Potential fastener problem

A. If high strength steels (and some other metals) are exposed to atomic hydrogen, and it is not promptly baked out, it invades the crystal lattice and embrittles it. The embrittlement will then be permanent, such that baking a week later or a month later will not restore the strength.

It is common to try to fracture 'notched samples' to see if hydrogen embrittlement is present.

Common sources include acid treatments, which by their nature, release hydrogen at the surface of the part, and electroplating processes, which do the same.

I don't believe that the effects of the hydrogen can be seen on an atomic level; and because the embrittlement can persist even after the hydrogen is gone, I don't think you can tell that way either. But the way you can tell is through the circumstantial evidence of a brittle fracture, and brittle fracture is observable. This is a plater talking, so more info may come if a metallurgist signs on :-)

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. Hydrogen embrittlement has been discussed many times on this site, so please review the archives for more information regarding the definition, mechanism, etc. With regards to microscopic examination, the usual features that are detected during fractography are regions of flat fracture characterized by brittle, intergranular fracture. These regions are often called "rock-candy" fracture due their appearance. Final fracture may still occur in a ductile overload manner when hydrogen embrittlement is involved, so care must be taken to evaluate the entire fracture surface. Scanning Electron Microscopy is best suited for fractography, due to the exceptional depth-of-field resolution compared to light optical microscopy.