MIL-HDBK-1004/10
Section 3:
CRITERIA FOR CATHODIC PROTECTION
Introduction. Various methods are available for determining
3.1
whether the structure to be protected is being effectively protected through
the application of cathodic protection. The technical basis for corrosion and
cathodic protection is electrochemical. Electrochemical methods of
determining the effectiveness of cathodic protection systems are the most
widely used criteria for establishing the adequacy of the protection. In
addition to electrochemical methods, inspections to determine the actual
condition of the structure being protected can be used to determine whether or
not effective protection has been achieved in the past. If there is no attack
of the protected system in an aggressive environment, then the protective
system has been functioning adequately. For buried or submerged systems where
access is restricted, the electrochemical criteria are most widely applied.
Electrical Criteria. For submerged and buried structures, criteria
3.2
based upon the electrochemical potential of the surfaces of the structure to
be protected are the most widely used criteria for determining whether or not
the structure is being effectively protected. In making these electrochemical
used to measure the difference in potential between the structure and a
reference electrode placed in contact with the electrolyte. For buried
structures, the copper/copper sulphate reference electrode is the reference
electrode most commonly used for this purpose. For structures submerged in
seawater the silver/silver chloride reference electrode is commonly used.
Other reference electrodes can be used when appropriate. Potential readings
obtained using any given reference electrode can be related to readings
obtained with other reference electrodes. In order to the assure that the
potential readings obtained are properly interpreted, the reference electrode
used should always be noted. Readings should be reported as "XX.XX V versus
YYY" where YYY is the reference electrode used to measure the structure
potential.
As these potential measurements are most commonly used to measure
the potential of buried pipelines they are commonly called "pipe-to-soil
potentials" even though they may refer to the wall of a water storage tank in
contact with potable water. The more precise term for these measurements is
"structure-to-electrolyte potential."
Interpretation of Structure-to-Electrolyte Potential Readings. In
3.3
order to determine whether or not a given surface is being adequately
protected, structure-to-electrolyte measurements are taken at various
locations surrounding the structure. Based upon a combination of corrosion
theory, experimental and laboratory tests, and more importantly, upon actual
field experience with a large number of protected structures, criteria for
interpreting these structure-to-electrolyte potentials have been developed.
3.3.1
National Association of Corrosion Engineers (NACE) Standard RP-01-
69. The most widely used criteria for evaluating structure-to-electrolyte
potentials have been included in the NACE Standard RP-01-69, Recommended
Practice for Control of External Corrosion on Underground or Submerged Piping
Systems.
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