Page 174 - Corrosion Engineering Principles and Practice

148 C h a p t e r 6 R e c o g n i z i n g t h e F o r m s o f C o r r o s i o n 149

4. Velocity effects include erosion–corrosion, a form of attack
caused by high velocity flow; cavitation caused at even higher
flow by the collapse of bubbles formed at areas of low pressure
in a flowing stream; and fretting that is caused by vibratory
relative motion of two surfaces in close contact under load
(erosion–corrosion, cavitation, fretting).
5. Intergranular corrosion at the grain boundaries in the metal
structure (intergranular, exfoliation).
6. Dealloying corrosion due to the selective dissolution of one
component of an alloy.
Group III: Corrosion specimens for these types should usually be
verified by microscopy of one kind or another.
7. Cracking phenomena includes corrosion fatigue, a mechani-
cal phenomenon enhanced by nonspecific corrosive envi-
ronments, and environmental cracking, in which a brittle
failure is induced in an otherwise ductile material under
tensile stress in an environment specific for the alloy system
(stress corrosion cracking, fatigue).
8. High-temperature corrosion (scaling, internal attack).
9. Microbial effects caused by certain types of bacteria or mi-
crobes when their metabolism produces corrosive species in
an otherwise innocuous environment, or when they produce
deposits which can lead to corrosion attack.
In this widely distributed guide, Paul Dillon acknowledged
microbial effects as a ninth broad type of corrosion attack. It could also
be argued that many of the forms in the previous list are more families
or multiple forms of corrosion damage. It is obvious that pitting and
crevice corrosion, for example, are quite distinct in how they occur,
have very dissimilar triggering mechanisms, and would be prevented
by totally different methods. Pitting and crevice corrosion were indeed
treated as two distinct forms of corrosion by Fontana and Greene in
their 1967 manual and by many others since then.
Similarly the velocity effects group (erosion–corrosion, cavita-
tion, and fretting corrosion) is a relatively artificial arrangement of
corrosion types that are in fact quite different in appearance. So, if we
count the actual forms of corrosion in Dillon’s document, we have a
total closer to fifteen than to eight! If we add to these forms the ninth
form of corrosion discussed in Fontana and Greene, that is, hydrogen
damage either as blistering or embrittlement, we now have a total of
seventeen corrosion forms!
As Fontana indicated in 1967, “This listing is arbitrary but covers
practically all corrosion failures and problems.” As this statement
clearly acknowledged, the listing popularized by Fontana is an attempt
to summarize all known corrosion types in recognizable categories.