API 653 Corrosion & Damage Mechanisms 5 — Questions and Answers
Question 1: What is the primary cause of external corrosion under insulation (CUI) on insulated storage tanks?
- The insulation material reacts chemically with the tank shell steel
- Water penetrates damaged insulation and becomes trapped against the tank shell surface (Correct answer)
- Insulation prevents heat dissipation causing elevated corrosion temperatures
- Cathodic protection cannot reach insulated surfaces
Correct answer: Water penetrates damaged insulation and becomes trapped against the tank shell surface
CUI occurs when water infiltrates through damaged, improperly installed, or deteriorated insulation and becomes trapped against the steel surface, creating a persistently wet environment that drives corrosion.
Question 2: Which API 653 inspection finding would most urgently require immediate evaluation for continued safe operation?
- Minor external rust staining on the upper shell courses
- Bottom plate pitting with measured remaining thickness below the calculated minimum (Correct answer)
- Slight discoloration of the tank coating on the roof
- Minor surface oxidation on the vent pipes
Correct answer: Bottom plate pitting with measured remaining thickness below the calculated minimum
Bottom plate thickness below the calculated minimum per API 653 criteria indicates the tank floor has insufficient remaining thickness for safe operation and requires immediate repair or retirement evaluation.
Question 3: In API 653 inspections, what is 'selective seam corrosion' and why is it significant?
- Corrosion that occurs only at welded seams due to electrochemical differences between weld metal and base metal (Correct answer)
- Corrosion selected for sampling during inspection
- Corrosion along seams caused by product leakage
- A type of crevice corrosion at flanged connections
Correct answer: Corrosion that occurs only at welded seams due to electrochemical differences between weld metal and base metal
Selective seam corrosion occurs preferentially along weld seams due to microstructural differences, residual stresses, or flux inclusion that make weld areas anodic relative to the base metal.
Question 4: What does 'dealloying' (also called selective leaching) involve when it occurs in tank components?
- Removal of alloying elements from the steel surface, leaving a porous, weakened structure (Correct answer)
- Loss of the protective alloy coating from a clad tank component
- Selective removal of chromium from stainless steel welds
- Dissolution of the zinc coating from galvanized components
Correct answer: Removal of alloying elements from the steel surface, leaving a porous, weakened structure
Dealloying involves the selective removal of one element (e.g., zinc from brass, or graphite from gray cast iron) from an alloy, leaving a porous, mechanically weak structure that retains the original shape but lacks structural integrity.
Question 5: A tank inspector finds orange-brown deposits on the internal shell of an atmospheric storage tank containing crude oil. What initial corrosion assessment should be performed?
- Assume the deposits are cosmetic rust and document only
- Sample and analyze deposits to distinguish between iron oxide rust, iron sulfide (MIC indicator), and other corrosion products (Correct answer)
- Immediately shut down the tank for full repair
- Apply epoxy coating over the deposits to stop further corrosion
Correct answer: Sample and analyze deposits to distinguish between iron oxide rust, iron sulfide (MIC indicator), and other corrosion products
Deposit sampling and analysis is essential to differentiate between relatively benign iron oxide rust and more aggressive corrosion products like iron sulfides that indicate MIC or H2S-related attack requiring different remediation approaches.
Question 6: What is the relationship between soil resistivity and the external corrosion rate of an unprotected tank bottom?
- Higher soil resistivity always means higher corrosion rates
- Lower soil resistivity (more conductive soil) generally correlates with higher external corrosion rates (Correct answer)
- Soil resistivity has no relationship to corrosion rate
- Corrosion rates are highest in soils with resistivity above 10,000 ohm-cm
Correct answer: Lower soil resistivity (more conductive soil) generally correlates with higher external corrosion rates
Lower soil resistivity indicates a more electrically conductive soil that facilitates galvanic and stray current corrosion cells, generally resulting in higher external corrosion rates on unprotected tank bottoms.
Question 7: According to API 653 principles, how does elevated operating temperature affect the corrosion rate of tank materials in service?
- Higher temperatures always decrease corrosion rates by drying out the corrosive medium
- Higher temperatures generally increase corrosion rates by accelerating electrochemical reactions (Correct answer)
- Temperature has no significant effect on corrosion rates in atmospheric tanks
- Elevated temperatures only affect corrosion rates in tanks above 200°F
Correct answer: Higher temperatures generally increase corrosion rates by accelerating electrochemical reactions
Higher operating temperatures accelerate electrochemical corrosion reactions and may activate additional damage mechanisms (e.g., naphthenic acid corrosion), so temperature is a critical factor in corrosion rate assessment and remaining life calculations.
API 653 Corrosion & Damage Mechanisms 5
What is the primary cause of external corrosion under insulation (CUI) on insulated storage tanks?