© 2006 by Taylor & Francis Group, LLC

Introduction 3
• A dissolution process at the anode
• A cathode site
• An electrolytic path between the anode and cathode
Any of these items could potentially be rate controlling. A coating that can
suppress one or more of the items listed above can therefore limit the amount of
corrosion. The main protection mechanisms used by organic coatings are:
• Creating an effective barrier against the corrosion reactants water and
oxygen
• Creating a path of extremely high electrical resistance, thus inhibiting
anode-cathode reactions
• Passivating the metal surface with soluble pigments
• Providing an alternative anode for the dissolution process
The last two protection mechanisms listed above are discussed extensively in Chapter 2.
This section will therefore concentrate on the first two protection mechanisms in the list
above.
It must be noted that it is impossible to use all these mechanisms in one coating.
For example, pigments whose dissolved ions passivate the metal surface require the
presence of water. This rules out their use in a true barrier coating, where water
penetration is kept as low as possible.
In addition, the usefulness of each mechanism depends on the service environment.
Guruviah studied corrosion of coated panels under various accelerated test
methods with and without sodium chloride (salt). Where salt was present, electrolytic
resistance of the coatings was the dominant factor in predicting performance. However,
in a generally similar method with no sodium chloride, oxygen permeation
was the rate-controlling factor for the same coatings [2].
1.2.1 DIFFUSION OF WATER AND OXYGEN
Most coatings, except specialized barrier coatings such as chlorinated rubber, do not
protect metal substrates by preventing the diffusion of water. The attractive force
for water within most coatings is simply too strong. There seems to be general
agreement that the amount of water that can diffuse through organic coatings of
reasonable thickness is greater than that needed for the corrosion process [2–8]. Table 1.1
shows the permeation rates of water vapor through several coatings as measured by
Thomas [9,10].
The amount of water necessary for corrosion to occur at a rate of 0.07 g
Fe/cm 2 /year is estimated to be 0.93 g/m 2 /day [9,10]. Thus, coatings with the lowest
permeability rates might possibly be applied in sufficient thickness such that water
does not reach the metal in the amounts needed for corrosion. Other coatings must
provide protection through other mechanisms. Similar results have been obtained
by other studies [2,11]. However, the role of water permeation through the coating
cannot be completely ignored. Haagan and Funke have pointed out that, although
water permeability is not normally the rate-controlling step in corrosion, it may be
the rate-determining factor in adhesion loss [11].
© 2006 by Taylor & Francis Group, LLC