SPED Materials Selection & Specification 3 — Questions and Answers
Question 1: The maximum allowable hardness for carbon and low-alloy steel in NACE MR0175 sour service is:
- HRC 22 (approximately 237 HBW) (Correct answer)
- HRC 30 (approximately 286 HBW)
- HRC 15 (approximately 170 HBW)
- HRC 40 (approximately 375 HBW)
Correct answer: HRC 22 (approximately 237 HBW)
NACE MR0175/ISO 15156 limits carbon and low-alloy steel hardness to HRC 22 (≈237 HBW) to prevent sulfide stress cracking.
Question 2: Galvanic corrosion risk is greatest when two dissimilar metals are:
- Separated by a large distance in dry air
- In electrical contact in the presence of an electrolyte (Correct answer)
- Both coated with the same organic paint system
- Operating above their respective transition temperatures
Correct answer: In electrical contact in the presence of an electrolyte
Galvanic corrosion requires both electrical continuity between the metals and an electrolyte to complete the electrochemical circuit.
Question 3: SA-335 P91 alloy steel pipe is primarily used in which service?
- Cryogenic service below −100°F
- High-temperature, high-pressure steam service above 1000°F (Correct answer)
- Concentrated sulfuric acid handling
- Seawater cooling systems
Correct answer: High-temperature, high-pressure steam service above 1000°F
SA-335 P91 (9Cr-1Mo-V) is a creep-resistant alloy specifically developed for high-temperature steam piping in power generation applications.
Question 4: Which type of corrosion involves selective dissolution of one component of an alloy, commonly seen in brass in ammonia service?
- Pitting corrosion
- Crevice corrosion
- Dealloying (dezincification) (Correct answer)
- Erosion-corrosion
Correct answer: Dealloying (dezincification)
Dezincification is the selective leaching of zinc from brass, leaving a porous copper matrix; it commonly occurs in stagnant or slow-moving water and in ammonia environments.
Question 5: When specifying a material for piping in hydrofluoric acid (HF) alkylation service, which alloy family is most commonly selected?
- Austenitic stainless steel (316L)
- Monel (Ni-Cu alloy, UNS N04400) (Correct answer)
- Carbon steel with aluminum lining
- Titanium Grade 2
Correct answer: Monel (Ni-Cu alloy, UNS N04400)
Monel alloys (Ni-Cu) are the industry-standard material for anhydrous HF service due to their excellent corrosion resistance to this highly aggressive acid.
Question 6: What does the 'L' designation signify in stainless steel grades such as 304L and 316L?
- Low temperature rated
- Low carbon content (≤0.030% C) to minimize sensitization (Correct answer)
- Longitudinally welded pipe
- Laser-inspected material
Correct answer: Low carbon content (≤0.030% C) to minimize sensitization
The 'L' grade has a maximum carbon content of 0.030%, which reduces chromium carbide precipitation at grain boundaries (sensitization) during welding.
Question 7: Erosion-corrosion damage in piping is most likely to occur at which location?
- Long straight horizontal runs with uniform flow
- Deadlegs and stagnant pockets
- Elbows, tees, and reducers with flow direction changes (Correct answer)
- Buried pipe sections with cathodic protection
Correct answer: Elbows, tees, and reducers with flow direction changes
Flow direction changes at elbows and tees create turbulence and impingement that strip protective oxide films, accelerating erosion-corrosion.
SPED Materials Selection & Specification 3
The maximum allowable hardness for carbon and low-alloy steel in NACE MR0175 sour service is: