Stress corrosion cracking requires three variables to occur:
Applied or residual stresses
Elevated temperatures
An aqueous corrosive media (Chlorides and Hydrogen Sulphide are common)
Without all three factors, SCC cannot proceed. When they’re present, however, experts propose a range of models and mechanisms to explain stress corrosion cracking.
Commonly accepted models include
Adsorption model: Specific chemical species adsorbs on the crack surface and lowers the fracture stress.
Embrittlement model: Hydrogen embrittlement is a major mechanism of SCC for steels and other alloys such as titanium. Hydrogen atoms diffuse to the crack tip and embrittle the metal.
Pre-existing active path model: Pre-existing paths, such as grain boundaries where intermetallics and compounds are formed, are expanded and exaggerated leaving metal weaker.
Film rupture model: Stress ruptures the passive film locally and sets up an active-passive cell. The newly formed passive film is ruptured again under stress and the cycle continues until failure.
On top of all these variables, the environmental conditions required to encourage SCC will also vary by the metal or alloy in question.
High-strength steels can fall victim to hydrogen sulphides.
Austenitic stainless steels are particularly susceptible to hot, concentrated chloride solutions and chlorine-contaminated steam.
However, duplex steels -- with their blend of austenitic and ferritic metallurgic composition -- can typically withstand higher temperatures before succumbing to SCC attack. This makes them an excellent choice for use in high-temperature processes with a risk of SCC.
It’s also important to consider that the applied stress of a process or installation might not be the only stress in play when determining the overall risk for SCC in a given design.
Residual stress often plays a significant role in the development of SCC scenarios.