HTHA is the damage mechanism that most tests the honesty of an inspection programme. It does not start as a crack, it does not thin the wall, and the methods that find it late are reliable while the methods that find it early are not.
At temperature and under hydrogen partial pressure, atomic hydrogen diffuses into steel and reacts with carbides to form methane. Methane molecules are far too large to diffuse back out, so they collect at grain boundaries and build pressure until they open microscopic voids.
That is the whole problem. The early damage is a population of cavities a few microns across, distributed through the thickness. It is not a crack. It removes no metal. It does not break the surface. Every routine inspection a plant performs is blind to it.
| Method | What happens |
|---|---|
| UT thickness | Full wall. HTHA removes no thickness. |
| Visual | Nothing. Damage is internal and microscopic. |
| MT / PT | Nothing until fissures reach the surface, which is very late. |
| Conventional UT flaw detection | Micro-cavities are far below the wavelength. No reflector to find. |
| Radiography | No density change worth imaging. |
By the time a method on that list finds something, the micro-fissures have linked into macro cracking. At that point the component is close to failure and the assessment is not really an assessment any more.
HTHA susceptibility is governed by temperature, hydrogen partial pressure and steel chemistry, and the industry has mapped that relationship for decades in the form of the Nelson curves published in API RP 941. Carbon steel is vulnerable at conditions where 1.25Cr or 2.25Cr steel is not.
Which makes positive material identification genuinely part of HTHA management. A carbon steel component substituted into an alloy line during a past repair is one of the most common routes to HTHA, and PMI finds it in seconds.
Equally, know your actual operating conditions rather than your design ones. Units get pushed. A reactor running fifteen degrees hotter than design for years may have crossed a curve nobody re-checked.
Grind, polish and etch the surface in place, lift the microstructure onto a film and examine it. This can show decarburisation and cavitation before there is anything a UT probe could detect.
The limit is severe: it examines the surface microstructure at the exact spot polished. HTHA is often worse mid-wall or at the process surface, so a clean replica from the outside says very little about the wall behind it.
This is where the specialist work is, and it is a combination rather than a single technique.
Used together by an experienced practitioner these detect HTHA earlier than anything else non-destructive. Used carelessly they generate false calls from ordinary microstructural variation, and both errors are expensive.
Once micro-fissures have linked, there is a real reflector. PAUT and TOFD then find and size it reliably — and by then you are sizing a crack for a Part 9 assessment and deciding whether to replace the component.
A negative HTHA inspection is not proof that HTHA is absent. Early-stage detection is genuinely difficult and the techniques have real limits of detection. Any vendor who tells you their method rules it out categorically is overselling. This is not a reflection on the practitioners — it is the nature of trying to detect a distributed population of micron-scale voids through 50 mm of steel.
Because inspection cannot reliably clear a component, HTHA is managed as much by engineering as by inspection:
Once HTHA has produced crack-like damage it is assessed under Part 9 like any other planar flaw. The difficulty is the input data: the assessment needs flaw dimensions and material toughness, and a component with diffuse hydrogen attack has degraded toughness that is hard to establish without destructive sampling.
In practice, confirmed HTHA in a pressure component usually leads to replacement rather than a fitness-for-service case for continued operation. That is a reasonable outcome rather than a failure of the standard: the uncertainty is genuinely large, and the consequence of being wrong is a hydrogen-filled vessel.
HTHA, hydrogen blistering and HIC compared in the library — what each does to the wall, and which method finds it.
Open the mechanism libraryWho writes this. A mechanical engineer with twelve years in oil and gas — in-line inspection, fired heater and furnace inspection, and pipeline integrity. What is here comes from the published standards and from what those years in the field actually looked like. It is not written by an API-certified inspector.
This is not an assessment. Nothing on this site may be used to justify a decision about real equipment. Assessing plant requires the current editions of the applicable codes, data from a licensed source, and a competent engineer who signs for the answer. · Integrity Field Guide