Page 483 - Corrosion Engineering Principles and Practice

450 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 451

For instance, the buildup of scale within storage tanks and piping
may result in situations where corrosive agents can accumulate and
accelerate a very vicious hidden corrosion process. One particular
example of such conditions that became extremely important after
the energy crisis of the 1970s is what is called corrosion under
insulation (CUI). However, the same problem exist in maybe a less
severe fashion for structural details used in the construction of a
system including lap joints, gaskets, and enclosed places that cannot
be accessed.
Designers should thoroughly analyze the operating conditions
the entire system will be subjected to and determine the potential
combinations of design details, temperature, humidity, and chemical
exposure that may be present at all points. The designer should also
consider the conditions experienced by a system throughout its life-
cycle including maintenance, storage, and transportation.
A comprehensive analysis of the operating environment can turn
out to be an extensive undertaking. Therefore, depending upon the
time and resources available, the extent of the analysis could range
from general to detailed using a tiered approach. It is most important
to first define the general operating conditions (e.g., atmospheric, im-
mersed in liquid, buried, and so forth) which can be further defined
with more precision as the information becomes available. A detailed
environmental assessment could include specifics on pH, pollutants
present, temperature, and other relevant factors. Design details should
not be overlooked since they certainly can initiate the microenviron-
ments responsible for triggering specific corrosion modes.

11.4.4 Evaluate Materials Based on Potential Corrosion
Failure Modes
Once the candidate materials and the environmental conditions have
been determined, the analyst must investigate the potential for one or
more of the many forms of corrosion to become active. At this point
in the process the designer should have at least one candidate material
to consider and a listing of potential corrosion modes in the anticipated
environmental conditions. Since the scope of the analysis has been
defined, one can assess whether any of the potential forms of corrosion
may become active when the candidate material is subjected to the
operational environment.
The most cost-economical way to investigate both of these issues
is to perform an analysis of the literature followed by carrying a test
program if facilities are available and time permits. Following the
completion of the literature analysis and/or testing program one
should have adequate information to reject any problematic materials,
thus yielding a listing of candidate materials that meet performance
requirements while simultaneously possessing acceptable corrosion
resistance.