Page 259 - Corrosion Engineering Principles and Practice

232 C h a p t e r 7 C o r r o s i o n F a i l u r e s , F a c t o r s , a n d C e l l s 233

FIGURE 7.24 Cracking of concrete cover in marine environment.

7.4.3 Differential Aeration: Oxygen Concentration Cells
The oxygen content of any solution ranks high on the list of factors
influencing the corrosion of iron and numerous other metals.
Elimination of oxygen by deaeration is a practical means of reducing
corrosion, as in the case of steam boilers which are operated with
deaerated feed water.
Differential aeration cells can be caused by crevices, lap joints,
dirt and debris, and moist insulation. Under these conditions, the
oxygen starved areas are anodic, while the areas with free access to
oxygen are cathodic. Common terms for this type of corrosion include
crevice corrosion, oxygen screening, and poultice action.
Oxygen not only enables a corrosion reaction by maintaining a
cathodic reaction, but it can promote one. This occurs where there is
a difference in the concentration of dissolved oxygen (DO) between
two points of the same metal surface. Since the direction of the
reaction is toward equilibrium, the only way that equilibrium can be
approached by corrosion will be to reduce the concentration of oxygen
where it is highest. Such reduction can be done by consuming the
oxygen as shown in Eq. (7.1).

O + 2H O + 4e → 4OH (7.1)
−
−
2
2
The end result is that whenever there is a difference in the
concentration of DO at two points on a metal surface, the surfaces in