7.13.7T ypes of Corrosion underO rganic Coatings
For corrosiontotake place on ametal surfaceunder acoating, it is necessary
for an electrochemical double layer to be established. In order for this to take
place, it is necessary for the adhesion between the substrate and coating to be
broken. This permits aseparate thin water layer to be formed at the interface
from water that has permeated the coating. As previously mentioned,a ll
organic coatings are permeable to water to some extent. Water permeation
occurs underthe influence of various driving forces:
1. Ac oncentration gradient, e.g., during immersion or exposure to a
humidatmosphere,resulting in true diffusion through the polymer
2. Capillary forces in the coating because of poor curing, improper
solvent evaporation, entrapment of air during application, or bad
interaction between binder and additives
3. Osmosisb ecause of impurities or corrosionp roducts at the
interface between the metal and coating
H 2 OH 2 OH 2 OH 2 O
OH
–
OH
–
OH
–
OH
–
OH
–
Fe
Fe
2+
Fe
2+
C
Corrosion initiation
Permeation
AC
AC
CACA
C
Fe
OH
–
O 2
O 2
O 2 H 2 OH 2 O
O 2
H 2 OO 2
O 2
O 2
H 2 OH 2 O
O 2
H 2 O
H 2 OH 2 OH 2 O
H 2 O
H 2 O
FIGURE 7.24
Introduction of ablister under an intact coating.
Coatings
479
CAT8247—CHAPTER 7—17/10/2006—15:04—TRGANESH—XMLM ODEL B–pp. 403–485