Environmental Engineering Reference
In-Depth Information
Figure 3.23
Intergranular corrosion and its absence respectively, in a pump impeller
in sensitized (right) and solution-annealed (left) condition.
3.7 SELECTIVE LEACHING
Selective leaching is the removal of one of the components of the alloy by corro-
sion. It is usually the less noble component, e.g., zinc in brasses. The selective
removal of zinc from brass is known as
dezincification
, which is the most com-
mon example of selective leaching. Preferential dissolution of iron from gray
cast iron has been named
graphitic corrosion
. The phenomenon also takes place
in some other alloy systems. Selective leaching is the general term to describe
these processes. Table 3.2 lists the combinations of alloys and environments
where selective leaching is encountered, some of which belong to high-tempera-
ture corrosion.
3.7.1 Dezincification
Dezincification occurs in brasses containing more than 15% zinc. It may take
place either uniformly all over the surface, called
layer type
, or in localized area
on the metal surface, called
plug type
(Fig. 3.24). Both are easily recognized by
the naked eye because of the coppery appearance of the attacked areas in contrast
to the normal yellow color of brasses. Sections of the affected areas, viewed
under microscope, reveal porous or spongy structure. The strength of the material
is impaired and the layer-type attack may lead to rupture of pipe with an increase
in water pressure; the plug type may lead to hole formation.
The layer-type attack is more prevalent in alloys of high zinc content; low-
zinc-containing alloys favor plug-type attack, although exceptions have been re-
ported. The chemistry of the environment seems to have a greater effect in de-
termining the type of attack. Uniform attack is produced by slightly acidic water,
