anodes or zinc work equally well
depending on assumed condensate resis- \Q^Q\a