High-temperature hydrogen attack: what it means for the inspector

How it works

At high temperature, hydrogen atoms can enter carbon and low-alloy steel and move through it. Inside, they react with the carbon in the steel, forming methane. Methane molecules do not diffuse out easily, so the gas collects at grain boundaries and builds up pressure. The result is tiny voids and then fissures, and in advanced cases visible cracks. The steel also loses strength and toughness in the damaged zone.

Because it needs heat, hydrogen at partial pressure and time, HTHA belongs to hot hydrogen service, as in hydroprocessing units. It is not the same as damage that happens at ambient or moderate temperatures. Steel chemistry matters: adding chromium and molybdenum improves resistance, and the question of which steels are safe in which conditions is handled in industry guidance rather than a rule of thumb, so use the source documents for specific limits.

Where it shows and how it is found

HTHA can appear in the base metal and at or near welds. The difficulty is that the early damage is below the surface.

• Surface methods are not enough. Visual examination, magnetic particle and penetrant testing work on the surface and will miss early internal damage.
• Advanced UT. Specialized ultrasonic techniques, applied by trained and qualified examiners, are the usual way to look for internal damage. Ordinary thickness readings do not find it, because the wall thickness has not changed.
• Process records. Knowing the temperature and hydrogen partial pressure history, and the steel's material, tells you where the risk is concentrated and which components need the specialised work.

Mix-ups to avoid

• HTHA versus hydrogen blistering. Blistering and stepwise cracking come from hydrogen in wet, low-temperature sour service, and they are covered under wet H2S damage. HTHA is hot and dry and starts as internal voids.
• HTHA versus sulfidation. Both are hot, but sulfidation removes metal from the surface and HTHA damages the metal inside.
• HTHA versus thinning. Thickness may be unchanged. A normal thickness reading does not clear a component of HTHA in hot hydrogen service.

On the API exam

API's effectivity sheet for September 2026 to May 2027 limits RP 571 questions to Section 2 plus 15 mechanism paragraphs. High-temperature hydrogen attack is RP 571 paragraph 3.36 and is one of the 15. Note that paragraph 3.35, high-temperature H2/H2S corrosion, is a separate entry on the sheet, so keep the two apart. Stems often hinge on the combination of hot hydrogen service, carbon or low-alloy steel, no obvious surface damage and a method that can see inside the wall. The standard teaches the damage forms and what finds them, so build your notes from RP 571 and RP 572 in your own copy.

Practice it

Check you can pick HTHA out from the other hydrogen-related mechanisms on the API 510 damage mechanism quiz. Compare it with Sulfidation and Wet H2S damage, or browse every term in the damage mechanism glossary.

Know the mechanisms? Check the rest of the Body of Knowledge

Take the free 10-question check. It is spread across the Body of Knowledge, needs no signup and explains every answer, so you see which domains to open first. The Long Seam course then adds a lesson for each domain, timed practice exams and a study plan from your exam date.

Sources

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• API 510 Exam Publications Effectivity Sheet, September 2026, January 2027 and May 2027, American Petroleum Institute
• API 510 Body of Knowledge, September 2026, January 2027 and May 2027, American Petroleum Institute
• API 510 Pressure Vessel Inspector: exam information, American Petroleum Institute
• API RP 571, Damage Mechanisms Affecting Fixed Equipment in the Refining Industry, 3rd edition, March 2020, paragraph 3.36. Cited by number only; read it in your own copy.