Technically reviewed by Vincent, Founder at HEXVARO
Last reviewed
Quick answer
Quick answer: how to reduce the risk
Avoid electroplating very hard fasteners where you can. Zinc flake coatings are the usual alternative for class 12.9 and other hard parts because they avoid acid cleaning and electroplating. If you must electroplate, specify baking and control the process.
| Fastener | Risk | Why |
|---|---|---|
| Class 10.9 and SAE Grade 8 | Moderate | Upper hardness range is close to 39 HRC |
| Class 12.9 | High | Hardness is about 39 HRC or more |
How hydrogen embrittlement happens
- Susceptible material: high hardness is the root cause.
- Hydrogen source: acid cleaning and electroplating add hydrogen, and corrosion can add more in service.
- Sustained tensile stress: tightening supplies the stress, and hydrogen moves to stress points over time.
Internal and environmental failures
Internal embrittlement comes from hydrogen trapped during processing. The supply is finite, so failures usually occur within 24 to 72 hours of tightening. Environmental embrittlement comes from corrosion in service. It can appear from a week to several years after installation.
Which fasteners are at risk
| Fastener | Risk | Why |
|---|---|---|
| Class 8.8 and SAE Grade 5 | Low | Hardness is well below the usual threshold |
| Class 10.9 and SAE Grade 8 | Moderate | Upper hardness range is close to 39 HRC |
| Class 12.9 | High | Hardness is about 39 HRC or more |
| ASTM A490 | Moderate to high | High strength, so hot-dip galvanizing is not allowed |
| Spring washers and hardened clips | High | Hard parts held under stress |
Coating choices
- Zinc flake: applied without acid cleaning or electroplating, and generally treated as free of added hydrogen.
- Mechanical zinc: an alternative where a zinc coating is required, but confirm the process with your supplier.
- Electroplated zinc: common and low cost, but it carries risk on hard parts.
- Hot-dip galvanizing: avoid it on class 12.9 and A490 parts.
- Plain with oil: a simple choice where the service environment is protected.
What baking does and does not do
Baking after plating drives out some of the hydrogen. ASTM F1941 sets relief requirements for coated quenched and tempered fasteners from 39 to 44 HRC, including a minimum bake of 14 hours. Zinc-plated through-hardened parts are commonly baked at 190 to 220 °C. Baking lowers the risk but does not remove it.
What to specify in an RFQ
- Property class and required hardness range.
- Coating type, and whether electroplating is acceptable.
- Baking requirement and test evidence, if plating is allowed.
- Service environment, including salt, acids and sour service.
Frequently asked questions
What is hydrogen embrittlement in bolts?
It is a brittle fracture caused by hydrogen and sustained stress in hard steel. The bolt can break without warning after it is tightened.
Can 12.9 bolts be zinc plated?
They can, but electroplating carries real risk. Zinc flake is the safer choice for most class 12.9 parts.
How long after tightening can a bolt fail?
Internal hydrogen failures usually occur within 24 to 72 hours. Corrosion-driven failures can take from a week to several years.
At what hardness does the risk begin?
Fasteners above about 39 HRC are generally treated as susceptible. Some applications set a limit of 35 HRC or lower.
Does baking remove all the risk?
No. Baking reduces the risk, but it does not remove it. Material, process control and service conditions still matter.
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