© 2006 by Taylor & Francis Group, LLC

134 Corrosion Control Through Organic Coatings
8.2 EVALUATION AFTER ACCELERATED AGING
After the accelerated aging, samples should be evaluated for changes. By comparing
samples before and after aging, one can find:
• Direct evidence of corrosion
• Signs of coating degradation
• Implicit signs of corrosion or failure
The coatings scientist uses a combination of techniques for detecting macroscopic
and submicroscopic changes in the coating-substrate system. The quantitative and
qualitative data this provides must then be interpreted so that a prediction can be
made as to whether the coating will fail, and if possible, why.
Macroscopic changes can be divided into two types:
1. Changes that can be seen by the unaided eye or with optical (light)
microscopes, such as rust-through and creep from scribe
2. Large-scale changes that are found by measuring mechanical properties,
of which the most important are adhesion to the substrate and the ability
to prevent water transport
Changes in both the adhesion values obtained in before-and-after testing and in the
failure loci can reveal quite a bit about aging and failure mechanisms. Changes in
barrier properties, measured by electrochemical impedance spectroscopy (EIS), are
important because the ability to hinder transport of electrolyte in solution is one of
the more important corrosion-protection mechanisms of the coating.
One may be tempted to include such parameters as loss of gloss or color change
as macroscopic changes. However, although these are reliable indicators of UV
damage, they are not necessarily indicative of any weakening of the corrosionprotection
ability of the coating system as a whole, because only the appearance of
the topcoat is examined.
Submicroscopic changes cannot be seen with the naked eye or a normal laboratory
light microscope but must instead be measured with advanced electrochemical
or spectroscopic techniques. Examples include changes in chemical structure of the
paint surface that can be found using Fourier transform infrared spectroscopy (FTIR)
or changes in the morphology of the paint surface that can be found using atomic
force microscopy (AFM). These changes can yield information about the coating-metal
system, which is then used to predict failure, even if no macroscopic changes have
yet taken place.
More sophisticated studies of the effects of aging factors on the coating include:
• Electrochemical monitoring techniques: AC impedance (EIS), Kelvin probe
• Changes in chemical structure of the paint surface using FTIR or x-ray
photoelectron spectroscopy (XPS)
• Morphology of the paint surface using scanning electron microscopy
(SEM) or AFM
© 2006 by Taylor & Francis Group, LLC