Page 170 - CW E-Magazine (29-7-2025)

Special Report Special Report

Monitoring cathodic protection (CP) be diffi cult in the case of a complex There are some points that should in areas where different soil resisti-
system by using Coupons piping network where a large num- be taken into consideration when pur- vity, soil chemistry, moisture content,
A CP Coupon is a piece of bare ber of current sources are operating chasing or designing a CP Coupon: current density, coating conditions
metal that should be manufactured and must be interrupted simulta-  The associated fi tting and soil and temperature are expected to be
from the same material as the structure neously. In this case CP Coupons can access tube (if used) should be made found. An example of these locations
under test. The Coupon is buried in the be momentarily disconnected from of non-metallic materials. is on top of a dry rocky hill, a low
same electrolyte near the structure to be the structure and its instant discon-  The diameter of the soil access tube wet valley, mid-span between CP
tested and connected to it through a test nect potential measurement com- should be large enough to allow the current sources, and at suction & dis-
station. This allows the Coupon to be pared to relevant standards in order placement of a common reference charge points of compressor stations.
connected to the structure’s CP system to assess its CP status. Moreover, if electrode like the ones in the CP Coupons should be installed in
and to be polarised in a similar manner the 100-mV shift polarisation crite- market. each of these locations in order to
as the structure. rion is adopted, and there is a need  The lead wires should be terminated assess the effectiveness of impressed

to obtain the rest potential or native in an accessible test station to have current CP system for each of these
Coupons have been used since the potential of a Coupon, then the CP easy access to them. In addition, the areas.
1930s in North America and Europe Coupons can be disconnected for a test station should be equipped with
by corrosion pioneers and are a proven longer period of time, thus allowing a disconnect switch or a similar How to install corrosion Coupons:
way to determine the polarisation level it to depolarise completely. This can feature in order to allow rapid dis- In order to acquire meaningful
of a structure and to determine the insu- be easily done without the need to connection of the Coupon from the readings a Coupon must have a good
lation resistance (IR) drop error that is disconnect the CP system for an structure. electrical contact between itself and
always associated with the structure to extended period of time.  The Coupon materials should be the surrounding environment. This is
soil potential measurement(4).  The presence of foreign rectifi er of the same material or alloy as the accomplished by compacting the soil
units can’t be interrupted or their structure under test. This will allow around the Coupon to remove any air Fig. 6: Direct reading ultrasonic thickness gauge/meter
Why Coupons are used to measure a location is unknown, resulting in it to have the same polarisation voids that will degrade the electrical According to NACE SP0104-2014, the pipeline and initiation of cracks
structure’s polarisation level? an IR error that will be present in behaviour as the structure and to connection between the Coupon and it is recommended that the lateral dis- or flaws and other pipeline flaws or
In structure to electrolyte potential instant off -potential measurements. have meaningful data from the CP the surrounding environment. Several tance between the Coupon and the defects. Figure-6 shows a very handy
measurement, an inevitable error is  The presence of directly connected Coupon potential readings. methods are available for Coupon pipeline shall range from 10-30 cm portable ultrasonic thickness gauge.

always introduced, which is known as sacrifi cial anodes can’t be interrup-  The size of the Coupon should be installation, including: and shall be installed in the lower half An ultrasonic transducer (piezoelec-
the IR drop error. This error depends on ted, resulting in an IR error in off - chosen based on the largest holi-  Excavation activities during of the pipe (between 3 o’clock and 9 tric crystal) is used to transmit ultra-
several factors such as burial depth, soil potential measurements. day size expected on the structure structure investigation; o’clock positions) (5). sonic wave (signal) into the pipe-
resistivity, coating conditions and the  The presence of stray currents might under test. Commercially available  Auguring; line section under test. The signal

presence of stray currents. To eliminate aff ect off -potential measurements. Coupon sizes range from 650 mm  Vacuum excavation; Ultrasonic crack inspection travels through the thickness of the
2
this error, the instant off -potential mea-  Rapid IR spikes that immediately to 10,000 mm . For bare or poorly  Hand digging; and Ultrasonic technology uses ultra- pipe and gets reflected back, which
2
surement method has been adopted to appear after current interruption can coated structures, a large Coupon  Installation of the Coupon during sonic sound waves to detect thick- is received by a receiver. The thick-
measure the true polarisation potential aff ect off -potential measurement. should be chosen; however, for the structure’s installation ness reduction in critical sections of ness of the pipe is directly digitally
level of a structure.  Multiple pipelines in the same right- a well-coated structure a smaller
of-way interfere with one another Coupon size should be selected where
All current sources aff ecting the that make individual measurement only small holidays are expected.
structure under test should be interrup- for each pipeline diffi cult.  Coupon lead wires should be
ted synchronously in order to obtain an securely connected to the Coupon
accurate instant off -potential measure- CP Coupon design such that a low-resistance electrical
ment. In some cases, this is too diffi - Several types of Coupons are avail- continuity is maintained throughout
cult, and it would be easier to assess the able(4): the design life. This can be done
CP level of the structure using the Cou-  Two wires Coupons. through silver soldering, exother-
pons instant disconnect potential. Some  Coupons with a built-in integral mic welding, mechanical connec-

of the cases where the CP Coupon can reference electrode. tion or any other suitable method.
be used instead of the structure to elec-  Coupons with a stationary electrode The connection should be isolated
trolyte potential are when: permanently buried near the from the electrolyte by encapsulat-
 The structure under test is under the Coupon. ing it in a protective coating.
infl uence of several impressed cur-  Dual Coupons with the same geo-
rent sources that must be synchro- metry and size for measuring both Where to install Coupons:
nously interrupted together. It may CP and rest potential. Coupons should be installed Fig. 7: Thickness monitoring of tubes & pipes by ultrasonic gauge (D-meter)

170 Chemical Weekly July 29, 2025 Chemical Weekly July 29, 2025 171

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