Magnetic Flux Leakage Testing for Inspecting Steel Structures and Bridge Cables

Non-destructive testing has one fast way to read the corrosion hidden inside pipe walls, tank floors and bridge cables without cutting into them: magnetic flux leakage testing, which finds it long before any damage shows. Knowing how its readings work lets an asset owner trust a scan report and know when a second check has to confirm what it flags before a repair is planned.

Every result starts from one principle, which the first section unpacks: where corrosion has thinned a magnetized wall or wire, magnetic flux leakage appears above it, and the sensors turn it into a signal.

What Is Magnetic Flux Leakage Testing?

Within non-destructive testing (NDT), magnetic flux leakage testing is the technique that drives a ferromagnetic component close to magnetic saturation and uses Hall-effect sensors or induction coils to measure the field that leaks out at flaws. Sound material keeps the flux inside, while a corrosion pit, wall loss or a broken wire pushes some of it out into the air: that escape is the magnetic flux leakage the test is named after.

What Does Magnetic Flux Leakage Testing Measure?

The reading shows where corrosion or wear has removed material, not the thickness that remains: a Hall-effect sensor or coil records each finding as an indication tied to a location, so a scan report reads as a map of suspect areas rather than a thickness chart. On wire rope and strand, testing looks at 2 kinds of loss: local flaws, such as a snapped wire or a pit, and the loss of metallic cross-sectional area along the rope.

Which Products Can Magnetic Flux Leakage Inspect?

Carbon steel plate, pipe, tank floors, wire rope, cables and tendons all suit the technique, with no couplant and no need to strip the coating. Each of them magnetizes readily, the one material property the test depends on, so aluminum, copper or austenitic stainless steel falls outside its scope.

Who Performs Magnetic Flux Leakage Testing?

NDT technicians, data analysts and integrity engineers share the work on a bridge, a tank or a pipeline, each covering one stage of the inspection:

• NDT technicians: set up the magnetizer and the scanner, run the calibration scan on a reference sample and record every pass under the written procedure.
• Data analysts: compare each signal with the calibration flaws, filter out noise and grade every indication by its estimated metal loss.
• Integrity engineers: turn the graded indications into decisions, such as a confirming ultrasonic reading, a shorter inspection interval, targeted maintenance or a repair.

How Does Magnetic Flux Leakage Testing Work?

In 3 steps, testing goes from magnetizing the steel to a map of defects:

1. A magnetizer, built from permanent magnets or an electromagnet, drives the steel close to saturation.
2. A defect forces some of that flux into the air as magnetic flux leakage.
3. Sensors read the leakage, and the system maps each signal to a position along the component.

How Does Magnetization Saturate a Ferromagnetic Component?

Saturation is reached when a magnetizer pushes so much flux into the steel that the material can’t carry more. A yoke with permanent magnets or an electromagnet sits on the wall or plate, and its 2 poles close a magnetic circuit through the component.

Near saturation, the steel’s permeability drops, so the flux spreads evenly through sound material and a reduced cross-section can’t take any more: the excess has to leave the metal. Weak magnetization would let the flux route around a small flaw instead of leaking, which is why the magnetizer is matched to the wall or the strand bundle it scans.

Why Do Defects Cause Magnetic Flux Leakage?

A flaw removes material that was carrying flux. A corrosion pit, wall thinning or a crack leaves less cross-section at that spot, and the saturated steel around it can’t absorb the extra flux, so some of the field bulges out into the air. That magnetic flux leakage takes the size and shape of the flaw, which is what makes it readable.

How Do Sensors Turn Leakage Into Defect Data?

Each sensor converts the escaping field into an electrical signal at every point of the scan. Hall-effect sensors read the strength of the magnetic flux leakage, and inductive coils react to the change in leakage as they pass over it. The system stores each signal with its position as scan data, and analysis of the amplitude and shape against calibration flaws helps identify the defect type and estimate how much material is gone.

Which Types of Damage Can Magnetic Flux Leakage Testing Detect?

Corrosion and pitting, cross-sectional metal loss, and wire breaks and cracks are the 3 types of damage that magnetic flux leakage reveals in ferromagnetic steel. Each one removes material from the flux path, so detection comes down to reading that loss as a leakage signal.

Corrosion and Pitting

Corrosion pitting and general corrosion are the main targets on plates, pipe walls and tank floors, and each leaves its own signature. Uniform corrosion over wide areas gives a broad, low signal, while a pit gives a sharp, narrow one, and the scanner grades each indication as an estimated percentage of wall loss. The technique also reads the far side of a plate, so a floor scan shows soil-side corrosion while the floor surface still looks sound from inside the tank.

Cross-Sectional Metal Loss

For a cable or a tendon, cross-sectional metal loss is the share of steel area it has lost, and testing by magnetic flux leakage quantifies it by relating the leakage amplitude to the missing area. This is the number integrity engineers need for these members: the section left to carry the load, more than the depth of any single pit.

Wire Breaks and Cracks

A broken wire or a crack gives a short, sharp signal because the gap cuts the flux across a short length of wire. In a multi-strand bundle, breaks in the outer wires are easier to detect than internal breaks near the core.

