API 510 Corrosion Mechanisms & Damage Assessment 4 — Questions and Answers
Question 1: What is 'creep' damage in the context of API 510 pressure vessel inspection?
- Slow progressive plastic deformation under sustained stress at elevated temperature (Correct answer)
- Rapid brittle fracture due to cyclic loading
- Localized corrosion at grain boundaries
- Hydrogen diffusion causing internal blistering
Correct answer: Slow progressive plastic deformation under sustained stress at elevated temperature
Creep is the time-dependent plastic deformation that occurs when metal is subjected to sustained stress at temperatures above its creep threshold.
Question 2: An API 510 inspector identifies 'blistering' on the interior of a carbon steel vessel in wet H₂S service. What is the most likely cause?
- Thermal fatigue from temperature cycling
- Hydrogen pressure buildup at internal laminations or inclusions (Correct answer)
- Crevice corrosion beneath scale deposits
- Galvanic attack from dissimilar metal contact
Correct answer: Hydrogen pressure buildup at internal laminations or inclusions
In wet H₂S service, atomic hydrogen diffuses into steel and collects at voids or laminations, building sufficient pressure to form surface blisters.
Question 3: Which corrosion rate formula is used in API 510 to calculate the short-term corrosion rate?
- (Previous thickness - Current thickness) / Total years in service
- (Previous thickness - Current thickness) / Years between inspections (Correct answer)
- (Design thickness - Current thickness) / Total years in service
- (Current thickness - Minimum thickness) / Years since last inspection
Correct answer: (Previous thickness - Current thickness) / Years between inspections
The short-term corrosion rate uses only the most recent inspection interval: (previous measured thickness − current measured thickness) ÷ years between the two inspections.
Question 4: Intergranular corrosion in austenitic stainless steel is most commonly caused by:
- Exposure to caustic soda at elevated temperatures
- Sensitization from heating in the range of 800°F to 1500°F (Correct answer)
- Contact with hydrofluoric acid solutions
- Pitting in chloride-containing solutions
Correct answer: Sensitization from heating in the range of 800°F to 1500°F
Sensitization occurs when chromium carbides precipitate at grain boundaries, depleting adjacent zones of chromium and making them susceptible to intergranular attack.
Question 5: What is 'crevice corrosion' and where is it most likely to occur on a pressure vessel?
- Uniform attack on large flat surfaces exposed to stagnant liquid
- Localized attack in confined spaces where stagnant solution becomes aggressive, such as under gaskets or deposits (Correct answer)
- Corrosion at the interface of two dissimilar metals
- Surface oxidation on external insulated surfaces
Correct answer: Localized attack in confined spaces where stagnant solution becomes aggressive, such as under gaskets or deposits
Crevice corrosion occurs in confined spaces where stagnant solution becomes depleted of oxygen or enriched in corrosive ions, creating a differential concentration cell.
Question 6: In API 510, when evaluating a vessel with general corrosion, the inspector should compare the measured thickness to:
- The original design thickness specified on the nameplate
- The calculated minimum required thickness (t_min) based on MAWP and code formulas (Correct answer)
- The nominal thickness from the original material certification
- The average of all previous thickness measurements recorded
Correct answer: The calculated minimum required thickness (t_min) based on MAWP and code formulas
API 510 requires comparing actual measured thickness to the minimum thickness calculated from the MAWP using the applicable code formula, not the original design thickness.
Question 7: What is the primary difference between 'erosion' and 'corrosion' as damage mechanisms in pressure vessels?
- Erosion is chemical, corrosion is mechanical
- Erosion is mechanical removal of material, corrosion is electrochemical or chemical attack (Correct answer)
- Erosion only occurs in gas service, corrosion only in liquid service
- Erosion requires high temperature, corrosion occurs only at ambient conditions
Correct answer: Erosion is mechanical removal of material, corrosion is electrochemical or chemical attack
Erosion involves the mechanical removal of metal by solid particles, fluid flow, or cavitation, while corrosion involves chemical or electrochemical reactions that degrade the metal.
API 510 Corrosion Mechanisms & Damage Assessment 4
What is 'creep' damage in the context of API 510 pressure vessel inspection?