Introduction
When you're running a close interval survey, you're already on the pipe, taking readings, and stopping at test stations. If that route crosses or parallels a high voltage AC transmission corridor, a few additional steps can tell you whether AC induced corrosion could be a concern on that section.
Most of the groundwork is already in place. The additional data adds knowledge about a corrosion mechanism that standard CP readings won't surface on their own. If the results come back clean, you move on. If they don't, you've identified a problem before it finds you at an excavation. And you have the data to support a formal AC mitigation evaluation.
This article covers what additional data to consider gathering when a CIS passes through an AC corridor, how to interpret what you find, and when the results should prompt a next step.
How AC Gets to a Buried Pipeline
Three mechanisms transfer energy from overhead power systems to buried metallic pipelines. Inductive coupling is the dominant mechanism for operating pipelines. The alternating magnetic field generated by power line current induces a voltage along any parallel buried conductor. The longer the parallel exposure and the higher the line load, the greater the induced voltage. Conductive coupling is primarily a fault-condition concern: when a line-to-ground fault occurs, current injected through tower footings raises local soil potential and can stress the coating beyond its dielectric limit. Capacitive coupling is mainly a construction hazard when pipe sections are on wooden skids before burial, and generally not a concern for buried operating systems.
For field assessment purposes: inductive coupling at steady-state is what you're measuring.
Why Standard CP Readings Don't Tell the Full Story
The –850 mV vs. CSE instant-OFF criterion does not protect against AC induced corrosion. Corrosion failures have been documented on pipelines fully meeting this criterion. The AC corrosion mechanism is electrochemically distinct from DC driven corrosion, and standard protection thresholds weren't developed to address it.
You can have a log full of readings more negative than –850 mV on a pipe that's actively corroding. Standard CP criteria weren't built to catch AC-induced corrosion.
The measurement itself has a built-in artifact. At a CP level near –850 mV CSE, induced AC current causes a negative potential shift — the pipe reads more negative than its true electrochemical state, appearing better protected than it actually is. At more negative CP levels (around –1,000 mV), the shift reverses. In either case, the AC influenced reading doesn't reflect actual corrosion conditions at the pipe surface.
For AC pipe-to-soil measurements during a CIS, the reading uses the same direct pipe connection already in use: the test station lead connected to the structure with the CSE reference electrode on the surface as it is for any other reading. What doesn't work is attempting to assess AC interference without that pipe connection. The measurement requires the complete circuit between pipe and reference electrode.
There is also a direct effect on CP performance itself. At high AC current densities (above approximately 500 A/m²), induced AC blocks cathodic protection current from reaching beneath disbonded coating. At elevated levels, AC interference can degrade the CP system's effectiveness at the exact locations where coating integrity is already compromised.
During a CIS: Additional Assessment Steps for AC Corridors
When a close interval survey crosses or parallels an AC transmission corridor, the following additional measurements are worth considering. An assessment zone of 500 ft before the corridor entry point and 500 ft beyond the exit point gives useful context on the extent of influence.
Conditions That Suggest Gathering Additional Data
Consider additional measurements when soil resistivity is low (less than 1,000 Ω-cm) and any of the following conditions are present:
AC pipe-to-soil (P/S) potential greater than 2 V AC vs. CSE
Touch potential at an above-grade appurtenance (valve station, crossing) of 15 V AC or greater vs. CSE, per NACE SP0177-2019
Prior history of AC-induced corrosion on the segment
The pipeline enters and exits an AC corridor — corridor crossing or parallel approach
High-performance, low-conductance pipe coating — FBE, polyethylene, or similar
High-performance coatings warrant specific attention. Because their high dielectric properties limit the overall current passing through the coating, any current that does reach a defect concentrates at that single point. Small holidays under FBE or PE coatings can see significantly higher AC current densities than the same holiday under a lower-resistance coating. This makes both the likelihood and the calculated severity greater at the same induced voltage.
