A tank bottom corrosion survey is the inspection that maps corrosion across the floor of an above-ground storage tank while the tank is emptied and out of service. The floor is the hardest part of a tank to see and the part most exposed to trapped water and soil-side corrosion, so a dedicated scan is used to find and measure the pitting and metal loss hidden in the plates. This guide explains how the floor scan works, why it is tied to out-of-service tank inspection, and how it connects the corrosion, coatings, and cathodic protection defenses used on storage tanks.
Tank Bottom Survey in one line: A tank bottom corrosion survey is an out-of-service inspection in which the empty tank floor is scanned - most commonly with magnetic flux leakage floor scanners - to locate and grade areas of corrosion and pitting on both the product side and the underside of the floor plates. It is a core part of storage-tank integrity work under API 653, because the floor is inaccessible in normal operation and is the plate most prone to trapped-water and soil-side corrosion, so the survey drives repair, patching, or floor-replacement decisions during the shutdown.
The floor of a large storage tank is a field of welded steel plates resting on a foundation, wetted by product above and exposed to moisture and soil below. It cannot be inspected while the tank holds product, so the survey is done when the tank is taken out of service, drained, cleaned, and made safe for entry. Inspectors then walk the floor with scanning equipment, covering the plates systematically to find where metal has been lost.
The workhorse technique is magnetic flux leakage, or MFL, scanning. An MFL scanner magnetizes the floor plate as it passes over it; where the steel is full-thickness the magnetic field stays contained, but at an area of corrosion or a pit the reduced metal lets magnetic flux leak out, and sensors detect that leakage and flag the spot. MFL is well suited to rapidly screening large floor areas for corrosion because it senses metal loss on both the top and, importantly, the underside of the plate, which is otherwise impossible to see.
MFL is a screening tool - it tells inspectors where the metal loss is. The flagged areas are then measured more precisely, typically with ultrasonic thickness readings, to quantify how deep the pitting or thinning actually is. Together the scan and the follow-up measurements produce a map of the floor's condition, distinguishing sound plate from areas that need repair or replacement.
The floor survey is inseparable from the broader out-of-service inspection of an above-ground tank, which is the framework storage-tank integrity is managed under. A tank cannot be entered or its floor scanned while in service, so these inspections are scheduled for the periodic internal shutdown when the tank is empty. The floor scan is one part of that internal inspection, alongside checks of the shell, roof, and appurtenances.
The floor gets special attention because it is both the most vulnerable and the least accessible plate in the tank. Water and sediment settle to the bottom and sit against the floor, and the underside is in contact with the foundation and any moisture there, so corrosion attacks the floor from both faces. Because the floor is what keeps product from reaching the ground, a floor leak is an environmental as well as an integrity concern, which raises the stakes of finding corrosion before it perforates.
The survey results drive concrete decisions during the outage. Areas of acceptable metal loss are recorded and monitored, more severe local thinning may be patched or the plate replaced, and widespread loss can justify replacing the whole floor or adding measures such as a new coating or an under-tank cathodic protection system. The findings also help set when the tank should next be opened for inspection.
Tank floor integrity is defended by the same layered approach used elsewhere in the field: control the corrosive environment, put a coating barrier on the steel, and back it with cathodic protection on the soil side. A tank bottom survey is how those defenses are verified. Persistent water bottoms and sediment feed product-side corrosion, an internal floor coating is the primary barrier, and cathodic protection under the tank protects the soil-side face; the floor scan measures how well that combined system has held up over the inspection interval.
A cloud SCADA platform such as Merobix contributes on the operating side, between shutdowns. Continuous tank monitoring - level, and in particular water-bottom management through draw-off - helps limit the standing water that drives internal floor corrosion, addressing the environment before it attacks the plate. Where a tank has soil-side cathodic protection, monitoring rectifier output and structure potentials remotely gives an ongoing indication that the under-tank protection is functioning between the widely spaced internal inspections.
The floor scan itself is a periodic, out-of-service measurement and is not something SCADA performs. But by helping keep water bottoms down and by watching the cathodic protection that guards the underside, continuous monitoring reduces how fast the floor corrodes and provides some visibility into its protection during the long intervals when the tank is in service and the floor is out of sight. Monitoring manages the conditions; the survey quantifies the result.
The floor is the least accessible plate in a tank - it cannot be seen or scanned while the tank holds product - and it is attacked from both sides, by settled water and sediment above and by soil-side moisture below. Because a floor leak lets product reach the ground, finding corrosion before it perforates is critical, which is why the floor gets a dedicated scan during out-of-service inspection.
MFL stands for magnetic flux leakage. An MFL scanner magnetizes the floor plate as it moves across it, and where corrosion or a pit has reduced the metal, magnetic flux leaks out and sensors detect it, flagging the spot. It is used to rapidly screen large floor areas and can sense metal loss on the underside of the plate, which is otherwise impossible to see.
It is performed during a scheduled out-of-service internal inspection, when the tank has been emptied, cleaned, and made safe for entry, since the floor cannot be scanned while the tank is in service. The survey is one part of the internal inspection under storage-tank integrity practice, and its findings help determine repairs and set when the tank should next be opened.
This page references the standards, specifications, and official documentation published by the organizations below. Editions, product capabilities, and documentation change over time - confirm current requirements and specifications directly with the source.
Last reviewed: July 27, 2026. Merobix is not affiliated with, endorsed by, or sponsored by these organizations; their names are used only to identify the standards and products discussed.
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