Page 190 - Corrosion Engineering Principles and Practice

164 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 165

washers and gaskets that do not prevent the entry of liquid between
them and the metal surface. Crevices may proceed inward from a
surface exposed to air, or may exist in an immersed structure.
The series of events leading to the formation of a severe crevice
can be summarized in the following three stages. Firstly, crevice
corrosion is believed to initiate as the result of the differential
aeration mechanism mentioned earlier. Dissolved oxygen in the
liquid which is deep in the crevice is consumed by reaction with the
metal [Fig. 6.21(a)]. Secondly, as oxygen diffusion into the crevice is
restricted, a differential aeration cell tends to be set up between the

Metal

Crevice

Water

(a)

Metal

Cathode

Fe 2+

e –
Anode

(b)
FIGURE 6.21 Schematic description of the stages of a crevice formation:
(a) first stage; (b) second stage; (c) third stage.