API 510 Corrosion, Welding & Material Defects 3 — Questions and Answers
Question 1: Stress corrosion cracking (SCC) in pressure vessels requires which three conditions to occur simultaneously?
- High temperature, high pressure, and low pH
- Susceptible material, tensile stress, and corrosive environment (Correct answer)
- Oxygen, moisture, and cyclic loading
- Dissimilar metals, electrolyte, and anodic current
Correct answer: Susceptible material, tensile stress, and corrosive environment
SCC requires a susceptible material, a specific corrosive environment (even in trace amounts), and sufficient tensile stress (residual or applied) — all three must be present simultaneously.
Question 2: According to API 510, what is the minimum required thickness formula variable 'C' used for?
- Corrosion rate in mpy
- Corrosion allowance added to calculated minimum thickness (Correct answer)
- Circumferential stress coefficient
- Coefficient of thermal expansion
Correct answer: Corrosion allowance added to calculated minimum thickness
In API 510 calculations, 'C' represents the corrosion allowance — the extra material thickness added to accommodate future corrosion over the vessel's remaining service life.
Question 3: Which type of cracking occurs in the heat-affected zone (HAZ) of welds in high-strength steels due to hydrogen embrittlement?
- Hot cracking
- Hydrogen-induced cracking (HIC) (Correct answer)
- Solidification cracking
- Lamellar tearing
Correct answer: Hydrogen-induced cracking (HIC)
Hydrogen-induced cracking (also called cold cracking or delayed cracking) occurs in the HAZ when atomic hydrogen diffuses into the steel lattice, and combined with residual stress, causes cracking after the weld cools.
Question 4: What type of corrosion is most commonly found in stagnant or low-flow areas, crevices, and under deposits on a pressure vessel?
- Galvanic corrosion
- Crevice corrosion (Correct answer)
- Erosion corrosion
- High-temperature oxidation
Correct answer: Crevice corrosion
Crevice corrosion occurs in confined spaces where stagnant solution becomes depleted in oxygen and enriched in corrosive ions, creating an aggressive local environment.
Question 5: Which weld defect is caused by gas pockets or voids trapped in the solidifying weld metal?
- Slag inclusion
- Porosity (Correct answer)
- Incomplete penetration
- Overlap
Correct answer: Porosity
Porosity consists of cavities or pores formed when gas (hydrogen, oxygen, or nitrogen) becomes trapped in the molten weld pool and cannot escape before solidification.
Question 6: High-temperature hydrogen attack (HTHA) in pressure vessels primarily affects which property of carbon steel?
- Yield strength
- Tensile strength and ductility through decarburization (Correct answer)
- Fatigue resistance
- Corrosion resistance
Correct answer: Tensile strength and ductility through decarburization
HTHA causes decarburization as atomic hydrogen reacts with carbon in the steel to form methane gas, leading to internal fissuring, blistering, and significant loss of tensile strength and ductility.
Question 7: A liquid penetrant test (PT) can ONLY detect which category of defects?
- Subsurface defects up to 1/4 inch deep
- Surface-open discontinuities (Correct answer)
- Internal volumetric defects
- Laminar flaws within the material
Correct answer: Surface-open discontinuities
Liquid penetrant testing (PT) relies on capillary action drawing penetrant into open surface discontinuities; it cannot detect defects that are not open to the test surface.
API 510 Corrosion, Welding & Material Defects 3
Stress corrosion cracking (SCC) in pressure vessels requires which three conditions to occur simultaneously?