Recommended Measurement Locations
Consider collecting data at a minimum of four points within the assessment zone:
One point in the 500 ft zone before the AC corridor entry
At least two points between the corridor entry and exit
One point in the 500 ft zone after the corridor exit
At each point, consider collecting:
Soil resistivity — Wenner four-electrode method, per ASTM G57. Electrode spacing should approximate pipe depth for a depth-targeted measurement. Check out Levi’s article on Soil Resistivity for assistance.
AC pipe-to-soil potential — measured from a test station connection with the CSE reference on the surface, as with any CIS reading.
If no test station is available near the location, consider installing one to support future monitoring if the assessment indicates risk.
Calculating AC Current Density
With AC pipe-to-soil voltage and soil resistivity collected, calculate AC current density at a coating holiday using:
iAC = (8 × VAC) / ((ρsoil / 100) × π × Ddefect)
Where:
iAC = AC current density (A/m²)
VAC = Measured AC pipe-to-soil voltage (V)
ρsoil = Soil resistivity (Ω-cm)
Ddefect = Diameter of coating defect (m)
When no prior information exists on defect sizes, use a default Ddefect of 0.01 m (1 cm). AC corrosion initiates preferentially at small pinholes where current concentrates (not at large bare areas) so the 1 cm default is a realistic and conservative starting assumption.
If an external corrosion coupon is installed at pipe depth on the segment — a 1 cm² or 10 cm²steel coupon electrically connected to the pipeline, the calculation can be bypassed. With an ammeter connected in series with the coupon circuit (or a voltmeter and calibrated shunt), AC current density is measured directly from the coupon's known surface area. Direct measurement is more reliable than the calculated approach and adds DC current density and instant-OFF potential to the data set simultaneously. If the assessment raises concerns, adding a coupon station at that location is a practical next step for ongoing monitoring.
A practical illustration of the calculated approach: 50 V AC at a test station, soil resistivity of 1,600 Ω-cm, 1 cm default defect diameter yields approximately 800 A/m². A pipeline documented in the technical literature showed –1,170 mV CSE (well protected by DC criteria) and was found during excavation to have active pitting corrosion under exactly those conditions.
At –1,170 mV CSE, the pipeline looked protected. The excavation told a different story. The current density calculation was the only thing that explained it.
Risk Thresholds
Apply the following thresholds based on DC current density at the pipe surface (per AMPP SP21424):
When DC current density exceeds 1 A/m²:
iAC < 30 A/m² — low risk
iAC 30 to 100 A/m² — elevated risk; warrants further evaluation
iAC > 100 A/m² — high risk; AC mitigation project recommended
When DC current density is less than 1 A/m²:
iAC < 100 A/m² — caution zone; corrosion unpredictable
iAC > 100 A/m² — high risk; AC mitigation project recommended
When DC current density is unknown: use 30 A/m² as the conservative action threshold.
The DC context matters because adequate cathodic protection provides some resistance to AC corrosion initiation. When DC current density at the pipe surface is sufficient, the threshold at which AC corrosion becomes likely is somewhat higher. When DC is low, as can occur under disbonded coating or in areas with inadequate CP coverage, even moderate AC current densities carry real risk.
Note: CIS spot readings give a useful snapshot of AC conditions on a given day. A formal AC interference assessment involves a minimum of 7 continuous days of monitoring to capture seasonal and daily load peaks. Power line loading typically peaks during summer months and during evening demand hours. Spot readings are a screening tool; if they indicate risk, a comprehensive monitoring and assessment program is the appropriate next step before designing mitigation.
When the Assessment Indicates Risk: Next Steps
If calculated AC current densities fall in the elevated, high, or severe range, the appropriate response is to engage a corrosion specialist to evaluate whether a formal AC mitigation program is warranted. AC mitigation design is an engineering level specialty. It involves system modeling, grounding system design, utility coordination, coupon monitoring program development, and ongoing verification — well beyond what a field survey assessment is meant to determine on its own.
