Aboveground Storage Tank Shell Corrosion - Assessing the Controlling Thickness at Corroded Areas

Introduction

Corrosion in carbon-steel tank shells can develop in multiple forms and with varying severity. Material loss may occur uniformly across broad areas of the shell or may be concentrated in more localized zones. Localized attack, including pitting, is also common. Because each corrosion pattern behaves differently, every affected area must be evaluated individually, and a detailed inspection is required to characterize the corrosion mechanism and its extent before any repair strategy is developed. In most situations, isolated pitting does not significantly compromise the structural performance of the shell unless the pits are deep, clustered, or closely spaced. The following sections outline the acceptance criteria for assessing both general metal loss and pitting in carbon-steel tank shells and provides example automated ultrasonic testing C-scan, phased array ultrasonic testing (PAUT) semi-encoded scans, and manual ultrasonic thickness testing results.

Figure 1:  Aboveground storage tank shell corrosion showing different degrees of localized corrosion.

Steel Aboveground Storage Tank External Corrosion

External corrosion on the outside diameter of insulated carbon-steel tank shells is primarily driven by Corrosion Under Insulation (CUI), a mechanism that develops when moisture becomes trapped beneath the insulation system and remains in prolonged contact with the steel surface. Water intrusion through damaged jacketing, failed seals, or weather-related degradation creates a localized electrolyte environment that promotes both general atmospheric corrosion and accelerated pitting, especially within the temperature range of approximately 25°C to 120°C, where wet–dry cycling is most aggressive. Because insulation retains moisture and prevents evaporation, corrosion progresses largely unseen and often develops into irregular thinning patterns, deep localized pits, or widespread metal loss before detection.

High-risk zones include shell-to-bottom junctions, penetrations, attachment points, and areas where insulation is compressed or mechanically damaged. As documented in API 571: Damage Mechanisms Affecting Fixed Equipment in the Refining Industry and NACE SP0198: Control of Corrosion Under Thermal Insulation and Fireproofing Materials, each occurrence of CUI must be evaluated as a unique case through targeted insulation removal, surface inspection, and nondestructive testing to accurately characterize damage, determine remaining wall thickness, and establish appropriate repair or mitigation strategies.

Figure 2: API-653 local area corrosion controlling thickness ultrasonic thickness measurements.

In API 653, the controlling thickness for a locally corroded area of a tank shell is determined by combining the minimum measured point thickness with an area-averaged thickness evaluation to ensure the remaining shell section can safely resist design loads. First, the inspector identifies the corroded region and determines the least remaining thickness, t2 , in that area, excluding isolated pits that meet the separate pitting criteria. Using this t2 value and the nominal tank diameter (in feet), the required evaluation length (in inches) along the shell is calculated from L=3.7√(Dt2) , limited to a maximum of 40 in. Along several inspection planes (vertical lines) through the corroded region, thickness readings are taken over this length L; for each plane, an average thickness tavg is computed, and the lowest of these averages is defined as t1 .

The controlling thickness for the corroded area is effectively this worst-case averaged value t1, provided it also satisfies the minimum allowable thickness calculated from API 653’s pressure-and-wind/seismic formulas (which account for product height, specific gravity, joint efficiency, and material properties). If t1 is greater than or equal to the required minimum shell thickness for all applicable load cases, and the local minimum t2 also meets the code’s pit/local metal-loss limits, the corroded area is acceptable; otherwise, repair or replacement is required. This approach, described in API 653 Section 4.3 and associated figures/tables, ensures that the evaluation reflects both localized thinning and the structural behavior of the shell as a plate element rather than relying on a single spot reading.

