Envision Sustainability Professional (ENV SP)

Correct: API 653 emphasizes that external inspections must focus on areas where moisture can be trapped. In insulated tanks, Corrosion Under Insulation (CUI) is a significant risk. The inspector must prioritize checking the weatherproofing, caulking, and areas like support rings or nozzles where the insulation envelope is breached, as these are the primary locations where water ingress leads to localized external corrosion.

Incorrect: Taking measurements at the geometric center of plates is less effective for external corrosion, which is typically localized at moisture traps or coating failures. Internal floating roof seals are related to product integrity and emissions rather than external shell corrosion. Vacuum box testing is a leak detection method for welds, typically used for the tank bottom or during construction, and is not the primary tool for assessing general external atmospheric corrosion or CUI.

Takeaway: Effective external inspection for corrosion requires targeted visual assessment of moisture-trapping components and insulation interfaces rather than randomized thickness sampling.

Correct: According to API 653, internal inspections must be thorough enough to ensure the continued integrity of the tank bottom. Crevice corrosion is a localized mechanism that often occurs under deposits or at lap joints. MFL (Magnetic Flux Leakage) is the industry-standard screening tool for detecting such localized thinning across large areas of the tank floor, but it requires a clean surface to be effective. Targeted UT (Ultrasonic Testing) is then used to quantify the specific metal loss identified by the MFL scan.

Incorrect: Extrapolating from previous data is insufficient because crevice corrosion is often non-linear and localized, making historical averages unreliable. Vacuum box testing only identifies existing leaks and does not assess the remaining thickness or the risk of future failure due to thinning. Cathodic protection is primarily effective for the soil-side (underside) of the tank bottom and does not mitigate internal crevice corrosion caused by product-side deposits or moisture trapped in lap joints.

Takeaway: Effective inspection of crevice corrosion requires thorough surface cleaning and a combination of area-wide screening (MFL) and localized quantification (UT) to ensure structural integrity.

Correct: According to API 653, specifically Section 4.4 regarding tank bottom evaluation, the primary concern with pitting is the potential for a leak to occur before the next inspection. The inspector must calculate the minimum remaining thickness at the end of the next service interval (MRT) based on the current measured thickness and the anticipated corrosion rate. If the MRT is less than the minimum required thickness specified by the standard (typically 0.1 inches for bottoms with leak detection or 0.05 inches with a reinforced liner), repairs or a shortened inspection interval are required.

Incorrect: Immediate replacement is not mandatory simply because pitting exceeds 50% of nominal thickness; API 653 allows for engineering evaluations and remaining life calculations. Applying a coating does not bypass the requirement to evaluate the structural integrity of the remaining metal or perform the necessary thickness calculations. Averaging thickness over a 12-inch area is a technique used for general corrosion, but for pitting, the deepest point of the pit is the critical measurement to prevent perforation and leakage; furthermore, shell plate criteria are not applicable to bottom plate assessments.

Takeaway: API 653 requires that tank bottom pitting be evaluated based on the remaining thickness necessary to prevent leakage until the next scheduled internal inspection.

Correct: Vibration-induced fatigue is a localized degradation mechanism that typically manifests as surface-breaking cracks at stress concentrators. According to API 653 principles, when cyclic loading is a concern, visual inspection is often insufficient. Magnetic Particle (MT) or Liquid Penetrant (PT) testing are the preferred non-destructive examination (NDE) methods for identifying these fatigue cracks at critical junctions like nozzles and structural attachments before they lead to catastrophic failure.

Incorrect: Ultrasonic thickness measurements are designed to monitor for general or localized corrosion (thinning), not for detecting fine fatigue cracks. Vacuum box testing is a leak-detection method for floor plates and is not an effective way to assess shell fatigue. Measuring verticality and roundness is used to assess structural stability and settlement issues, which are distinct from the microscopic crack initiation and propagation associated with high-frequency vibration.

Takeaway: In high-vibration environments, API 653 integrity management requires surface-specific NDE (MT or PT) at high-stress weld points to detect fatigue cracking that thickness monitoring would miss.

Correct: According to API 653 and API 651, soil-side corrosion is primarily an electrochemical process influenced by the environment beneath the tank bottom. The effectiveness and historical continuity of a cathodic protection (CP) system are the most critical factors in preventing this. If the CP system has been intermittent and the foundation cushion contains moisture or corrosive contaminants (like chlorides), the probability of significant underside pitting increases substantially, making these the most reliable risk indicators.

Incorrect: Internal product temperature and filling cycles relate more to internal corrosion mechanisms or fatigue rather than soil-side corrosion. The shell-to-bottom weld and internal linings protect the tank from internal product-side corrosion but do not mitigate or indicate the status of corrosion occurring on the soil-side of the bottom plates. Structural stability against wind and secondary containment liners are safety and environmental features that do not directly indicate the electrochemical corrosion rate of the bottom plates.

Takeaway: The integrity of the cathodic protection system and the condition of the foundation cushion are the primary determinants of soil-side corrosion risk for storage tank bottoms.

