JANUARY 31 2003
CAUTION: Polarized potentials that result
in excessive generation
of hydrogen should
be avoided on all metals, particularly higher strength steel, certain grades of stainless
steel, titanium, aluminum alloys, and pre-stressed concrete pipe.
Note: The earth current technique is often meaningless in multiple pipe rights-of-way,
in high resistivity surface soil, for deeply buried pipe, in stray current areas, or where
local corrosion cell action predominates.
Aluminum Piping. The following criterion applies:
A minimum of 100 mV of cathodic polarization between the structure
surface and a stable reference electrode contacting the electrolyte. The
formation or decay of this polarization can be used in this criterion.
CAUTION: Excessive Voltages--Notwithstanding the minimum criterion in paragraph
6-3, if aluminum is cathodically protected at voltages more negative than -1200 mV
measured between the pipe surface and a saturated copper/copper sulfate reference
electrode contacting the electrolyte, and compensation is made for the voltage drops
other than those across the pipe-electrolyte boundary, it may suffer corrosion as the
result of the buildup of alkali on the metal surface. A polarized potential more negative
than -1200 mV should not be used unless previous test results indicate that no
appreciable corrosion will occur in the particular environment.
CAUTION: Alkaline Conditions--Aluminum may suffer from corrosion under high pH
conditions, and application of cathodic protection tends to increase the pH at the metal
surface. Therefore, careful investigation or testing should be conducted before applying
cathodic protection to stop pitting attack on aluminum in environments with a natural pH
in excess of 8.0.
Copper Piping. The following criterion applies:
a) A minimum of 100 mV of cathodic polarization between the structure
surface and a stable reference electrode contacting the electrolyte. The formation or
Dissimilar Metal Piping. A negative voltage between all pipe surfaces and
a stable reference electrode contacting the electrolyte equal to that required for the
protection of the most anodic metal should be maintained.
CAUTION: Amphoteric materials that could be damaged by high alkalinity created by
cathodic protection should be electrically isolated and separately protected.
Determining Voltage Drops. Methods for determining voltage drop(s) shall