API 571 Stress Corrosion Cracking 2 — Questions and Answers
Question 1: Wet H2S damage in API 571 encompasses which specific damage forms?
- PTASCC and CSCC only
- HIC, SOHIC, SSC, and stepwise cracking (SWC) (Correct answer)
- Cl-SCC and high-temperature hydrogen SCC
- LME and thermal fatigue
Correct answer: HIC, SOHIC, SSC, and stepwise cracking (SWC)
API 571 groups hydrogen-induced cracking (HIC), stress-oriented HIC (SOHIC), sulfide stress cracking (SSC), and stepwise cracking (SWC) under the wet H2S damage category.
Question 2: Hydrogen-induced cracking (HIC) preferentially initiates at which microstructural features?
- Grain boundaries near the surface
- Non-metallic inclusions and laminations within the steel plate (Correct answer)
- Weld fusion lines only
- Machined external surfaces
Correct answer: Non-metallic inclusions and laminations within the steel plate
HIC blisters and planar cracks initiate at MnS inclusions and laminations where atomic hydrogen recombines into molecular hydrogen, building up internal pressure.
Question 3: How does SOHIC (Stress-Oriented HIC) differ from standard HIC?
- SOHIC only occurs in high-alloy stainless steels
- SOHIC cracks align perpendicular to applied stress and can link to form through-wall cracks (Correct answer)
- SOHIC occurs at much higher operating temperatures
- SOHIC requires no hydrogen — it is purely mechanical
Correct answer: SOHIC cracks align perpendicular to applied stress and can link to form through-wall cracks
SOHIC involves HIC cracks that orient with applied or residual stress, potentially linking through the vessel wall thickness to create a serious through-thickness crack path.
Question 4: Sulfide stress cracking (SSC) most preferentially affects which type of steel?
- Soft, low-strength carbon steel with low hardness
- High-strength steels with hardness exceeding HRC 22 (Correct answer)
- Austenitic stainless steels in all conditions
- Copper alloys in H2S service
Correct answer: High-strength steels with hardness exceeding HRC 22
SSC preferentially attacks high-strength, high-hardness steels because hardness directly correlates with susceptibility to hydrogen embrittlement in H2S environments.
Question 5: NACE MR0175/ISO 15156 provides requirements for which application?
- High-temperature oxidation resistance of alloys
- Material selection for equipment in H2S-containing sour service (Correct answer)
- Cathodic protection system design for pipelines
- Inspection intervals for pressure vessels
Correct answer: Material selection for equipment in H2S-containing sour service
NACE MR0175 specifies material requirements, hardness limits, and heat treatment requirements for equipment exposed to H2S-containing environments to prevent SSC.
Question 6: Carbonate cracking in refinery systems is associated with which type of environment?
- Crude oil distillation overhead systems
- Alkaline carbonate/bicarbonate solutions in FCC and amine units (Correct answer)
- Amine regenerator overhead at elevated temperature
- Hydrogen reformer effluent streams
Correct answer: Alkaline carbonate/bicarbonate solutions in FCC and amine units
Carbonate cracking is an alkaline SCC mechanism affecting carbon steel exposed to carbonate and bicarbonate solutions at elevated temperatures in refinery processes.
API 571 Stress Corrosion Cracking 2
Wet H2S damage in API 571 encompasses which specific damage forms?