Criteria for Steel. The criteria options for the cathodic
protection of steel and cast iron in soil and water are as follows:
a) -850 mV or more negative with respect to a copper/copper
sulfate reference cell. This potential is measured with the protective
current applied. For valid interpretation, the potential measurements must be
corrected for IR drop through the electrolyte and metallic paths.
b) 100 mV or greater negative polarization shift measured between
the pipe surface and a stable reference electrode contacting the electrolyte.
The formation or decay of this polarization can be used in this criterion.
c) A potential at least as negative as the potential established
by the E log I curve method.
d) A net protective current from the electrolyte into the surface
of the structure as determined by an earth current technique.
100 mV or greater negative polarization
Criteria for Aluminum.
shift (refer to para. 126.96.36.199).
Excessive Voltages: If cathodically protected
at voltages more negative than -1.20 V measured
between the structure surface and a saturated
copper-copper sulfate reference electrode
contacting the electrolyte and compensated for
the voltage (IR) drops other than those across
the structure-electrolyte boundary, may suffer
corrosion as the result of the build-up of
alkali on the metal surface. A voltage more
negative than -1.20 V should not be used unless
previous test results indicate no appreciable
corrosion will occur in the particular
Alkaline Soil Conditions: Since aluminum may
suffer from corrosion under high pH conditions
and since application of cathodic protection
tends to increase the pH at the metal surface,
careful investigation or testing should be made
before applying cathodic protection to stop
pitting attack on aluminum structures in
environments with a natural pH in excess of
Criteria for Copper.
(refer to para 188.8.131.52).
Criteria for Dissimilar Metal Structures. A negative potential
equal to that required for the most anodic materials should be maintained.
The potential should not exceed the maximum allowable potential for any
material (such as for aluminum) in the system.
Other Electrical Criteria. Criteria evaluation of the structure-
to-electrolyte potentials on other materials have been developed but are not
included in NACE RP-01-69. The same measurement techniques and precautions
are applicable to these criteria as for those in NACE RP-01-693.