Reversible and irreversible coating impedance loss as an indicator of barrier efficiency and permanent coating damage

 
Online low-frequency impedance monitoring of coatings exposed to the atmosphere produces impedance modulus patterns that can be associated with the reversible influence of humidity/temperature fluctuations in the short term and the irreversible changes imparted by weathering in the long term. A new and intact coating system provides a dielectric barrier that effectively isolates the metal substrate from the environment. The baseline impedance can be calculated from the thickness and approximate relative dielectric constant and/or measured on the coating when dry. The loss of impedance from the baseline value and the recovery of the baseline value is observed when the coating is exposed to different humidity/temperature regimes, artificial or natural weathering. It will be presented that the intensity of the reversible impedance variations during weathering as well as the irreversible impedance loss resulting from accelerated exposure can be related to the responsiveness of the dry coating to the temperature rise, the isothermal response of the coating to moisture, the accelerated electroosmotic water uptake and the visual appearance of the coating after accelerated exposure.