Mechanism of SCC

The mechanism of SCC is a widely debated topic and several volumes have been written on this subject. A discussion of mechanisms is beyond the scope of this presentation. However, a summary of important mechanism is given below.

• The plastic deformation of a metal provides increased active sites on an active metal from which the passive has been dissolved. The active sites promote the propagation of cracks. A schematic of film rupture model is shown in the figure below.

• Cracks are promoted by repeater formation and rupture of a brittle film growing with the metal at the crack tip. The stress acts to open the crack and rupture the protective work. The freshly exposed metal dissolves rapidly resulting in crack extension.

• Adsorption model: It is referred generally as stress sorption model. It assumes the adsorption of environmental species lowers the interatomic bond strength and the stress required for cleavage fracture. The crack propagates continuously and is dependant on the rate of arrival of species at the crack tip (Petch and Uhlig). A picture of cleavage fracture in iron is shown below.

Typical cleavage fracture in iron.

A sample picture is presented below. The figure shows the rupture of the film on alloy surface by tensile forces and advancement of the crack.