The risk table below is a general reference for how AC voltage and current density map to risk level. Use it to frame the conversation with the engineer or specialist reviewing the findings:
Risk Level | AC Voltage | AC Current Density |
Low | < 5 V | < 20 A/m² |
Moderate | 5–15 V | 20–50 A/m² |
High | ~20 V | 50–100 A/m² |
Severe | > 20 V | > 100 A/m² |
The CIS based assessment is a screening tool. It tells you whether the risk level justifies a more detailed investigation and design effort. Taking mitigation action directly based on screening results (without a proper engineering study) risks both inadequate protection and unintended effects on the existing CP system.
AC Mitigation and the CP System: A Planning Note
Installing AC grounding without DC decouplers doesn't just fall short — it can turn your grounding electrodes into the anode of your CP system.
If a formal mitigation project moves forward and AC mitigation components are installed; grounding ribbon, gradient control mats, deep ground electrodes; every ground connection must include a DC decoupling device. This is a design requirement, not an add-on consideration.
Without decouplers, grounding systems provide an unintended drain path for cathodic protection current. The CP system works harder to compensate, protection levels drop, and in the worst case the grounding electrodes become the anode in the CP circuit — creating a new corrosion problem in the process of solving the AC interference. Any engineer or contractor designing an AC mitigation system for a cathodically protected pipeline should be specifying decouplers as a standard component. If they're not in the design, that's the question to ask before installation begins.
Personnel Safety
NACE SP0177-2019 sets the threshold: 15 V AC steady-state at any above-grade appurtenance constitutes a shock hazard, independent of corrosion risk. Human body resistance drops significantly with moisture: wet skin resistance can fall to approximately 1,000 ohms, putting dangerous current levels within reach at voltages that may seem manageable in dry conditions.
Field protocols on AC-affected pipelines:
Dead-front construction required at test stations in AC-affected areas — no exposed energized leads accessible at grade
Treat the pipeline as a live conductor until AC testing confirms safe levels. This applies during CP surveys, maintenance, and valve operations.
Grounding cable sequence: connect to the ground facility first, then to the pipeline; disconnect from the pipeline first, then from the ground
Avoid surveys and maintenance during elevated fault risk conditions — electrical storms, high winds, and scheduled power line switching events
Check out our full list of articles here: The Archive
Summary and Key Takeaways
A CIS that crosses an AC corridor is an opportunity to add a few measurements that can identify whether AC corrosion could be a concern on that segment.
The –850 mV CSE criterion does not protect against AC-induced corrosion. Failures have been confirmed on fully protected pipelines.
Calculate iAC using measured AC voltage, soil resistivity (Ω-cm), and a 1 cm default defect diameter. If an external corrosion coupon is installed, direct measurement is preferred.
Risk thresholds per AMPP SP21424: 30 A/m² (when DC current density > 1 A/m²) or 100 A/m² (when DC < 1 A/m²) as primary action levels.
CIS spot readings are a screening tool, not a full assessment. Elevated results should prompt a formal AC interference evaluation and mitigation design — an engineering level project.
Any AC mitigation installed on a cathodically protected pipeline must include DC decouplers at all ground connections. Without them, mitigation degrades existing CP performance.
15 V AC at any above-grade appurtenance = personnel shock hazard per NACE SP0177-2019.
Referenced Standards & Technical Resources
NACE SP0177-2019 — Mitigation of Alternating Current and Lightning Effects on Metallic Structures and Corrosion Control Systems (AMPP)
AMPP SP21424 — Alternating Current Corrosion on Cathodically Protected Pipelines: Risk Assessment, Mitigation and Monitoring
Peabody's Control of Pipeline Corrosion, 2nd ed. (R.L. Bianchetti, ed.) — Chapter 17: AC Interference
AC Corrosion of Pipelines (L.V. Nielsen et al.) — Chapters 6, 7, and 9
ASTM G57 — Standard Test Method for Field Measurement of Soil Resistivity Using the Wenner Four-Electrode Method
IEEE Standard 80 — Guide for Safety in AC Substation Grounding
49 CFR Part 192 — Transportation of Natural and Other Gas by Pipeline: Minimum Federal Safety Standards
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