The Honest Holiday

You pull up to a test station at a road crossing inside an HCA. Routine check. You drop your copper sulfate cell, clip onto the test lead, and the pipe-to-soil on-potential reads −1.140 V CSE. By the −0.850 V criterion, that's comfortable. You could write it down, close the box, and drive to the next one.

But you know what's baked into that number. Every on-potential carries IR drop — the voltage the meter picks up from CP current pushing through the soil between your reference cell and the pipe. The reading looks like protection. Some of it is just current finding its way through dirt.

So how much of that −1.140 V is real polarization, and how much is IR drop you can't see? On a single buried line, the honest answer could be: interrupt the whole system and take an instant-off. Current interrupters on every rectifier and bond affecting that segment, all synchronized. Some have RMUs. An educated assumption that nearby foreign systems have no effect and nothing new has been installed or adjusted. A decent amount of work for "I'm pretty sure that's all the current".

An external corrosion coupon gets you that number without shutting anything down.

An external corrosion coupon is a small piece of bare metal — usually carbon steel, matched to the pipe — buried in the native soil right next to the pipeline. It has a known surface area, commonly 1, 10, or even 100 cm². Two wires run from it to the test station: one connects it electrically to the pipe so it picks up CP current like the structure does, and one is a test lead so you can read it.

Think of it as a coating holiday you built on purpose. The pipe is coated; current only reaches steel where the coating is broken. The coupon is a known, measurable patch of bare steel sitting in the same soil at the same depth, taking on CP current the same way a real holiday would. The difference is you can disconnect it whenever you want.

That last part is the whole point.

When you throw the coupon's disconnect switch, current stops flowing through that coupon instantly. The IR drop in its measurement circuit goes to zero the moment the switch opens. The potential you read in that first instant — the coupon instant-off — is the true polarized potential of bare steel in that soil, with no IR drop to argue about. You got an instant-off reading at that location and never touched a rectifier.

This is the method behind AMPP/NACE SP0104, "The Use of Coupons for Cathodic Protection Monitoring Applications," and it leans on the same potential-measurement fundamentals as NACE TM0497. The standards give you the framework. The coupon gives you the data point.

This is the bread and butter. Instead of synchronizing interrupters across a whole segment to kill IR drop, you disconnect one coupon at one test station and read its instant-off. The main system keeps running. Every other structure on that segment stays protected while you take your measurement.

It's especially useful where interruption is impractical — long networks, shared corridors, foreign rectifiers you don't control, segments where someone else's CP affects your readings. The coupon doesn't care who owns the current. It only reports what's reaching bare steel at that spot.

Because a coupon has a known surface area, it does something a pipe can't easily do: it lets you measure current density directly. Wire the coupon through a shunt, or read it with a zero-resistance (RMS) ammeter, and you get the magnitude and direction of current flowing to or from that known area. Divide current by area and you have A/m².

That matters most for AC. On a line sharing a corridor with high-voltage transmission, AC discharge concentrates at coating defects, and AC corrosion can run even when your DC potentials look fine. The coupon is your stand-in defect. ISO 18086 frames the risk by AC current density — broadly, below about 30 A/m² is low risk, 30 to 100 A/m² is the zone where the DC density and other factors decide it, and above 100 A/m² is high risk. You cannot read any of that off a pipe-to-soil potential. You read it off a coupon.

The same logic flags DC interference. A meaningful gap between the pipe's behavior and the coupon's instant-disconnect potential, or current measured discharging off the coupon, points to stray current — rail systems, foreign CP, telluric activity. Pair that with your CIS or DCVG data and the picture fills in.

The −0.850 V instant-off criterion is one way to prove protection. The 100 mV of cathodic polarization criterion is the other, and on bare or poorly coated steel it's often the realistic one. Proving it means measuring decay: read the polarized potential, disconnect, and watch how far it relaxes back over hours to days.

You don't have to leave a whole CP system off for two days to watch a pipe depolarize. You will leave one coupon disconnected. Read its instant-off, walk away, come back, and read where it settled. The shift is your polarization, measured cleanly on a known surface.

There's always a question hanging over indirect coating surveys: the %IR or dB number says the holiday is this big — but how big is that really, on the pipe? A coupon is a known size. Run your DCVG or ACVG over the coupon and you get a real-world anchor — a measured indication tied to a defect of known area, in that soil, at that depth. Holidays of similar depth and size nearby get a lot easier to read.

A coupon only tells the truth if it sits in conditions that match the structure. Most bad coupon data traces back to a bad install, so this is where the work is.

A few rules hold everywhere:

Put coupons where protection is most likely to drift, not where the digging is easy:

On large-diameter pipe, coupons set at different clock positions around the circumference will show you how much soil resistivity and coating condition change around a single joint.

