Page 473 - Corrosion Engineering Principles and Practice

440 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 441

11.3 Selection Compromises
One common reason designers pay little attention to corrosion
prevention and control (CPAC) is that they have minimal experience
in considering corrosion issues. Engineers must not only balance
potential benefits against other performance parameters, but must
also consider the longer-term attributes such as service life, anticipated
maintenance and repair, and disposal.
As it is becoming obvious, selecting the best material, from a
corrosion standpoint may not be a simple task. Numerous factors
need to be considered and there is no single path to making an
informed selection. Ultimately, designers must rely upon their best
engineering judgment to select the optimum material, considering
corrosion resistance in balance with other performance requirements,
as well as other tangible factors, including cost, availability, and
maintainability.
The choice of a material is the result of several compromises. For
example, the technical appraisal of an alloy will generally be a com-
promise between corrosion resistance and some other properties such
as strength and weldability. And the final selection will be a compro-
mise between technical competence and economical considerations.
In specifying a material, the task usually requires three stages:
1. Listing the requirements
2. Selecting and evaluating the candidate materials
3. Choosing the most economical material
Some particular requirements and typical selection consider-
ations are presented in Table 11.2. The material selection process is
also influenced by the fact that the materials are either considered for
the construction of a new system, or for the modification or repairs in
an existing facility. For the construction of new equipment, the selec-
tion procedure should begin as soon as possible and definitively
much before the design is finalized since the optimum design for cor-
rosion resistance may greatly affect how the system will be assembled
or constructed.
In a repair application, there is usually less opportunity for
redesign, and the main decision factors will be centered on delivery
time and ease of fabrication in the field. It is also advisable to estimate
the remaining life of the equipment so that the repair is not grossly
overdesigned in terms of its corrosion allowance.
It is important to realize that one will seldom find the perfect
choice, since most affordable commercial materials will corrode
under some particular conditions. Instead, designers should identify
the material and associated corrosion prevention and control practices
that meet budgetary constraints specific to each program.