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CP BLOG Continued from page 29
z A relatively high amount of anodes are installed relatively close (in the order of 10 to 20 ft [3 to 6 m]) to the pipeline (impressed current).
z The pipeline does not need to be isolated from other metal objects (no insulation joints necessary).
This method has been applied in situa-
tions where many underground pipelines are installed in a small area where it is very difficult and expensive to install insulation joints (e.g., water treatment works). The contractor calls it "hot spot" CP, although I was under the impression that hot-spot protection meant protecting areas along a pipeline that are most prone to corrosion caused by corrosive soils or damaged coating. I would like to know if anyone is famil-
iar with this CP method. Which criteria can I use to determine if this CP method is effective? I am not sure if I can use the criteria from NACE SP0169, "Control of External Corrosion on Underground or Submerged Metallic Piping Systems."
sible to provide adequate CP to buried metallic structures. Generally, the method of choice is
A NACE International, Vol. 51, No. 4 A
to protect all buried metallic structures. This typically means installing impressed current CP (ICCP) through the use of rectifiers. This electrically bonds all structures together. The use of external buried coupons to check the adequacy of CP on critical structures can probably JM R][\QÅML I would not recommend just trying to
protect the steel pipes (as your contractor suggests) and ignoring the effects that the ICCP may be having on any adjacent metallic structures. Your question can be very complex and knowledgeable corro- sion control professionals should study each site carefully.
In the event you do not have ac- cess to or control over the adja- cent pipelines, and their owners do not want them to be affected,
April 2012 MATERIALS PERFORMANCE 31
Those present in complex envi- ronments such as petrochemical ZMÅVMZQM[ IVL _I\MZ IVL _I[\M- water treatment plants, it is pos-
one solution is to use polymeric continu- ous anodes for the portion of the pipeline that shows the hot spot. Generally these anodes are low-power and cause no inter- ference with adjacent lines. They are close enough to the structure to mitigate, and in many cases solve, interference from the outside. This has been a solution for cases with multiple owners in a right-of-way.
sites. To be effective, the current should be distributed evenly by appropriate an- ode placement and all the structures po- larized to 850 mV "off" potential. Conti-
A Continued on page 32
This technique is actually distrib- uted ICCP, commonly used in plants, compressor stations, and even underground storage tank