Magnetic Flux Leakage Testing (MFL) — Tank Floor & Pipeline Inspection
Magnetic Flux Leakage (MFL) testing is a high-speed NDT method for ferromagnetic materials. A powerful magnet drives carbon steel to near magnetic saturation; at corrosion pits, thinning, or wall loss the reduced cross-section cannot contain the flux, which "leaks" from the surface and is detected by Hall effect sensor arrays. MFL is the standard screening method for API 653 storage tank floors and the backbone of pipeline integrity management through in-line inspection (ILI) pigging — as well as well casing and wire rope inspection.
How MFL Testing Works
Three principles make MFL work: steel can be magnetically saturated; flux stays confined in intact, full-thickness steel but escapes at metal-loss locations; and Hall effect sensors convert that leakage field into a measurable voltage. Signal amplitude scales with defect depth and area, and signal shape helps discriminate internal from external metal loss. Key constraints: MFL only works on ferromagnetic steels (not austenitic stainless or aluminium); scan speed must stay within system limits; calibration against reference defects of known depth is required; and MFL indicates metal loss rather than measuring exact remaining wall — ultrasonic testing provides the precise thickness measurement at flagged locations.
MFL Applications
- Tank floor inspection (API 653 Appendix C) — full-floor scanning during internal inspections with near-100% coverage.
- In-line inspection (ILI / intelligent pigging) — MFL pigs assess internal and external metal loss in live transmission pipelines per API 1163 and ASME B31.8S.
- Well casing inspection — wireline MFL tools detect corrosion and perforation in steel casing per NORSOK D-010 well integrity programs.
- Wire rope inspection — magnetic testing of crane, hoist, and bridge ropes for broken wires and loss of metallic area per ASME B30 and EN 12385.
Tank Floor MFL — API 653 Appendix C Workflow
1) Empty, degas, and clean the tank. 2) Calibrate the scanner on reference plates with machined defects of known percentage wall loss. 3) Walk the scanner in overlapping passes for 95–100% coverage. 4) Supplement the edge zone with manual UT — MFL cannot reliably inspect within 50–75 mm of the shell-to-floor weld. 5) Confirm every flagged indication with UT or PAUT against API 653 floor retirement criteria. 6) Report calibration, coverage map, indications, UT confirmations, and fitness-for-service evaluation.
MFL Pipeline Pigging (ILI)
MFL pigs travel with the product flow at up to 3 m/s while the line stays in service. Standard MFL detects axially oriented metal loss; transverse field MFL (TFI) detects axial cracking and seam-weld anomalies. High-resolution tools find roughly 5% wall loss with about ±10% depth-sizing accuracy, versus approximately 20% for standard-resolution tools. ILI programs are governed by API 1163, NACE SP0102, and ASME B31.8S.
MFL vs UT for Tank Floors
| Characteristic | MFL scanner | Manual UT |
|---|---|---|
| Coverage per shift | Entire floor (100%) | Partial grid scan |
| Output | Approximate metal-loss indication | Precise wall thickness (mm) |
| Edge/near-shell zone | Cannot inspect (50–75 mm) | Inspects to the edge |
| Small pit sensitivity | Moderate, resolution-dependent | Excellent |
Best practice per API 653 Appendix C: MFL for 100% screening, UT confirmation of all flagged areas, and a UT grid in the edge zone. Where volumetric weld examination is also in scope, radiographic testing or PAUT covers the welds MFL cannot assess.
MFL Services & Digital Twin Integration
Atlantis NDT provides ASNT Level III consulting and inspection support for MFL programs — procedure development per API 653 Appendix C, scanner qualification, ILI data review per API 1163, and fitness-for-service evaluation. Every floor map and confirmation reading can be published to a tank-floor MFL digital twin overlay, so corrosion history, repair patches, and next-inspection projections live on a navigable 3D model of the asset rather than in static reports.
Frequently Asked Questions
What is magnetic flux leakage testing?
An NDT method that magnetises ferromagnetic steel to near saturation and detects the flux leaking from the surface at corrosion pits and wall-loss locations using Hall effect sensors — used for rapid screening of tank floors, pipelines, well casings, and wire ropes.
How is MFL used for storage tank floor inspection?
Per API 653 Appendix C, an MFL scanner is rolled across the cleaned floor in overlapping passes, achieving full coverage far faster than manual UT. Flagged locations are confirmed with UT, and the near-shell edge zone gets supplemental UT.
What is MFL intelligent pigging?
In-line inspection (ILI) sends a self-propelled MFL tool through a pipeline in service. Sensor arrays record leakage signals from internal and external metal loss along the entire line, which analysts size and rank so operators can excavate and repair the worst locations. API 1163 governs tool qualification.
Can MFL tell internal from external corrosion?
Approximately, yes. Defects nearer the sensors produce sharper signals, and analysis software uses signal-shape algorithms to discriminate internal versus external metal loss. Precise discrimination and sizing still rely on follow-up UT.
What are the limitations of MFL?
It works only on ferromagnetic steel, gives relative rather than absolute thickness data, may miss shallow pitting below roughly 10% wall loss, needs controlled scan speed and full saturation, and cannot inspect the tank-floor edge zone — all reasons MFL is paired with UT.
What standards govern MFL testing?
API 653 Appendix C for tank floors; API 1163, NACE SP0102, and ASME B31.8S for pipeline ILI; API 1110 for liquid pipelines; ASME B30 / EN 12385 for wire rope.
Atlantis NDT Products & Services
Atlantis NDT supports inspection organisations end to end: NDT inspection management software — Atlantis ERP for work orders, certification tracking and audit-ready records; a digital twin platform for asset integrity that overlays inspection history on a navigable 3D model; modern NDT reporting software; NDT training & certification backed by a 96% first-attempt pass rate and ASNT Level III authorship; ASNT Level III consulting; and 3D laser scanning services. Affordable. Accessible. Fully customizable. Book a free consultation — pricing varies by region and scope, and we return a tailored quote within 24 hours.