Nace CP 1, Руководство по эксплуатации

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TM0101-2001
NACE International 21
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Appendix A: Using Coupons to Determine the Adequacy of Cathodic Protection
A.1 Coupons, particularly when accompanied by other
engineering tools and data, have been used to evaluate
whether cathodic protection at a test site complies with a
given criterion.
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A.2 The small size of a coupon may reduce the effect of
voltage drops due to currents from other sources.
The
magnitude of these voltage drops can be determined by
interrupting cathodic protection current sources while the
coupon is disconnected and noting whether there is a shift
in the coupon-to-electrolyte potential.
A.3 These methods use coupons to assess the adequacy
of cathodic protection applied to a selected test site.
A
cathodic protection coupon is a metal sample representing
the tank at the test site and used for cathodic protection
testing. The coupons used for these tests should be:
(a) Of the same material and with the same or nearly the
same properties as the tank;
(b) Known not to interfere with determining the adequacy
of the cathodic protection system;
(c) On a coated tank, the size of the coupon should
represent the largest anticipated coating defect;
(d) Placed at tank depth in the same backfill as the tank;
(e) Prepared with all mill scale and foreign materials
removed from the surface; and
(f)
Placed at a location of an ineffective coating, if known.
A.4 During normal operations, a coupon has an insulated
test lead brought above ground and connected to a tank
test lead. The coupon receives cathodic protection current
and represents the tank at the test site.
For testing
purposes, this connection is opened, and the polarized
potential of the coupon is measured.
A.4.1 The time the connection is open to measure the
coupon’s instant-off potential should be minimized to
avoid significant depolarization of the coupon.
The
coupon is then allowed to depolarize.
A.4.2 If possible, coupon current direction and
magnitude should be verified using a current clip gauge
or resistor permanently placed in series with the
coupon lead.
Measurements showing discharge of
current from the coupon should be reason to question
the validity of using a coupon at the test site.
A.5 The basic test equipment for both of these tests is the
same:
A.5.1 Voltmeter with adequate input impedance.
Commonly used digital instruments have a nominal
impedance of 10 megohms. An analog instrument with
an internal circuit resistance of 100,000 ohms/volt may
be adequate in certain circumstances if the circuit
resistance is low. A potentiometer circuit may be
necessary in other circumstances.
A.5.2 Meter leads with insulated wire and terminal
connections suitable for making reliable electrical
contact with the tank and reference electrode. Color-
coded meter leads are suggested to avoid confusion of
polarity of the measured value.
A.5.3 A CSE or other standard reference electrode
may be used.
Reference electrodes that may be
substituted for the CSE are described in Paragraph 5.5.
A.6 Cathodic Protection Coupon Test Method A—for
Negative 850-mV Polarized Tank-to-Electrolyte Potential of
Steel Tank
A.6.1 This method uses a cathodic protection coupon
to assess the adequacy of cathodic protection on a
steel tank according to the criterion stated in NACE
Standard RP0285:
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“A negative polarized potential
of at least 850 mV relative to a
saturated copper/copper sulfate
reference electrode.”
A.6.2 Comparison with Other Methods
A.6.2.1 Advantages
(a) May provide a polarized coupon-to-electrolyte
potential, free of voltage drop, with a minimum of
specialized equipment, personnel, and vehicles;
and
(b) May
provide a more comprehensive
evaluation of the polarization at the test site than
conventional
tank-to-electrolyte potential
measurements that may be influenced by the
location, size, and number of coating holidays, if
the tank is coated.
A.6.2.2 Disadvantages
(a) May have high initial costs to install coupons,
especially for existing tank.
A.6.3 Procedure
A.6.3.1 Before the test, verify that:
(a) Cathodic protection equipment has been
installed and is operating properly; and
(b) Coupon is in place and connected to a tank
test lead.