Correct: According to API 653, the inspection of tank bottoms must provide sufficient data to determine the minimum remaining thickness. Magnetic Flux Leakage (MFL) is the industry standard for scanning large areas of the tank bottom for soil-side or internal corrosion, while Ultrasonic Testing (UT) is used to verify and quantify the specific depth of the indications found. This combination is essential for evaluating localized crevice corrosion in lap-welded joints where visual inspection cannot reach.

Incorrect: Visual examination and hammer testing are subjective and cannot accurately quantify the remaining thickness of the plates within a lap joint. Applying a coating without first measuring the remaining thickness violates API 653’s requirement to ensure the bottom meets minimum thickness standards (typically 0.1 inches) before returning to service. Hydrostatic testing is a proof test for structural integrity and leak-tightness but does not provide the quantitative thickness data required to calculate the remaining life or the next inspection interval.

Takeaway: API 653 requires quantitative data from NDE methods like MFL and UT to assess localized corrosion and ensure the tank bottom meets minimum thickness requirements for continued service.

Correct: Galvanic corrosion occurs when two dissimilar metals, such as carbon steel and stainless steel, are in electrical contact within an electrolyte. According to API 653, when a corrosion mechanism is identified, the inspector must use appropriate NDE (Non-Destructive Examination) methods to quantify the damage. Ultrasonic thickness (UT) measurements are the primary tool for determining the remaining thickness of the carbon steel (the anode in this pair), while verifying electrical isolation addresses the root cause of the galvanic cell.

Incorrect: Increasing the frequency of external visual inspections is ineffective because galvanic corrosion between internal piping and the shell occurs on the internal surface and may not manifest externally until a through-wall failure occurs. Cathodic protection systems are typically designed for soil-side corrosion or general internal protection and may not be sufficient to overcome the localized high-current density of a direct galvanic couple. Vacuum box testing is a leak-detection method for welds and does not provide data on the thinning or metal loss associated with galvanic corrosion at piping supports.

Takeaway: To assess galvanic corrosion, inspectors must use quantitative methods like ultrasonic testing at the contact points of dissimilar metals and verify the effectiveness of electrical isolation.

Correct: According to API 653, when assessing shell integrity for uniform corrosion, the inspector must determine the actual minimum thickness of each shell course. This is typically achieved through ultrasonic thickness (UT) measurements at enough locations to accurately represent the condition of the plate. This measured value is then compared against the calculated minimum thickness (t-min) required for the specific height and product gravity to ensure the tank is safe for continued operation.

Incorrect: Relying on theoretical corrosion rates or original nominal thickness is insufficient because it does not account for actual environmental or operational changes that may have accelerated metal loss. Visual inspection alone cannot quantify uniform thinning, which may not be apparent without precision measurement tools. Averaging thickness across the entire shell is prohibited because API 653 requires each individual shell course to meet its own specific minimum thickness requirements based on the hydrostatic pressure at that level.

Takeaway: API 653 requires the quantification of actual metal loss through physical measurement of each shell course to ensure the remaining thickness meets or exceeds the calculated minimum requirements for safe operation.

Correct: API 653 requires that internal inspections specifically address localized corrosion mechanisms, such as pitting or microbial induced corrosion, which are often more aggressive than general atmospheric thinning. The inspector must consider the effectiveness of internal linings (per API 652) and the actual measured metal loss on the bottom plates, as these areas are not accessible during external inspections and do not follow the same degradation patterns as the external shell.

Incorrect: Extrapolating internal conditions from external shell UT is insufficient because bottom plate corrosion is often soil-side or product-side and independent of shell condition. Assuming uniform corrosion is a common error; internal corrosion is frequently localized, requiring a more detailed statistical or worst-case analysis. Prioritizing atmospheric corrosion for internal components is incorrect because the product chemistry and vapor space moisture are the primary drivers of internal degradation.

Takeaway: Internal integrity assessments must focus on localized corrosion and the specific protective measures applied to the tank bottom and vapor space, as these differ significantly from external shell corrosion patterns.

Correct: Hydrogen embrittlement and hydrogen-induced cracking (HIC) typically manifest as tight, surface-breaking or near-surface cracks, especially in high-strength steels and weld heat-affected zones (HAZ). API 653 and industry best practices for damage mechanisms (like API RP 571) indicate that standard UT thickness gauging is ineffective for detecting or monitoring cracks. Wet Fluorescent Magnetic Particle Testing (WFMT) is the most sensitive method for detecting these cracks. Once identified, a fitness-for-service (FFS) assessment per API 579-1/ASME FFS-1 is required to determine the structural integrity and remaining life of the asset.

Incorrect: Ultrasonic thickness measurements are intended to monitor metal loss due to corrosion or erosion and cannot reliably detect or characterize tight environmental cracks. While internal linings can prevent future hydrogen exposure, they do not address the structural risk of existing cracks already present in the steel. Radiographic testing (RT) is a volumetric method that is often less sensitive to tight, surface-breaking cracks than surface NDE methods like WFMT or ACFM, and it is not the preferred method for monitoring environmental cracking in service.

Takeaway: Managing hydrogen-related cracking requires specialized surface-sensitive NDE methods and fitness-for-service evaluations rather than standard volumetric or thickness-based inspection techniques.