Electrochemical and physical evaluations of coil coatings on metal-coated steels for roofing applications

Abstract

Four types of commercial organic coil coatings, applied over galvanized and Galvalumed steel, were evaluated for applicability as roofing materials using electrochemical and physical test methods. Scribed candidate coatings were subjected to 13 weeks of cyclic salt-fog and UV exposures via ASTM D 5894 (with modifications). This controlled laboratory exposure approach has been suggested as a practical accelerating alternative to long-term outdoor exposure testing. Electrochemical impedance spectroscopy (EIS), sample potentials and coating disbonding at the scribe lines were used for evaluation. Unscribed samples were also subjected to 13 weeks of continuous immersion in the same test electrolyte composition and evaluated physically and via EIS. The complexity of the coating systems as well as the testing protocol introduced several evaluation difficulties, especially in the electrochemical testing. The relative merits of the coatings as well as the testing approaches are presented and discussed.