API 653 Corrosion & Damage Mechanisms 3 — Questions and Answers
Question 1: According to API 653, what is the significance of measuring both the minimum thickness and the average thickness in a corrosion rate calculation?
- Average thickness is used for structural calculations while minimum thickness determines remaining life (Correct answer)
- Both values are averaged together to get a single corrosion rate
- Only minimum thickness is relevant for API 653 calculations
- Average thickness determines the inspection interval only
Correct answer: Average thickness is used for structural calculations while minimum thickness determines remaining life
API 653 uses minimum measured thickness to calculate remaining corrosion allowance and remaining life, while average thickness may be used for certain structural integrity assessments.
Question 2: What is 'channeling corrosion' as it applies to tank floors?
- Corrosion that follows the direction of product flow
- Linear corrosion paths running parallel to weld seams caused by differential aeration (Correct answer)
- Corrosion in the drainage channels under the tank
- Corrosion caused by flow-induced turbulence at inlet nozzles
Correct answer: Linear corrosion paths running parallel to weld seams caused by differential aeration
Channeling corrosion on tank floors appears as linear corrosion paths, often running parallel to lap weld seams, caused by differential aeration cells created by the overlapping plate configuration.
Question 3: In the context of crude oil storage tanks, what damage mechanism is associated with naphthenic acids at elevated temperatures?
- Stress corrosion cracking
- Naphthenic acid corrosion (Correct answer)
- Hydrogen embrittlement
- Sulfide stress cracking
Correct answer: Naphthenic acid corrosion
Naphthenic acid corrosion is a high-temperature corrosion mechanism that attacks carbon steel and low-alloy steels when crude oils containing naphthenic acids are processed above approximately 220°C (428°F).
Question 4: What is the primary indicator that a tank shell has experienced settlement-induced stress damage?
- Uniform thinning of the shell plates
- Out-of-roundness or shell distortion visible during inspection (Correct answer)
- Increased corrosion rate on the tank bottom
- Discoloration of the external paint system
Correct answer: Out-of-roundness or shell distortion visible during inspection
Differential settlement causes shell distortion and out-of-roundness, which can be measured during inspection and may indicate overstressing of the shell plates and welds.
Question 5: Which of the following best describes 'underdeposit corrosion' in storage tanks?
- Corrosion occurring beneath the tank foundation
- Accelerated corrosion occurring under sediment, sludge, or scale deposits (Correct answer)
- Corrosion of the tank's underground piping
- Corrosion beneath cathodic protection anodes
Correct answer: Accelerated corrosion occurring under sediment, sludge, or scale deposits
Underdeposit corrosion occurs when sludge, sediment, or scale deposits on the tank floor create localized anaerobic conditions and concentration cells that dramatically accelerate local corrosion rates.
Question 6: What type of damage is indicated when weld heat-affected zones in a carbon steel tank show preferential corrosion compared to the base metal?
- Crevice corrosion
- Galvanic corrosion between weld and base metal
- Weld HAZ sensitization or residual stress-driven corrosion (Correct answer)
- Erosion corrosion from turbulent flow
Correct answer: Weld HAZ sensitization or residual stress-driven corrosion
Preferential corrosion in weld heat-affected zones results from microstructural changes, residual stresses, and potential sensitization introduced during welding, making the HAZ more susceptible to corrosive attack.
Question 7: During an API 653 inspection, corrosion product analysis reveals black iron sulfide scale on the tank floor. What damage mechanism does this most likely indicate?
- Oxygen concentration cell corrosion
- Microbiologically influenced corrosion from sulfate-reducing bacteria (Correct answer)
- Naphthenic acid corrosion
- Galvanic corrosion from a copper component
Correct answer: Microbiologically influenced corrosion from sulfate-reducing bacteria
Black iron sulfide (FeS) scale is a characteristic byproduct of microbiologically influenced corrosion (MIC) by sulfate-reducing bacteria, which convert sulfate to hydrogen sulfide that reacts with iron.
API 653 Corrosion & Damage Mechanisms 3
According to API 653, what is the significance of measuring both the minimum thickness and the average thickness in a corrosion rate calculation?