API 571 Corrosion Mechanisms 2 — Questions and Answers
Question 1: What pH range is generally associated with aggressive acid corrosion in refinery process systems?
- 7–9
- 4–7
- Below 4 (Correct answer)
- Above 9
Correct answer: Below 4
Acid corrosion becomes significant at pH below 4, where hydrogen ion concentration is high enough to drive aggressive metal dissolution.
Question 2: Atmospheric corrosion is most severe in which type of environment?
- Arid desert climate
- Marine or coastal environment (Correct answer)
- High-altitude location
- Cold arctic environment
Correct answer: Marine or coastal environment
Marine environments combine high relative humidity with chloride ions from sea spray, dramatically accelerating atmospheric corrosion rates.
Question 3: Corrosion under insulation (CUI) is primarily driven by what mechanism?
- Galvanic coupling at the metal surface
- Moisture penetrating insulation and concentrating corrosive species against the pipe (Correct answer)
- Microbiological activity inside insulation
- Oxygen exclusion under the insulation
Correct answer: Moisture penetrating insulation and concentrating corrosive species against the pipe
CUI occurs when water infiltrates degraded insulation and remains in contact with the metal surface, concentrating chlorides and other corrosives.
Question 4: What is the primary damage mechanism for carbon steel in amine treating systems?
- High-temperature acid corrosion
- Alkaline corrosion from fresh amine
- Amine corrosion from degradation products and stress corrosion cracking (Correct answer)
- High-temperature oxidation
Correct answer: Amine corrosion from degradation products and stress corrosion cracking
Carbon steel in amine service suffers general corrosion from degraded amine compounds and is susceptible to SCC if residual welding stresses are present.
Question 5: Microbiologically influenced corrosion (MIC) is most commonly caused by which organisms?
- Aerobic bacteria only
- Sulfate-reducing bacteria (SRB) (Correct answer)
- Nitrate-reducing bacteria
- Fungi and mold
Correct answer: Sulfate-reducing bacteria (SRB)
Sulfate-reducing bacteria produce hydrogen sulfide as a metabolic byproduct, directly attacking metal surfaces and promoting pitting corrosion.
Question 6: Which factor does NOT directly influence the electrochemical corrosion rate of carbon steel in water service?
- Temperature
- pH level
- Water color or turbidity (Correct answer)
- Dissolved oxygen content
Correct answer: Water color or turbidity
Water color is an aesthetic property and not an electrochemical parameter; temperature, pH, and dissolved oxygen all directly affect corrosion kinetics.
API 571 Corrosion Mechanisms 2
What pH range is generally associated with aggressive acid corrosion in refinery process systems?