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

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?

Accepted Answer

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

Answer

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?

Accepted Answer

Chromium and molybdenum additions

Answer

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?

Accepted Answer

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

Answer

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?

Accepted Answer

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

Answer

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.

API 571 - API 571 Corrosion and Materials Practice Test

API 571 - API 571 Corrosion and Materials Practice Test

API 571 Hydrogen Damage