Corrosion electrochemical characteristics of red iron oxide pigmented epoxy coatings on aluminum alloys

Red iron oxide pigmented epoxy coatings were prepared on aluminum alloys and were characterized by electrochemical impedance spectroscopy (EIS) immersed in NaCl solution. The evolutions of impedance models of coated metals were obtained by the fitting analysis of experimental data using suitable equivalent electrical circuits (EEC). The results indicated that the composite electrode system could be fitted by three impedance models. At the initial immersion stage, coatings acted as a barrier layer and only performed a simple circuit consisting of a coating resistance (Rc) parallel to a coating capacitance (Cc). After a certain time of exposure, water and (or) oxygen arrived at the metal surface through the coating, leading to the formation of electrochemical corrosion sites at metal interface and thereby the appearance of double-layer capacitance (Cdl) and charge transfer resistance (Rct). After intensive attacking of metal substrates, the mass transfer of corrosion products was in difficulty. In this case, the diffusion elements were added to the EEC. It was found that due to the presence of inert pigment particles, the mass transfer behaviors were interestingly different from those of the varnish polymer coatings. Electrical parameters were also obtained from the EIS data. lnCc–time curve showed a Case II water sorption kinetics, typical non-Fickian diffusion for water uptake.