Page 219 - Handbook of Adhesion Promoters

212 Adhesion and Corrosion Protection

and morphological structure of a coating as well as its permeability to oxygen
and moisture)
• effect of deleterious substances (water, chlorides, solvents can cause the forma-
tion of discontinuities in coating, such as pinholes, cracks, swollen sections,
which will eventually lead to a loss of the barrier functions)
• mechanical damage
1
In the general terms, adhesion loss is a result of
• wet adhesion loss (moisture penetrates coating and displaces it from the interface
forming hydrogen bonds with substrate and coating; water hydrates oxide layer
of substrate creating a weak layer of hydrates at an interface)
• cathodic disbondment (oxygen and water, which have penetrated coating catodi-
cally, generate alkalinity reacting with polymer of coating and causing disbond-
ment)
• anodic undercutting (corrosion products − oxides − are deposited under the coat-
ing and they lift and eventually debond coating from substrate during subsequent
periods of wetting and drying)
• physical, mechanical, and chemical damage (as listed above).
From the above short description, it is evident that the relationship between adhesion
of coating and its protective values is a complex system of numerous mechanisms which
can unlikely form uniform comprehensive mechanism but the relationship can more likely
be described by the existing experiences from studies of different angles of the application
and deterioration of coatings as discussed below.
It is generally accepted that the water diffuses through a coating in a quantity that is
orders of magnitude higher than required to support a corrosion reaction with a rate com-
2
parable to the corrosion rate of uncoated materials. The protective action is, therefore, not
2
determined solely by the ability of coating to provide a good barrier to oxygen or water.
The ionic species diffusion is remarkably reduced by the polymer matrix and it is
recognized as one of the fundamental mechanisms of corrosion protection of organic coat-
2
ings. The ion permeability is reduced by addition of pigments, increased degree of cross-
linking, and by the physical properties of the polymer matrix (e.g., the glass transition
2
temperature).
The wet adhesion is one of the dominant mechanisms for the protective performance
2
of the organic coatings. The coating which has the ability to maintain chemical-physical
2
bonds with a substrate prolongs corrosion initiation time and opposes its propagation. On
the other hand, the decreased permeability of coating caused by a crosslink density or
properties of matrix polymer slows down the diffusion of deleterious chemicals to the
interface.
The osmotic pressure gradient across the coating thickness is generated by the highly
2
concentrated salt solution. The water-soluble ionic impurities at the interface may cause
2
blistering and promote the underpaint corrosion.
There is a critical threshold concentration, depending on the characteristics of the
film and the corrosion mechanism, beyond which both blistering and corrosion is
2
observed.
An ineffective surface cleaning and preparation process (chemical conversion treat-
ment, sanding, and abrasive treatments) and hydrophilic solvents retained in the film can