Application of EIS to the initial stages of atmospheric zinc corrosion

This paper reports an experimental study of the initial stage of atmospheric zinc corrosion using ex situ electrochemical impedance spectroscopy (EIS) in methanol electrolyte. Compared with the traditional techniques for studying atmospheric corrosion, such as gravimetry, the EIS technique significantly reduced the exposure time for detectable corrosion at any relative humidity from several days to a few hours. The samples were first exposed to synthetic atmospheres with careful control of O-2 and CO2 concentrations, relative humidity and temperature. EIS was then used to measure the polarization resistance (R-p) of the exposed samples. The corrosion products were analysed by a combination of grazing-angle X-ray diffraction, Fourier transform infrared spectroscopy and photoelectron spectroscopy. Several interesting phenomena occurring in the initial stage of corrosion were demonstrated by studying the electrochemical properties of the surface layer formed on the zinc. At high values of relative humidity (RH 95-100%), with CO2 > 40 ppm, the R-p of the surface film formed increased monotonically with time and relative humidity. At intermediate values of relative humidity (RH 50-85%) in the presence of CO2 (40-500 ppm), R-p first increased with time, reached a maximum, then fell from the maximum value before again rising slowly. A brief description of the mechanism of atmospheric zinc corrosion is proposed.