The inspection shows where a wire broke but not how deep a crack runs, a job ultrasonic testing does better. On a cable-stayed bridge, a cluster of broken wires found this way often leads to stay cable maintenance such as individual strand replacement, a repair suited to multi-strand cables where the damage is limited to a few strands.

What Equipment Does a Magnetic Flux Leakage Test Require?

A magnetizer, sensors that read the leakage and a data acquisition system that records each signal with its position make up the equipment. A scanner carries all 3, whether as a cart for storage tank floors and plates, a ring around a cable or a tool that travels inside a pipeline.

Magnetizers and Yokes

A magnetizer or yoke supplies the field, either from permanent magnets or from an electromagnet driven by electric current. Magnet-based heads need no electrical supply, which suits portable equipment, whereas an electromagnetic yoke lets the operator adjust the field strength and switch it off to reposition the head. Width is the other trade-off: a wider yoke covers more surface per pass, but it needs a stronger magnet to saturate the same wall.

Hall-Effect Sensors and Inductive Coils

Both sensor types capture magnetic flux leakage, and each responds to a different property of the field:

Data Acquisition and Scan Speed

Together, the sampling rate and the scan speed set the resolution of the results. A scan that runs too fast for the sampling rate lets a short pit fall between 2 samples and escape detection, while a steady pace gives each defect a consistent signal shape for sizing. Wheel encoders tie every sample to a distance, so the inspection report places each indication in feet and inches along the pipe, plate or cable.

Where Is Magnetic Flux Leakage Testing Used?

Oil and gas pipelines and above-ground storage tanks, the cables of cable-stayed and suspension bridges, and post-tensioning tendons are the main uses of magnetic flux leakage testing. What these assets share is corrosion that develops inside a pipeline, a duct or under a floor plate, where no visual inspection reaches.

Pipelines and Storage Tank Floors

On pipelines and storage tank floors, the technology maps corrosion over long runs of pipe wall and wide areas of plate. Tank floor scanners cover the plates of above-ground storage tanks and detect soil-side corrosion before it turns into leaks, while pipe and oil well tubing are also screened with flux leakage equipment before and during service.

In a pipeline, the technology works from the inside, through an in-line inspection tool, often called a smart pig, that the flow of oil or gas pushes along the pipeline. Its parts each play one role:

• Magnets and steel brushes: in contact with the pipe wall, they magnetize it as the tool travels.
• Sensor ring: set between the magnet poles, it records the magnetic flux leakage all around the pipe.
• Odometer wheels: they tie each indication to a distance along the pipeline, so a dig crew can find it.

The operator gets a metal-loss map of the whole pipeline without digging it up, as long as it has launch and receive traps and bends wide enough for the tool. Most tools magnetize the pipe along its axis, which is well suited to detecting corrosion but can miss narrow features that run along the pipe, such as axial cracks; tools with a circumferential field are used for those. Speed matters as well, since a tool pushed too fast by the flow loses detection sensitivity.

Bridge Stay Cables and Suspension Cables

Stay cables and suspension cables hide their steel inside ducts, sheaths and filler, where corrosion and broken wires stay out of sight, which is why bridge inspectors use magnetic scans to examine them. Each stay endures high fatigue loads and typically holds 10 to 127 strands, so a ring-shaped head that travels along it reads the whole bundle without opening it. Suspension cables get the same ring scan, which looks for wire fractures and lost section.

A magnetic scan does not replace every inspection on a stay cable: on multi-strand stays, one strand can be removed and tested after 10 to 20 years of service, to verify its condition and confirm that its residual fatigue life matches expectations.

Post-Tensioning and Prestressing Tendons

In grouted internal tendons and in external ones, magnetic flux leakage testing screens for fractured wires and section loss. Each post-tensioning tendon typically carries 4 to 37 strands inside a duct, and since the grout or the duct hides corrosion from the eye, a magnetic scan lets inspectors assess the strands without opening the duct. On a bridge, these scans are one part of post-tensioning tendon inspection, alongside visual checks of the anchor heads and sonic testing of the ducts.

Magnetic flux leakage has a track record on corroded tendons. An FHWA techbrief on corrosion-induced tendon failures describes the Mid-Bay Bridge in Florida: after a failed tendon was found in 2000, inspectors put every tendon through magnetic flux leakage testing to locate section loss, the scans found corrosion and wire fractures in 2 tendons, and by the end of the inspections 11 of the 846 tendons had been replaced.

What Are the Strengths and Limitations of Magnetic Flux Leakage Testing?

Fast, repeatable screening of ferromagnetic steel is the strength; wall thickness and less precise depth sizing than ultrasonic testing are the limits. Repeatability matters in structural integrity management, where an asset owner compares inspection results over the years, in a condition assessment, to see whether section loss is growing.

Main Advantages of Magnetic Flux Leakage Testing

Speed, no couplant, repeatability and coating tolerance are its main advantages:

• Speed: a scanner covers a whole tank floor, pipe run, or cable length in passes, instead of one thickness reading at a time.
• No couplant: the field crosses the air gap, so the head needs no gel or water against the steel surface.
• Repeatability: repeated scans of the same tendon or cable give consistent results, so a later scan can be compared with an earlier one to track change.
• Coating tolerance: paint and other non-magnetic coatings provide no barrier to the field, up to the liftoff limit of the sensors, beyond which small flaws fade from the signal.

