API 510 Corrosion, Welding & Material Defects 5 — Questions and Answers
Question 1: What is the primary purpose of a corrosion allowance specified in the original design of a pressure vessel?
- To provide extra thickness for mechanical damage during installation
- To allow for predictable metal loss over the vessel's intended service life (Correct answer)
- To compensate for manufacturing tolerances in plate thickness
- To provide additional strength for pressure surges
Correct answer: To allow for predictable metal loss over the vessel's intended service life
The corrosion allowance is extra wall thickness deliberately included in the design to accommodate expected corrosion over the vessel's intended service life without falling below the calculated minimum required thickness.
Question 2: Which weld discontinuity is described as non-metallic material, such as flux or oxide, trapped within the weld metal?
- Porosity
- Slag inclusion (Correct answer)
- Cold lap
- Crater crack
Correct answer: Slag inclusion
Slag inclusions are non-metallic solid materials (flux residue, oxides) entrapped in the weld metal or between weld passes, typically caused by inadequate interpass cleaning or improper welding technique.
Question 3: Hydrogen blistering in pressure vessels typically occurs in which environment?
- High-temperature steam service above 750°F
- Wet hydrogen sulfide (H2S) or other sour service environments (Correct answer)
- Chloride-containing cooling water at ambient temperature
- Concentrated caustic (NaOH) at elevated temperatures
Correct answer: Wet hydrogen sulfide (H2S) or other sour service environments
Hydrogen blistering occurs in sour (wet H2S) service where atomic hydrogen generated by the sulfide corrosion reaction diffuses into the steel and combines at internal voids or laminations to form H2 gas, causing blisters.
Question 4: According to API 510, which organization's welding standards are typically referenced for repair welding procedures on pressure vessels?
- AWS D1.1
- ASME Section IX (Correct answer)
- API Standard 582
- NACE MR0175
Correct answer: ASME Section IX
API 510 specifies that repair welding must be performed in accordance with ASME Section IX, which governs welding and brazing qualifications including procedure and performance qualifications.
Question 5: What type of corrosion damage is characterized by small, deep, cup-shaped cavities on the metal surface while the surrounding metal remains relatively unattacked?
- General corrosion
- Crevice corrosion
- Pitting corrosion (Correct answer)
- Erosion corrosion
Correct answer: Pitting corrosion
Pitting corrosion creates localized, deep cavities (pits) on the metal surface while surrounding areas remain largely unaffected, making it particularly dangerous because the small surface area masks the true depth of attack.
Question 6: A weld with 'overlap' or 'cold lap' is a condition where:
- The weld metal extends beyond the weld groove but is not fused to the base metal (Correct answer)
- The weld metal overflows from the joint but bonds properly to the base metal
- Insufficient weld metal is deposited in the joint groove
- The weld bead surface is excessively convex
Correct answer: The weld metal extends beyond the weld groove but is not fused to the base metal
Overlap (cold lap) occurs when weld metal rolls over onto the base metal surface without fusing to it, creating a mechanical notch at the weld toe that acts as a stress concentrator.
Question 7: Which damage mechanism is associated with wet hydrogen sulfide service and involves crack propagation through steel driven by hydrogen pressure?
- Stress corrosion cracking (SCC)
- Hydrogen-induced cracking (HIC) (Correct answer)
- High-temperature hydrogen attack (HTHA)
- Caustic stress corrosion cracking
Correct answer: Hydrogen-induced cracking (HIC)
Hydrogen-induced cracking (HIC) in wet H2S service occurs when atomic hydrogen diffuses into the steel and recombines as molecular H2 at traps (inclusions, laminations), building pressure that causes internal cracks to initiate and propagate.
API 510 Corrosion, Welding & Material Defects 5
What is the primary purpose of a corrosion allowance specified in the original design of a pressure vessel?