EIS of Zinc Under Thin Solution Films with Various Thicknesses Analyzed By a Transmission Line Equivalent Circuit
Authors: Somphotch Chulaluk , Ooi Azusa , Tada Eiji , Nishikata Atsushi 
Published: February 2020

In this research, an influence of corrosion products on corrosion behavior of zinc under thin solution films with various thicknesses was studied by electrochemical impedance spectroscopy (EIS). The corrosion products evaluated in this work were formed during periods of exposure corresponding to 1, 3, 6, and 12 months in a natural environment (Japan Weathering Testing Center, Japan) compared with the non-exposed (polished zinc). A solution of 0.2 M NaCl was used as the thin film under conditions varying thickness from 5 to 800 µm. For the films thicker than 50 μm, the solution was directly placed whole electrode cell surface with a micropipette. However, For the films thinner than 25 μm, the solution was prepared by mixing with ethanol on the cell surface.

The EIS was carried out to monitor impedance behavior at an open circuit potential (OCP) from the frequency range 10 kHz to 10 mHz. The impedance for the interface of metal and electrolyte (Z
total) was obtained by dividing the measured impedance with the two-electrode cell by two. A transmission line model equivalent circuit (TML model) as shown in figure 1 was employed to describe the electrochemical impedance behavior.

The EIS bode plots show different behaviors when the thickness of thin solution film changes. From the θ vs. log f plot in exposed zinc, all the shapes show two-time constants, one at high frequencies (HFL) and another at low frequency (LFL) where the phase shift does not exceed -45o. It means current distribution remains non-uniform even in a whole range of frequency in all cases. All electrochemical parameters were successfully obtained from the fitting method by applying the TML equivalent circuit of a metal-thin electrolyte layer interface. The appropriate initial values were optimized before input into the fitting software with the two-time constant. Some parameters of corrosion products such as resistance Rf* and capacitance CPEf* at thickness 800 μm were fixed for all thickness in the same exposure periods, where residual parameters in TML circuit were free.

Charge transfer resistance R
ct obtained from the fitting results (TML model) was used to study the corrosion behavior. The corrosion current density (i
corr) can be calculated from the Stern-Geary equation, k= 13 mV and 4 mV for polished and exposed zinc respectively. For polished zinc, the results of polished zinc show that its corrosion behavior was dependent on a varying of the thin solution films. The i
corr tends to increase while the thickness was reduced approximately to 25 µm. This behavior can be described by the diffusion limited current density (i
lim) is the important parameter affected by the electrolyte layer thickness in cathodic reaction, i.e., an enhancement of diffusion process of O2 through the water layer. Then, the corrosion behavior is cathodically control. Further reduce of the thickness decreased the i
corr due to a lack of amount of water needed for hydration of dissolved metal ions. Therefore, in a transition area, the corrosion rate is anodically control.

On the other hand, the i
corr of the exposed zinc have the approximate value and the corrosion behavior was independent of the thin solution films for whole thickness range. From these results, they might not be controlled by mass transport (O2 diffusion) but they must be controlled by the charge transfer process for both cathodic and anodic reaction. The cathodic and anodic polarization of the exposed and polished zinc were also measured in 0.2 M NaCl bulk solution. Before measurement, open circuit potential (E
ocp) was monitored. The polarization curves show the deviation of Tafel slope for exposed zinc (1, 3, 6 and 12 months) compared to non-exposed one (polished zinc). E
ocp of exposed zinc approaches to the positive values. It can be preliminary implied that the charge transfer process of both cathodic and anodic reaction was suppressed by zinc corrosion products which were identified as Zincite (ZnO) and Simonkolleite (Zn5(OH)8Cl2.H2O) whereas the charge transfer process of anodic reaction was more suppressed.

Figure 1