Page 348 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

326 CONSEQUENCES OF CORROSION

Defect: departure Fault: inability of a
of a system from system to perform
specification according to
System
specification

Failure mode:
mechanism
leading to
failure
Failed state: state
of a system Failure: termination of
unable to perform the ability of a system
according to to perform according
specification to specification

Figure 5.1 Interrelation among defects, failures, and faults (16).

A universal representation describing the interactions between defects, faults, and
failures of a system is shown in Figure 5.1 (16):
The arrows in the figure imply that quantifiable relations possibly exist between
a defect, a fault, and a consequent failure. A defect in materials science is any
microstructural feature representing a disruption in the perfect periodic arrangement
of atoms in a crystalline material. The fundamental defects and their distribution in a
given material can have a significant impact on the overall properties of the material.
While such defects do not constitute flaws in the normal sense of the word they
can nonetheless serve as anchors for the initiation of actual faults and subsequent
failures. There are four fundamental defect types:

1. Point defects
2. Line defects
3. Planar and surface defects
4. Bulk defects

5.3.1 Point Defects

Point defects or sites are vacancies that are usually occupied by an atom, but are
presently unoccupied. If a neighboring atom moves to occupy the vacant site, the
vacancy moves in the opposite direction to the site that used to be occupied by
the moving atom. The stability of the surrounding crystal structure guarantees that
the neighboring atoms will not simply collapse around the vacancy. In some
materials, neighboring atoms actually move away from a vacancy, because they can
form better bonds with atoms in other directions.