Nace CP 1, Course Manual

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Underground Corrosion Control 3:23
CP 1 – Cathodic Protection Tester Course Manual
©
NACE International, 2000
02/01/05
influence the cathodic protection circuit resistance. In some cases, constant
current rectifiers are needed to maintain cathodic protection.
Oxygen can easily penetrate porous soils. Oxygen acts as a depolarizer,
thus increasing current requirements.
Oxygen concentration cells may develop at points of contact between
gravel and the structure. Cathodic protection current may not penetrate into
the point of contact. As a result, it may be difficult to protect structures
backfilled with or resting on gravel or crushed rock.
Glacial Till
Glacial till is similar to gravel and the concerns noted above apply also to
this type of soil.
Temperature
Corrosion rates tend to increase with temperature. Consequently, in warmer
electrolytes, current requirements will be higher than in cooler ones. Heat
exchanger water boxes and oil heater-treater vessels are good examples of
higher temperature environments. Gas pipelines downstream of a
compressor station normally require a high current density for the increase
in temperature caused by the compressed gas.
Increasing temperature acts to reduce polarization. This also increases
current requirements.
As mentioned in the discussion of galvanic anode materials, elevated
temperatures can affect the performance of aluminum and zinc anodes.
Oxygen Content
As oxygen levels increase, polarization tends to decrease. Thus, in
oxygenated environments, high current requirements are to be expected.
Movement of Structure and Electrolyte
The relative movement between a protected structure and the electrolyte
affects current requirement. As the relative velocity increases, so does the
current requirement due to decreasing polarization.
Ship hulls are a good example of this phenomenon. At anchor, the current
requirement for the hull is low. When under way, however, the current