Atomic hydrogen enters steel during acid pickling and electroplating, or later from corrosion in service, and collects at grain boundaries under stress. In fasteners harder than about 39 HRC it can cause sudden brittle fracture hours or days after tightening, usually at the head fillet or the first engaged thread, with a grainy intergranular fracture face. Prevention relies on avoiding electroplating on hard parts, relief baking and coatings such as zinc flake.
Three of the 12.9 screws popped their heads overnight with nobody touching them, that's hydrogen embrittlement until proven otherwise.
Heads popping off days after assembly, with no load change, is the classic hydrogen signature.
Also heard as
- HE
- hydrogen-assisted cracking
- delayed fracture
Do not mix it up with
- Fatigue: Fatigue needs repeated loading and leaves beach marks; hydrogen embrittlement can crack a statically loaded bolt with a brittle, grainy face.
Related terms
Bake
To heat plated high-strength fasteners soon after plating so absorbed hydrogen escapes before it cracks them.
Zinc plating
An electroplated zinc layer, usually with a clear, blue or yellow passivate, the cheapest common fastener finish.
Zinc flake
A baked-on coating of zinc and aluminum flakes in a binder, giving high corrosion resistance without plating hydrogen.
Grade 8
Shop shorthand for the strongest common inch bolt, and loosely for anything overbuilt; six lines on the head.
Fatigue
Cracking that grows a little with every load cycle until a bolt breaks far below its static strength.