Page 467 - Corrosion Engineering Principles and Practice

434 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 435

aircraft in which rivets have been known to fracture due to the high
tensile stresses resulting from pillowing [4].
11.2.2 Multiple Material/Environment Combinations
There are a vast number of material/operational environment
combinations, and each can form the basis for potential corrosion
problems to occur. Of the 103 plus elements in a Periodic Table in App.
B, approximately 80 are metals, each possessing different mechanical,
chemical, and physical properties. Although all these metals can corrode,
they do it in many different manners. Furthermore, most of these metals
have been alloyed to make tens of thousands of different alloys.
From a purely technical standpoint, an obvious answer to
corrosion problems would be to use more resistant materials. In many
cases, this approach is an economical alternative to other corrosion
control methods. Table 11.1 lists the questions that should be answered
in order to estimate the corrosion behavior of materials either in
service or considered for such usage [5].
A common form of representing the corrosion resistance of
materials is what is known as isocorrosion diagrams. The use of the
prefix “iso” refers to lines (or regions) of constant corrosion behavior
across variations in concentration and temperature. The diagram
shown in Fig. 11.2 illustrates how some stainless steels compare to

• Corrosive Agent Variables
Main constituents (identity and amount)
Impurities (identity and amount)
Temperature
pH
Degree of aeration
Velocity or agitation
Pressure
Estimated range of each variable
• Type of Application
What is the function of part or equipment?
What effect will uniform corrosion have on serviceability?
Are size change, appearance or corrosion product a problem?
What effect will localized corrosion have on usefulness?
Will there be stresses present?
Is SCC a possibility?
Is design compatible with the corrosion characteristics of the material?
What is the desired service life?

TABLE 11.1 Information Required for Estimating the Corrosion Performance of
a Material