11612Mechanism of SCC Growth

SCC is currently thought to occur by one of three possible mechanisms.

Preexisting Active Path Mechanisms This mechanism presupposes the existence of regions within the metallurgical structure that are chemically reactive and susceptible to corrosion, for example, grain boundaries and elemental depleted zones in heat-treated materials. This can result in anodic dissolution at such active sites. The importance of stress in such instances is twofold: first, that it ensures the crack remains open and, second, it can result in the fracture of an existing protective surface film.

Film-Induced Cleavage This mechanism is based on the formation of a brittle film, which results from corrosion reactions, and its subsequent fracture. Once the film is broken, localized corrosion, that is, dissolution may occur allowing the crack to propagate for a short distance before arresting due to repassivation. This mechanism may repeat itself, allowing continued crack growth.

Adsorption-Related Mechanisms These mechanisms rely heavily on the stress component and associate crack propagation with a decrease in the integrity, in the region of the crack tip, due to the adsorption of chemical species. The most common failure type being that of hydrogen embrittlement, this process is discussed in more detail later.

0 0

Post a comment