Page 471 - Corrosion Engineering Principles and Practice

438 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 439

corrosion test for all purposes. Corrosion data are also quite specific
to materials and environments where they were taken. The type of
corrosion and the rate at which it occurs may be highly influenced by
even small changes in a material composition/processing history or
by minor changes in the testing environment. For these reasons, the
most reliable predictor of practical performance is service experience,
followed closely by field testing, because both are based on the actual
environment and its complex variations.
Many investigators have traditionally avoided the use of statistical
techniques because the added reliability did not seem to offset the
effort and time required to become familiar with the methodology or
to perform the necessary calculations. Most often, the results must be
interpreted in terms of relative rankings. Such interpretations can be
subjective and depend much on the judgment of the investigator.

11.2.4 Complexity of Materials/Performance Interactions
Data may simply not be available for all potential forms of corrosion
that might pertain to a specific application/environment combina-
tion. Additionally, the inherent corrosion resistance of a material is
largely determined by its elemental composition, by its processing
history, surface morphology, geometry, and in some cases its size. The
following example illustrates how exfoliation corrosion cracking, a
particular form of intergranular corrosion associated with high-
strength aluminum alloys, may be minimized during the manufac-
turing process by accepting a deliberate materials loss.
Alloys that have been extruded or otherwise worked heavily, with
a microstructure of elongated, flattened grains, are particularly prone
to this damage. Figure 11.6 illustrates the anisotropic grain structure

LT

SL

ST
FIGURE 11.6 Schematic representation of the anisotropic grain structure of
wrought aluminum alloys.