Storage tank bottom plate corrosion assessment is a critical non-destructive evaluation process used to quantify wall thickness loss, pitting severity, and remaining service life of aboveground storage tank floors. By combining techniques such as magnetic flux leakage scanning, ultrasonic thickness mapping, and laboratory corrosion analysis, accredited third-party laboratories help operators detect hidden underside corrosion, comply with tank integrity standards, and make reliable repair-versus-replace decisions.
Technical Overview
The bottom plate is the most corrosion-vulnerable component of an aboveground storage tank. It is permanently exposed to two aggressive environments at once: the product side, where free water, sediment, chlorides, and microbiologically influenced corrosion (MIC) accumulate in the lowest part of the tank, and the soil side, where moisture, differential aeration cells, chloride-bearing backfill, stray currents, and ineffective cathodic protection drive hidden metal loss. Because this damage develops out of sight, a structured corrosion assessment is the only reliable way to establish the true condition of the tank floor.
A comprehensive bottom plate corrosion assessment typically includes high-coverage floor scanning, ultrasonic thickness verification, visual examination of internal surfaces, and laboratory analysis of plate samples and deposits. Results are evaluated against recognized references such as ASTM E797 and EN 14127 for ultrasonic thickness measurement, ASTM G46 for pitting evaluation, and construction and acceptance codes such as GB 50128 for vertical cylindrical steel welded tanks. The final deliverable quantifies remaining wall thickness, corrosion rates, and remaining life, and supports fitness-for-service conclusions.
Testing Samples
Bottom Plate Sections and Coupons
Plate sections removed during repair or floor replacement are examined in the laboratory to verify field thickness readings, measure weight-loss corrosion rates, and characterize pit morphology through cross-sectional metallography.
Corrosion Products, Scale, and Sludge
Deposits collected from the tank floor are analyzed by techniques such as X-ray diffraction (XRD) and scanning electron microscopy with energy-dispersive spectroscopy (SEM/EDS) to distinguish electrochemical corrosion from microbiologically influenced corrosion.
Water Bottoms and Residual Product Samples
Accumulated free water and residual liquid are tested for pH, chloride and sulfate content, dissolved oxygen, and sulfate-reducing bacteria (SRB) to characterize the corrosivity of the internal environment.
Soil and Groundwater Samples
For underside corrosion evaluation, soil beneath or adjacent to the tank is tested for resistivity, moisture content, pH, redox potential, and chloride or sulfate concentration to assess soil corrosivity and cathodic protection effectiveness.
In-Service Field Measurement Data
Ultrasonic thickness grid readings, magnetic flux leakage scan records, and historical inspection data are treated as assessment data sets used to calculate corrosion rates and project future wall loss.
Testing Items
- Remaining wall thickness and minimum thickness verification of bottom and annular plates
- General (uniform) corrosion wall loss expressed as a percentage of nominal thickness
- Pitting depth, pit density, and pitting factor evaluation per ASTM G46
- Short-term and long-term corrosion rate calculation
- Remaining useful life estimation based on corrosion allowance
- Differentiation of product-side corrosion from soil-side corrosion
- Microbiologically influenced corrosion (MIC) screening and confirmation
- Coating condition assessment, blistering, and rust grading per ISO 4628 and ASTM D610
- Bottom weld seam and lap-joint integrity, including leak-path detection
- Soil corrosivity parameters relevant to underside attack
- Fitness-for-service conclusions: continued operation, local repair, or plate replacement
Testing Methods
Visual and Close Internal Inspection
After cleaning and gas-freeing, inspectors examine the entire floor for rust staining, pitting, coating breakdown, and weld deterioration. Surface conditions are graded using recognized pictorial standards, providing a baseline for all subsequent instrument-based measurements.
