Page 39 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

LOCALIZED CORROSION 17

(i) Underground structures: the bottom of a metallic pipe buried in earth, with a
relatively limited surface of the metal poorly aerated, tends to become anodic
in relation to the large aerated surface of the rest of the metal.
(ii) Mill scale (rust) and pitting: three layers of iron oxide scales formed on steel
during rolling results in mill scale, which, when placed in an electrolyte, under-
goes corrosion. The defects in the mill scale act as anode with the remaining
area acting as a large cathode. An electric current is easily produced between
the steel and the mill scale leading to the corrosion of steel without affecting the
mill scale. A galvanic cell with an emf of 200–300 mV similar to copper/steel
couple is created.

Oxygen comes from the mill scale or diffuses from the air to the steel surface.
The coating breaks down because of the movement of the electrolyte, oxygen, and
moisture through the film to the pores. Water and gases passing through the mill scale
film dissolve ionic material and cause osmotic pressure. Water diffusion and visual
blistering occur. The osmotic pressure, thermal agitation, and vibration of the coating
film molecules lead to electroendosmatic gradient between the corroding area and the
protected areas in electrical contact.

1.4.2 Poultice Corrosion
This is a special case of localized corrosion because of differential aeration, which
usually appears as pitting. This form of corrosion occurs when an absorptive material
such as paper, wood, asbestos, sacking, cloth, is in contact with a metallic surface that
periodically becomes wet. Because of drying periods, adjacent wet and dry regions
develop. Near the edges of wet ones and because of limited amounts of dissolved
oxygen differential aeration cell develops, and this leads to pitting corrosion. An
example of this form of corrosion is the extensive damage of aluminum surface of
fuel tanks in aircraft because of bacterial and fungal growth in jet fuel. Design to
avoid contact of absorptive material with metallic surface or painting can prevent
poultice corrosion (19).

1.4.3 Crevice Corrosion

This form of corrosion occurs in the presence of stagnant corrosive solution near a
hole, under a deposit, or any geometric shape that can form a crevice. It is also known
as cavernous corrosion or underdeposit corrosion. This form of corrosion results
from a concentration cell formed between the electrolyte within the crevice, which is
oxygen starved, and the electrolyte present outside the crevice where plenty of
oxygen is present. The metal within the crevice acts as anode, and the metal outside
the crevice functions as the cathode. The difference in aeration produces a different
equilibrium potential, given by the Nernst equation applied to the reaction

+
−
1 ∕ 2O + 2H + 2e → H O
2 2