Magnetic Flux Leakage (MFL)

MFL is an electromagnetic NDT method that magnetizes a ferromagnetic component to saturation and detects flux leakage at metal-loss features using Hall-effect or coil sensors, widely used for tank-floor and pipeline pig inspection.

Definition

Magnetic Flux Leakage (MFL) inspection magnetizes a ferromagnetic component, typically to near magnetic saturation. Where wall thickness is locally reduced — by pitting, corrosion, or grooving — magnetic flux 'leaks' from the surface and is sensed by a Hall sensor or coil array. The leakage amplitude correlates with metal loss.

Technical Context

MFL is the dominant technique for storage-tank floor scanning and in-line pipeline inspection (ILI pigs). It is fast and reliable for general wall loss but less sensitive to circumferentially oriented cracks.

When It Is Used

API 653 tank-floor inspection
Pipeline ILI pigging
Tube and wire rope inspection

Related Standards

API 653, API 1163, ASTM E570.

How it works

A ferromagnetic wall is driven near magnetic saturation. Metal loss reduces the cross-section available to carry that flux, so flux is forced out of the wall and into the surrounding space, where sensors detect it. Signal amplitude relates to the volume of metal missing, not to depth alone.

What it finds

Volumetric metal loss — corrosion, pitting, gouging — quickly and over large areas. Tank floors and pipelines are the classic applications, and in-line inspection tools rely on it.

What it will not find

Tight planar flaws such as cracks aligned with the flux, which remove almost no metal and therefore leak little. Very gradual general thinning can also under-respond because the flux gradient is what generates the signal.

How it is actually done

Wall thickness limits apply, since saturation must be achievable; lift-off, scanning speed and coating thickness all modulate signal amplitude and have to be controlled or compensated. Results are usually confirmed by ultrasonic thickness at the indications found.

Governing codes and standards

API 653 for tank floor examination; API 1163 for in-line inspection system qualification; ASTM E570 for flux leakage examination of ferromagnetic tubing.

Where it goes wrong

Reading amplitude as depth. Signal responds to lost volume, so a broad shallow patch and a narrow deep pit can produce comparable amplitudes — which is exactly why MFL screens and ultrasonics confirms.

Where Magnetic Flux Leakage fits in an inspection programme

A term is only useful when it connects to a decision. Magnetic Flux Leakage appears in written procedures, in technique sheets, and in the records an owner or accreditation body reviews afterwards — which means the way it is defined in your documentation has to match the way it is applied on site. Where the two drift apart, audits find it. Atlantis writes and reviews procedures against the governing codes, trains inspection personnel to apply them, and builds the record-keeping that makes the evidence retrievable years later. Procedure development and code consulting · NDT training and certification · Ask us about your programme.

Related terms

Storage Tank — A storage tank is a low-pressure, large-volume welded container for the storage of liquids — typically atmospheric (API 650), low pressure (API 620), or for water (AWWA D100) — inspected in service per API 653.
API 653 Tank Inspection, Repair, Alteration, and Reconstruction — API 653 is the inspection, repair, alteration, and reconstruction code for above-ground welded steel storage tanks built to API 650, defining periodic external (5 yr), ultrasonic (10 yr), and internal (typically 10 yr) inspection intervals.
Pipeline — A pipeline is a long-distance pipe system for transporting fluids (crude oil, refined products, natural gas, water, chemicals), inspected by in-line inspection (ILI/pigging), direct assessment, and hydrostatic testing under API 1163, ASME B31.4, and B31.8.

More method terms

Ultrasonic Testing · Radiographic Testing · Magnetic Particle Testing · Penetrant Testing · Eddy Current Testing · Visual Testing · Acoustic Emission Testing · Leak Testing · Thermography / Infrared Testing · Microwave Testing

Where this comes up in practice

Terms like this one appear in three places that matter commercially: the written practice that governs how your personnel are qualified, the procedures and technique sheets that define how an examination is actually performed, and the evidence an auditor or client asks for when they want to know why an inspection was accepted. Getting the terminology right is the easy part; being able to produce the qualification record, the calibration traceability and the procedure revision that applied on the day of the inspection is the part that decides audits.

Atlantis NDT provides NDT training and certification against ASNT SNT-TC-1A and ISO 9712, ASNT Level III consulting for written practices and procedure approval, inspection management software that holds qualification, calibration and procedure-revision evidence in recoverable form, and an asset integrity platform that binds inspection results to the asset they describe. Browse the full NDT glossary or ask a Level III directly.