API 571 Stress Corrosion Cracking 1 — Questions and Answers
Question 1: Stress corrosion cracking (SCC) requires which three factors to be simultaneously present?
- Heat, pressure, and dissolved oxygen
- Tensile stress, corrosive environment, and susceptible material (Correct answer)
- Compressive stress, high pH, and low temperature
- Fatigue loading, hydrogen gas, and alkaline conditions
Correct answer: Tensile stress, corrosive environment, and susceptible material
SCC initiates and propagates only when tensile stress, a specific corrosive environment, and a susceptible material are all present at the same time.
Question 2: Polythionic acid SCC (PTASCC) in refineries affects which material during which condition?
- Carbon steel exposed to caustic during normal operation
- Sensitized austenitic stainless steels during shutdowns in sulfur service (Correct answer)
- Duplex stainless steel in seawater during operation
- Nickel alloys at high operating temperature
Correct answer: Sensitized austenitic stainless steels during shutdowns in sulfur service
PTASCC occurs in sensitized stainless steels when polythionic acids form on surfaces during equipment shutdown and air exposure in sulfur-containing service.
Question 3: Caustic stress corrosion cracking of carbon steel is historically known by what other name?
- Hydrogen embrittlement
- Caustic embrittlement (Correct answer)
- Wet H2S cracking
- Amine SCC
Correct answer: Caustic embrittlement
Caustic embrittlement is the historical term for caustic SCC, first recognized in boiler steels exposed to concentrated sodium hydroxide at elevated temperatures.
Question 4: What post-weld treatment is most commonly applied to prevent stress corrosion cracking?
- Solution annealing at 2000°F
- Stress relief post-weld heat treatment (PWHT) (Correct answer)
- Normalizing
- Water quenching after welding
Correct answer: Stress relief post-weld heat treatment (PWHT)
Stress relief PWHT reduces residual welding stresses below the threshold required to drive SCC initiation in susceptible environments.
Question 5: Liquid metal embrittlement (LME) can occur when carbon steel or other metals contact which substances?
- Liquid mercury or molten zinc (Correct answer)
- Dilute hydrochloric acid
- Concentrated sodium hydroxide
- Hydrogen sulfide gas
Correct answer: Liquid mercury or molten zinc
LME occurs when susceptible metals contact certain liquid metals such as mercury or molten zinc, causing catastrophic brittle failure at very low applied stresses.
Question 6: Amine stress corrosion cracking in carbon steel is best prevented by which measure?
- Increasing amine concentration to reduce corrosivity
- Reducing operating temperature below 100°F
- PWHT of all carbon steel welds in amine service (Correct answer)
- Using plastic or FRP pipe liners
Correct answer: PWHT of all carbon steel welds in amine service
PWHT eliminates residual weld stresses, which are the primary driver of amine SCC in carbon steel, making it the standard industry prevention method.
API 571 Stress Corrosion Cracking 1
Stress corrosion cracking (SCC) requires which three factors to be simultaneously present?