Stress Corrosion Cracking

Stress Corrosion Cracking (SCC) is a common issue with many specialty alloys. SCC is often associated with chlorides, but can also occur with caustics 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: an applied stress, an aqueous corrosive media, and an elevated temperature. Stress may be from an external source, such as that in a pressurized vessel, or an internal source, such as stresses from a weldment that has not been properly stress relieved. Temperatures typically need be in excess of 120oF for SCC in stainless steel.

When we consider chloride induced SCC in specialty alloys, the nickel content is a primary factor in the susceptibility of alloys. The following curve suggests the probability of an alloy to crack, based upon the nickel content of that alloy.

Carbon steel and ferritic stainless steels, with little to no nickel, do not experience Cl- SCC, nor do high nickel alloys (those with Ni over 30% are much less prone to cracking). Duplex stainless steels, because of the ferrite in their microstructure (see next figure), can be equally resistant. While austenite would permit SCC, the ferrite microstructure pins the cracks before they can propagate.

Alloys like AL-6XN®, with only 25% nickel, still do not typically experience SCC, due to the molybdenum content. Molybdenum, like high nickel, increases the resistance to SCC.

Another way to look at alloy performance in resistance to stress corrosion cracking, is to consider the temperature at which SCC begins. The following shows some of those temperatures for alloys we typically see associated with SCC.

316L should not be used when chloride concentrations exceed 1000 ppm. 22Cr (2205) duplex materials, in non-acidic environments, can tolerate chloride levels as high as 8-10,000 ppm. Super duplex materials, such as our Zeron® 100, and super austenitic alloys like Rolled Alloy’s AL-6XN®, can be used in environments with up to 50,000 ppm chlorides. Super duplex and super austenitic stainless steels are considered virtually immune to chloride stress cracking.

One note of caution: Chlorides are easily concentrated. They can increase many fold when the solvent liquid can evaporate and chlorides remain behind. Chlorides of a few hundred ppm routinely increase to several thousand; chlorides of a few thousand can rise to several tens of thousands. When measuring chlorides, measure those in the system as it operates, not the ppm in the in-coming fluids.

The following are macro and micro examples of typical SCC appearance.

When in doubt, call a Rolled Alloys metallurgical specialist, we can help. 1-800-521-0332 (ask for a metallurgist), or

- March 2017