Hydrogen Embrittlement in High-Strength Fasteners: What the Plating Line Controls
Hydrogen embrittlement is not a coating problem. It is a steel problem caused by the plating process. A bolt can pass every dimensional and coating inspection and still fracture in service days or weeks after installation, with no warning and no visible corrosion.
How hydrogen gets in
Pickling and electroplating generate atomic hydrogen at the steel surface. Most of it recombines and leaves as gas; a small fraction diffuses into the steel lattice and collects at grain boundaries, inclusions and high-stress regions. There it reduces ductility. Under sustained load — which is exactly the condition a tightened bolt lives in — a crack initiates and propagates.
Which parts are at risk
- Hardness above roughly 320 HV (about class 10.9 and above, spring steel, some case-hardened parts).
- Tensile strength above about 1,000 MPa.
- Parts under sustained tensile load: tightened fasteners, springs, retaining rings.
- Any part that has been stripped and replated more than once.
Low-carbon, low-strength parts are far less susceptible, which is why the same line can run for years without an incident and then fail when a higher grade is introduced.
The bake window
| Parameter | Typical practice |
|---|---|
| Start of bake | Within 1–4 hours of plating. The sooner the better; hydrogen keeps diffusing while the part sits wet. |
| Temperature | 190–220 °C for zinc and zinc-alloy coatings. Above about 230 °C, tempering of the base steel becomes a risk. |
| Time | 4 h minimum for thin coatings; 8–24 h for thick, high-hardness parts. Follow the customer specification where one exists. |
| Atmosphere | Air-circulating oven. Steam or inert atmospheres are used in some specifications but are not a substitute for time and temperature. |
Where the sealer fits
A sealer does not remove hydrogen and cannot repair an embrittled part. Its role is indirect:
- If the process route is seal → bake, the sealer must survive the bake. Otherwise the part is corrosion-protected in name only.
- If the route is bake → seal, the sealer must not require a cure temperature that undoes the bake, and the parts must be kept dry and clean in between.
- Sealers that need a high-temperature cure after a completed bake can re-introduce a thermal cycle the specification did not intend. Check the cure condition against the bake condition.
Verification
Process control is verified with a bake audit, not with a certificate:
- Thermocouple in a representative part, not in the oven air.
- Recorded time from plating to oven entry, and time at temperature.
- Load pattern recorded: parts per batch, position, and whether the batch was split.
- Periodic mechanical testing (ASTM F1940 or F519, or the customer's method) on a sampling basis.