Pulsed Eddy Current (PEC) Testing: How It Works, Applications & Limitations

1. Introduction
2. Pulsed Eddy Current (PEC) Testing – An explanation
3. How Pulsed Eddy Current Testing Works
4. PEC vs Conventional Eddy Current Testing
5. Applications of Pulsed Eddy Current Testing
6. Limitations of Pulsed Eddy Current Testing
7. Using PEC Correctly: Screen Broadly, Confirm Selectively
8. Conclusion
9. Frequently Asked Questions (FAQs)

Introduction

Insulation and/or fireproofing are typically applied to industrial pipelines, pressure vessels, storage tanks and structural steel to enhance energy efficiency and equipment protection. These protective layers prolong asset life, but may allow corrosion to go undetected until substantial wall loss has occurred. The cost, time and effort involved in removing insulation for routine checks is often unnecessary.

Pulsed Eddy Current (PEC) is a viable alternative. It is an electromagnetic non-destructive testing (NDT) method that can be used to check the carbon steel components for corrosion without stripping away the insulation, coatings, or fireproofing. Identification of potential wall loss areas means that detailed inspections are only made where necessary, reducing the inspection cost, minimising downtime and improving maintenance planning.

The article will describe the principles behind pulsed eddy current testing, how it differs from traditional eddy current testing, typical applications, and the limitations inspectors should consider when assessing corrosion under insulation.

Pulsed Eddy Current (PEC) Testing – An explanation

Pulsed Eddy Current (PEC) testing is an electromagnetic non-destructive testing (NDT) screening technique that can identify wall loss in carbon steel components without removing insulation, coatings, or fireproofing. It is extensively utilized for corrosion under insulation inspection due to its ability to quickly detect general corrosion, which reduces the need for removing insulation or causing disturbance.

PEC can be used to inspect insulated metal assets, whereas ultrasonic thickness testing is typically done with direct access to the metal surface. It does not give a point measurement, but estimates the average remaining wall thickness in the area which is being inspected by the probe. Therefore, PEC is mainly used as a screening tool for scanning areas that need to be inspected with other techniques, like ultrasonic testing (UT).

How Pulsed Eddy Current Testing Works

PEC testing works based on the principle of electromagnetic induction. A magnetic field is applied by the probe, which breaks through the insulation and reaches the carbon steel. The transitory reaction that results from suddenly turning off this field is measured. The magnetic field interacts with the steel, creating eddy currents which can be distributed throughout the thickness of the wall. The rate at which these currents decay over time provides information on the remaining wall thickness.

PEC differs from traditional eddy current testing, which relies on a steady alternating current. Instead, PEC examines the material's transient response following each excitation. This enables it to both identify and map concealed corrosion under non-metallic barriers without exposing the metal.

Pulse vs Continuous Excitation 

The standard eddy current testing method is based on the continuous sinusoidal signal at one or more frequencies. This method works well for finding small surface and near surface defects like fatigue cracks or localized corrosion in conductive materials that can be accessed.

PEC operates on a different principle. Rather than transmitting a continuous signal, the probe delivers a broadband electromagnetic excitation. Each excitation contains a wide frequency spectrum, so the magnetic field penetrates the steel to a range of depths within a single measurement cycle. 

PEC assesses the entire wall thickness rather than surface-breaking defects since the complete transient response is recorded in a single excitation cycle. This makes it especially well suited for screening insulated carbon steel parts for general loss in the wall.

Diffusion Through the Wall and the Decay Curve

Once the pulse enters the carbon steel it creates eddy currents throughout the material. Upon termination of the excitation pulse, these currents become weaker over time, which leads to a decay of the magnetic field over time. This changing magnetic response is recorded by the probe, and is called the decay curve.

The decay curve is determined primarily by the amount of steel under the probe. Thicker sections retain the induced energy longer, so the signal decays more slowly. Where wall thickness has been reduced by corrosion, less material is available to sustain the eddy currents, so the signal decays faster and produces a steeper decay curve.

Modern PEC instruments use calibration data and signal-processing algorithms to estimate the average remaining wall thickness within the area being inspected by analyzing this decay. This method measures total metal loss from corrosion, rather than searching for individual cracks.

