Page 634 - Corrosion Engineering Principles and Practice

596 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 597

blistering or delamination process taking place. This delay can be
partly explained by the time required to set up a steady-state diffusion
of oxygen, water, and associated ions through the film or along the
paint/metal interface [7].
The process starts with the initiation of blisters around the region
of the point of damage. These blisters become larger as time progresses
until they eventually coalesce. Once this has occurred, the delamination
process takes over and corrosion progresses rapidly. As the coating
delaminates from a fault, the pH at the interface behind it falls as the
process continues.
These processes occurring under the film may be cyclic in nature,
starting with an initiation time where there is no apparent activity.
Figure 14.5 illustrate such a step process coating failure as it gradually
occurred on a car door exposed to repeated winters of deicing salt
exposure.
Blistering and delamination act in conjunction with each other in
the failure of organic coatings. As blisters develop and grow, they
coalesce to form a large disbonded region in the vicinity of a fault in
the coating. This disbonded area provides the site for the delamination
process to initiate and propagate.

FIGURE 14.5
Beach marks
indicative of the
cyclic progress of
a car door coating
failure.