At a new station, build it in from the start. Locate the line and excavate to pipe depth with hand digging, augering, or air-vacuum. Set the coupon 4 to 12 inches off the pipe at grade-matched depth, lower half. Route the current-carrying lead and the test lead to the station and put a disconnect switch in the box so instant-off is a flip, not a re-wire. Drop a nonmetallic soil-access tube (PVC, 2-inch or larger) down to coupon depth so a portable reference cell reads close to the coupon instead of up at the surface — that alone strips a lot of IR error out of your potentials. Document everything.

Retrofitting an existing station is the cost-effective upgrade. Locate the pipe, verify depth, pick an adjacent spot, and use localized excavation so you're not exposing the line. Set the coupon the same way, route the leads through existing conduit, add the disconnect switch if you can, and extend a soil-access tube. Backfill, baseline, document. You've turned a plain test post into a real monitoring point for a fraction of a new install.

At your important stations — HCAs, interference zones, remote points you don't want to revisit often — bury a permanent copper/copper sulfate reference cell next to the coupon. It gives you a stable, weatherproof benchmark that doesn't drift with surface moisture or season. Keep it at least 6 inches from the coupon so its electrolyte doesn't change the soil chemistry the coupon is reading. Route its lead to the station, record serial numbers and positions, and log a baseline.

Back to that −1.140 V CSE on-potential at the road crossing. Here's what the coupon does to it.

You throw the disconnect switch and read the coupon instant-off: −0.742 V CSE.

That's the real polarized potential of bare steel in that soil. It's short of −0.850 V. The roughly 400 mV you "had" in the on-reading was IR drop and nothing more. By the instant-off criterion, that location is not protected — and the on-potential never would have told you.

Now you leave the coupon disconnected and come back the next day. It has depolarized to −0.620 V CSE. The shift from −0.742 to −0.620 is 122 mV — past the 100 mV criterion.

So which is it? Both readings are real, and both criteria are legitimate under SP0169. This spot fails the −0.850 V instant-off criterion but meets the 100 mV polarization criterion. That's not a contradiction — it's exactly the call CP criteria exist to let you make, and the coupon is the only thing on site that let you make it honestly. Without it you had one inflated number and a guess. With it you have two clean readings and a defensible decision to document.

This is also where we'll point you back to "Mixed Metal Potentials" from the last issue of Field Notes — coupons do similar duty when dissimilar metals muddy your potentials, and the disconnect trick is the same.

Everything above is about working with a coupon by hand — drive out, throw the switch, take your reading, drive to the next one. We ran into two products at this year's AUCSC conference that build on that same idea, and they're worth knowing about. We don't sell either one. We met the people who make them, they looked genuinely useful, and a few jobs we have coming up are exactly the kind of complicated, hard-to-read situations these were built for. One of them has great surface potentials and active corrosion staring back at us from the ditch. Investigate, test, remediate — and a coupon is going in the ground before we're done.

We don't sell these. We're just passing along good tools when we find them.

Tinker & Rasor's Multi-Stick Multi-Metal Coupon Reference Cells (developed with Chapman Engineering). This is a coupon and a permanent reference cell built into one unit. The install we described back in "Add a permanent reference cell," done in a single stick. The interesting part is the multi-metal design: carbon steel, copper, and (on some models) 316 stainless coupons on the same device, each with its own current-measurement wire, plus a small 1 cm² steel coupon set aside just for AC current-density work. If you read "Mixed Metal Potentials" last month, this is the tool for that problem.

The white paper is a good read on its own — linked below.

Contact Stephanie Decker at [email protected] for ordering information and pricing.

Borin's G-DART remote monitoring and datalogging system. If the Multi-Stick is a better coupon to put in the ground, the G-DART is what reads it for you after you've driven away. It drops into a standard 3-inch test-station pole — the same retrofit-an-existing-station upgrade we talked about — and logs the exact readings this article is built on: coupon instant-off, AC and DC current density, and the 100 mV depolarization shift. It takes them every second. That's the part a annual visit can't match: telluric swings, dynamic stray current, AC that rises and falls with the load on the line next door. The interference we've covered in past issues hides between site visits; a logger like this is how you catch it.

Contact Gregg Steele at [email protected] for ordering information and pricing.

We haven't run either one long enough to give you our own field verdict yet. When we do, you'll read about it here.

Established in 2011, Roberts Corrosion Services, LLC delivers comprehensive, turn-key cathodic protection and corrosion control solutions nationwide. Our end-to-end expertise encompasses design and inspection, installation and repair, surveys and remedial work. We provide drilling services for deep anode installations and a full laboratory for analysis of samples and corrosion coupons, as well as custom CP Rectifier manufacturing.

While our initial focus was on the Appalachian Basin area, we complete field work all over the US. We are a licensed contractor in many states and can complete a wide range of services.

Our biggest strength is in our flexibility for our clients. Solutions and Results.

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