API 510 Material Evaluation 5 — Questions and Answers
Question 1: An inspector is evaluating a vessel that operates in polythionic acid service during shutdowns. Which material is most resistant to polythionic acid stress corrosion cracking (PASCC)?
- Type 304 stainless steel
- Type 347 stainless steel (stabilized grade) (Correct answer)
- Type 316 stainless steel
- Type 321 stainless steel with sensitized HAZ
Correct answer: Type 347 stainless steel (stabilized grade)
Type 347 SS, stabilized with niobium, resists sensitization and is therefore the most resistant grade to PASCC among standard austenitic grades.
Question 2: A vessel exhibits localized corrosion in a pattern corresponding to the original weld bead geometry. The base metal shows minimal attack but the weld metal is heavily corroded. What type of corrosion is occurring?
- Crevice corrosion at the weld root
- Galvanic corrosion due to electrochemical potential difference between weld and base metal (Correct answer)
- Flow-accelerated corrosion on the weld profile
- Pitting corrosion concentrated at weld inclusions
Correct answer: Galvanic corrosion due to electrochemical potential difference between weld and base metal
Galvanic corrosion occurs when the weld metal has a different electrochemical potential than the base metal in the same electrolyte, causing preferential attack of the anodic member.
Question 3: When performing a material evaluation for a pressure vessel in cyclic thermal service, which material property is most critical to assess for resistance to thermal fatigue?
- Ultimate tensile strength at maximum temperature
- Coefficient of thermal expansion and thermal conductivity (Correct answer)
- Charpy impact energy at minimum operating temperature
- Hardness at room temperature
Correct answer: Coefficient of thermal expansion and thermal conductivity
Thermal fatigue is driven by differential thermal strains; materials with lower thermal expansion coefficients and higher thermal conductivity (reducing temperature gradients) are most resistant.
Question 4: An API 510 inspector reviews a vessel data report and finds the design code listed is ASME Section VIII, Division 1, Edition 1965. The vessel requires a major repair using current materials. What is the material evaluation requirement?
- Use only materials listed in the 1965 code edition
- Use current ASME Section II materials that are acceptable under current code and were permissible under the original code (Correct answer)
- Any commercially available steel may be used without restriction
- Apply Division 2 material requirements since a major repair is involved
Correct answer: Use current ASME Section II materials that are acceptable under current code and were permissible under the original code
API 510 requires repairs to use current ASME Section II materials; the material should be acceptable under the current code and equivalent to what was allowable under the original construction code.
Question 5: A material evaluation for a new vessel nozzle fabricated from SA-105 (carbon steel forging) reveals a hardness of 165 HB. The vessel will operate in wet H2S service. Is this material acceptable per NACE MR0103?
- No; SA-105 is not permitted in any wet H2S service
- Yes; 165 HB is well below the 200 HB limit and acceptable for wet H2S service (Correct answer)
- Yes; the hardness is acceptable but post-weld heat treatment is still mandatory
- No; forgings require a minimum hardness of 200 HB for structural adequacy
Correct answer: Yes; 165 HB is well below the 200 HB limit and acceptable for wet H2S service
NACE MR0103 permits carbon steel with hardness ≤200 HB (22 HRC) in wet H2S service, so 165 HB is acceptable for the base metal.
Question 6: During evaluation of a vessel shell, an inspector finds that the material has experienced liquid metal embrittlement (LME). Which service condition is consistent with this finding?
- Long-term exposure to dilute sulfuric acid at ambient temperature
- Contact of carbon steel with molten zinc during fire or hot-dip galvanizing operations (Correct answer)
- High-cycle fatigue from pressure pulsations in a pump discharge vessel
- Thermal shock from emergency water injection into a hot vessel
Correct answer: Contact of carbon steel with molten zinc during fire or hot-dip galvanizing operations
Liquid metal embrittlement occurs when a susceptible solid metal contacts a liquid metal—common examples include zinc from galvanized coatings contacting carbon or stainless steel at elevated temperatures.
Question 7: An inspector is evaluating weld overlay cladding on a pressure vessel. The cladding is Alloy 625 over carbon steel. Which analytical concern is specific to the transition zone between cladding and base metal?
- Formation of brittle sigma phase in the cladding
- Dilution of the nickel alloy by iron from the base metal, potentially reducing corrosion resistance below the specified alloy composition (Correct answer)
- Carburization of the base metal adjacent to the cladding
- Solidification cracking in the base metal HAZ
Correct answer: Dilution of the nickel alloy by iron from the base metal, potentially reducing corrosion resistance below the specified alloy composition
Dilution during weld overlay cladding can reduce the nickel and chromium content of the first layer below that required for corrosion resistance, often necessitating two-layer application.
API 510 Material Evaluation 5
An inspector is evaluating a vessel that operates in polythionic acid service during shutdowns.
Which material is most resistant to polythionic acid stress corrosion cracking (PASCC)?