hydrogen embrittlement

A strong, tough steel bolt can shatter like glass hours or days after it was tightened, under a load it carried without complaint at first — with no obvious corrosion, no warning. The culprit is often hydrogen: individual hydrogen atoms, small enough to squeeze between the metal's own atoms, seep into the steel and rob it of its toughness, so it fails in a brittle way at stresses it should easily survive. It is a poisoning of the metal from the inside.

Where does the hydrogen come from? Atomic hydrogen is produced right at the metal surface by ordinary processes — acid pickling, electroplating, welding with damp electrodes, the cathodic reaction of corrosion itself, or over-aggressive cathodic protection. Being the smallest atom, it diffuses into the lattice and collects at high-stress spots, crack tips, and internal interfaces. Once concentrated there it weakens the atomic bonds and pins or tangles the dislocations that normally let the metal deform gently, so instead of bending the metal cracks. High-strength steels are the most vulnerable — roughly the harder and stronger the steel, the more prone it is — which is a cruel irony, because the strongest fasteners are exactly the ones that snap. Failure is typically delayed, appearing only after hydrogen has had time to migrate and pile up.

It is close cousin to stress-corrosion cracking and the two overlap, but the emphasis differs: here the villain is hydrogen entering the metal, not the metal dissolving away. Defenses aim at keeping hydrogen out or getting it back out: avoid or carefully control acid pickling and plating, use low-hydrogen welding practice and dry electrodes, and 'bake' plated high-strength parts at a couple hundred degrees C for hours to let the trapped hydrogen diffuse out before it does harm. Choosing a slightly lower-strength, tougher steel is often the surest cure.