Page 53 - Handbook of Adhesion Promoters

46 Mechanisms of Adhesion Loss

• there is no significant delamination in the absence of metal cation
• the rate of delamination strongly depends on the electrolyte cation and on the cat-
alytic activity of the surface in oxygen reduction
• the rate increases with applied potential and temperature
• there is an induction period for the beginning of delamination for thick coatings.
Considering the above environmental factors, it becomes pertinent that before corro-
sion may take place there have to be pathways for electrolyte cations and oxygen to pene-
trate interface of preventive coating, such as interfacial bond degradation, chemical
3
degradation of one or more coating layers. The ultimate defense against corrosion degra-
dation is the establishment of an impermeable barrier to water, oxygen, and an electrolyte
3
which has consistent properties under conditions of the performance.
Mechanical forces may contribute to damage of substrate protection and stresses
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may contribute to increased diffusion rate of moisture, oxygen, and electrolyte. Processes
that cause damage to coating, such as etching, weathering, swelling, etc. similarly lead to
3
protection loss.
In the studies of adhesion loss between epoxy and steel, five mechanisms of adhe-
sion loss were singled out as the processes influencing the outcome:
• the combination of moisture and temperature (high temperature causes a momen-
tary loss of adhesion, but after cooling, adhesion can be regained; moisture alone
produces loss of adhesion over time and high temperature helps to accelerate the
disbondment process)
• wet adhesion loss (displacement of epoxy by water forming hydrogen bonds dis-
placing bonds between epoxy and metal; oxide layer deterioration by hydration −
oxide hydrates have poor adherence to their base metals, therefore, mechanical
adhesion is reduced by the presence of a weak layer of hydrates at the interface)
• cathodic disbondment (anodic reaction occurs at a coating defect. It is usually
coupled with a nearby cathodic reaction beneath the coating. Oxygen and water
migrate through the coating and support the cathodic reaction producing alkalin-
ity causing damage to the polymer)
• anodic undercutting (the mechanism is also known as oxide lifting. The corrosion
products generated by the anodic reaction are deposited under the epoxy film
during subsequent periods of wetting and drying. They cause lifting or debonding
of the coating from the substrate)
• fabrication (during bending the shearing, stresses generated at the coating/steel
interface weaken the adhesion of the epoxy film by mechanical action)
Adhesion loss occurred when the corrosive electrolyte reached the coating/metal
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interface leading to the interfacial bonds hydrolysis and under-film corrosion. Zinc alu-
3-
2+
minum phosphate can release solubilized species, such as PO and Zn on the exposure
4
4
to corrosive electrolytes. The solubilized species react with hydroxyl ions produced at
cathodic regions thus preventing adhesion bonds hydrolysis and, therefore, zinc aluminum
4
phosphate can reduce adhesion loss. Zinc aluminum phosphate reduces adhesion loss by
4
80%.