Page 206 - Corrosion Engineering Principles and Practice

180 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 181

its physical appearance at the microscopic level is quite similar for
most systems (Figs. 6.16 and 6.17). The effects of this form of attack on
mechanical properties may be extremely harmful.
In these cases, the driving force is a difference in corrosion
potential that develops between the grain boundary and the bulk
material in adjacent grains. The difference in potential may be caused
by a difference in chemical composition between the two zones. This
can develop as a result of migration of impurity or alloying elements
in an alloy to the grain boundaries. If the concentration of alloying
elements in the grain boundary region becomes sufficient, a second
phase or constituent may separate or precipitate. This may have a
corrosion potential different from that of the grains (or matrix) and
cause a local cell to form.
The constituent may be anodic, cathodic, or neutral to the base
metal or adjacent zone. Examples of anodic constituents are the
intermetallic phases Mg Al and MgZn in aluminum alloys and Fe N
8
4
2
5
in iron alloys. Examples of cathodic constituents are FeAl and CuAl
3
2
in aluminum alloys and Fe C in iron alloys. Examples of neutral
3
constituents are Mg Si and MnAl in aluminum alloys and Mo C and
6
2
6
W C in wrought Ni-Cr-Mo alloys.
6
6.3.5 Dealloying
Another type of localized corrosion involves the selective removal by
corrosion of one of the elements of an alloy by either preferential
attack or by dissolution of the matrix material. The various kinds of
selective dissolution have been named after the alloy family that has
been affected, usually on the basis of the element dissolved (except in
the case of graphitic corrosion).
Dezincification
Dezincification refers to the selective leaching of the zinc phase in
alloys such as brasses that contain more than 15 percent Zn. The gross
appearance and size of a part that has suffered dezincification is often
unchanged except for the formation of a copper hue. The part,
however, will have become weak and embrittled, and therefore
subject to failure without warning. To the trained observer,
dezincification is readily recognized under the microscope, and even
with the unaided eye, because the red copper color is easily
distinguished from the yellow of brass.
Conditions generally conducive to dezincification include the
presence of an electrolyte (e.g., seawater), slightly acidic conditions,
presence of carbon dioxide or ammonia species, and appreciable
oxygen. There are two general types of dezincification. The most
common is layer dezincification that proceeds uniformly, as shown in
Fig. 6.35. The second type is referred to as the plug dezincification
and occurs at localized areas. The site of plugs may be recognized by
the presence above them of a deposit of brownish-white zinc-rich
corrosion product.