Electrical and electrochemical behavior of a zinc-rich epoxy coating system with carbon nanotubes as a diode-like material
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2019 Elsevier Ltd Non-conventional responses in the impedance spectra of a zinc-rich coating system were observed in electrical and electrochemical impedance measurements that have previously been considered to have no effect on the system. Despite the impressive progress in zinc-rich coating systems to control corrosion in metal structures through galvanic effects, some physical properties influencing the responses of electrochemical systems remain unexplored. Electrochemical and non-electrolytic cells on zinc-rich (55 vol%) coatings with multi-wall carbon nanotube systems were analyzed to determine the intrinsic response of the coating with electrical passive and active analogs under electrical conditioning cycles. In the dry condition, the impedance spectra of the carbon nanotube-added zinc-rich coating showed increased electrical resistance of the film under consecutive cyclic testing. The non-conventional response of the increase in impedance was studied by determining the impedance responses under various temperatures and bias potentials and performing a current-voltage method. The non-conventional phenomenon was found to be caused by an increased barrier height between the semiconducting zinc oxide material and metal, which is susceptible to an external electrical field. During immersion, the change in impedance of the zinc-rich primer (ZRP) film under consecutive electrochemical sequences differed from that under natural degradation. This non-conventional response of the ZRP coating was influenced by the applied bias potential. The capacitive behavior influenced by the zinc corrosion products in the coating had a longer relaxation time, thus decreasing the open circuit potential. To understand this phenomenon, we propose a charging-relaxation potential model. In addition, the potential exhibited a change in the electrical resistance of the ZRP film during consecutive electrochemical testing.