API 571 Acidic Process Stream Corrosion 2

Question 1

CO2 corrosion ('sweet corrosion') of carbon steel produces what primary corrosive species when CO2 dissolves in water?

Accepted Answer

Carbonic acid (H2CO3)

Answer

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?

Accepted Answer

Iron carbonate (FeCO3)

Answer

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?

Accepted Answer

Below 140°F (60°C)

Answer

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?

Accepted Answer

Monel 400 (Ni-Cu alloy)

Answer

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?

Accepted Answer

3 ft/s (0.9 m/s)

Answer

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.

API 571 - API 571 Corrosion and Materials Practice Test

API 571 - API 571 Corrosion and Materials Practice Test

API 571 Acidic Process Stream Corrosion 2