What causes environmental cracking? Environmental 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
environmental 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 environmental cracking for steels and
other alloys such as titanium. Hydrogen atoms diffuse to the
crack p and embrittle the metal.
