Page 101 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

Scoring the corrosion potential 4/79
Corrosion Indeer imum negative polarization voltage shift of 100 millivolts. This
Atmospheric is the amount of shift in potential between the polarized
Internal pipeline (after current has been applied for some time) and the
Subsurface buried pipeline without a protective current applied-
Subsurface environment the native state. Many corrosion control experts believe that the
Coating 100-mV shift criterion is the most conclusive measure of CP
effectiveness. Unfortunately, the 100 mV is also often the most
I
ICathodic protection (25 P~S) costly measurement to obtain. requiring a polarization survey,
whereby the pipeline is depolarized over hours or days and
Effectiveness (15 pts) comparative pipe-to-soil measurements are made. The 0.85-
Interference potential (10 pts) volt criterion is normally adequate because it encompasses the
100-mV shift in almost every case since native potentials are
Cuthodicprotection is the application of electric currents to a normally less than 700 mV.
metal in order to offset the natural electromotive force of corro- A criterion for excessive CP currents is also often appropri-
sion. Chemical reactions occur at the anode and the cathode as ate. Excessive currents might cause hydrogen evolution that
corrosion occurs and ions are formed. Some form of CP system can cause coating disbondment.
is normally used to protect a buried or submerged steel pipeline The actual practice of ensuring adequate levels of cathodic
as one part of the common two-part defense against corrosion- protection is often more complex than the simple application of
coating and CP. The exceptions to CP use might be instances criteria. Readings must be carefully interpreted in light of the
where temporary lines are installed in fairly non-corrosive soil measurement system used. Too much current may damage the
and where conditions do not warrant cathodic protection. coating. Higher levels ofprotection may be required when there
Nonmetal lines may not require corrosion protection. is evidence ofbacteria-promoted corrosion. A host of other fac-
In this evaluation, the effectiveness of the CP system is tors must often similarly be considered by the corrosion engi-
assessed in general and in terms of possible interferences. The neer in determining an adequate level of protection.
effectiveness is judged by the existence of a system that meets In the interpretation of all pipe-to-soil measurements, atten-
the following general criteria:
tion must be paid to the resistances that are part of the pipe-to-
Enough electromotive force is provided to effectively negate soil reading. The reading that is sought, but difficult to obtain,
any corrosion potential. is the electric potential difference between the outside surface
Enough evidence is gathered, at appropriate times. to ensure ofthe pipe and apoint in the adjacent soil a short distance away.
that the system is working properly. In actual practice, a reading is taken between the pipe surface
(via the test lead) and apoint at the ground surface, usually sev-
In assessing interference potential, points are awarded based eral feet above the pipe. The soil component of the circuit is a
on the potential for any of three sources of interference and nonmetallic current path. Consequently, this model is not
measures taken to mitigate such interference. directly analogous to a simple electrical circuit. The measured
circuit is completed at the ground surface by contacting the soil
with a reference electrode (a half cell, usually copper electrode
CP svstem effectiveness in a copper sulfate solution). Therefore, the normal pipe-to-soil
reading measures not only the piece of information sought. but
To ensure that the CP system can be effective, the evaluator also all resistances in the electric circuit, including wires, pipe
should seek records of the initial cathodic protection design. steel, instruments, connectors, and, the largest component. the
Are the design parameters appropriate? What was the projected several feet of soil between the buried pipe wall and the ground
life span ofthe system? Is the system functioning according to surface. The knowledgeable corrosion engineer will take read-
plan? ings in such a way as to separate the extraneous information
The evaluator should then inspect documentation of the most from the needed data. The industry refers to this technique as
recent checks on the system. Anode beds can become depleted, compensating for the IR drop.
conditions can change, equipment can malfunction. Will the There is some controversy in the industry as to exactly how
operator become aware of serious problems in a timely man- the readings should be interpreted to allow for the IR drop. An
ner? Although cathodic protection problems can be caught dur- instant-off pipe-to-soil measurement, where the reading is
ing normal test lead readings and certainly during close interval taken immediately as the current source is interrupted is often
surveys, problems such as malfunctioning rectifiers (or worse, taken as a reading that is relatively IR free. Therefore. some
rectifiers whose electrical connections have been reversed! ) operators use a more conservative adequacy criterion of “at
should ideally be found even quicker. least 850 mV interrupted (or instant-ow pipe-to-soil potential
instead of “at least 850 mV with the current applied.”
Effectiveness criteria The presence of adequate protective In some cases the controversy is more theoretical because
currents is normally determined by measurement of the voltage government regulations mandate certain techniques. The evalu-
(potential) difference between the pipe metal and the elec- ator should be satisfied that sufficient expertise exists in the
trolyte. By some common practices and regulatory agency interpretation of readings to give valid answers.
requirements, a pipe-to-soil potential of at least -0.85 volts
(-850 millivolts), as measured by a copper-copper sulfate ref- Equipment One aspect of the adequacy of protection will be
erence electrode, is the general criterion indicating adequate the maintenance of the associated cathodic protection equip-
protection from corrosion. Another common criterion is a min- ment. For impressed current CP systems. equipment such as