Hydrogen embrittlement relief
Hydrogen-relief heat treatment between 150 °C and 250 °C, immediately after galvanic coating, in ovens calibrated to API 6A.
Most electrochemical processes generate hydrogen in an ionic state, which diffuses into the metal and produces internal stress, embrittlement or hydride precipitation. For high-hardness fasteners, hydrogen embrittlement relief is what guarantees the part will not embrittle — and oven calibration is what makes that demonstrable in an audit.
What makes hydrogen embrittlement dangerous is not intensity: it is delay. The part leaves the bath approved, assembles at the right torque, passes inspection — and fails days or weeks later, under static load, without warning. The failure does not show in a dimensional test or a visual inspection, because it has not yet happened when the part is inspected.
Relief depends on three things, and the third is the one that gets forgotten. Temperature between 150 °C and 250 °C; time compatible with the hardness and thickness of the part; and the timing — immediately after galvanic coating. Hydrogen that has already diffused and lodged does not come out with the same treatment.
Oven calibration is what turns this into proof. A temperature uniformity survey shows that every point of the load reached the range, not only the control thermocouple — because the part that sat in the cold corner is exactly the one that will fail in the field. That is why the oil and gas standard asks for the survey, not the nominal temperature.
Technical data
- Temperature
- 150 °C to 250 °C
- Timing
- Immediately after galvanic coating
- Ovens
- Calibrated with a temperature uniformity survey (TUS)
What each standard sets for Hydrogen embrittlement relief.
The chip at the top of the page names the standard. This table says what it requires — which is what the specifier checks before closing the drawing.
| Standard | What it covers | What it sets here |
|---|---|---|
| API Specification 6A | Oven calibration | Wellhead equipment requirements, including the temperature uniformity of the furnace used in heat treatment. |
| ISO 10423 · 21st ed., Annex M | Temperature uniformity | The annex that defines the uniformity survey — it is the one that requires proving temperature across the whole load, not only at the control point. |
| CQI-9 · 3rd ed. | Heat treatment assessment | AIAG heat treatment process assessment. It governs furnace, instrument and record — and it is a heat treatment standard, not a plating one. |
| ASTM B849 | Stress relief, before the bath | Heat treatment prior to coating. The standard and the class come from the customer’s specification — it is the step Petrobras cites together with B850. |
| ASTM B850 | Hydrogen relief | Post-coating treatment guide to reduce the risk of embrittlement in high-strength steel. |
| ASTM F1940 | Qualification test | Process test to verify whether the fastener coating induces hydrogen embrittlement. |
The applications that come here.
High-hardness fasteners
Class 10.9 and above, and high-strength steel in general — the range where embrittlement stops being a hypothesis.
Wellhead equipment
Components under API Specification 6A, with the oven uniformity survey as part of the dossier.
Aerospace
Aircraft parts coated in low-embrittlement cadmium, where relief is part of the coating specification.
Automotive
Safety fasteners in a supply chain that runs under heat treatment assessment.
What specifiers usually ask.
What is hydrogen embrittlement?
It is the loss of steel ductility when atomic hydrogen diffuses into the metal and lodges in the microstructure. The part fails under a load well below its nominal strength, in a brittle way and with no prior deformation. In electroplating, the hydrogen comes from acid pickling and from electrodeposition itself.
When is hydrogen embrittlement relief mandatory?
For high-hardness steel fasteners and components — in practice, class 10.9 and above — whenever there is an electrochemical process or acid pickling. Several oil and gas and aerospace specifications require it in writing, with a maximum interval between the bath and the oven.
Why does the oven uniformity survey matter?
Because the control thermocouple measures one point, and the whole load must have reached the range. A part that sat in a colder region of the furnace did not get complete relief and is precisely the one that will fail in the field. The survey maps the chamber and is what the audit asks to see.
How soon after the bath must relief be done?
As soon as possible, and most specifications set a maximum interval. Hydrogen keeps diffusing and lodging while the part waits; past the point where it has settled into the microstructure, the same heat treatment no longer removes it all.