Page 349 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

CORROSION DAMAGE, DEFECTS, AND FAILURES 327

5.3.2 Line Defects
Line defects are dislocations around which some of the atoms of the crystal lattice are
misaligned. There are two types of dislocations: the edge dislocation and the screw
dislocation. Dislocations are caused by the termination of a plane of atoms in the
middle of a crystal. In such a case, the surrounding planes are not straight, but instead
bend around the edge of the terminating plane so that the crystal structure is perfectly
ordered on either side.

5.3.3 Planar and Surface Defects
An important example of planar defect is the grain boundaries that occur where the
crystallographic direction of the lattice abruptly changes. This occurs commonly
when two crystals begin growing separately and then meet. Any types of corrosion to
be discussed later are directly related to the nature and geometry of grain boundary
structures.

5.3.4 Bulk Defects

An example of bulk defects are voids where there are simply no structural atoms.
Another example consists of impurities that can cluster together to form small regions
of a different phase such as precipitates. Impurities and precipitates also play an
important role in the corrosion resistance of metals.

5.3.5 Fault
The growth of a defect into what becomes a fault or a faulty component really depends
on many factors, which is predominantly the type of corrosion that is progressing. In
the fault-tree analysis context, the fault event of a component is defined as a state
transition from the normal state to a faulty state of that component. These state tran-
sitions are irreversible, which means that a faulty state does not return to the intended
state even if the influences that caused the fault event in the first place disappear.
Corrosion phenomena are irreversible by nature as they change a metal into more
stable oxidized states. In fact, the corrosion products can be converted into metals
only by complicated and energetically expensive processes that eventually result in
molten metals. However, not all corrosion processes lead to undesirable processes and
products if, for example, a corrosion allowance is included in the system at the design
stage. Some well-known examples of corrosion faults are encountered in electronic
components where even very small amounts of surface corrosion can drastically alter
the intended behavior of the components.

5.3.6 Connector Corrosion
Connector corrosion is well known and understood as a familiar problem that con-
tributes significantly to electrical wiring failures. Connector corrosion also plays a