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.