© 2006 by Taylor & Francis Group, LLC

Introduction 7
to the metal is a necessary condition. However, good adhesion alone is not enough;
adhesion tests in isolation cannot predict the ability of a coating to control corrosion [36].
1.2.3.2 Wet Adhesion
A coating can be saturated with water, but if it adheres tightly to the metal, it can
still prevent sufficient amounts of electrolytes from collecting at the metal surface
for the initiation of corrosion. How well the coating clings to the substrate when it
is saturated is known as wet adhesion. Adhesion under dry conditions is probably
overrated; wet adhesion, on the other hand, is crucial to corrosion protection.
Commonly, coatings with good dry adhesion have poor wet adhesion [37-41].
The same polar groups on the binder molecules that create good dry adhesion can
wreak mischief by decreasing water resistance at the coating-metal interface — that
is, they decrease wet adhesion [42]. Another important difference is that, once lost,
dry adhesion cannot be recovered. Loss of adhesion in wet conditions, on the other
hand, can be reversible, although the original dry adhesion strength will probably
not be obtained [16, 43].
Perhaps it should be noted that wet adhesion is a coating property and not a
failure mechanism. Permanent adhesion loss due to humid or wet circumstances also
exists and is called water disbondment.
Relatively little research has been done on wet adhesion phenomena. Leidheiser
has identified some important questions in this area [43]:
1. How can wet adhesion be quantitatively measured while the coating is wet?
2. What is the governing principle by which water collects at the organic
coating-metal interface?
3. What is the thickness of the water layer at the interface, and what determines
this thickness?
Two additional questions could be added to this list:
4. What makes adhesion loss under wet circumstances irreversible? Is there
a relationship between the coating property, wet adhesion, the failure
mechanism, and water disbondment?
5. Why does the reduction of adhesion on exposure to water not lead to
complete delamination? What causes residual adhesion in wet circumstances?
As a possible answer to the last question above, Funke has suggested that dry
adhesion is due to a mixture of bond types. Polar bonding, which is somewhat
sensitive to water molecules, could account for reduced adhesion in wet circumstances,
whereas chemical bonds or mechanical locking may account for residual
adhesion [16]. Further research on wet adhesion could answer some of the aforementioned
questions and increase understanding of this complex mechanism.
1.2.3.3 Important Aspects of Adhesion
Two aspects of adhesion are important: the initial strength of the coating-substrate
bond and what happens to this bond as the coating ages.
© 2006 by Taylor & Francis Group, LLC