API 510 Corrosion Mechanisms and Monitoring 2 — Questions and Answers
Question 1: What type of corrosion occurs when two dissimilar metals are in electrical contact in the presence of an electrolyte?
- Crevice corrosion
- Galvanic corrosion (Correct answer)
- Pitting corrosion
- Intergranular corrosion
Correct answer: Galvanic corrosion
Galvanic corrosion occurs when dissimilar metals are electrically connected in an electrolyte, causing the more anodic metal to corrode preferentially.
Question 2: Which monitoring technique uses the rate of change in electrical resistance of a metal probe to measure corrosion rate?
- Linear polarization resistance (LPR)
- Electrical resistance (ER) probes (Correct answer)
- Ultrasonic thickness measurement
- Coupon weight loss method
Correct answer: Electrical resistance (ER) probes
Electrical resistance (ER) probes measure corrosion rate by detecting changes in the probe's cross-sectional area as metal is lost to corrosion.
Question 3: Polythionic acid stress corrosion cracking (PTA SCC) primarily affects which material in refineries?
- Carbon steel in high-pressure service
- Sensitized austenitic stainless steel (Correct answer)
- Duplex stainless steel at elevated temperatures
- Copper alloys in sour water service
Correct answer: Sensitized austenitic stainless steel
PTA SCC affects sensitized austenitic stainless steel when exposed to polythionic acids formed from sulfur compounds, water, and oxygen during shutdowns.
Question 4: What is the primary driving force for under-deposit corrosion in pressure vessels?
- Elevated temperature differentials across the vessel wall
- Concentration of aggressive species beneath accumulated solids (Correct answer)
- High fluid velocity causing erosion of the deposit layer
- Differential oxygen concentration creating a galvanic cell
Correct answer: Concentration of aggressive species beneath accumulated solids
Under-deposit corrosion is driven by the concentration of corrosive species (chlorides, acids, sulfur compounds) trapped beneath deposits, creating a localized aggressive environment.
Question 5: In API 510 inspection practice, which corrosion mechanism is most commonly associated with crude oil distillation overhead systems?
- High-temperature sulfidic corrosion
- Naphthenic acid corrosion
- Hydrochloric acid (HCl) dew point corrosion (Correct answer)
- Caustic stress corrosion cracking
Correct answer: Hydrochloric acid (HCl) dew point corrosion
HCl dew point corrosion is the dominant mechanism in crude distillation overhead systems, where HCl condenses from hydrolyzed chloride salts and attacks carbon and low-alloy steels.
Question 6: Which parameter is most critical in determining the susceptibility of carbon steel to wet H₂S damage (hydrogen blistering)?
- pH of the aqueous phase and H₂S partial pressure (Correct answer)
- Operating temperature above 150°F (65°C)
- Flow velocity exceeding 10 ft/s
- Carbon equivalent of the steel above 0.43
Correct answer: pH of the aqueous phase and H₂S partial pressure
Wet H₂S damage susceptibility depends primarily on the pH of the water phase and H₂S partial pressure, with damage most severe at low pH and elevated H₂S concentrations.
Question 7: A pressure vessel inspector finds localized deep pits on the internal surface of a carbon steel vessel in cooling water service. What corrosion mechanism is most likely responsible?
- General/uniform corrosion from dissolved CO₂
- Microbiologically influenced corrosion (MIC) (Correct answer)
- Erosion-corrosion from high fluid velocity
- Caustic gouging from concentrated alkalis
Correct answer: Microbiologically influenced corrosion (MIC)
MIC in cooling water systems commonly produces localized deep pitting due to the action of sulfate-reducing or acid-producing bacteria creating aggressive microenvironments.
API 510 Corrosion Mechanisms and Monitoring 2
What type of corrosion occurs when two dissimilar metals are in electrical contact in the presence of an electrolyte?