Page 466 - Corrosion Engineering Principles and Practice

432 C h a p t e r 1 1 M a t e r i a l s S e l e c t i o n , Te s t i n g , a n d D e s i g n C o n s i d e r a t i o n s 433

to be considered individually. The challenge that designers face is
balancing the degree of detail put forth in the analysis against their
available resources including time and budgetary concerns [2].

11.2 Complexity of Corrosion Conscious
Materials Selection
Unanticipated corrosion problems will always occur, and correcting
them may require a reactive approach using the service of a trained
specialist. However, corrosion should ideally be minimized by
proactively considering corrosion resistance early during system
design, when material selection has not been “locked in.” However,
because of its complexity, corrosion analysis is really more a practice
or discipline, such as reliability engineering, than a simple process of
designing fatigue resistance into a structure. The impact of improper
considerations given to corrosion is also similar to improper
consideration of reliability: unanticipated problems that result in
reduced readiness and increased life-cycle costs [2].
Unfortunately, materials selection is rarely taught to designers
with a focus on corrosion prevention and control. One might wonder
why a practical, design-based material selection process that considers
corrosion prevention and control has not already been developed.
The following list identifies some of the factors that contribute to the
difficulty in developing such a simplified, readily understood
corrosion conscious materials selection process.

11.2.1 Multiple Forms of Corrosion
There are multiple and often competing forms of corrosion to consider
for every design. As described previously in Chap. 6, corrosion
damage can take many forms that may be triggered, as explained and
illustrated in Chap. 7, by often relatively complex invisible cells. One
commonly encountered situation is the stress corrosion cracking
(SCC) or the corrosion fatigue of a component following the formation
of corrosion pits in the stressed area. In such cases the initiation of a
corrosion process is governed by a pitting mechanism while the
subsequent propagation phase is governed by a cracking mode.
Another common occurrence of multiple forms of corrosion is
what is called pack rust in civil engineering circles and pillowing in
aerospace engineering. Both types of corrosion attack start by the
formation of a special crevice corrosion environment that is often
exacerbated by successive wetting and drying cycles. The end result
is a continuous buildup of internal stresses that were never considered
during the initial design. Forces as high as 70 kPa have been measured
in the case of pack rust affecting steel components [3]. The buildup of
voluminous corrosion products on aluminum parts may also lead to
an undesirable increase in stress levels near critical fastener holes in