API 510 Corrosion Assessment 3 — Questions and Answers
Question 1: What is the primary driving factor for stress corrosion cracking (SCC) to occur?
- High flow velocity only
- Tensile stress, susceptible material, and corrosive environment simultaneously (Correct answer)
- Compressive stress and neutral pH
- Elevated temperature alone
Correct answer: Tensile stress, susceptible material, and corrosive environment simultaneously
SCC requires all three conditions to be present: a susceptible material, tensile stress, and a specific corrosive environment.
Question 2: A vessel shell has a measured thickness of 0.320 inches and a required minimum thickness of 0.285 inches. What is the corrosion allowance remaining?
- 0.285 inches
- 0.605 inches
- 0.035 inches (Correct answer)
- 0.320 inches
Correct answer: 0.035 inches
Remaining corrosion allowance = measured thickness minus required minimum = 0.320 minus 0.285 = 0.035 inches.
Question 3: Hydrogen-induced cracking (HIC) in pressure vessels is most commonly associated with which process environment?
- High-pressure steam service
- Wet hydrogen sulfide (H2S) service (Correct answer)
- Caustic soda at low temperature
- High-velocity liquid water service
Correct answer: Wet hydrogen sulfide (H2S) service
HIC is strongly associated with wet H2S environments where atomic hydrogen is generated and diffuses into the steel.
Question 4: When calculating remaining life using API 510, which formula is correct?
- RL = (t_actual minus t_min) times corrosion rate
- RL = (t_actual minus t_min) divided by corrosion rate (Correct answer)
- RL = corrosion rate divided by (t_actual minus t_min)
- RL = t_actual divided by corrosion rate
Correct answer: RL = (t_actual minus t_min) divided by corrosion rate
Remaining life = (actual thickness minus minimum required thickness) divided by the corrosion rate in thickness per year.
Question 5: Crevice corrosion is most likely to initiate in which location on a pressure vessel?
- Open flat surfaces with high flow
- Tight gaps under gaskets or lap joints (Correct answer)
- Areas of high turbulence
- Elevated temperature zones above 600 degrees F
Correct answer: Tight gaps under gaskets or lap joints
Crevice corrosion initiates in occluded, low-oxygen areas such as under gaskets, lap joints, or threaded connections where stagnant electrolyte concentrates.
Question 6: Which inspection technique is MOST effective for detecting and sizing pitting corrosion on a vessel shell?
- Visual inspection from the exterior
- Ultrasonic thickness scanning (C-scan) (Correct answer)
- Magnetic particle testing
- Pressure testing
Correct answer: Ultrasonic thickness scanning (C-scan)
Ultrasonic C-scan mapping provides area coverage to detect, size, and map pitting patterns on vessel shells.
Question 7: In API 510, what is the typical next inspection interval limit for a pressure vessel in normal service based on remaining life?
- No more than one-half of the remaining life, not to exceed 10 years (Correct answer)
- No more than the full remaining life, not to exceed 5 years
- No more than one-quarter of the remaining life, not to exceed 15 years
- No more than one-half of the remaining life, not to exceed 5 years
Correct answer: No more than one-half of the remaining life, not to exceed 10 years
API 510 limits the next inspection interval to no more than half the calculated remaining life and not to exceed 10 years for internal inspections.
API 510 Corrosion Assessment 3
What is the primary driving factor for stress corrosion cracking (SCC) to occur?