API Corrosion & Damage Mechanisms 1 — Questions and Answers
Question 1: According to API 571, which damage mechanism is characterized by cracking that occurs in high-strength steels exposed to hydrogen-containing environments under sustained tensile stress?
- Hydrogen-induced cracking (HIC)
- Sulfide stress cracking (SSC) (Correct answer)
- Stress corrosion cracking (SCC)
- Caustic embrittlement
Correct answer: Sulfide stress cracking (SSC)
Sulfide stress cracking (SSC) occurs in high-strength steels under tensile stress in hydrogen sulfide (H₂S) environments, and is a primary concern addressed by NACE MR0175/ISO 15156.
Question 2: Which type of corrosion preferentially attacks grain boundaries in a metal, often as a result of sensitization in austenitic stainless steels?
- Galvanic corrosion
- Intergranular corrosion (Correct answer)
- Crevice corrosion
- Pitting corrosion
Correct answer: Intergranular corrosion
Intergranular corrosion attacks along grain boundaries, commonly occurring in sensitized austenitic stainless steels where chromium carbides precipitate and deplete chromium from the grain boundary regions.
Question 3: In API 571 terminology, which high-temperature damage mechanism involves the diffusion of carbon from a steel into an adjacent higher-alloy steel or weld material?
- Decarburization (Correct answer)
- Carburization
- Carbon migration (creep)
- Graphitization
Correct answer: Decarburization
Decarburization involves the loss of carbon from the surface layer of steel at elevated temperatures, reducing hardness and strength in the affected zone.
Question 4: Which damage mechanism described in API 571 is caused by the condensation of ammonium bisulfide in refinery overhead systems, resulting in aggressive corrosion?
- Amine corrosion
- Naphthenic acid corrosion
- Ammonium bisulfide corrosion (Correct answer)
- Hydrofluoric acid corrosion
Correct answer: Ammonium bisulfide corrosion
Ammonium bisulfide (NH₄HS) corrosion occurs in refinery crude unit overhead and hydroprocessing systems when ammonium bisulfide deposits and attacks carbon and alloy steels.
Question 5: According to API 571, at what minimum temperature (°F) does high-temperature oxidation of carbon steel typically become a significant concern?
- 600°F (315°C)
- 900°F (482°C) (Correct answer)
- 1100°F (593°C)
- 1400°F (760°C)
Correct answer: 900°F (482°C)
High-temperature oxidation of carbon steel becomes significant above approximately 900°F (482°C), where the oxidation rate increases substantially and protective oxide scales may break down.
Question 6: Which API 571 damage mechanism results from the absorption of atomic hydrogen into steel during corrosion reactions, leading to blistering, stepwise cracking, or stress-oriented cracking?
- Hydrogen-induced cracking (HIC)
- Creep
- Thermal fatigue
- Wet H₂S damage (Correct answer)
Correct answer: Wet H₂S damage
Wet H₂S damage is a broad term covering hydrogen blistering, hydrogen-induced cracking (HIC), stress-oriented hydrogen-induced cracking (SOHIC), and sulfide stress cracking (SSC), all caused by hydrogen absorption in wet H₂S service.
Question 7: In API 571, naphthenic acid corrosion is most commonly associated with which process condition?
- Low-temperature crude processing with high water content
- High-temperature crude distillation above 450°F with high total acid number (TAN) crudes (Correct answer)
- Hydroprocessing in the presence of hydrogen sulfide
- Atmospheric distillation of sweet crudes with low sulfur content
Correct answer: High-temperature crude distillation above 450°F with high total acid number (TAN) crudes
Naphthenic acid corrosion occurs at elevated temperatures (typically 450–750°F) when processing high TAN crude oils, affecting carbon steel and even some stainless steels in distillation units.
API Corrosion & Damage Mechanisms 1
According to API 571, which damage mechanism is characterized by cracking that occurs in high-strength steels exposed to hydrogen-containing environments under sustained tensile stress?