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Fundamentals of Corrosion
7.4.4 Cathodic Delamination
When cathodic protection is applied to a coated metal, loss of adhesion
between the substrate and the paint film, adjacent to defects, often takes
place. This loss of adhesion is known as cathodic delamination. Such delamination can also occur in the absence of applied potential. Separate anodic
and cathodic reaction sites under the coating result in a driving force, as
during external polarization. The propagation of a blister due to cathodic
delamination under an undamaged coating on a steel substrate is illustrated
in Figure 7.3. Under an intact coating, corrosion may be initiated locally at
sites of poor adhesion.
A similar situation develops in the case of corrosion under a defective
coating. When there is a small defect in the coating, part of the substrate
is directly exposed to the corrosive environment. Corrosion products are
formed immediately that block the damaged site from oxygen. The defect in
Corrosion initiation
Blocking of pore
H 2 O
H 2 O
H 2 O
O 2
O 2
H 2 O
H 2 O
H 2 O
O 2
O 2
H 2 O
C
C
Anodic
Cathodic delamination
H 2 O
H 2 O
O 2
O 2
FigurE 7.3
Blister initiation and propagation under a defective coating (cathodic delamination).