Ultrasonic Thickness Measurement and Corrosion Mapping
Magnetic Flux Leakage (MFL) Floor Scanning
MFL saturates the ferromagnetic plate with magnetic flux and detects leakage fields caused by wall loss, allowing rapid detection of both top-side and underside thinning, including isolated pits. Scan coverage typically exceeds 80 to 90 percent of the floor area, with anomalous indications verified by spot ultrasonic measurements.
Advanced Electromagnetic Methods
Low-frequency eddy current (LFET) and pulsed eddy current (PEC) techniques extend screening capability through coatings and sediment, and can be deployed on robotic crawlers to assess floors without draining the tank.
Acoustic Emission Testing
Acoustic emission monitoring detects the activity of ongoing corrosion and possible through-wall leakage from the outside of an in-service tank, providing a cost-effective screening tool between internal inspections.
Vacuum Box Testing and Weld Surface NDT
Vacuum box testing locates through-wall leak paths in bottom weld seams, while magnetic particle or liquid penetrant testing confirms the integrity of suspect welded joints and repairs.
Laboratory Analysis
Laboratory programs follow ASTM G1 for cleaning and weight-loss evaluation of coupons, ASTM G46 for pitting characterization, and combine metallography, SEM/EDS, XRD, and microbiological assays to identify active corrosion mechanisms.
Corrosion Rate and Remaining Life Calculation
The corrosion rate is calculated as the difference between original and current thickness divided by the exposure interval. Remaining life equals the difference between current thickness and the minimum required thickness divided by the corrosion rate, and is benchmarked against design and construction requirements such as GB 50128 and the applicable tank integrity inspection standard.
Applications
- Crude oil, refined product, and chemical storage tanks at terminals, tank farms, and refinery sites
- Petrochemical and chemical plant storage vessels requiring turnaround integrity verification
- Firewater, process water, and wastewater tanks in industrial facilities
- Aviation fuel and low-point water drainage systems where internal corrosion concentrates
- Risk-based inspection programs and statutory or insurer-driven inspection cycles
- Planning of floor repairs, partial plate replacement, or full bottom replacement during outages
- Environmental compliance and secondary containment leak-prevention programs
- Due-diligence condition assessments for asset transactions and life-extension studies
FAQ
Why do tank bottom plates corrode faster than the shell?
The floor traps free water and sediment on the product side while remaining in permanent contact with moist soil on the underside. Both sides can suffer general thinning and deep localized pitting, including microbiologically influenced corrosion, long before any external symptom is visible.
How often should a bottom plate corrosion assessment be performed?
Internal inspections are commonly scheduled every five to ten years, with the exact interval driven by the measured corrosion rate, corrosion allowance, containment features, and service history. In-service techniques such as robotic floor scanning and acoustic emission testing can supplement or defer internal entry.
Can the bottom be assessed without taking the tank out of service?
Yes. Robotic magnetic flux leakage and ultrasonic crawlers can scan the floor through the stored liquid, and acoustic emission monitoring can screen for active corrosion and leakage from outside the tank. Manual ultrasonic measurement and visual inspection still require internal entry after cleaning.
What thickness triggers repair of a bottom plate?
Acceptance limits depend on the applicable design code and tank integrity standard. Many widely adopted standards require evaluation or repair when remaining floor thickness approaches approximately 2.5 mm, with stricter criteria applied to annular plates and more tolerant criteria applied to isolated pits verified by pit-depth measurement.
How is microbiologically influenced corrosion confirmed?
MIC is identified by combining pit morphology from metallography, chemical and mineralogical analysis of corrosion products using SEM/EDS and XRD, and microbiological testing of water bottoms and deposits for sulfate-reducing and acid-producing bacteria.
Does cathodic protection eliminate underside corrosion?
No. Cathodic protection significantly reduces soil-side attack, but shielding from sand bedding, scale, coatings, or a poorly conductive path to remote areas of large floors can leave local regions unprotected. Periodic bottom assessment remains necessary to verify protection effectiveness.
Why Choose ZKGX?
- State-of-the-art analytical equipment
- Highly qualified scientific team
- Fast turnaround time
- Competitive pricing