Inhomogeneity of organic coatings and its effect of protection
It has been revealed that when metal is protected by an organic coating, corrosion proceeds through the ionic conduction pathways within the coating layer. An important aspect of polymeric coatings from the protection point of view is their inhomogeneous nature. Phenomena such as lack of cross-linking, inappropriate pigmentation, quick solvent evaporation etc may introduce local micro defects. The rate of diffusion of ions at these micro defects is proportional to the ionic concentration of the solution at which the coating is immersed and therefore they have been known as "D" or direct type areas. On the other hand much of the coating is composed of the highly resistive "I" type area where the ionic resistance increases with increasing the ionic concentration of environment. Traditionally it is believed that a certain level of adhesion is a fundamental requirement to avoid anodic and cathodic areas connecting underneath the coating and this adhesion will be reduced at D area Therefore the ionic resistance and specifically D type areas play an important role to determine the anti-corrosive properties of paint. In the present study a set of alkyd coatings in the form of detached coatings has been examined using DC technique to determine the ratio of D to I types. The significance of thickness, curing condition, solvent type, nano particle incorporation and multi-layer application on D area formation has been investigated. Subsequently a small group of these coatings were applied on steel panels and the long term corrosion behaviour was looked at. Also the adhesion strength of this group of coatings was examined by pull-off method. Results indicate that various factors affect the D to I ratio and these factors therefore will also influence the protection efficiency of the layer but is relatively insensitive to the adhesion strength. Copyright (2012) by the Australasian Corrosion Association.
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