API-580 Corrosion Mechanisms

Question 1

Which type of corrosion occurs in the heat-affected zone (HAZ) of welded equipment?

Accepted Answer

Intergranular corrosion

Answer

Intergranular corrosion occurs along grain boundaries in the HAZ where sensitization can occur during welding, particularly in austenitic stainless steels where chromium carbides precipitate at grain boundaries.

Question 2

What is the primary driver of stress corrosion cracking (SCC)?

Accepted Answer

The combination of tensile stress, a susceptible material, and a corrosive environment

Answer

SCC requires three simultaneous conditions: a susceptible material, sustained tensile stress (residual or applied), and a specific corrosive environment. Removing any one factor prevents SCC.

Question 3

What is the mechanism behind microbiologically influenced corrosion (MIC)?

Accepted Answer

Bacteria create localized corrosive conditions on metal surfaces

Answer

MIC occurs when bacteria colonize metal surfaces and create biofilms that produce localized acidic or anaerobic conditions, accelerating corrosion through metabolic byproducts such as sulfuric acid or hydrogen sulfide.

Question 4

Which corrosion mechanism is most common in carbon steel piping carrying acidic crude oil?

Accepted Answer

Naphthenic acid corrosion

Answer

Naphthenic acid corrosion occurs in carbon steel and low-alloy steel processing units handling crude oils with high Total Acid Number (TAN), particularly at temperatures between 220-400°C.

Question 5

What is hydrogen-induced cracking (HIC) and where does it typically occur?

Accepted Answer

Cracking caused by hydrogen atoms diffusing into steel, common in wet H2S environments

Answer

HIC occurs when atomic hydrogen generated by corrosion reactions diffuses into steel and recombines at inclusions or laminations, creating internal pressure that causes stepwise cracking, commonly found in wet H2S service.

API-580 Exam Practice Test

API-580 Exam Practice Test

API-580 Corrosion Mechanisms