Transcript Coupon Technology - West Virginia Corrosion Control

Coupon Technology
An Overview
Frank Ansuini
electrochemical devices, inc.
NACE International
2009 Eastern Area Conference
© EDI 2009
What are coupons?
… simulated holidays
A piece of bare metal electrically
connected to the structure
… similar material,
… similar environment,
… similar behavior?
Types of Pipeline Coupons
Weight loss coupon
bonded directly to pipe
A. W. Peabody
Control of Pipeline Corrosion, 1967
CP coupon
bonded through test station
Renewed interest generated by
SP0169
… voltage (IR) drops other than
those across the structure to
electrolyte boundary must be
considered ...
What Is IR Drop ?
EM
EM = EP + VIR
reference electrode
EP
VIR
voltage drop caused by current
flowing through an electrolyte
anode
redrawn from: R. A. Gummow
Materials Performance, August 1998
Why are IR Drop-free
readings important?
IR Drop is an error in the reading
 Consideration or elimination is required
by today’s regulations (SP0169)
 Ensure that the structure is protected

Why not place the reference
electrode next to the pipe?
reference
IR Drop
damaged coating
Then, why not place the reference right
next to a holiday?
Percentage of total
voltage drop to remote
earth vs. distance from
a holiday.
2.5 cm, 80%
1.5 cm, 70%
1 cm, 50%
5 cm, 90%
coating
1 cm dia. holiday
IR Drop fields develop rapidly near a holiday.
redrawn from: R. A. Gummow
Materials Performance, August 1998
A CP coupon creates an artificial holiday
right next to the reference electrode



it is electrically bonded to the pipe and
reaches the same potential as the pipe
when it is disconnected from the system, a
measurement should be taken before the
coupon depolarizes (instant disconnect)
the residual voltage drop in this type
measurement is usually insignificant
What CP Coupons Do

Allow measurement of instant
disconnect (IR drop free) potentials
without having to interrupt cathodic
protection current to structure
Some Currents Cannot Be
Interrupted
Sacrificial anodes
 Telluric currents
 Stray currents
 Galvanic currents
 Foreign rectifiers

In these circumstances, CP coupons may be the
only way to measure potentials free of IR Drop
error.
Industry Discussions Areas
 Size
 Material
 Shape
 Orientation
 Placement
TG210 meeting at NACE TCC
Coupon Size

Goldilocks
criterion
If it is too large . . .
Sac. anode system may not supply sufficient current
Current distribution to the structure may be affected

If it is too small . . .
It could depolarize too quickly to get a valid reading

Ideal size ...
Same as the largest expected holiday
Coupon Material

Ideally …
identical to pipe

Realistically ... carbon steel
“age” without CP a couple of months
then, let it polarize a couple of months
Coupon Shape

Available shapes
Cylindrical
Ring
Disc
Plate
Rod

Which is best?
… mine
Coupon Location and
Orientation

Readings are affected by
proximity to anode
soil environment
shielding by structure
shielding by foreign objects
Coupons can be stand-alone
Place coupon very close to the
reference and in the same
backfill as the structure.
reference
coupon
structure
Coupons can be attached to
permanent reference electrodes
Place coupon very close to the
reference and in the same
backfill as the structure.
reference
coupon
structure
Reference Electrode Array
Anode
Rod Coupon
Ref.: Wilkin, Dimond & Ansuini
NACE Conference Paper 03200
Two ways of making IR Drop
Free Readings
-1.1
Cell #1
Coupon #7
Disconnect the
rectifier (Cell #1)
Volts
-0.9
-0.7
Disconnect the
coupon (Coupon #7)
-0.5
-0.3
24-May
13-Jul
Ref.: Wilkin, Dimond & Ansuini
NACE Conference Paper 03200
1-Sep
21-Oct
10-Dec
Coupons can be attached
to test stations
reference
coupon
structure
A permanent reference electrode can
be installed in the test station riser.
Concentric Coupon
 Coupon assembly is fitted to a test
station riser. Reference electrode is
inside the riser and measures potential
through the slot membrane.
 Tank used for
prototype coupon
testing.
Ref.: Ansuini & Dimond
NACE Conference Paper 05039
Comparison of Rod and Concentric Coupons
1/2" rod coupon @ 4"
1/8 coupon @ 4" facing structure
-1.20
-1.20
-1.10
-1.10
-1.00
-1.00
-0.90
-0.90
-0.80
-0.80
-0.70
-0.70
-0.60
Coupon
interrupted
Rectifier
interrupted
Current-on readings with a rod coupon
contained about 300 mV IR Drop.
Disconnecting the coupon eliminated
about a third of it.
Ref.: Ansuini & Dimond
NACE Conference Paper 05039
-0.60
Current-on readings with a concentric
coupon contained about 30 mV IR Drop.
Disconnecting the coupon eliminated
virtually all of it.
Interrupting CP Current
Connect switch
between coupon
and structure leads
Hold magnet
next to switch to
interrupt current
Limitations of CP Coupons
Coupons may not give reliable data when
used for:
1) Native potential
2) CP Current Density
3) Depolarization
because the coupon may not be the same
material,
it may not be in the same electrolyte and
it may not be the same distance from the
anode.
Conclusions



Cathodic protection coupons can be used to
obtain instant-disconnect (IR drop free)
potentials of any structure.
Concentric CP coupons can make IR drop free
potential measurements without disconnecting
either the coupon or the rectifier.
Coupons should not be used for depolarization
without additional qualification studies.
Contact
Information
[email protected]
www.edi-cp.com