What initiates hydrogen stress cracking?

Hydrogen stress cracking, often referred to as hydrogen embrittlement, is primarily initiated by the diffusion of atomic hydrogen into steel. When atomic hydrogen penetrates the steel, it can enter the crystal lattice structure of the metal. This occurrence creates high internal stresses within the material, leading to a reduction in ductility and an increased likelihood of cracking under applied loads.

The atomic hydrogen generated from various sources such as electrochemical reactions, welding, or corrosion processes can accumulate in the steel, particularly in areas of high stress or weld zones. With sufficient hydrogen concentration, these internal stresses can overwhelm the material's ability to deform plastically, resulting in brittle fracture.

The other options do not initiate hydrogen stress cracking in the same way that atomic hydrogen does. For instance, while rust formation may affect the overall integrity of steel, it does not directly lead to the diffusion of atomic hydrogen required for embrittlement. Thermal stress can impact the structural integrity of materials but is unrelated to hydrogen ingress. Lastly, absorption of metal ions from the environment may affect corrosion resistance, but it does not directly relate to hydrogen stress cracking. Thus, the diffusion of atomic hydrogen is the correct mechanism that initiates this form of cracking.