Evaluation Barrier Properties of Organic Coatings by Water Permeation and Electrochemical Methods

By P.C. Inone, C.M. Garcia, A Ruvolo-Filho

Organic coatings protect metals against corrosion by various mechanisms, which can be summa­rized as follows: (a) suppression of the anodic and/or cathodic reaction, (b) the introduction of a high electrical resistance into the circuit of the corrosion cell, and (c) a barrier to aggressive species (oxygen, water, and ions). Measurement of ionic resistance by the polarization method has been used successfully for many rears as a method to characterize paints and varnishes. Briefly, good performance is recognized if a high d.c. resistance is observed when the coating is exposed to an elec­trolyte solution. In these studies, it was stated that coat­ings frequently contain areas of low ionic resistance named D-type areas and areas of high ionic resistance named I-type areas. The predominance of either kind of these areas determines the total ionic resistance of a coating.

Today, electrochemical impedance spectroscopy (EIS) is preferred to evaluate the performance of organic coat­ings because it allows for a more complete characteriza­tion. For instance, the uptake of ions into the coatings has been related to the increase in d.c. resistance of the coatings, while the uptake of water has been related to the increasing capacitance of the coatings. Also, EIS has been used to estimate the degree of delamina­tion.