Case Study #1 Aboveground Storage Tank Controlling Thickness – Pass

In the passing case, the tank shell has a diameter of 20 ft and a nominal thickness of 0.375 in. Thickness measurements taken within the corroded region show the least remaining thickness, t2, to be 0.22 in. Per API 653, the evaluation length is calculated using L=3.7√(Dt2) , which yields L=7.8i in when using a 20-ft diameter. Ultrasonic thickness readings were taken along five inspection planes (a through e) over this length L. Across these planes, the corrosion profile included both gradual thinning and a localized low point at 0.22 in. The plane with the lowest area-average thickness was Plane c, where the averaged value, t1, was 0.252 in. When compared to the required minimum shell thickness of 0.25 in for this example, the controlling average thickness remained slightly above the acceptance limit. Since both the area-average thickness t1 and the minimum point thickness t2 satisfy API 653 criteria, this corroded region is considered acceptable without repair.

Figure 3:  Establishing the measurement area for API-653 controlling thickness for given tank diameter and minimum thickness inside corrosion area – Pass Example.

Figure 4:  API-653 inspection tank shell corrosion measurement showing grid measurements, control plan average thickness measurements, minimum thickness and PASS result.

Case Study #2 Aboveground Storage Tank Controlling Thickness – Fail

In the failing example, the same 20-ft diameter tank shell exhibits more severe corrosion. The minimum thickness measured within the affected area is t2=0.18in, producing an evaluation length of L=7.0in using the API 653 formula. Ultrasonic measurements collected along the five inspection planes again showed localized thinning, but with more pronounced metal loss in the central portion of the corrosion patch.

In this case, Plane c contained the deepest metal loss, including a minimum reading of 0.18 in, and its average thickness over the evaluation length was only 0.210 in. Because this controlling average thickness t1=0.210in falls below the required minimum allowable thickness of 0.25 in, the shell section does not meet API 653 acceptance criteria. Even though the other inspection planes exhibited higher average values, API 653 requires the lowest area-average thickness to govern the evaluation; therefore, this corroded area must be repaired or replaced before the tank can be returned to service.

Figure 5:  Establishing the measurement area for API-653 controlling thickness for given tank diameter and minimum thickness inside corrosion area – Pass Example.

Figure 6:  API-653 inspection tank shell corrosion measurement showing grid measurements, control plan average thickness measurements, minimum thickness and FAIL result.

Non-destructive Testing Methods for Tank Shell Controlling Thickness

Automated UT C-scan corrosion mapping is a powerful standalone method for evaluating large corroded regions on steel tank shells, particularly where a comprehensive visual of wall-loss distribution is needed. In this technique, an encoded scanner moves a conventional or dual-element probe across the surface to generate a true plan-view, pixel-based thickness map. Because the coverage is continuous and automated, C-scan inspection is highly effective for detecting subtle patterns of thinning, broad general corrosion, and irregular wastage that may not be visible from isolated spot readings. Its greatest advantages lie in its speed, data density, and the ability to produce digital corrosion maps suitable for trending over multiple inspection cycles.

Figure 7: Tank shell automated ultrasonic C-scan for corrosion mapping.   Automated ultrasonic C-scans are superimposed onto 1^st shell course at high resolution.

Phased-array ultrasonic testing using a dual-linear-array (PAUT-DLA) probe represents a different standalone technology designed for applications where corrosion morphology must be characterized with higher fidelity. Unlike C-scan, which is fundamentally a mapping technique, PAUT-DLA provides advanced imaging of the sound beam’s interaction with the material, offering superior sensitivity to near-surface pitting, undercutting, and complex geometries. The dual-array configuration improves focusing and resolution, enabling inspectors to visualize corrosion shapes, detect small deep pits, and understand the depth profile of metal loss with greater precision. PAUT-DLA is particularly valuable when evaluating areas where material behavior is critical—such as nozzles, weld-to-shell junctions, and zones prone to stress concentration—because it provides diagnostic insight rather than purely thickness mapping.

Figure 8:  Phased array ultrasonic testing (PAUT) dual element probe semi-automated encoded scan of tank shell corrosion.

