What is hydrogen embrittlement and which materials are at risk?

• A. Hydrogen diffuses into metal causing embrittlement and cracking; high-strength steels and some nickel alloys are at risk.
• B. Hydrogen forms a protective oxide layer on aluminum alloys.
• C. It only affects cast iron at low temperatures.
• D. It increases toughness in copper alloys.

Answer: (A)

Hydrogen embrittlement is the phenomenon where hydrogen atoms diffuse into a metal’s lattice, weakening its cohesive bonds and making it more prone to cracking under tensile stress. Hydrogen moves through the metal and tends to gather at defects such as grain boundaries, dislocations, and inclusions, where it reduces the local bond strength. Under load, these hydrogen-rich regions become initiation sites for cracks that can propagate with little plastic deformation, leading to sudden, brittle failure.

Materials most at risk are those with high strength and more open diffusion paths for hydrogen, notably high-strength steels and some nickel-based alloys. In these metals, the combination of strong metallic bonding and the ease with which hydrogen can diffuse leads to a greater tendency for decohesion and crack growth under service conditions or during processing that introduces hydrogen.

Hydrogen embrittlement is not about forming a protective oxide layer, so aluminum alloys don’t gain protection from hydrogen in this context. It is not limited to cast iron or low temperatures, and it does not increase toughness in copper alloys; in fact, it generally makes metals more brittle and prone to cracking under stress.