Page 40 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

18 INTRODUCTION AND FORMS OF CORROSION

Using the activities of the dissolved species in water and assuming water activity
to be 1, we can write

0
+ 2
E = E + 0.0592 log (0 ) 1∕2 (H ) ∕H O
2
2
2
∘
= 1.23–0.0592 pH (volts at 25 C)
Thus localized corrosion ensues because of the difference in potential, which is
in turn a result of the difference in oxygen concentration. The more aerated surfaces
act as cathodes because of their more noble potential. Differences in metal ion con-
centrations can result in localized corrosion where the crevice rich in ions acts as the
cathode.
Crevices result by design or accident. Crevices by design occur at gaskets, flanges,
rubber O-rings, washers, bolt holes, rolled tube ends, threaded joints, riveted seams,
overlapping screen wires, lap joints, underneath coatings (filiform corrosion), or insu-
lation (poultice corrosion) and any point where close-fitting surfaces are present (9).
Oxygen differential cells can be established between oxygenated seawater outside
or at the opening of the crevice surfaces such as inside crevice anodic areas. It is nec-
essary that the crevice is large enough to allow the entry of the corrosive solution and
narrow enough to form a stagnant state and hold the solution with the desired corro-
sive properties. The opening of the crevice is generally of the order of 50–200 μm.
The narrow space present between two metals or a metal and a nonmetal is a favor-
able site for crevice corrosion. The hydrolysis reaction of iron and acid formation
within the crevice produces change in pH and chloride concentration in the crevice
environment. The space between the two materials is less aired, has a weak surface,
and contains a concentrated salt solution. In some special cases such as in magnesium
and magnesium alloys, crevice corrosion is initiated by hydrolysis reaction and acid
formation, and oxygen does not play the conventional major role (14).
Crevice corrosion and pitting corrosion are similar in that both involve autocat-
alytic propagation. The two modes differ in causes of initiation; the morphology and
the degree of penetration of pitting are different (17).

1.4.4 Filiform Corrosion

This is a special form of filamentary corrosion occurring on metallic surfaces and
is related to crevice corrosion also known as underfilm corrosion. This form of cor-
rosion is generally apparent under painted body of some used cars. It appears as a
blister under the paint. The filament propagation underfilm may appear split or joined
together, as they propagate in direct lines, some of them reflecting because of obsta-
cles such as adhesive parts of the organic film to the substrate that become trapped in
a very narrow place (17).
The filament occurs on metals covered by an organic film and because of a cer-
tain discontinuity in the film, air, and water penetrate through the coating and reach
the underlying metal. This humid layer becomes saturated or rich in corrosive ions
from soluble salts and forms a zone known as the active head of the filament. The