What Does Film-Induced Cleavage Mean?
Film-induced cleavage is a mechanism of stress corrosion cracking (SCC). It is one of the most common anodic mechanisms of SCC failure.
Cracks usually propagate by alternate film growth and (brittle) film fracture, followed by rapid film formation over exposed metal.
Corrosionpedia Explains Film-Induced Cleavage
In film-induced cleavage, there is no special property of the face-centered cubic substrate through which the crack jumps; the special property lies in the film itself, which must be able to trigger a crack with a rate of hundreds of meters per second within less than 100 nm.
In this mechanism, stress corrosion cracks initiated in a brittle surface film may propagate (over a microscopic distance) into underlying more ductile material, before being arrested by ductile blunting of the crack tip. If the brittle film reforms over the blunted crack tip (under the influence of corrosion processes), the process can be repeated. The brittle films that are best-established as causing film-induced cleavage are de-alloyed layers (such as in brass). The film-induced cleavage process normally causes a transgranular fracture.
The process of film-induced cleavage can be described as:
- The walls and tip of the crack are covered by a brittle film (either an oxide film or a de-alloyed layer).
- The film at the crack tip is ruptured by the plastic strain.
- The brittle crack moves from the film into the matrix.
- The crack is blunted by plastic strain.
- For the crack to grow further, the surface film must reform at the crack tip surface.
This cycle of crack propagation and blunting repeats itself, causing crack advancement.