Stress-Corrosion Cracking

Stress corrosion is an environmentally induced cracking phenomenon that sometimes occurs when a metal is subjected to a tensile stress and a corrosive environment simultaneously. This is not to be confused with similar phenomena such as hydrogen embrittlement, in which the metal is embrittled by hydrogen, often resulting in the formation of cracks. Moreover, SCC is not defined as the cause of cracking that occurs when the surface of the metal is corroded resulting in the creation of a nucleating point for a crack. Rather, it is a synergistic effort of a corrosive agent and a modest, static stress. Another form of corrosion similar to SCC, although with a subtle difference, is corrosion fatigue, and is discussed in Section 2.9.1. The key difference is that SCC occurs with a static stress, while corrosion fatigue requires a dynamic or cyclic stress.

Stress corrosion cracking (SCC) is a process that takes place within the material, where the cracks propagate through the internal structure, usually leaving the surface unharmed. Furthermore, there are two main forms of SCC, intergranular and transgranular. For the intergranular form, the cracking progresses mostly along grain boundaries, whereas, in transgranular SCC, the cracking does not strictly adhere to the grain boundaries, instead it can penetrate grains. Most cracks tend to propagate in a direction that is perpendicular to the direction of applied stress. Aside from an applied mechanical stress, a residual, thermal, or welding stress along with the appropriate corrosive agent may also be sufficient to promote SCC. Pitting corrosion, especially in notch-sensitive metals, has been found to be one cause for the initiation of SCC.

SCC is a dangerous form of corrosion because it can be difficult to detect, and it can occur at stress levels which fall within the range that the metal is designed to handle. Furthermore, the mechanism of SCC is not well understood. There are a number of proposed mechanisms that attempt to explain the phenomenon of SCC, but none have done so with complete success. Figure 16 shows pictures of the two types of stress corrosion cracking.