Reading Remaining Wall Thickness

PEC does not measure thickness directly, as does an ultrasonic thickness gauge. Rather, it determines wall thickness by comparing the recorded decay characteristics to a reference signal obtained from a known-good region of the same component. Instead of reporting the results as an absolute value, they are usually expressed as a % of that reference wall.

The slower the decay rate the thicker the wall, and the faster the decay rate the thinner the wall. These signal characteristics are translated into an estimated remaining wall thickness or percentage of wall loss so that inspectors can easily identify areas that may need further inspection.

Although these estimates are sufficiently accurate for screening purposes, they should not be considered a substitute for detailed thickness measurements. Ultrasonic measurements are normally used to confirm areas of substantial wall loss prior to making maintenance decisions.

The Probe Footprint: Why PEC Measures an Average 

Instead of measuring a single point, PEC probes measure a comparatively vast area; the footprint size is determined by insulating standoff, wall thickness, and probe design. Therefore, the reported thickness is an average of the thickness of the steel under the probe.

The larger footprint allows inspectors to scan large areas much faster than the point-by-point inspection techniques, and is therefore very effective in large pipelines, pressure vessels, and storage tanks.

However, averaging means that isolated defects have only a limited influence on the reported value. While widespread corrosion throughout the footprint results in a clearly observable change, corrosion impacting only a small area of the footprint is unlikely to significantly alter the total response. 

This is why pulsed eddy current is ideal for determining overall wall loss and not for the detection of localized pitting or small defects. If areas of concern are detected, further testing is required, such as ultrasonic thickness testing to give more detailed measurements that can confirm the extent of the corrosion.

PEC vs Conventional Eddy Current Testing

Both methods use electromagnetic induction, but have different inspection purposes. Conventional Eddy Current Testing (ECT) is used to identify surface and near surface defects in conductive parts, whereas Pulsed Eddy Current (PEC) testing is used to survey carbon steel parts for general loss of wall thickness due to insulation or other non-metallic barriers.

Therefore PEC is viewed as a corrosion screening technique and conventional ECT as a flaw detection technique.

The conventional ECT is more sensitive to localized defects while the PEC test is able to speed up the inspection of the insulated assets by identifying areas to be further inspected.

For a deeper look at how standard eddy current methods detect surface and near-surface flaws, see our detailed guide on conventional eddy current testing.

→ Read: Eddy Current Testing Explained

Applications of Pulsed Eddy Current Testing

The principal benefit of pulsed eddy current testing is the ability to assess carbon steel through insulation, coatings, and fireproofing without disturbing these protective layers. This means that it is a useful way to scan big industrial applications and save time and money on inspection.

Corrosion Under Insulation (CUI) 

The most common application of PEC testing is corrosion under insulation. The moisture between the damaged insulation and the steel surface can slowly corrode the metal surface without being seen. Using PEC, inspectors can screen insulated pipelines and equipment for wall loss without removing insulation, making CUI inspection significantly faster and more cost-effective.

Corrosion Under Fireproofing (CUF)

Fireproofing materials not only shield structural steel from high temperatures but can also trap moisture against the steel surface, promoting corrosion. Routine inspection by removing fire-proofing is costly and disruptive. PEC can assess the condition of carbon steel under fireproofing and determine areas that need specific follow-up inspection.

Corrosion Under Pipe Supports 

At pipe supports, moisture and debris are often trapped, which promotes corrosion. Because these contact locations are hard to reach, PEC provides a useful screening solution: specialized pipe-support probes can evaluate the pipe wall without removing insulation or disassembling the support. Readings obtained at or close to contact points necessitate careful interpretation and an appropriate method because supports indicate a geometric discontinuity.

Insulated Pressure Vessels and Storage Tanks

Insulation is used for large pressure vessels and heated storage tanks as a measure to increase thermal efficiency. Removing insulation to gain access to every area is impractical, costly, and disruptive. PEC significantly reduces the amount of insulation that needs to be removed by quickly screening huge surface areas and identifying areas of wall weakening.

