API 510 Corrosion Mechanisms & Damage Assessment 3 — Questions and Answers
Question 1: What is the primary concern with high-temperature hydrogen attack (HTHA) in pressure vessels?
- Surface scaling that reduces thermal efficiency
- Decarburization and fissuring that weaken the steel permanently (Correct answer)
- Hydrogen embrittlement causing brittle fracture at low temperatures
- Formation of hydrogen blisters on the outer surface
Correct answer: Decarburization and fissuring that weaken the steel permanently
HTHA causes decarburization and internal fissuring as atomic hydrogen reacts with carbides in the steel, permanently reducing strength and ductility.
Question 2: An inspector observes a vessel operating in wet H₂S service. Which damage mechanism should be the primary concern?
- High-temperature oxidation
- Sulfide stress cracking (SSC) (Correct answer)
- Crevice corrosion
- Intergranular corrosion
Correct answer: Sulfide stress cracking (SSC)
Wet H₂S service promotes sulfide stress cracking, especially in high-strength or hardened steels, and is a critical concern per API 510 and NACE standards.
Question 3: What is the significance of the 'corrosion allowance' in pressure vessel design and inspection?
- It is the minimum wall thickness required for structural support only
- It is extra metal thickness added to account for expected corrosion over the vessel's service life (Correct answer)
- It defines the maximum operating pressure the vessel can sustain
- It is the tolerance allowed in the original manufacturing process
Correct answer: It is extra metal thickness added to account for expected corrosion over the vessel's service life
Corrosion allowance is the additional thickness beyond the calculated minimum to accommodate anticipated metal loss during the design service life.
Question 4: In API 510, what calculation determines the remaining corrosion allowance for an in-service vessel?
- Nominal thickness minus actual measured thickness
- Actual measured thickness minus required minimum thickness (Correct answer)
- Design thickness minus corrosion allowance
- Measured thickness divided by corrosion rate
Correct answer: Actual measured thickness minus required minimum thickness
Remaining corrosion allowance equals the actual measured thickness minus the required minimum thickness (t_minimum), indicating how much additional metal loss is acceptable.
Question 5: Galvanic corrosion between two dissimilar metals is most likely to occur when:
- The metals are separated by an insulating gasket
- The metals are in electrical contact in the presence of an electrolyte (Correct answer)
- The metals are exposed to high-temperature dry gas
- Both metals are from the same alloy family
Correct answer: The metals are in electrical contact in the presence of an electrolyte
Galvanic corrosion requires electrical contact between dissimilar metals and an electrolyte (conductive liquid) to complete the electrochemical cell.
Question 6: Which material condition makes a vessel most susceptible to brittle fracture during startup in cold weather?
- High residual compressive stresses from heat treatment
- Low Charpy impact toughness at low operating temperatures (Correct answer)
- High carbon content with full annealing treatment
- Thick walls with fine grain structure
Correct answer: Low Charpy impact toughness at low operating temperatures
Low Charpy impact toughness at low temperatures means the steel cannot absorb sufficient energy to prevent crack propagation, making brittle fracture likely.
Question 7: In API 510, 'localized corrosion' detected during inspection is typically evaluated using:
- Average thickness readings across the entire shell course
- Point thickness measurements in the corroded area compared to t_minimum (Correct answer)
- Visual inspection rating charts only
- Hardness testing to determine remaining strength
Correct answer: Point thickness measurements in the corroded area compared to t_minimum
Localized corrosion is assessed by comparing the minimum thickness measured within the affected area to the calculated minimum required thickness (t_minimum).
API 510 Corrosion Mechanisms & Damage Assessment 3
What is the primary concern with high-temperature hydrogen attack (HTHA) in pressure vessels?