API 571 Acidic Process Stream Corrosion

Question 1

At what Total Acid Number (TAN) does naphthenic acid corrosion typically become a significant concern in crude oil processing?

Accepted Answer

0.5 mg KOH/g

Answer

A TAN above 0.5 mg KOH/g is the widely recognized threshold where naphthenic acid corrosion becomes a significant concern requiring enhanced monitoring and material upgrades.

Question 2

What temperature range is associated with the highest naphthenic acid corrosion rates in refinery distillation equipment?

Accepted Answer

450°F–750°F (232°C–399°C)

Answer

Naphthenic acid corrosion is most severe between 450°F and 750°F (232°C–399°C), where the acids are liquid and thermally active but have not yet decomposed.

Question 3

Which alloying element in stainless steel provides the greatest improvement in resistance to naphthenic acid corrosion?

Accepted Answer

Molybdenum

Answer

Molybdenum content above approximately 2.5% (as found in 316L SS) significantly improves resistance to naphthenic acid by stabilizing the passive film against acid attack.

Question 4

What is the primary corrosion mechanism in refinery amine treating units that causes attack on carbon steel equipment?

Accepted Answer

Acidic conditions from absorbed CO2 and H2S at elevated temperatures

Answer

Amine systems absorb acid gases (CO2 and H2S), and at high temperatures or heavy acid gas loading, these create corrosive acidic conditions that attack carbon steel.

Question 5

Carbon steel welds in amine service are susceptible to amine stress corrosion cracking when hardness exceeds which threshold?

Accepted Answer

HRC 22 (235 HB)

Answer

Carbon steel welds and HAZ regions with hardness above HRC 22 (approximately 235 HB) are susceptible to amine SCC, which is why PWHT is required to soften these areas.

API 571 - API 571 Corrosion and Materials Practice Test

API 571 - API 571 Corrosion and Materials Practice Test

API 571 Acidic Process Stream Corrosion