What Is Tank Floor MFL Inspection?
Magnetic Flux Leakage (MFL) inspection is the standard method for screening the floor plates of an aboveground storage tank (AST) for corrosion. We deploy a motorised, robotic floor scanner that runs across the tank bottom under its own power, carrying a powerful magnetic bridge and an array of sensors. As the scanner passes over a plate, its magnet saturates the steel with magnetic flux. Wherever corrosion, pitting or metal loss has thinned the plate, the flux cannot all stay inside the steel — some of it leaks out at the surface, and the sensors read that leakage as a signal.
The reason MFL is so valuable is that a tank floor corrodes from two directions at once. The topside sits under product, water bottoms and settled sludge; the underside, or soil side, rests directly on a sand or oil-water pad and is exposed to ground moisture, microbial activity and cathodic conditions that the eye can never reach. MFL detects metal loss from both faces of the plate in a single pass, which is exactly why it has become the workhorse screening technique for floor-plate integrity across the oil, gas and petrochemical sector.
It is worth being clear about the technique. MFL floor scanning is a floor-specific method that uses a crawler-type robotic scanner running on the tank bottom — it is not a flying drone. It sits within our wider industrial inspection capability alongside aerial and confined-space methods, but the floor itself is scanned by a ground-running scanner because that is the only way to magnetically couple the sensor head to the steel.
Why the Tank Floor Is the Highest Integrity Risk
Of all the components on a storage tank, the floor carries the greatest integrity risk, and the reason is simple: most of its deterioration is invisible. Shell courses can be measured from outside, roofs can be walked and viewed, but the underside of the floor is permanently sealed against the foundation. Soil-side corrosion can progress for years, hollowing out the plate from beneath, while the topside a cleaning crew can see still looks sound. When a floor finally perforates, the result is loss of containment — product escaping into the foundation, the bund and potentially the environment.
That risk is why API 653, the industry code for tank inspection, repair, alteration and reconstruction, makes floor evaluation a central requirement of every internal inspection. The code expects the floor to be assessed for corrosion and its remaining thickness compared against a minimum, with the inspection interval driven by the measured corrosion rate and by whatever leak-detection, release-prevention or cathodic-protection systems the tank has. Oil and gas operators in Singapore's tank farms and refineries plan these internal inspections precisely so that a floor problem is found on schedule rather than discovered as a leak.
MFL is the method that makes a code-compliant floor inspection practical. Manually taking ultrasonic readings across thousands of square metres of floor would be impossibly slow and would still miss most of the surface. MFL screens the whole floor quickly, tells you where the metal loss is concentrated, and lets the detailed measurement effort be focused where it actually matters.
What MFL Floor Scanning Detects
Our floor scans are built around the specific ways a tank bottom degrades. Every indication is located, graded and prepared for ultrasonic confirmation and floor mapping.
Topside Corrosion
Metal loss on the upper face of the floor plate, driven by water bottoms, aggressive product, settled solids and sludge. Often patchy and concentrated where water collects, and readily flagged by the scanner.
Underside / Soil-Side Corrosion
The hidden threat — metal loss on the concealed underface where the plate meets the pad. Invisible to any visual check, this is the corrosion MFL exists to find before it perforates the floor.
Pitting & Localised Metal Loss
Sharp, isolated pits and clusters that penetrate far faster than general thinning. MFL is sensitive to these concentrated losses and separates deep pitting from broad, shallow wastage.
Weld-Adjacent Thinning
Corrosion tracking along lap and butt welds and around the critical zone near the shell-to-floor connection, where floor plates are most highly stressed and leaks are most consequential.
How the MFL Inspection Works
An MFL floor survey follows a disciplined sequence, because the quality of the data depends entirely on the condition of the floor when we scan it.
First, the tank must be prepared. It is taken out of service, drained, cleaned and gas-freed so that the floor is safe to enter and the plate surface is dry and reasonably free of sludge, scale and loose debris. Coating and residue reduce the scanner's sensitivity, so surface preparation is not optional — it directly determines how small a defect we can reliably detect.
