Page 631 - Corrosion Engineering Principles and Practice

594 C h a p t e r 1 4 P r o t e c t i v e C o a t i n g s 595

(b)
FIGURE 14.3 (continued)

All of these obviously produce a more rapid failure of the coating and
decrease its service life.
In given conditions of humidity and oxygen permeation, the
initial corrosion site may begin to move in a random manner as the
corrosion product reduces the oxygen content at the surface and the
area becomes highly anodic to the surrounding cathode area of
oxygen saturation. The worm track of corrosion which then occurs is
termed filiform corrosion (Fig. 14.4).
When a coating is exposed to an aggressive environment, such
as that found in a marine application, the associated failure
mechanisms of blistering and delamination are amongst the most
important to consider. These two failure modes are often treated
separately. However, from a mechanistic point of view, these two
modes are quite similar and only differ in their degree of
aggressiveness [6].
Due to the presence of imperfections in the coating, the steel
substrate is directly exposed to its surroundings and may start
corroding as described in Eq. (14.1). In order to maintain electroneutrality
within the system, this reaction is balanced by at least one cathodic
reaction. In most naturally occurring situations, this reaction will be
the reduction of oxygen from ambient air, as illustrated in Eq. (14.2).