A physical model for anticorrosion behavior of duplex coatings

Abstract

Duplex coatings combine an "active" protection provided by a metal deposit, playing the role of sacrificial anode to cathodically protect the substrate with "passive" protection of an organic layer, playing the role of a diffusional barrier to hinder transport of ionic species. The corrosion behavior of these duplex coatings was studied in a chlorinated solution for long immersion times. The electrochemical impedance spectroscopy was measured during the immersion and an original physical model was proposed. This model considers first, for short immersion times, a porous electrode behavior corresponding to impedance proportional to the square root of plane electrode impedance. For longer immersion times, the development of secondary pores in the walls of the initial pores was assumed. Consequently, an impedance proportional to Z0.25 was obtained, where Z is the impedance of a plane electrode. This model agrees with the experiment and needs only three unknown parameters for short immersion times and five for long immersion times.