Onset of Failure in Corrosion Protective Barrier Coatings

Corrosion protection offered by barrier coatings, i.e. the topcoat in most systems, is often measured by electrochemical impedance spectroscopy, EIS, wherein the resistive component of the coating’s impedance, at low frequencies ~0.01 Hz, is monitored to provide a sensitive measure of how well the coating is protecting its metal substrate.  A competent barrier coating starts with a very large resistance which eventually falls 3 or 4 orders of magnitude, to become characteristic only of the oxidized metal surface.  During service a coating is exposed to an environment that includes moisture, aggressive ions, ultraviolet radiation and temperature changes.  The objective here is to understand coating resistance change in terms of overall deterioration, and link it to deterioration in other, common use properties, e.g. gloss.  The model relates the barrier protection to coating thickness and the progress of degradation.  Aggressive ions cannot penetrate an intact crosslinked coating, so local failure in corrosion protection, leading to penetration by water, and aggressive ions, and thus rust spots etc. must be the result of very local thinning or a channel percolating through the coating thickness.  The fatigue life, at which exposure cycles cause final failure, is determined by a combination of the degradation rate and how the coating responds to the mechanical stresses caused by the environment.  Although they may occur, there is no need to invoke adventitious defects to explain the first, scattered appearance of corrosion.