API 510 Corrosion, Welding & Material Defects 4 — Questions and Answers
Question 1: In API 510, the 'remaining life' of a pressure vessel is calculated by dividing which two values?
- Actual thickness by allowable stress
- Actual thickness minus required thickness by the corrosion rate (Correct answer)
- Corrosion allowance by design pressure
- Inspection interval by corrosion rate
Correct answer: Actual thickness minus required thickness by the corrosion rate
Remaining life = (actual measured thickness − required minimum thickness) ÷ corrosion rate, giving the estimated years before the vessel wall reaches the retirement thickness.
Question 2: Which form of corrosion attacks along grain boundaries of the metal, often caused by chromium depletion in stainless steels?
- Pitting corrosion
- Intergranular corrosion (Correct answer)
- Erosion corrosion
- Galvanic corrosion
Correct answer: Intergranular corrosion
Intergranular corrosion in stainless steels is typically caused by sensitization — chromium carbide precipitation at grain boundaries depletes chromium in adjacent zones, making them anodic relative to the grain interior.
Question 3: What is 'lamellar tearing' in the context of weld defects?
- A crack that forms in the weld metal during solidification
- A subsurface cracking in base metal parallel to the surface caused by through-thickness stresses (Correct answer)
- A surface crack caused by excessive heat input
- A defect formed by entrapment of welding flux
Correct answer: A subsurface cracking in base metal parallel to the surface caused by through-thickness stresses
Lamellar tearing occurs in rolled plate when through-thickness welding stresses exceed the low ductility of the plate in the short-transverse direction due to non-metallic inclusions aligned parallel to the plate surface.
Question 4: Which API 510 inspection approach involves using risk assessment to prioritize and set inspection intervals?
- Time-based inspection (TBI)
- Risk-based inspection (RBI) (Correct answer)
- Condition-based monitoring (CBM)
- Failure mode and effects analysis (FMEA)
Correct answer: Risk-based inspection (RBI)
Risk-based inspection (RBI) uses the probability and consequence of failure to establish inspection priorities and optimize intervals, allowing resources to be focused on higher-risk equipment.
Question 5: Erosion-corrosion in pressure vessels is characterized by accelerated metal loss caused by:
- Biological organisms attacking the metal surface
- Combined mechanical abrasion and corrosive attack from flowing fluid (Correct answer)
- Hydrogen diffusion into the steel lattice
- Selective leaching of alloying elements
Correct answer: Combined mechanical abrasion and corrosive attack from flowing fluid
Erosion-corrosion occurs when the mechanical action of a flowing fluid (especially with entrained solids or at turbulent zones) removes protective surface films, exposing fresh metal to corrosive attack in a continuous cycle.
Question 6: What does the term 'sensitization' refer to in austenitic stainless steels?
- Increased sensitivity to hydrogen embrittlement at low temperatures
- Chromium carbide precipitation at grain boundaries reducing corrosion resistance (Correct answer)
- Formation of sigma phase causing brittleness
- Increased susceptibility to pitting in chloride environments
Correct answer: Chromium carbide precipitation at grain boundaries reducing corrosion resistance
Sensitization occurs when austenitic stainless steel is heated to 800–1500°F (425–815°C), causing chromium to diffuse to grain boundaries and form chromium carbides, depleting adjacent zones of the chromium needed for corrosion resistance.
Question 7: An incomplete fusion defect in a weld occurs when:
- The weld metal fails to penetrate through the full joint thickness
- The weld metal does not bond completely to the base metal or previous weld pass (Correct answer)
- Slag is trapped between weld passes
- Excessive heat causes burning through the base metal
Correct answer: The weld metal does not bond completely to the base metal or previous weld pass
Incomplete fusion (lack of fusion) is a weld discontinuity where the weld metal fails to fuse completely with the base metal or adjacent weld beads, creating a planar unbonded interface that is highly detrimental under stress.
API 510 Corrosion, Welding & Material Defects 4
In API 510, the 'remaining life' of a pressure vessel is calculated by dividing which two values?