API 571 Hydrogen Damage

Question 1

What is the fundamental mechanism of High Temperature Hydrogen Attack (HTHA)?

Accepted Answer

Atomic hydrogen reacts with carbides in steel to form methane, causing internal fissuring and decarburization

Answer

Surface hydrogen blistering from dissolved gas

Answer

Hydrogen absorption causing brittleness at ambient temperature

Answer

Galvanic acceleration of corrosion by hydrogen evolution

Question 2

API 941 Nelson curves define the safe operating limits for which type of equipment?

Accepted Answer

Carbon and alloy steels operating in high-temperature, high-pressure hydrogen service

Answer

Atmospheric storage tanks in humid environments

Answer

Stainless steel equipment in chloride-containing environments

Answer

Fired heaters operating at high radiant heat flux

Question 3

Which alloying additions most effectively extend Nelson curve limits and improve resistance to HTHA?

Accepted Answer

Chromium and molybdenum additions

Answer

Nickel and copper additions

Answer

Silicon and manganese additions

Answer

Sulfur and phosphorus content

Question 4

Hydrogen embrittlement (HE) at ambient temperature primarily affects which material property?

Accepted Answer

Reduces ductility and fracture toughness, enabling brittle fracture at low stress

Answer

Permanently reduces yield strength of the material

Answer

Has no significant effect if loading rates are slow

Answer

Only affects non-ferrous metals like copper and brass

Question 5

Hydrogen blistering in carbon steel typically occurs in which service environment?

Accepted Answer

Low-strength carbon steel in wet H2S environments at ambient to moderate temperature

Answer

High-temperature service above 700°F in reformers

Answer

High-alloy austenitic stainless steel in amine service

Answer

Titanium equipment in acid service

Question 6

What operational change most effectively reduces the risk of HTHA in process equipment?

Accepted Answer

Lowering operating temperature and/or hydrogen partial pressure below the Nelson curve

Answer

Increasing hydrogen partial pressure to purge the system

Answer

Adding H2S to the stream to suppress hydrogen activity

Answer

Increasing fluid velocity through the equipment