Storage tank bottom plate testing is a critical integrity assessment process that detects hidden corrosion, pitting, and metal loss in above-ground storage tank floors before catastrophic leaks occur. This guide covers the principal testing methods, including magnetic flux leakage (MFL) scanning and ultrasonic thickness measurement, along with applicable standards such as API 653 and EEMUA 159, common defect types, acceptance criteria, and inspection intervals for facilities in the oil, gas, chemical, and water industries.
Technical Overview
The bottom plate of an above-ground storage tank is the component most vulnerable to corrosion-driven failure. While the shell and roof of a tank are largely visible and accessible, the tank floor operates in a concealed environment: the top surface is exposed to potentially corrosive stored products, water bottoms, and sludge, while the bottom surface contacts a sand or bitumen-sand foundation that can retain moisture, chlorides, and oxygen. This combination of product-side and soil-side attack makes the tank bottom the leading source of leaks in storage facilities.
Because underside corrosion is invisible during routine walkdowns, dedicated bottom plate testing is required to quantify remaining wall thickness, locate and size pitting, and assess weld integrity. A leak from a tank bottom can contaminate soil and groundwater, trigger regulatory action and remediation costs, and interrupt operations for months. Bottom plate testing is therefore a core element of storage tank mechanical integrity programs, tank lifecycle management, and risk-based inspection (RBI) strategies.
Modern bottom plate inspection combines several complementary non-destructive testing (NDT) techniques. Magnetic flux leakage (MFL) floor scanning provides rapid, full-coverage corrosion mapping, while ultrasonic testing (UT) delivers precise thickness verification at suspicious indications. Supplementary methods—vacuum box testing, acoustic emission monitoring, and robotic in-service scanning—extend coverage to weld seams and to tanks that cannot readily be taken out of service. The governing inspection logic for steel tanks is defined in standards such as API 653, API 650, API RP 575, EEMUA 159, and relevant national codes (for example, GB series standards in China), which also establish minimum remaining thickness criteria and out-of-service intervals.
Testing Samples
Bottom plate testing programs are applied to a defined population of plate components within the tank floor assembly. The scope of sampling depends on tank design, service history, and corrosion risk.
Bottom Plate Field
The primary sample is the rectangular or square butt-welded (or lap-welded) plate field that forms the tank floor, typically 6 mm to 12 mm (1/4 in to 1/2 in) nominal thickness in carbon steel. Testing covers both sketch plates and filler plates, with the plate field divided into a grid to record the location of every indication.
Annular Ring Plates
Annular plates around the tank perimeter experience high stress from the shell-to-bottom junction and are frequently the thinnest and most corrosion-prone components. They receive prioritized scanning and detailed ultrasonic thickness measurement, particularly the zone within 300 mm of the shell.
Shell-to-Bottom Weld and Interior Welds
The fillet weld connecting the shell to the bottom, along with internal bottom plate seams, sump attachments, and reinforcing pads, is examined for cracks, undercut, and pinhole leaks using visual, magnetic particle, and vacuum box techniques.
Sumps, Trenches, and Internal Appurtenances
Water draw-off sumps, internal drainage trenches, mixers, heater supports, and floating roof landing areas on the bottom plate are sampled because they create local corrosion cells and crevices that accelerate metal loss.
Testing Items
A comprehensive bottom plate assessment evaluates the following measurable characteristics:
- Remaining wall thickness of each bottom plate and annular ring, compared against nominal thickness and corrosion allowance
- General (uniform) corrosion: area-averaged metal loss and corrosion rate calculation from previous inspection data
- Localized pitting corrosion: pit depth, pit diameter, pit spacing, and clustering evaluation
- Edge corrosion at plate laps and weld toes, including preferential weld-zone attack
- Underside (soil-side) corrosion blisters and bulging detected through MFL signal amplitude
- Coating and lining condition on the product side, including holidays, disbondment, and under-film corrosion
- Weld integrity: surface cracks, porosity, incomplete penetration, and through-wall leakage paths
- Bottom settlement and foundation distortion that may pond water and accelerate corrosion
- Cathodic protection effectiveness beneath the tank, where installed
- Sludge and water bottom characterization for corrosivity assessment
Testing Methods
Method selection balances coverage, sensitivity, tank access condition (in service versus out of service), and budget. The most widely applied techniques are described below.
Magnetic Flux Leakage (MFL) Floor Scanning
MFL is the industry-standard technique for full tank bottom coverage. The scanner magnetizes the plate locally, and sensors detect flux leakage caused by wall loss on either surface. Modern MFL floor scanners record C-scan corrosion maps in real time, covering a typical production rate of several hundred square meters per shift. MFL detects both top-side and bottom-side defects but does not by itself distinguish which surface is corroded; indications must be verified with ultrasonic testing after local cleaning.
Ultrasonic Thickness (UT) Measurement
Conventional zero-degree UT probes provide point-by-point thickness readings at MFL indications, on grid patterns, and at critical zones such as annular rings. Thickness values are recorded to the nearest 0.01 mm and used to confirm minimum remaining thickness, calculate corrosion rates, and establish remaining life in accordance with API 653. Advanced approaches such as B-scan and corrosion mapping add profile data across pitted areas.