Manual ultrasonic thickness testing, performed with instruments such as the Evident DL38, serves as an independent and widely recognized technique for routine thickness measurement and corrosion assessment. Manual UT is valued for its simplicity, portability, and ability to obtain reliable thickness readings in locations where automated scanners or phased-array probes cannot be easily deployed, such as tight clearances, irregular surfaces, or areas with coating disruptions. The DL38 enhances manual UT by offering integrated A-scan storage, waveform capture, and onboard data logging, allowing each reading to be documented with verifiable signal data, identifiers, and operator notes. This transforms manual UT from a simple spot-measurement tool into a method capable of producing traceable, audit-ready records suitable for regulatory compliance or long-term inspection history.

Figure 9: Example manual ultrasonic thickness measurement using a standard thickness gage with B-scan capabilities.

Conclusion

Corrosion assessment on steel tank shells can be approached effectively through several distinct ultrasonic inspection technologies, each offering unique strengths depending on the inspection objective and site conditions. Automated UT C-scan mapping excels when broad-area coverage and high-resolution corrosion imaging are required, providing comprehensive thickness maps that reveal the overall distribution and severity of metal loss. Phased-array dual-linear-array (PAUT-DLA) inspection, by contrast, is best applied when detailed characterization of corrosion morphology is needed, offering superior near-surface resolution and diagnostic insight into complex thinning profiles, localized pitting, and geometrically sensitive regions. Meanwhile, manual ultrasonic thickness testing, especially when performed with modern instruments such as the Evident DL38, remains a practical and reliable method for routine measurement and verification, with digital waveform capture and onboard data storage enabling traceable and auditable records. Each technique stands on its own as a valid and capable tool within an API 653 corrosion assessment program, and selecting the most appropriate method depends on the required level of detail, inspection access, and the criticality of the area being evaluated.

API & Industry Standards

1. API 653 – Tank Inspection, Repair, Alteration, and Reconstruction.

   
   

   American Petroleum Institute. (Latest Edition).

   
   

   Provides the governing methodology for calculating required shell thickness, determining controlling thickness in corroded areas, and evaluating localized and general metal loss.

2. API 571 – Damage Mechanisms Affecting Fixed Equipment in the Refining Industry.

   
   

   American Petroleum Institute.

   
   

   Defines corrosion mechanisms relevant to carbon-steel tank shells, including atmospheric corrosion, CUI, and uniform thinning.

3. NACE SP0198 – Control of Corrosion Under Thermal Insulation and Fireproofing Materials.

   
   

   NACE International.

   
   

   Covers corrosion under insulation (CUI) mechanisms, risk factors, and inspection considerations for carbon-steel tanks.

Ultrasonic Testing & Corrosion Mapping Technologies

4. TechKnowServ Corporation.

   
   

   “Automated Ultrasonic Testing (AUT) C-Scan – Ship Hull Survey.”

   
   

   https://www.techknowserv.com/post/automated-ultrasonic-testing-aut-c-scan-ship-hull-survey

   
   

   Discusses high-resolution automated UT corrosion mapping and encoded scanning methods applicable to large steel structures.

5. TechKnowServ Corporation.

   
   

   “A Review of Non-Destructive Testing Methods for Aboveground Storage Tank Floor Inspection.”

   
   

   https://www.techknowserv.com/post/a-review-of-non-destructive-testing-methods-for-aboveground-storage-tank-floor-inspection

   
   

   Summarizes AUT C-scan, phased-array techniques, dual-element scanning, and UT applications for tank floors and shells.

6. Evident Scientific (formerly Olympus).

   
   

   DL38 Ultrasonic Thickness Gauge – Product Specifications and Application Notes.

   
   

   Evident Scientific.

   
   

   Details capabilities of DL38, including dual-element probes, A-scan capture, waveform storage, and digital data logging.

7. Evident Scientific / Olympus NDT.

   
   

   “Advantages of Dual Linear Array (DLA) Probes for Corrosion Inspection.”

   
   

   Describes PAUT-DLA probe behavior, focusing, near-surface resolution, and corrosion imaging benefits.