What initiates a crack formation in hydrogen stress corrosion cracking?

Crack formation in hydrogen stress corrosion cracking is primarily initiated by pressure accumulation around grain boundaries. This is a key factor because, during the corrosion process, hydrogen can diffuse into the metal, particularly at the grain boundaries, where it can lead to the buildup of internal pressure. This pressure can create localized stress concentrations that exceed the material's yield strength, resulting in the initiation of cracks.

The interaction of hydrogen with the metal at grain boundaries also disrupts the bonding between atoms, making it easier for cracks to form under tensile stress. Essentially, the accumulation of hydrogen at these critical locations weakens the integrity of the metal structure and makes it more susceptible to cracking under stress.

This understanding emphasizes the importance of controlling hydrogen embrittlement in alloys susceptible to stress corrosion cracking, particularly in environments where hydrogen is present or generated.