⚡ Key Takeaways
- What it is: high-strength steel absorbs hydrogen atoms and then fails under sustained tensile stress — a delayed fracture that happens hours to weeks after assembly.
- Risk threshold: tensile strength ≥1000 MPa (roughly class 10.9 and above) or hardness ≥320 HV requires hydrogen embrittlement control.
- Three sources: acid pickling, electroplating (zinc or zinc-nickel) and phosphating.
- Standard test: GB/T 3098.17-2000, the preload test with parallel bearing faces — 200 hours without fracture.
- Prevention: bake within 4 hours of plating at 190–220°C for 4–8 hours.
- Alternative processes: Dacromet, Geomet and mechanical zinc plating, none of which involve electrolysis.
1. Why hydrogen embrittlement is so dangerous
2. Where the hydrogen comes from
3. The GB/T 3098.17 verification test
4. Prevention and control
5. Selection FAQ
1. Why hydrogen embrittlement is so dangerous
Hydrogen embrittlement (HE) is delayed fracture: the bolt is perfectly fine after assembly, then fails suddenly under static load a few hours to a few weeks later, with no visible plastic deformation.
Typical features:
- A flat, crystalline fracture face (intergranular failure);
- Occurs under tensile stress — compressive stress does not cause it;
- The harder the steel, the more sensitive it is: below 1000 MPa it rarely happens, above 1200 MPa the risk climbs steeply.
2. Where the hydrogen comes from
| Process | Risk | Why |
|---|---|---|
| Acid pickling | High | Acid reacts with iron and releases hydrogen atoms that diffuse into the steel |
| Zinc / zinc-nickel plating | High | Hydrogen evolves at the cathode during electrolysis |
| Phosphating | Medium | Acid attack plus reaction hydrogen |
| Welding / flame cutting | Medium | Moisture dissociates and releases hydrogen |
| Dacromet / Geomet | Low | Dip-spin plus curing, no electrolysis |
3. The GB/T 3098.17 verification test
Standard: GB/T 3098.17-2000 / ISO 15330, mechanical properties of fasteners — preload test for hydrogen embrittlement with parallel bearing faces.
Method:
- Run the sample into a dedicated fixture and apply the proof load (preload to 80% of yield);
- Hold it with parallel bearing faces for 200 hours;
- Inspect for fracture at intervals;
- Release the load after 200 hours and check for cracks.
Acceptance: no fracture within 200 hours, and after release the thread must run free with no visible cracks.
4. Prevention and control
4.1 Post-plating bake
- Into the oven within 4 hours of electroplating;
- 190–220°C for 4–8 hours, adjusted to hardness;
- The harder the part, the longer the bake.
4.2 Process substitution
- For high-strength parts, prefer Dacromet or Geomet — no electrolysis, no hydrogen;
- Stainless A2-70 and A4-80 are inherently less sensitive;
- Avoid long pickling in strong acid.
4.3 At the design stage
- Use class 8.8 where class 10.9 is not needed;
- Avoid abrupt changes in thread cross-section;
- Avoid re-plating — every rework cycle reintroduces hydrogen.
5. Selection FAQ
Q: Does a class 8.8 zinc-plated bolt need a bake? Generally not (hardness below 320 HV), but automotive safety parts should still be baked.
Q: Is Dacromet really hydrogen-free? The coating itself involves no electrolysis, but pre-treatment still uses a short pickle, so a bake remains advisable.
Q: How do you check bolts already in service? Non-destructive testing will not find it; only batch testing gives an answer.
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