Nace CP 1, Course Manual

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TM0497-2002
18 NACE International
NACE Publication 35201 (latest revision). “Technical Report
on the Application and Interpretation of Data from
External
Coupons Used in the Evaluation of
Cathodically Protected Metallic Structures.” Houston,
TX: NACE, 2001.
NACE Publication 54276. “Cathodic Protection Monitoring
for Buried Pipelines.” Houston, TX: NACE, 1990.
Peabody’s Control of Pipeline Corrosion. 2
nd
ed. R.
Bianchetti, ed. Houston, TX: NACE, 2001.
Parker, M.E. Pipeline Corrosion and Cathodic Protection.
2nd ed. Houston, TX: Gulf Publishing, 1962.
Stephens, R.W. “Surface Potential Survey Procedure and
Interpretation of Data,” in Proceedings of the
Appalachian Corrosion Short Course, held May 1980.
Morgantown, WV: University of West Virginia, 1980.
West, L.H. “Fundamental Field Practices Associated with
Electrical Measurements,” in Proceedings of the
Appalachian Corrosion Short Course, held May 1980.
Morgantown, WV: University of West Virginia, 1980.
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Appendix A: Reference Electrodes
A1 Pipeline metals have unstable electrical potentials
when placed in an electrolyte such as soil or water.
However, a half-cell that has a stable, electrochemically
reversible potential characterized by a single, identifiable
half-cell reaction is a reference electrode. The stability of a
reference electrode makes it useful as an electrical
reference point or benchmark for measuring the potential of
another metal in soil or water.
When connected by a
voltmeter to another metal in soil or water, the reference
electrode becomes one half of a corrosion cell.
The
reference electrodes used for measuring potentials on
buried or submerged pipelines have voltage values that are
normally positive with respect to steel.
A2 Pipeline potentials are usually measured using either a
saturated copper/copper sulfate (CSE), a silver/silver
chloride (Ag/AgCl), or a saturated potassium chloride (KCl)
calomel reference electrode.
CSEs are usually used for
measurements when the electrolyte is soil or fresh water,
and less often for salt water. When a CSE is used in a
high-chloride
environment, the stability (i.e., lack of
contamination) of the electrode must be determined before
the readings may be considered valid. Ag/AgCl reference
electrodes are usually used for seawater environments.
The KCl calomel electrodes are more often used for
laboratory work because they are generally less rugged,
unless specially constructed, than the other two reference
electrodes.
A2.1 The voltage equivalents (at 25°C [77°F]) to
negative 850 mV referred to a CSE are:
A2.1.1 Ag/AgCl seawater reference electrode
used in 25 ohm-cm seawater: -800 mV,
3
and
A2.1.2 Saturated KCl calomel reference
electrode: -780 mV.
A2.2 A CSE is composed of a pure copper rod
immersed in a saturated solution of distilled water and
copper sulfate (CuSO
4
). The pure copper rod extends
from one end of the reference electrode, providing a
means of connection to a voltmeter. The other end of
the reference electrode has a porous plug that is used
to make an electrical contact with the pipeline
electrolyte.
Undissolved CuSO
4
crystals in the
reference electrode should always be visible to ensure
the solution is saturated. The reference is reasonably
accurate (within 5 mV when measured against a
reference electrode known to be free of contamination).
The advantages of this reference electrode are low
cost and ruggedness.
A2.3 Ag/AgCl reference electrodes are used in marine
and soil environments.
The construction and the
electrode potential vary with the application and with
relation to the potential of a CSE reference electrode.
The electrolytes involved may be natural seawater,
saturated KCl, or other concentrations of KCl.
The
user shall utilize the manufacturer’s recommendations
and potential values for the type of Ag/AgCl cell used.
The Ag/AgCl reference electrode has a high accuracy
(typically less than 2 mV when handled and maintained
correctly) and is very durable.
A2.4 A saturated KCl calomel reference electrode for
laboratory use is composed of a platinum wire in
contact with a mercury/mercurous chloride mixture
contacting a saturated KCl solution enclosed in a glass
container, a voltmeter connection on one end, and a
porous plug on the other end for contact with the
pipeline electrolyte.
For field use a more-rugged,
polymer-body, gel-filled KCl calomel electrode is
available, though modifications may be necessary to
increase contact area with the environment.
The
presence of mercury in this electrode makes it
environmentally less desirable for field use.
A2.5 In addition to these standard reference
electrodes, an alternative metallic material or structure
may be used in place of the saturated CSE if the
stability of its electrode potential is ensured and if its
voltage equivalent referred to a CSE is established.
A2.6 A permanently installed reference electrode may
be used; however, whether it is still accurate should be
determined.