Stress Corrosion Cracking

Stress Corrosion Cracking (SCC) is a cracking phenomenon that is crucial to consider when working with specialty alloys.

SCC is often associated with chlorides but can also occur with exposure to caustic and other corrosive media. The most common type of stress corrosion cracking in stainless steels occurs in chloride environments, which will be the focus of this discussion.

Three underlying factors must be present to cause SCC:

1) Stress

2) Aqueous Corrosive Media

3) Elevated Temperature

Stress could be from an external source, such as a pressurized vessel, or an internal source, such as a weldment containing residual stresses. Temperatures typically need to be higher than 120°F for SCC to occur in stainless steel. Under certain conditions, SCC can occur at much lower temperatures.

When we consider chloride-induced SCC in specialty alloys, the nickel content is one of the primary indicators of an alloy’s susceptibility. Below is a diagram called the Copson Curve, which plots an alloy’s nickel content on the x-axis and the probability of succumbing to SCC on the y-axis. This diagram shows that the more nickel in an alloy’s composition, the less susceptible it is to SCC. Additionally, on the far left of the Copson Curve, in the region of alloys containing little nickel, ferritic stainless steels and duplex stainless steels are shown to be far less susceptible to SCC than austenitic stainless steels. The reason for this is the ferrite in their microstructure. Ferrite effectively pins SCC before it can propagate into the base material.