Second, we scan the floor plate by plate. The robotic scanner runs systematic overlapping passes at a steady speed, covering the great majority of the floor area far faster than any point-by-point method. As it moves, it flags corrosion and metal-loss indications in real time, building a coverage map of where the plate is losing thickness and how severely.
Third, every significant indication is proven up. Because MFL grades an indication rather than giving an exact thickness, we return to each flagged location and take an ultrasonic thickness measurement to quantify the real metal loss and establish whether it is topside or underside. This screen-then-confirm approach is what gives the inspection both speed and accuracy.
Finally, the confirmed results are plotted onto a floor plan. The corrosion pattern, depth and location are mapped across the whole bottom so the owner and the API 653 inspector can see at a glance where the plate is sound, where it needs monitoring, and where it must be repaired.
- Whole-floor coverage — the scanner screens the entire accessible bottom, not the handful of grid points a manual UT survey could reach.
- Finds the hidden threat — soil-side corrosion that no visual inspection can see is detected from above the plate.
- Screen then confirm — fast MFL screening focuses slower ultrasonic measurement exactly where metal loss is real.
- Repair-ready mapping — every finding is located on a floor plan, so repairs target failed plate and leave sound plate untouched.
Where MFL Fits in an Out-of-Service Tank Inspection
Floor MFL is one part of a complete internal inspection, and it is planned to run alongside the other work that can only be done while the tank is open. Because opening a tank is costly and disruptive, operators consolidate the full scope into a single out-of-service window — often within a wider plant shutdown — so the tank returns to service once, fully assessed.
Within that window, MFL floor scanning is paired with ultrasonic shell thickness measurement to grade the wall courses, with roof-plate assessment, and with remote visual inspection of the tank interior, nozzles and internal fittings. Our broader tank inspection service brings these methods together, and where the interior must be viewed without putting people inside, confined-space drone inspection captures the shell, roof underside and internal structure. The floor scan supplies the one dataset none of the others can — a direct read of the plate on which the entire tank sits.
Standards We Work To
A defensible floor inspection is anchored to recognised codes, and our MFL surveys are structured around them.
API 653
The governing code for AST inspection, repair, alteration and reconstruction. It sets the requirement to evaluate the floor, assess remaining thickness against a minimum, and determine the inspection interval — the framework every floor scan feeds into.
API 575
The recommended practice for inspection of atmospheric and low-pressure storage tanks, giving guidance on damage mechanisms and inspection methods, including how floor examination fits the overall tank inspection strategy.
ASTM E570
The standard practice for flux leakage examination of ferromagnetic steel tubular products and, by extension, the electromagnetic principles that underpin MFL floor scanning and its sensitivity to metal loss.
Ultrasonic Prove-Up
MFL indications are quantified by ultrasonic thickness measurement, so the survey combines electromagnetic screening with direct thickness data — the pairing that turns a coverage map into an actionable floor assessment.
Why Choose SG Drone Inspections
We are a specialist inspection provider in Singapore, delivering complete, mapped floor-integrity data that owners and API 653 inspectors can rely on for repair and fitness-for-service decisions.
Screen-and-Confirm Discipline
We never treat MFL as the final word. Every significant indication is proven up ultrasonically, so what you receive is confirmed metal loss, not raw screening signals.
Full Floor Mapping
Findings are plotted onto a floor plan with location, depth and topside-versus-underside classification, giving a repair-ready picture of the whole tank bottom.
Integrated Tank Scope
Floor MFL is coordinated with shell UT, roof and internal visual inspection so a single out-of-service window delivers a complete tank assessment.
Code-Aligned Reporting
Data is structured to feed directly into the API 653 evaluation, so your inspector can move from our floor map to a fitness-for-service and interval decision without gaps.
Frequently Asked Questions
Get a Tank Floor MFL Inspection Quote
Send us your tank diameter, product and planned out-of-service window. We reply with a scope and quote within 24 hours — no obligation.