Coupon Technology An Overview Frank Ansuini electrochemical devices

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 SP 0169 … 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? n IR Drop is an error in the reading n Consideration or elimination is required by today’s regulations (SP 0169) n Ensure that the structure is protected

Why not place the reference electrode next to the pipe? damaged coating IR Drop reference

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 n when it is disconnected from the system, a measurement should be taken before the coupon depolarizes (instant disconnect) n the residual voltage drop in this type measurement is usually insignificant n

What CP Coupons Do n Allow measurement of instant disconnect (IR drop free) potentials without having to interrupt cathodic protection current to structure

Some Currents Cannot Be Interrupted n Sacrificial anodes n Telluric currents n Stray currents n Galvanic currents n Foreign rectifiers In these circumstances, CP coupons may be the only way to measure potentials free of IR Drop error.

Industry Discussions Areas n Size n Material n Shape n Orientation n Placement TG 210 meeting at NACE TCC

Coupon Size n Goldilocks criterion If it is too large. . . Sac. anode system may not supply sufficient current Current distribution to the structure may be affected n If it is too small. . . It could depolarize too quickly to get a valid reading n Ideal size. . . Same as the largest expected holiday

Coupon Material n Ideally … identical to pipe n Realistically. . . carbon steel “age” without CP a couple of months then, let it polarize a couple of months

Coupon Shape n Available shapes Cylindrical Ring Disc Plate Rod n Which is best? … mine

Coupon Location and Orientation n 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 Disconnect the rectifier (Cell #1) Disconnect the coupon (Coupon #7) Ref. : Wilkin, Dimond & Ansuini NACE Conference Paper 03200

Coupons can be attached to test stations reference coupon structure A permanent reference electrode can be installed in the test station riser.

Concentric 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 Coupon interrupted Rectifier interrupted Current-on readings with a rod coupon contained about 300 m. V IR Drop. Disconnecting the coupon eliminated about a third of it. Ref. : Ansuini & Dimond NACE Conference Paper 05039 Current-on readings with a concentric coupon contained about 30 m. V 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 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 n n n 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 frank@edi-cp. com www. edi-cp. com