API 653 Material Evaluation 3 — Questions and Answers
Question 1: Which type of corrosion produces a smooth, uniformly thinned surface and is generally easier to evaluate than localized corrosion?
- Pitting corrosion
- Crevice corrosion
- General (uniform) corrosion (Correct answer)
- Galvanic corrosion
Correct answer: General (uniform) corrosion
General corrosion thins the plate uniformly, making thickness measurements straightforward and corrosion rate projections more reliable.
Question 2: When a tank inspector discovers stress corrosion cracking (SCC) on a stainless steel shell, what is the most critical environmental factor to investigate?
- Presence of sulfur compounds
- Chloride ion concentration (Correct answer)
- High pH of stored product
- Low ambient temperature
Correct answer: Chloride ion concentration
Chloride ions are the primary environmental trigger for stress corrosion cracking in austenitic stainless steels.
Question 3: API 653 requires that shell repair welds be made using a minimum preheat temperature based on the:
- Shell plate thickness and carbon equivalent (Correct answer)
- Tank diameter and operating pressure
- Product specific gravity and flash point
- Inspection interval and corrosion rate
Correct answer: Shell plate thickness and carbon equivalent
Preheat requirements for repair welding are determined by the base metal's carbon equivalent and the plate thickness per API 653 welding provisions.
Question 4: A tank bottom plate exhibits a reddish-brown scale with deep pitting beneath it. This morphology is most characteristic of:
- Microbiologically influenced corrosion (MIC) (Correct answer)
- High-temperature oxidation
- Uniform general corrosion
- Erosion-corrosion
Correct answer: Microbiologically influenced corrosion (MIC)
MIC produces localized pitting under biological deposits (tubercles) that often appear as reddish-brown mounds on the plate surface.
Question 5: For a tank bottom, API 653 specifies the minimum thickness for fitness-for-service based primarily on:
- Hydrostatic pressure from liquid head above the bottom (Correct answer)
- Product corrosivity classification
- Original nominal thickness minus total corrosion allowance
- 0.10 inches for all steel bottoms
Correct answer: Hydrostatic pressure from liquid head above the bottom
Tank bottom minimum thickness per API 653 is set at 0.10 inches (2.5 mm) but is also governed by the ability to resist hydrostatic head from the stored liquid.
Question 6: Charpy V-notch (CVN) impact testing of tank shell material is most important when evaluating tanks for:
- High-temperature service above 200°F
- Low-temperature or cold-service operation (Correct answer)
- Product compatibility with carbon steel
- Fire resistance of the shell plate
Correct answer: Low-temperature or cold-service operation
CVN testing measures the fracture toughness at low temperatures, which is critical to ensure ductile behavior and prevent brittle fracture in cold-service tanks.
Question 7: When evaluating material for a tank shell replacement plate, the ASTM specification that requires the material to meet both chemical composition and mechanical property requirements is:
- ASTM A36 (Correct answer)
- ASTM E18
- ASTM G48
- ASTM A370
Correct answer: ASTM A36
ASTM A36 is a structural carbon steel specification commonly used for tank shells that defines both chemical composition limits and minimum mechanical properties.
API 653 Material Evaluation 3
Which type of corrosion produces a smooth, uniformly thinned surface and is generally easier to evaluate than localized corrosion?