API 571 Acidic Process Stream Corrosion 2 — Questions and Answers
Question 1: CO2 corrosion ('sweet corrosion') of carbon steel produces what primary corrosive species when CO2 dissolves in water?
- Sulfuric acid (H2SO4)
- Carbonic acid (H2CO3) (Correct answer)
- Hydrochloric acid (HCl)
- Formic acid (HCOOH)
Correct answer: Carbonic acid (H2CO3)
CO2 dissolves in water to form carbonic acid (H2CO3), which dissociates to lower the pH and directly attacks carbon steel through an electrochemical mechanism.
Question 2: What protective corrosion product can form on carbon steel during CO2 corrosion and reduce corrosion rates under certain conditions?
- Iron sulfide (FeS)
- Iron carbonate (FeCO3) (Correct answer)
- Iron oxide (Fe3O4)
- Iron hydroxide (Fe(OH)3)
Correct answer: Iron carbonate (FeCO3)
Iron carbonate (FeCO3, siderite) can precipitate as a protective scale on carbon steel during CO2 corrosion when temperatures and pH are favorable, significantly reducing corrosion rates.
Question 3: Below which temperature does protective iron carbonate (FeCO3) scale typically fail to form during CO2 corrosion, leaving carbon steel unprotected?
- Below 60°F (16°C)
- Below 140°F (60°C) (Correct answer)
- Below 212°F (100°C)
- Below 300°F (149°C)
Correct answer: Below 140°F (60°C)
Below approximately 140°F (60°C), FeCO3 precipitation kinetics are too slow to form a protective scale, leaving carbon steel exposed to high CO2 corrosion rates.
Question 4: In HF alkylation units, which material is the preferred choice for many components due to its resistance to hydrofluoric acid?
- 304 stainless steel
- Carbon steel with Alonized coating
- Monel 400 (Ni-Cu alloy) (Correct answer)
- Titanium Grade 2
Correct answer: Monel 400 (Ni-Cu alloy)
Monel 400 (a nickel-copper alloy) forms a protective nickel fluoride film in HF acid service and is the preferred material for HF alkylation unit components per API 751.
Question 5: What flow velocity limit is typically imposed on carbon steel piping in anhydrous HF acid service to prevent erosion of the protective iron fluoride scale?
- 1 ft/s (0.3 m/s)
- 3 ft/s (0.9 m/s) (Correct answer)
- 10 ft/s (3.0 m/s)
- 20 ft/s (6.1 m/s)
Correct answer: 3 ft/s (0.9 m/s)
Carbon steel in HF acid service is typically limited to approximately 3 ft/s to prevent velocity-enhanced erosion of the protective FeF2 scale that would expose fresh steel to rapid attack.
Question 6: Sour water corrosion in refinery overhead systems is primarily caused by which corrosive species?
- Ammonium chloride (NH4Cl)
- Ammonium bisulfide (NH4HS) (Correct answer)
- Sodium hydroxide (NaOH)
- Hydrogen fluoride (HF)
Correct answer: Ammonium bisulfide (NH4HS)
Ammonium bisulfide (NH4HS) formed by the reaction of H2S and NH3 in overhead water is the primary corrosive agent responsible for sour water corrosion in refinery overhead systems.
Question 7: Above what NH4HS concentration in refinery overhead water systems does carbon steel corrosion rate increase significantly, per API 932-A guidance?
- 0.5 wt%
- 2 wt% (Correct answer)
- 8 wt%
- 15 wt%
Correct answer: 2 wt%
API 932-A identifies 2 wt% NH4HS as the threshold concentration above which carbon steel corrosion rates increase significantly, especially when combined with high velocities.
API 571 Acidic Process Stream Corrosion 2
CO2 corrosion ('sweet corrosion') of carbon steel produces what primary corrosive species when CO2 dissolves in water?