What causes stress corrosion cracking? Stress corrosion
cracking results from the conjoint action of three
components: (1) a susceptible material; (2) a specific
chemical species (environment) and (3) tensile stress. For
example, copper and its alloys are susceptible to ammonia
compounds, mild steels are susceptible to alkalis and
stainless steels are susceptible to chlorides. There is no
unified mechanism for stress corrosion cracking in the
literature. Various models have been proposed which include
the following: • Adsorption model: specific chemical species
adsorbs on the crack surface and lowers the fracture stress.
• Film rupture model: stress ruptures the passive film
locally and sets up an active-passive cell. Newly formed
passive film is ruptured again under stress and the cycle
continues until failure. • Pre-existing active path model:
Pre-existing path such as grain boundaries where
intermetallics and compounds are formed. • 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.
