API 571 Acidic Process Stream Corrosion 1 — Questions and Answers
Question 1: At what Total Acid Number (TAN) does naphthenic acid corrosion typically become a significant concern in crude oil processing?
- 0.1 mg KOH/g
- 0.5 mg KOH/g (Correct answer)
- 1.5 mg KOH/g
- 3.0 mg KOH/g
Correct answer: 0.5 mg KOH/g
A TAN above 0.5 mg KOH/g is the widely recognized threshold where naphthenic acid corrosion becomes a significant concern requiring enhanced monitoring and material upgrades.
Question 2: What temperature range is associated with the highest naphthenic acid corrosion rates in refinery distillation equipment?
- 100°F–200°F (38°C–93°C)
- 200°F–400°F (93°C–204°C)
- 450°F–750°F (232°C–399°C) (Correct answer)
- 800°F–1000°F (427°C–538°C)
Correct answer: 450°F–750°F (232°C–399°C)
Naphthenic acid corrosion is most severe between 450°F and 750°F (232°C–399°C), where the acids are liquid and thermally active but have not yet decomposed.
Question 3: Which alloying element in stainless steel provides the greatest improvement in resistance to naphthenic acid corrosion?
- Chromium
- Molybdenum (Correct answer)
- Nickel
- Silicon
Correct answer: Molybdenum
Molybdenum content above approximately 2.5% (as found in 316L SS) significantly improves resistance to naphthenic acid by stabilizing the passive film against acid attack.
Question 4: What is the primary corrosion mechanism in refinery amine treating units that causes attack on carbon steel equipment?
- Galvanic attack between lean and rich amine streams
- Acidic conditions from absorbed CO2 and H2S at elevated temperatures (Correct answer)
- Oxidation of amine degradation products forming organic acids
- Microbiologically influenced corrosion in stagnant amine sections
Correct answer: Acidic conditions from absorbed CO2 and H2S at elevated temperatures
Amine systems absorb acid gases (CO2 and H2S), and at high temperatures or heavy acid gas loading, these create corrosive acidic conditions that attack carbon steel.
Question 5: Carbon steel welds in amine service are susceptible to amine stress corrosion cracking when hardness exceeds which threshold?
- HRC 10 (187 HB)
- HRC 22 (235 HB) (Correct answer)
- HRC 30 (286 HB)
- HRC 40 (371 HB)
Correct answer: HRC 22 (235 HB)
Carbon steel welds and HAZ regions with hardness above HRC 22 (approximately 235 HB) are susceptible to amine SCC, which is why PWHT is required to soften these areas.
Question 6: Caustic stress corrosion cracking (caustic embrittlement) in carbon steel is most likely to occur when NaOH concentrations exceed approximately:
- 1–3 wt%
- 5–7 wt%
- 10–30 wt% (Correct answer)
- 50 wt%
Correct answer: 10–30 wt%
Caustic SCC in carbon steel typically requires NaOH concentrations above about 10 wt% combined with temperatures above approximately 150°F (66°C) and tensile stress.
Question 7: Which mitigation method is most effective for preventing caustic corrosion and caustic SCC in carbon steel equipment?
- Using 304 stainless steel instead of carbon steel
- Limiting operating temperature and applying PWHT to welds (Correct answer)
- Coating internal surfaces with phenolic epoxy lining
- Injecting corrosion inhibitors into the caustic stream
Correct answer: Limiting operating temperature and applying PWHT to welds
Keeping operating temperatures below approximately 150°F (66°C) for concentrated caustic, combined with PWHT to reduce residual stresses in welds, is the primary mitigation for caustic corrosion and SCC.
API 571 Acidic Process Stream Corrosion 1
At what Total Acid Number (TAN) does naphthenic acid corrosion typically become a significant concern in crude oil processing?