Those advantages make magnetic flux leakage testing the first inspection pass on a large asset, with slower point methods kept for the areas it flags.

What Are the Limitations of Magnetic Flux Leakage Testing?

Wall thickness, the need for a magnetic material and sizing uncertainty set the limits of magnetic flux leakage testing:

• Thickness: the magnetizer has to saturate the wall, so thick sections are hard to test and each system has a maximum wall thickness it can handle.
• Ferromagnetic steel only: a material the magnetizer can’t saturate gives no leakage signal to read.
• Sizing: the signal depends on a defect’s length, width, depth, and orientation together, so depth estimates carry more uncertainty than a direct thickness reading.

How Does Magnetic Flux Leakage Testing Compare to Other Non-Destructive Testing Methods?

Non-destructive testing offers several techniques, and magnetic flux leakage testing is its fast screening tool: ultrasonic testing measures thickness precisely, magnetic particle testing finds surface cracks, and acoustic and guided-wave methods monitor or screen long lengths.

Magnetic Flux Leakage Testing Versus Ultrasonic Testing

Magnetic flux leakage covers ground faster, while an ultrasonic reading gives the remaining thickness at a point with more precision. The ultrasonic probe also needs a couplant and a bare, smooth surface, and magnetic flux leakage needs neither, which is why many inspections use both: the magnetic scan screens the storage tank floor or the pipeline, then a point reading confirms the depth at each indication.

Magnetic Flux Leakage Testing Versus Magnetic Particle Testing

Testing by magnetic flux leakage is used to detect volumetric metal loss through the wall, while magnetic particle testing shows cracks at or just below the surface. It magnetizes the component too, but it relies on iron particles that gather at the crack, so the inspector has to read them by eye on a clean surface; magnetic flux leakage is recorded electronically instead, which is why it can read the far side of a plate or the inside of a cable bundle.

Magnetic Flux Leakage Testing Versus Acoustic and Guided-Wave Methods

A magnetic scan covers the steel only where the head passes, while acoustic monitoring listens all the time and guided waves screen a range from a single probe position. Acoustic sensors on a cable or tendon pick up the energy a stressed wire releases when it breaks, and since that sound reaches nearby sensors at slightly different times, the arrival times locate the break. The guided-wave technique works differently again: it can detect corrosion or fractures within a limited distance, which makes it most useful in anchorage zones a scanner can’t reach.

For tendons at risk, the 2 approaches work together: a magnetic scan measures the section already lost, while continuous listening tracks each new break in real time and helps owners decide when post-tensioning repair can no longer wait.

Turn Magnetic Flux Leakage Testing Results Into a Repair Plan With Freyssinet USA

A magnetic flux leakage test locates corrosion and broken wires inside a stay cable or tendon, and each finding then calls for a decision on the cable.

At Freyssinet USA, our own crews carry those findings through maintenance, repair or strand replacement.

Request a stay cable inspection and repair plan from our team.

Frequently Asked Questions About Magnetic Flux Leakage Testing

What Unit Measures the Flux in Magnetic Flux Leakage Testing?

Magnetic flux is measured in webers, and the flux density a sensor actually reads is measured in teslas, or in gauss, still widely used in US practice (1 tesla equals 10,000 gauss). A Hall-effect sensor gives a reading proportional to the flux density at each point, while an induction coil responds to how fast the flux changes, so its signal depends on scan speed.

What Are the Steps of a Magnetic Flux Leakage Inspection Procedure?

Testing starts with surface preparation, removing loose scale and debris that would hold the scanner away from the steel, and with a calibration scan on a reference sample with known artificial flaws. The inspector then scans the component in overlapping passes, marks every indication above the reporting threshold and proves up the significant ones with ultrasonic testing before the report.

Is Magnetic Flux Leakage Testing Part of Routine Bridge Inspections?

No, routine inspections of stay cables and post-tensioning tendons stay visual, and magnetic flux leakage testing is used during in-depth and special bridge inspections, the close-up, hands-on inspections that look for defects a routine visual inspection can’t detect. Testing is called in when routine findings, the structure’s age or its exposure suggest hidden corrosion or broken wires.

What Qualifications Does a Magnetic Flux Leakage Technician Need?

Most US employers certify their NDT technicians themselves under a written practice, at Level I, II or III in each NDT method, after a set number of classroom hours, supervised field experience in the method and an eye exam. A Level II technician can set up the equipment, run the scan and interpret the results, while a Level III writes and approves the procedures the others follow.

How Often Should Bridge Cables and Tendons Be Inspected With Magnetic Flux Leakage Testing?

Bridge owners schedule testing from a risk assessment: the age of the cable or tendon, its exposure to water and de-icing salts, and what earlier inspections found. A stay cable or post-tensioning tendon with known corrosion or broken wires gets a shorter interval or continuous monitoring, while one in good condition waits for the next in-depth inspection.