API 510 Corrosion Assessment 2 — Questions and Answers
Question 1: When evaluating corrosion under insulation (CUI), which temperature range poses the HIGHEST risk for carbon steel?
- -20°F to 32°F (-29°C to 0°C)
- 25°F to 250°F (-4°C to 121°C) (Correct answer)
- 300°F to 450°F (149°C to 232°C)
- Above 500°F (260°C)
Correct answer: 25°F to 250°F (-4°C to 121°C)
CUI on carbon steel is most active in the 25°F to 250°F range where moisture can accumulate and not fully evaporate.
Question 2: A vessel exhibits uniform metal loss of 0.012 inches over a 2-year inspection interval. What is the calculated corrosion rate?
- 0.006 in/yr (Correct answer)
- 0.012 in/yr
- 0.024 in/yr
- 0.003 in/yr
Correct answer: 0.006 in/yr
Corrosion rate = total loss divided by years = 0.012 divided by 2 = 0.006 in/yr.
Question 3: Which type of corrosion is characterized by selective attack along grain boundaries of a metal?
- Pitting corrosion
- Galvanic corrosion
- Intergranular corrosion (Correct answer)
- Erosion corrosion
Correct answer: Intergranular corrosion
Intergranular corrosion attacks the grain boundaries preferentially, often due to sensitization in stainless steels.
Question 4: API 510 requires that remaining life calculations use which corrosion rate when both short-term and long-term rates are available?
- The long-term rate always
- The short-term rate always
- The more conservative (higher) rate (Correct answer)
- The average of both rates
Correct answer: The more conservative (higher) rate
API 510 specifies using the more conservative (higher) corrosion rate to ensure safe remaining life estimates.
Question 5: What phenomenon occurs when two dissimilar metals are in electrical contact in a corrosive electrolyte?
- Stress corrosion cracking
- Galvanic corrosion (Correct answer)
- Crevice corrosion
- Erosion corrosion
Correct answer: Galvanic corrosion
Galvanic corrosion occurs when dissimilar metals are in electrical contact and immersed in an electrolyte, causing the more active metal to corrode preferentially.
Question 6: During a pressure vessel inspection, localized thin spots are found. Per API 510, what minimum thickness concept is used to evaluate these areas?
- Nominal thickness
- Required minimum thickness (t_min) (Correct answer)
- Measured average thickness
- Design thickness
Correct answer: Required minimum thickness (t_min)
API 510 compares actual measured thickness against the required minimum thickness (t_min) calculated from applicable design codes.
Question 7: Which factor does NOT typically influence the rate of uniform corrosion in a pressure vessel?
- Process fluid temperature
- Fluid pH
- Vessel wall color (Correct answer)
- Concentration of corrosive species
Correct answer: Vessel wall color
Vessel wall color has no effect on corrosion rate; temperature, pH, and corrosive concentration are the primary influencing factors.
API 510 Corrosion Assessment 2
When evaluating corrosion under insulation (CUI), which temperature range poses the HIGHEST risk for carbon steel?