Corrosion Behavior of Zinc under Thin Solution Films of Different Thicknesses

Abstract

Zinc corrosion under 0.2 M NaCl solution films 5–800 μm in thickness was studied by electrochemical impedance spectroscopy (EIS). As-polished and rusted zinc plates obtained by exposure to a natural atmospheric environment for 1, 3, 6, and 12 months were used as working electrodes. The obtained EIS results were analyzed using a transmission line equivalent circuit to determine the charge transfer resistance (Rct), the reciprocal of which is proportional to the corrosion current density. For the as-polished zinc, the corrosion behavior is divided into three regions by the solution film thickness (Xf). In Region I (Xf: 800–200 μm), Rct−1 is independent of Xf and is approximately ten times smaller than that for carbon steel. In Region II (Xf: 200–25 μm), however, Rct−1 increases with decreasing solution thickness down to approximately 25 μm due to an enhancement of the oxygen diffusion (cathodic process) through the solution film. In Region III (Xf < 25 μm), further decreases in thickness reduce Rct−1 because the zinc dissolution (anodic process) is suppressed. In contrast, Rct−1 of the rusted zinc is independent of Xf because the charge transfer process is strongly suppressed by the zinc corrosion products, which were identified as simonkolleite and zinc oxide.