API 571 Hydrogen Damage 1 — Questions and Answers
Question 1: What is the fundamental mechanism of High Temperature Hydrogen Attack (HTHA)?
- Surface hydrogen blistering from dissolved gas
- Atomic hydrogen reacts with carbides in steel to form methane, causing internal fissuring and decarburization (Correct answer)
- Hydrogen absorption causing brittleness at ambient temperature
- Galvanic acceleration of corrosion by hydrogen evolution
Correct answer: Atomic hydrogen reacts with carbides in steel to form methane, causing internal fissuring and decarburization
HTHA occurs when atomic hydrogen at high temperature and pressure reacts with iron carbides in steel, forming methane gas that becomes trapped internally, creating fissures and decarburization.
Question 2: API 941 Nelson curves define the safe operating limits for which type of equipment?
- Atmospheric storage tanks in humid environments
- Carbon and alloy steels operating in high-temperature, high-pressure hydrogen service (Correct answer)
- Stainless steel equipment in chloride-containing environments
- Fired heaters operating at high radiant heat flux
Correct answer: Carbon and alloy steels operating in high-temperature, high-pressure hydrogen service
API 941 provides Nelson curves defining safe temperature-pressure combinations for various steel grades to prevent HTHA in hydrogen-containing process streams.
Question 3: Which alloying additions most effectively extend Nelson curve limits and improve resistance to HTHA?
- Nickel and copper additions
- Chromium and molybdenum additions (Correct answer)
- Silicon and manganese additions
- Sulfur and phosphorus content
Correct answer: Chromium and molybdenum additions
Chromium and molybdenum form stable carbides that resist the hydrogen-carbon reaction, which is the fundamental mechanism of HTHA.
Question 4: Hydrogen embrittlement (HE) at ambient temperature primarily affects which material property?
- Permanently reduces yield strength of the material
- Reduces ductility and fracture toughness, enabling brittle fracture at low stress (Correct answer)
- Has no significant effect if loading rates are slow
- Only affects non-ferrous metals like copper and brass
Correct answer: Reduces ductility and fracture toughness, enabling brittle fracture at low stress
Absorbed hydrogen reduces toughness and ductility while having minimal effect on tensile strength, making hydrogen embrittlement insidious and dangerous.
Question 5: Hydrogen blistering in carbon steel typically occurs in which service environment?
- High-temperature service above 700°F in reformers
- Low-strength carbon steel in wet H2S environments at ambient to moderate temperature (Correct answer)
- High-alloy austenitic stainless steel in amine service
- Titanium equipment in acid service
Correct answer: Low-strength carbon steel in wet H2S environments at ambient to moderate temperature
Hydrogen blisters form in low-strength carbon steel when atomic hydrogen from H2S-promoted cathodic reactions diffuses into the steel and recombines at internal inclusions.
Question 6: What operational change most effectively reduces the risk of HTHA in process equipment?
- Increasing hydrogen partial pressure to purge the system
- Lowering operating temperature and/or hydrogen partial pressure below the Nelson curve (Correct answer)
- Adding H2S to the stream to suppress hydrogen activity
- Increasing fluid velocity through the equipment
Correct answer: Lowering operating temperature and/or hydrogen partial pressure below the Nelson curve
Operating below the applicable Nelson curve limit, by reducing temperature and/or hydrogen partial pressure, is the primary strategy for HTHA prevention.
API 571 Hydrogen Damage 1
What is the fundamental mechanism of High Temperature Hydrogen Attack (HTHA)?