API 571 Hydrogen Damage 2 — Questions and Answers
Question 1: What is the primary mechanism by which H2S promotes hydrogen entry into steel in wet H2S service?
- H2S increases operating pressure, forcing hydrogen in
- H2S acts as a hydrogen recombination poison, allowing atomic hydrogen produced at the steel surface to be absorbed (Correct answer)
- H2S thermally decomposes at the surface to release hydrogen
- H2S oxidizes the steel surface, creating a porous entry path
Correct answer: H2S acts as a hydrogen recombination poison, allowing atomic hydrogen produced at the steel surface to be absorbed
H2S prevents atomic hydrogen from safely recombining into H2 gas at the steel surface, forcing the atomic hydrogen to absorb into the metal instead.
Question 2: Which standard test method specifically evaluates steel plate resistance to HIC?
- Charpy V-notch impact test per ASTM A370
- NACE TM0284 immersion test in simulated sour solution (Correct answer)
- Rockwell hardness test per ASTM E18
- Four-point bend test per NACE TM0177
Correct answer: NACE TM0284 immersion test in simulated sour solution
NACE TM0284 is the industry-standard test for evaluating HIC resistance of steel products intended for wet H2S (sour) service.
Question 3: Why does low sulfur content improve HIC resistance of steel plate?
- Sulfur increases steel hardness, which improves resistance
- Low sulfur means fewer MnS inclusions that serve as hydrogen trap and blister nucleation sites (Correct answer)
- Sulfur promotes formation of a protective sulfide scale
- Low sulfur raises the material's position on the Nelson curve
Correct answer: Low sulfur means fewer MnS inclusions that serve as hydrogen trap and blister nucleation sites
MnS inclusions are primary hydrogen trap sites; HIC-resistant steels minimize these inclusions through low sulfur content and calcium treatment to control inclusion shape.
Question 4: HF corrosion in alkylation units can lead to which type of hydrogen damage?
- High Temperature Hydrogen Attack (HTHA)
- Hydrogen-induced cracking (HIC) and hydrogen blistering (Correct answer)
- Metal dusting and carburization
- Thermal fatigue cracking
Correct answer: Hydrogen-induced cracking (HIC) and hydrogen blistering
HF acid corrosion generates significant atomic hydrogen as a cathodic byproduct, making HIC and blistering serious concerns in carbon steel equipment in HF alkylation units.
Question 5: Which alloy is the standard material choice for HF alkylation unit equipment due to its HF corrosion resistance?
- 304L austenitic stainless steel
- Alloy 400 (Monel — 67Ni-33Cu) (Correct answer)
- 316L austenitic stainless steel
- 2205 duplex stainless steel
Correct answer: Alloy 400 (Monel — 67Ni-33Cu)
Alloy 400 (Monel) is the industry-standard material for HF alkylation service due to its excellent resistance to hydrofluoric acid, though welds must be stress-relieved.
Question 6: HIC cracks in steel plate are generally oriented in what direction relative to the plate surface?
- Perpendicular to the pipe wall (through-wall)
- Parallel to the plate rolling direction, creating a laminar or stepwise pattern (Correct answer)
- At 45° to the plate surface in shear planes
- Randomly oriented with no preferred direction
Correct answer: Parallel to the plate rolling direction, creating a laminar or stepwise pattern
HIC cracks propagate parallel to the steel plate surface along elongated MnS inclusions oriented in the rolling direction, creating characteristic laminar blisters.
API 571 Hydrogen Damage 2
What is the primary mechanism by which H2S promotes hydrogen entry into steel in wet H2S service?