API-580 Corrosion Mechanisms — Questions and Answers
Question 1: Which type of corrosion occurs in the heat-affected zone (HAZ) of welded equipment?
- Intergranular corrosion (Correct answer)
- Uniform corrosion
- Galvanic corrosion
- Erosion corrosion
Correct answer: Intergranular corrosion
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)?
- The combination of tensile stress, a susceptible material, and a corrosive environment (Correct answer)
- High operating pressure alone
- Cyclic loading at low temperatures
- Exposure to atmospheric oxygen
Correct answer: The combination of tensile stress, a susceptible material, and a corrosive environment
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)?
- Bacteria create localized corrosive conditions on metal surfaces (Correct answer)
- Microbes directly consume metal atoms
- Fungi grow through metal grain boundaries
- Algae produce electrical currents in pipelines
Correct answer: Bacteria create localized corrosive conditions on metal surfaces
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?
- Naphthenic acid corrosion (Correct answer)
- Chloride stress corrosion cracking
- Ammonia stress corrosion cracking
- Caustic cracking
Correct answer: Naphthenic acid corrosion
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?
- Cracking caused by hydrogen atoms diffusing into steel, common in wet H2S environments (Correct answer)
- Cracking from hydrogen gas pressure in storage tanks
- Fractures caused by hydrogen embrittlement at high temperatures
- Corrosion from hydrogen peroxide cleaning solutions
Correct answer: Cracking caused by hydrogen atoms diffusing into steel, common in wet H2S environments
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.
Question 6: What distinguishes pitting corrosion from uniform corrosion?
- Pitting creates localized holes while uniform corrosion affects the entire surface evenly (Correct answer)
- Pitting occurs only at high temperatures
- Uniform corrosion is more dangerous than pitting
- Pitting only affects non-metallic materials
Correct answer: Pitting creates localized holes while uniform corrosion affects the entire surface evenly
Pitting corrosion creates small, localized cavities or holes in the metal surface, making it particularly dangerous because it can cause perforation with minimal overall metal loss, unlike uniform corrosion which affects the entire surface.
API-580 Corrosion Mechanisms
Which type of corrosion occurs in the heat-affected zone (HAZ) of welded equipment?