API 510 Corrosion, Welding & Material Defects 2 — Questions and Answers
Question 1: Which type of corrosion occurs when two dissimilar metals are in electrical contact in the presence of an electrolyte?
- Crevice corrosion
- Galvanic corrosion (Correct answer)
- Erosion corrosion
- Pitting corrosion
Correct answer: Galvanic corrosion
Galvanic corrosion occurs when two metals with different electrochemical potentials are in contact with each other and an electrolyte, causing the more active (anodic) metal to corrode preferentially.
Question 2: What is the primary purpose of post-weld heat treatment (PWHT) on pressure vessel welds?
- To increase tensile strength of the weld metal
- To relieve residual stresses and improve toughness (Correct answer)
- To eliminate porosity in the weld
- To improve surface appearance of the weld
Correct answer: To relieve residual stresses and improve toughness
PWHT is performed primarily to relieve residual stresses induced during welding and to improve notch toughness, which reduces the risk of brittle fracture.
Question 3: Which NDE method is MOST effective for detecting subsurface cracks in ferromagnetic pressure vessel materials?
- Liquid penetrant testing (PT)
- Magnetic particle testing (MT) (Correct answer)
- Visual inspection (VT)
- Eddy current testing (ET)
Correct answer: Magnetic particle testing (MT)
Magnetic particle testing (MT) can detect both surface and near-subsurface discontinuities in ferromagnetic materials by revealing flux leakage at defect locations.
Question 4: Under API 510, what term describes corrosion that occurs uniformly over the entire internal surface of a pressure vessel?
- Localized corrosion
- General corrosion (Correct answer)
- Pitting corrosion
- Intergranular corrosion
Correct answer: General corrosion
General (uniform) corrosion attacks the exposed surface at a relatively uniform rate, making it predictable and easier to manage through corrosion allowance calculations.
Question 5: A weld discontinuity that is open to the surface and runs parallel to the weld axis is MOST likely a:
- Crater crack
- Longitudinal crack (Correct answer)
- Transverse crack
- Undercut
Correct answer: Longitudinal crack
A longitudinal crack runs parallel to the weld axis and can occur in the weld metal, heat-affected zone, or base metal, often due to hot cracking or hydrogen-induced cracking.
Question 6: What is the term for the selective removal of one element from an alloy by corrosion, such as zinc from brass?
- Erosion
- Dezincification (Correct answer)
- Pitting
- Intergranular attack
Correct answer: Dezincification
Dezincification is a form of dealloying where zinc is selectively leached from brass, leaving a porous, weakened copper-rich structure.
Question 7: Which welding defect is characterized by a groove melted into the base metal adjacent to the weld toe that is not filled by weld metal?
- Overlap
- Undercut (Correct answer)
- Incomplete fusion
- Porosity
Correct answer: Undercut
Undercut is a groove or channel burned into the base metal at the weld toe or root that is left unfilled, reducing the cross-sectional thickness of the base metal.
API 510 Corrosion, Welding & Material Defects 2
Which type of corrosion occurs when two dissimilar metals are in electrical contact in the presence of an electrolyte?