Environmental Engineering Reference
In-Depth Information
Figure 3.12
Subsurface enlargement of pit in a type 321 stainless steel aircraft fresh-
water storage tank (95
).
nonmetal, tends to get corroded preferentially in a corrosive environment com-
pared to the area outside the crevice. This type of localized attack is known as
crevice corrosion
. Examples of crevices are lap joint, areas under bolts and rivets,
screw threads penetrating a metal or wood, area under a rubber gasket, and areas
under dirt or corrosion debris. The attachment of barnacles or other biofouling
organisms in marine applications provides crevice areas. The terms
gasket corro-
sion
and
deposit corrosion
are also used to describe crevice corrosion in the latter
cases.
Crevice corrosion may take place on any metal and in any corrosive environ-
ment. However, metals like aluminum and stainless steels, which depend on their
surface oxide film for corrosion resistance, are particularly prone to crevice corro-
sion, especially in environments containing chloride ions, such as seawater.
The gap defining a crevice is usually large enough for the entrapment of a
liquid but too small to permit flow of the liquid. The width is of the order of a
few thousandths of an inch, not exceeding 1/8 in. (3.18 mm).
3.5.1 Mechanism
To start with, the areas inside the crevice and outside it undergo corrosion in the
same manner in a corrosive environment. In a neutral chloride solution the anodic
dissolution is supported by the cathodic reduction of oxygen:
Anodic
M
→
M
n
n
e
(2.6)
