API 510 Corrosion Assessment 5 — Questions and Answers
Question 1: Which type of corrosion produces a series of small, deep cavities penetrating into the metal while leaving surrounding areas relatively unaffected?
- Uniform corrosion
- Galvanic corrosion
- Pitting corrosion (Correct answer)
- Erosion corrosion
Correct answer: Pitting corrosion
Pitting corrosion is a localized form that creates deep, narrow cavities while surrounding metal remains largely intact, making it particularly dangerous and hard to detect.
Question 2: High-temperature hydrogen attack (HTHA) in pressure vessels primarily causes which type of damage?
- Surface pitting
- Decarburization and fissuring of the steel (Correct answer)
- Uniform metal thinning
- Chloride cracking
Correct answer: Decarburization and fissuring of the steel
HTHA causes decarburization as hydrogen reacts with carbides in the steel, forming methane gas that creates fissures and significantly degrades mechanical properties.
Question 3: What is the most reliable method for an API 510 inspector to distinguish between active pitting corrosion and old, inactive pits?
- Visual observation of pit color
- Comparison of pit depth measurements between inspection intervals (Correct answer)
- Magnetic particle testing of pit surfaces
- Radiographic testing of the shell
Correct answer: Comparison of pit depth measurements between inspection intervals
Comparing pit depth measurements from successive inspections reveals whether pits are growing (active) or stable (inactive), providing direct evidence of activity.
Question 4: Under API 510, which assessment method may be used to evaluate pressure vessels with localized metal loss that does not meet minimum thickness requirements?
- API 579 Fitness-for-Service (FFS) assessment (Correct answer)
- Immediate vessel replacement
- Re-rating to a lower MAWP without analysis
- Increasing inspection frequency only
Correct answer: API 579 Fitness-for-Service (FFS) assessment
API 579 FFS assessment provides analytical methods to evaluate whether vessels with local metal loss can continue safe operation despite falling below nominal design thickness.
Question 5: A carbon steel vessel operating in a caustic (NaOH) service exhibits cracking along welds. Which corrosion mechanism is MOST likely responsible?
- Hydrogen blistering
- Caustic stress corrosion cracking (caustic embrittlement) (Correct answer)
- Chloride SCC
- Intergranular corrosion
Correct answer: Caustic stress corrosion cracking (caustic embrittlement)
Caustic embrittlement is the preferential cracking of carbon steel in NaOH environments, particularly at welds where residual tensile stresses are high.
Question 6: When applying the ASME Section VIII Division 1 formula for minimum required shell thickness under internal pressure, which variable directly represents the corrosion allowance?
- S (allowable stress)
- R (inside radius)
- C (corrosion allowance added to t_min) (Correct answer)
- P (design pressure)
Correct answer: C (corrosion allowance added to t_min)
The corrosion allowance C is added to the calculated minimum thickness t_min to establish the nominal design thickness, accounting for expected metal loss over the vessel life.
Question 7: Which non-destructive examination (NDE) method is BEST suited for detecting hydrogen-induced cracking (HIC) damage in a vessel shell plate?
- Dye penetrant testing (PT)
- Wet fluorescent magnetic particle testing (WFMT)
- Ultrasonic testing (UT) with straight beam or phased array (Correct answer)
- Visual testing (VT) from the exterior
Correct answer: Ultrasonic testing (UT) with straight beam or phased array
UT with straight beam or phased array can detect the internal, subsurface planar flaws characteristic of HIC without requiring surface access to the damage plane.
API 510 Corrosion Assessment 5
Which type of corrosion produces a series of small, deep cavities penetrating into the metal while leaving surrounding areas relatively unaffected?