Hydrogen Embrittlement: Damage Mechanism of the Week
Hydrogen embrittlement explained: how atomic hydrogen cracks hard or high-strength steel, its sources, why cooldown matters, and how to prevent it.
By Syed Hasan3 Nov 20263 min read
Week 12 of our Damage Mechanism of the Week series covers a mechanism that links several others. Hydrogen embrittlement is the loss of ductility caused by atomic hydrogen inside steel. It is behind cold cracking of welds, cracking of hard welds in sour service, and the careful cooldown procedures used on hydroprocessing reactors.
Short answer: Atomic hydrogen dissolved in steel makes it less ductile, especially near ambient temperature. With enough tensile stress and a susceptible, hard or high-strength material, cracks form. Hydrogen comes from service (wet H2S, HF, hot hydrogen), from welding, from overprotective cathodic protection or from plating. Prevention combines low-hydrogen welding, PWHT and hardness control, hydrogen bake-out before repair welding, and slow, controlled cooldowns that let hydrogen escape.
At a glance
Three ingredients
Why it often appears during cooldown
Hydrogen diffuses out of steel faster at moderate temperatures and is most damaging near ambient. Equipment in hydrogen-rich service can hold a lot of dissolved hydrogen at operating temperature. If it is cooled and pressurised quickly, that hydrogen is trapped while the steel becomes most sensitive. That is why hydroprocessing units use controlled shutdown sequences, holding at intermediate temperatures, before cooling and depressurising fully.
Where to watch
- Welds and heat-affected zones with high hardness, especially in sour or HF service; see wet H2S damage.
- High-strength bolting and components.
- Repair welds on equipment that has been in hydrogen service.
- Cathodically protected high-strength steels, where overprotection generates hydrogen.
Preventing it
- Low-hydrogen welding: dry consumables, preheat where required, and controlled interpass temperatures.
- PWHT and hardness limits on welds in susceptible service.
- Hydrogen bake-out before repair welding on equipment from hydrogen service.
- Controlled cooldown and depressurisation procedures.
- Material selection: avoid unnecessarily high-strength steels where hydrogen is present.
Learn more with EIM
Hydrogen embrittlement is one of the damage mechanisms in API RP 571 (3rd edition), the body of knowledge for the API 571 certification. EIM is an independent training provider; API certifies inspectors. EIM's self-paced API 571 preparation includes a question bank, practice mode with explanations, timed mock tests, study notes and flashcards, with one year of access, and EIM's live API 571 Damage Mechanisms - Exam Preparation course with Mr. Deepak Rawtal runs weekend batches. See the API 571 page This post is part of our weekly Damage Mechanism of the Week series.
Frequently asked questions
What is hydrogen embrittlement?
Loss of ductility in steel caused by atomic hydrogen, which can lead to cracking under tensile stress.
Which materials are most susceptible?
Higher-strength steels and hard welds or heat-affected zones.
Where does the hydrogen come from?
From service (wet H2S, HF, high-temperature hydrogen), welding with moist consumables, cathodic protection overprotection and electroplating.
Why does it matter at shutdown?
Steel is most sensitive near ambient temperature, so cooling quickly while still holding hydrogen increases the risk.
What is hydrogen bake-out?
Heating equipment to drive dissolved hydrogen out of the steel, often before repair welding.
Sources
- API RP 571, 3rd edition: hydrogen embrittlement. Summarised, not quoted.
- Checked 7 October 2026.
