API 571 High Temperature Corrosion 2 — Questions and Answers
Question 1: The Couper-Gorman curves extend McConomy curves by accounting for what additional factor?
- Effect of temperature on sulfidation
- Effect of hydrogen partial pressure on H2S corrosion rates (Correct answer)
- Effect of flow velocity on erosion-corrosion
- Effect of chlorides on alloy selection
Correct answer: Effect of hydrogen partial pressure on H2S corrosion rates
Couper-Gorman curves account for the accelerating effect of hydrogen partial pressure on H2S corrosion rates in hydrogen-containing environments.
Question 2: In which type of steel does sigma phase embrittlement occur, and at what temperature range?
- Carbon steels above 500°F
- Austenitic and duplex stainless steels between 600–1000°F (Correct answer)
- Low-alloy Cr-Mo steels in hydrogen service
- Aluminum alloys at any temperature
Correct answer: Austenitic and duplex stainless steels between 600–1000°F
Sigma phase forms in austenitic and duplex stainless steels during long-term exposure in the 600–1000°F range, causing severe embrittlement that manifests at ambient temperature.
Question 3: 885°F (475°C) embrittlement is a damage mechanism primarily affecting which steel family?
- Austenitic stainless steels
- Ferritic and duplex stainless steels (Correct answer)
- Carbon and low-alloy steels
- Nickel-base superalloys
Correct answer: Ferritic and duplex stainless steels
Ferritic and duplex stainless steels with high chromium content undergo alpha-prime phase precipitation near 885°F, causing severe ambient-temperature embrittlement.
Question 4: Which inspection method is most effective for detecting High Temperature Hydrogen Attack (HTHA)?
- Magnetic particle testing
- Dye penetrant testing
- Advanced ultrasonic testing (TOFD or phased array) (Correct answer)
- Hydrostatic pressure testing
Correct answer: Advanced ultrasonic testing (TOFD or phased array)
HTHA creates internal fissures and decarburization that require advanced volumetric UT methods like TOFD or phased array UT to reliably detect.
Question 5: Temper embrittlement in Cr-Mo steels occurs primarily during which condition?
- Rapid quenching from austenitizing temperature
- Slow cooling or extended hold in the 700–1050°F range (Correct answer)
- Welding at high heat input
- Service at temperatures above 1200°F
Correct answer: Slow cooling or extended hold in the 700–1050°F range
Temper embrittlement occurs when Cr-Mo steels are held or slowly cooled through the 700–1050°F range, causing tramp elements to segregate to grain boundaries and reduce toughness.
Question 6: Which of the following is NOT considered a high-temperature damage mechanism in API 571?
- Carburization
- Nitriding
- Chloride stress corrosion cracking (Correct answer)
- Fuel ash corrosion
Correct answer: Chloride stress corrosion cracking
Chloride SCC is a low-to-moderate temperature mechanism requiring liquid water and chlorides; it is not classified under high-temperature damage mechanisms in API 571.
API 571 High Temperature Corrosion 2
The Couper-Gorman curves extend McConomy curves by accounting for what additional factor?