Structural Steel

Insulated or fireproofed steel is commonly used in industrial facilities to hold up important pieces of machinery. PEC testing allows these members to be assessed for concealed corrosion and prioritised for repair before structural integrity is compromised.

Offshore Splash Zone Assets 

Offshore platforms, having their splash zone continuously exposed to seawater, humidity and salt spray, are highly vulnerable to corrosion. PEC can be used to inspect pipelines and structural items in such harsh conditions without extensive surface preparation.

High-Temperature In-Service Pipelines

In many process pipelines it is not desirable to shut down the production for inspection, as operation is required at temperatures where such inspections could not take place. By insulating in-service equipment at high temperatures, typically up to about 500°C with the right high-temperature probes, PEC can be carried out without the requirement for stoppage. For accurate findings, the proper temperature adjustment must be used because conductivity and permeability alter with temperature.

Ship Structures

Marine vessels contain insulated carbon steel structures that are difficult to inspect using conventional methods. PEC can be used to quickly scan these components for the presence of hidden corrosion, so that detailed inspections are only performed when necessary.

Learn more about the latest inspection advancements specifically developed for detecting corrosion under insulation.

→ Read: Advancements in NDT Inspection for Corrosion Under Insulation

Limitations of Pulsed Eddy Current Testing

Although PEC is a very efficient screening method for insulated carbon steel, it is not suitable for all inspection scenarios. Specifically, the signal can be greatly attenuated or distorted by wire mesh reinforcement, stainless steel cladding, and insulation backed by aluminum foil. Knowing its limits will ensure that it is used correctly and that the outcome of the inspection is interpreted correctly.

Probe Footprint Averages the Measurement

The PEC readings are not single-point readings, but are the average of the wall condition underneath the probe. This enables rapid coverage of large areas, but isolated pits and very small defects may be missed because they have little effect on the averaged reading. PEC is therefore a more effective means of identifying extensive corrosion rather than pitting corrosion.

Reduced Accuracy Near Edges and Complex Geometry 

The magnetic field distribution is less predictable in the vicinity of nozzles, flanges, welds, supports and other sudden geometric changes. These features may impact the decay signal and affect reliability of the measurement. As stated in the equipment protocol, readings should be obtained on consistent portions of the component and typically kept a minimum distance from welds, nozzles, flanges, and supports.

Lift-Off Affects Measurement Quality 

The distance between the probe and the steel surface (lift-off) directly affects the signal strength. Air gaps and variations in insulation thickness or weather jacketing can reduce measurement accuracy. These effects can be minimized by good calibration, but excessive lift-off can still cause a loss of sensitivity.

Limited to Ferromagnetic Materials 

PEC testing is applicable only to carbon steel and other ferromagnetic materials. It is not suitable for austenitic stainless steel, aluminium, copper, or other non-ferromagnetic alloys. While eddy currents are induced in these materials, they lack the magnetic permeability that produces the slow, measurable decay PEC relies on, so wall thickness cannot be reliably estimated.

Wall Thickness and Standoff Limits

At both ends of the thickness range, PEC has feasible working limits. While very thick sections (usually above 65 mm, or up to approximately 100 mm on high-performance systems) surpass the measurable response window, very thin walls (usually below about 6 mm) give a decay signal that is too fast to resolve satisfactorily. Similar to this, high standoff brought on by extremely thick insulation lowers resolution by increasing the effective footprint and decreasing the signal-to-noise ratio.

Cladding and Insulation Material Effects

PEC performance is significantly impacted by the kind of insulation and jacketing. Cladding made of aluminum and galvanized steel is usually suitable. Stainless steel cladding, wire mesh reinforcement, and insulation backed by aluminum foil all weaken or distort the field, making readings untrustworthy. Prior to scheduling an inspection, the type of insulation should always be verified.

Cannot Identify the Exact Location of Wall Loss 

PEC does not identify which surface of the wall, internal, external or throughout, is being affected by the corrosion. Further inspections are necessary if the exact location or shape of the corrosion needs to be known.

Requires Confirmation with Other NDT Methods 

PEC is a screening method, and any indication of significant wall loss should be confirmed by a quantitative technique. The follow-up method most commonly employed is ultrasonic thickness testing, which can give accurate thickness readings that aid in the repair and fitness for service decisions.

Using PEC Correctly: Screen Broadly, Confirm Selectively 

The best use of Pulsed Eddy Current (PEC) testing is in combination with other inspection techniques. PEC is not a replacement for other NDT methods but is an excellent tool for quickly pinpointing areas where corrosion is likely and hence detailed examination can be concentrated only on those areas.

A typical workflow begins with PEC screening of insulated carbon steel assets. Selected areas with apparent wall thinning are then removed of insulation, and undergo ultrasonic thickness (UT) testing to provide accurate point thickness measurements and to validate the amount of metal lost. This targeted approach avoids unnecessary insulation removal, reduces inspection costs, minimises downtime, and improves maintenance planning.

Rapid screening and accurate thickness verification can be combined to allow the inspection of larger assets more efficiently and with confidence in the inspection results.

Looking to scale up your PEC screening? Explore the PECA (Pulsed Eddy Current Array) probe for faster, higher-coverage inspection of large insulated assets.

→ Read: PECA Pulsed Eddy Current Array Probe

Conclusion

The Pulsed Eddy Current (PEC) Inspection is an important inspection method for general wall loss on insulated carbon steel assets as it does not require the removal of insulation, coatings or fireproofing. It is also highly effective in corrosion under insulation inspection because it can easily survey large areas within a short period of time, saving both time and cost while helping maintenance teams locate problem areas early.

However, rather than taking the place of quantitative thickness assessment, PEC must be used as a screening method. Each reading is an average measurement within the probe footprint, so if the defect is local and/or the geometry is complex, ultrasonic testing or other complementary NDT techniques may be needed to confirm.

When incorporated into a risk-based inspection (RBI) programme, PEC provides an effective tool for improved asset management; it can be used to specify inspection areas with a greater degree of specificity, thus avoiding unnecessary insulation removal and informed decisions on maintenance to maximize equipment reliability and plant safety.

Frequently Asked Questions (FAQs)

1. How thick an insulation layer can Pulsed Eddy Current (PEC) testing inspect through?

The efficiency of the inspection will vary with the equipment, probe design, insulation material, and calibration. Most modern PEC systems can inspect carbon steel through approximately 100–300 mm of non-metallic insulation with standard weather cladding, and some configurations exceed this. Signal strength decreases as standoff increases, so accuracy declines with very thick insulation. Metallic cladding also matters: galvanised or aluminium jacketing is generally acceptable, whereas stainless steel cladding and wire mesh can substantially reduce signal quality.

2. Is it possible to detect pitting corrosion with PEC testing?

PEC performs best when detecting general wall loss distributed across the probe footprint. Small, isolated pits may not make an appreciable difference in the averaged measurement, making them difficult to detect. If localized pitting is suspected, the area is usually confirmed by ultrasonic thickness testing or other high-resolution NDT method.

3. Is PEC testing a replacement for ultrasonic thickness testing? 

No. Ultrasonic testing provides accurate point-by-point thickness measurements, whereas Pulsed Eddy Current testing indicates areas of probable wall loss across a footprint. In practice, PEC is used in prioritization of inspection locations and UT is used to confirm corrosion prior to making maintenance decisions.

4. What is PECA?

Pulsed Eddy Current Array (PECA) is an enhanced version of PEC technology that incorporates multiple sensing elements into an array instead of a single probe. This enhances coverage of inspections, accelerates the rate of data collection, and creates more detailed maps of corrosion on large pipelines, vessels, and storage tanks.

5. Does PEC testing work on stainless steel? 

PEC is designed for ferromagnetic materials such as carbon steel. Austenitic stainless steel, aluminum, copper, and other non-ferromagnetic alloys do not typically provide the magnetic decay response that is necessary for PEC measurement.

6. Which industries commonly use Pulsed Eddy Current testing? 

PEC testing is used across the oil and gas, petrochemical, chemical processing, power generation, marine, and offshore industries. It is used quite often in insulated pipelines, pressure vessels, storage tanks, structural steel, and even fireproofed equipment, where hidden corrosion is considered to be a threat to long-term integrity of the apparatus.