API 510 Corrosion Mechanisms & Damage Assessment 5 — Questions and Answers
Question 1: Which API 510 inspection finding would most directly indicate active under-deposit corrosion?
- Smooth metal loss uniformly distributed across the shell
- Localized thinning beneath fouling deposits when deposits are removed (Correct answer)
- External surface rust staining with no measurable wall loss
- Blistering on the interior surface near nozzle connections
Correct answer: Localized thinning beneath fouling deposits when deposits are removed
Under-deposit corrosion occurs beneath scale, fouling, or biological deposits where trapped moisture and aggressive species concentrate, causing localized thinning.
Question 2: Thermal fatigue damage in pressure vessels is primarily caused by:
- Sustained high-pressure operation above the MAWP
- Cyclic thermal stresses from repeated heating and cooling (Correct answer)
- Single-event overpressure transients
- Long-term creep under steady-state high temperature
Correct answer: Cyclic thermal stresses from repeated heating and cooling
Thermal fatigue results from cyclic thermal stresses that occur when temperature fluctuations cause differential expansion and contraction, eventually initiating cracks.
Question 3: During API 510 fitness-for-service assessment of a corroded vessel, a 'remaining life' calculation requires which two key inputs?
- Design pressure and original wall thickness
- Remaining corrosion allowance and the applicable corrosion rate (Correct answer)
- Operating temperature and fluid composition
- Weld joint efficiency and mill undertolerance
Correct answer: Remaining corrosion allowance and the applicable corrosion rate
Remaining life is calculated as remaining corrosion allowance (actual thickness − t_minimum) divided by the applicable corrosion rate.
Question 4: What is 'amine stress corrosion cracking' and which vessel service is most at risk?
- Cracking from caustic solutions in steam boilers
- Cracking in carbon steel vessels handling amine solutions used in gas treating (Correct answer)
- Cracking from hydrofluoric acid in alkylation units
- Cracking from polythionic acid in refinery shutdown conditions
Correct answer: Cracking in carbon steel vessels handling amine solutions used in gas treating
Amine stress corrosion cracking occurs in carbon steel exposed to amine solutions (MEA, DEA, MDEA) used in gas sweetening, particularly in weld heat-affected zones under residual stress.
Question 5: Polythionic acid stress corrosion cracking (PTASCC) is a special concern during:
- Normal operation of vessels in high-temperature H₂S service
- Shutdown, maintenance, and startup when sensitized austenitic stainless steel contacts moisture and air (Correct answer)
- Hydrotest operations using chlorinated water
- External insulation removal on carbon steel vessels
Correct answer: Shutdown, maintenance, and startup when sensitized austenitic stainless steel contacts moisture and air
PTASCC forms when iron sulfide scale on sensitized austenitic stainless steel reacts with oxygen and moisture during shutdown, creating polythionic acids that attack grain boundaries.
Question 6: An API 510 inspector notices that wall thinning is concentrated in the bottom third of a horizontal vessel in aqueous service. What is the most likely explanation?
- External corrosion from insulation under insulation (CUI) at the bottom
- Water dropout and settling causing accelerated corrosion at the liquid-vapor interface and bottom (Correct answer)
- High-temperature oxidation concentrated at the lowest point
- Galvanic attack from the vessel support saddles
Correct answer: Water dropout and settling causing accelerated corrosion at the liquid-vapor interface and bottom
In horizontal vessels handling wet streams, water settles to the bottom, creating concentrated corrosive conditions and accelerated wall loss in that zone.
Question 7: Which condition is required for 'microbiologically influenced corrosion' (MIC) to occur in a pressure vessel?
- High-temperature operation above 250°F
- Presence of microorganisms, typically in stagnant or low-flow aqueous conditions (Correct answer)
- Exposure to strong oxidizing acids
- Direct contact between dissimilar metals in an electrolyte
Correct answer: Presence of microorganisms, typically in stagnant or low-flow aqueous conditions
MIC requires living microorganisms (bacteria, fungi) in an aqueous environment, typically in stagnant or low-flow conditions where colonies can establish and create aggressive local chemistry.
API 510 Corrosion Mechanisms & Damage Assessment 5
Which API 510 inspection finding would most directly indicate active under-deposit corrosion?