Vacuum Box Testing
For through-wall leak paths in bottom welds, a vacuum box with a soap solution film is placed over the seam. Applying a partial pressure differential of typically 21 kPa (about 5 psi or 0.2 bar) reveals leaks as forming bubbles. This technique is mandatory in many repair scopes and new construction acceptance tests per API 650.
Visual Inspection and Surface NDT
After cleaning (ideally to bare metal or near-white finish), the product side is visually inspected for pitting, coating breakdown, and deformation. Magnetic particle testing (MT) and penetrant testing (PT) are applied to welds where cracking is suspected. Visual examination is also essential at the shell-to-bottom corner weld and beneath the first shell course where access allows.
In-Service and Robotic Techniques
For tanks that cannot be emptied, ultrasonic thickness crawling robots operating under a liquid film measure bottom thickness from the product side, while the tank remains in service. Acoustic emission (AE) monitoring during controlled loading can rank tanks by active corrosion and leak probability, helping prioritize the out-of-service program. These methods do not replace full out-of-service inspection but reduce risk between internal inspections.
Supplementary and Emerging Methods
- Pulsed eddy current (PEC) for corrosion under insulation and through-coating screening at annular areas
- Low-frequency eddy current array scanning for shallow pitting on cleaned floors
- 3D laser scanning or photogrammetry for bottom settlement and buckling documentation
- Radiographic correlation of lap welds where underside access verification is required
Applications
Bottom plate testing programs are applied across industries that store liquids in vertical cylindrical steel tanks:
- Crude oil, refined product, and aviation fuel terminals, where soil-side corrosion and product-side water bottoms dominate risk
- Refinery process tanks, including sour water, caustic, and slop service with high corrosion severity
- Chemical and petrochemical storage, including acids, alkalis, and solvent tanks where lining integrity is coupled to floor condition
- Firewater, potable water, and wastewater tanks, including those with internal coatings subject to under-film corrosion
- Biofuel, ethanol, and edible oil storage where microbial-influenced corrosion (MIC) can perforate floors rapidly
- Terminal change-of-service evaluations, tank life-extension studies, and pre-purchase due diligence inspections
- Regulatory compliance inspections supporting environmental permits and secondary containment integrity programs
Accredited third-party laboratories and inspection bodies routinely perform these assessments, issuing documented results that owners use for fitness-for-service decisions, repair planning, and insurance or regulatory submissions.
FAQ
How often should tank bottom plates be inspected?
Under API 653, the internal (out-of-service) inspection interval is corrosion-rate driven but may not exceed 20 years, and many tanks are examined every 7 to 10 years depending on service severity and release-prevention measures. RBI analysis and acoustic emission screening results can justify extending or shortening the interval. External in-service inspections are typically performed every 5 years, with ultrasonic thickness measurements from the shell side to track corrosion rates.
What minimum thickness requires repair of a bottom plate?
API 653 generally requires repair or replacement when remaining thickness reaches the smaller of 50% of nominal bottom thickness or the minimum thickness defined in the corrosion allowance (commonly 0.1 in / 2.5 mm for bottom plates without a release-prevention barrier). Pits are evaluated by remaining thickness: pits with remaining wall below roughly 60% of nominal may require evaluation or repair depending on spacing and clustering, and any indication of a through-wall pit mandates immediate repair.
Can a tank bottom be tested while the tank is in service?
Yes, partially. Robotic ultrasonic crawlers can measure floor thickness from the product side in many services, and acoustic emission testing can rank corrosion activity without opening the tank. However, these in-service methods have coverage limitations and cannot fully replace an out-of-service MFL scan with visual inspection, which remains the reference approach at each internal inspection.
Why is MFL used first, followed by ultrasonic testing?
MFL delivers rapid full-area coverage and flags every region of possible metal loss, which would be impractical with point UT alone. UT then provides accurate quantification at each flagged location and determines whether the corrosion is on the top or bottom surface, information that dictates the repair method (plate patch versus full replacement) and the corrosion control strategy.
What preparation is required before bottom plate testing?
The tank must be emptied, gas-freed, and isolated under a documented safety plan. Sludge, scale, and loose coating must be removed—ideally by abrasive blast or high-pressure water jetting to achieve a clean surface, because MFL and UT sensitivity degrade significantly on corroded or coated surfaces. The floor is then gridded so that every indication can be precisely located for repair mapping.
Do standards other than API apply?
Yes. EEMUA 159 provides detailed tank inspection guidance widely used in Europe and beyond; API RP 575 addresses atmospheric and low-pressure tank inspection practices; API 650 governs new construction and repair welding acceptance; NACE/AMPP publications address underside corrosion control and coatings; and national codes such as the GB standards in China apply to locally regulated facilities. Owners should confirm the applicable jurisdictional requirements before finalizing the inspection scope.