API 510 Corrosion Assessment 4 — Questions and Answers
Question 1: What type of corrosion involves the preferential attack of one phase in a multi-phase alloy, such as the removal of zinc from brass?
- Pitting corrosion
- Dealloying (selective leaching) (Correct answer)
- Erosion corrosion
- Intergranular corrosion
Correct answer: Dealloying (selective leaching)
Dealloying, or selective leaching, involves the preferential removal of one element from an alloy, such as dezincification of brass.
Question 2: According to API 510, how should a corrosion rate be established for a new vessel with no prior inspection history?
- Assume zero corrosion until the first inspection
- Use published data, experience from similar services, or process engineering estimates (Correct answer)
- Default to 0.050 in/yr for all services
- Use only the design corrosion allowance divided by design life
Correct answer: Use published data, experience from similar services, or process engineering estimates
For new vessels, API 510 allows use of published corrosion data, similar service experience, or process engineer estimates to establish an initial corrosion rate.
Question 3: Which condition is required for erosion-corrosion to be a significant degradation mechanism in a pressure vessel?
- Static fluid with low pH
- High-velocity flow, especially with entrained solids or vapors (Correct answer)
- Dry service with no moisture
- Uniform temperature distribution
Correct answer: High-velocity flow, especially with entrained solids or vapors
Erosion-corrosion requires high-velocity flow that mechanically removes protective oxide layers, with entrained solids or phase changes accelerating the damage.
Question 4: What is the purpose of establishing a short-term corrosion rate in API 510 assessments?
- To use a conservative 10-year average
- To capture recent changes in process conditions that may differ from historical rates (Correct answer)
- To replace the long-term rate in all calculations
- To estimate corrosion during vessel fabrication
Correct answer: To capture recent changes in process conditions that may differ from historical rates
Short-term rates reflect the most recent inspection interval, capturing changes in process conditions that may not be represented by historical long-term data.
Question 5: A pressure vessel inspector discovers blistering on the inner surface of a vessel in H2S service. What damage mechanism does this MOST likely indicate?
- Thermal fatigue
- Hydrogen blistering (Correct answer)
- Stress corrosion cracking
- Pitting corrosion
Correct answer: Hydrogen blistering
Hydrogen blistering occurs when atomic hydrogen diffuses into the steel and accumulates at inclusions, forming blisters visible on the surface.
Question 6: Which factor most significantly increases the susceptibility of austenitic stainless steel to chloride stress corrosion cracking?
- Low tensile stress and neutral pH
- Elevated temperature above approximately 140 degrees F (60 degrees C) (Correct answer)
- High compressive residual stress
- Alkaline environment with no chlorides
Correct answer: Elevated temperature above approximately 140 degrees F (60 degrees C)
Chloride SCC in austenitic stainless steel is significantly accelerated at elevated temperatures, typically above 140 degrees F, in the presence of chlorides and tensile stress.
Question 7: Per API 510, if a vessel's corrosion rate doubles after a process change, what is the MOST appropriate inspector action?
- Continue with the original inspection interval and note the change
- Revise the inspection interval based on the new, higher corrosion rate (Correct answer)
- Immediately take the vessel out of service
- Apply a corrosion inhibitor without notifying engineering
Correct answer: Revise the inspection interval based on the new, higher corrosion rate
A significant change in corrosion rate warrants revising the inspection interval using the new rate to ensure the vessel remains within safe limits.
API 510 Corrosion Assessment 4
What type of corrosion involves the preferential attack of one phase in a multi-phase alloy, such as the removal of zinc from brass?