What Is Hydrogen Embrittlement and Why Do High-Strength Bolts Suddenly Fracture?

Have you ever seen a batch of new bolts sit in storage for months, then suddenly fracture during installation — with a flat, shiny fracture surface and no obvious plastic deformation? That's hydrogen embrittlement.

Three Necessary Conditions

Hydrogen embrittlement requires all three:

1. High-strength material: Typically above 1000 MPa tensile (HRC 32+). Class 8.9 and below are generally safe; classes 10.9 and 12.9 are most at risk.

2. Hydrogen source: Electroplating (zinc, cadmium), pickling, welding, or environment (H2S, moisture).

3. Tensile stress: Including residual stress from preload and external working loads.

Why It Delays Fracture

Hydrogen atoms diffuse into the steel and accumulate at stress concentration sites (thread roots). This process takes time — hours to months. When hydrogen concentration reaches a critical threshold, cracks initiate and propagate suddenly, causing fracture at loads far below the rated strength.

Prevention

1. Bake immediately after plating: Within 4 hours after electroplating, bake at 190–220°C for 4–24 hours to diffuse out hydrogen.

2. Use lower strength if possible: Class 8.9 instead of 10.9.

3. Use mechanical plating: Zn-flake coatings (Dacromet/Geomet) don't introduce hydrogen.

4. Avoid acid pickling for high-strength parts.

5. Control preload: Higher preload means higher embrittlement risk.

Common Misconceptions

• "The bolt broke because of poor quality" — it may be hydrogen embrittlement, related to plating process
• "New bolts are safe" — embrittlement can develop during storage
• "Stainless steel is immune" — 17-4 PH and precipitation hardening grades are susceptible

Toprecision high-strength bolts use Zn-Ni flake coating (hydrogen-free process) with documented bake-out records.