Abstract
The mechanism and prevention of the zinc atmospheric corrosion are focused, especially in the initial stages. An experimental technique - ex-situ electrochemical impedance spectroscopy (EIS) in a nonaqueous electrolyte (methanol) - on studying the initial stages of the zinc atmospheric corrosion under any exposure condition is developed. 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 O2 and CO2 concentrations, relative humidity, temperature and concentrations of contaminated salts. EIS was then used to measure the polarization resistance (Rp) of these exposed samples. The corrosion products were analyzed by a combination of grazing-angle x-ray diffraction, Fourier transform infrared spectroscopy, photoelectron spectroscopy and extended x-ray absorption fine structure spectroscopy (EXAFS) measurements. Several interesting phenomena occurring in the initial stage of the zinc atmospheric corrosion were demonstrated by studying the electrochemical properties of the surface layer formed on zinc. At high values of relative humidity (RH 95-100%), with CO2 > 40 ppm, the Rp of the surface film formed on zinc increased monotonically with time and relative humidity. At intermediate values of relative humidity (RH 50-85%) in the presence of CO2 (40-500 ppm), Rp first increased with time, reached a maximum, then dropped from the maximum value before again rising sluggishly. A brief description of the mechanism of zinc atmospheric corrosion is suggested.Concerning the prevention against zinc atmospheric corrosion, environmentally compatible non-chromate treatment and electrochemical anodic coating (ANC) were investigated. Non-chromate conversion treatment was inspected and tested using the optimum test conditions developed for the determination of the protective value of a non-chromate conversion coating by exposing zinc specimens contaminated with 1.5 □g/cm2 NaCl particles to 80% RH at ambient temperature. The corrosion mechanism of ANC and a modified one (CoANC) by adding cobalt ions to the electrolyte in the anodic treatment was investigated using EIS. By comparing the EIS, the corrosion of both ANC-coated zinc (ANC-ZN) and CoANC-ZN was found changing from charge transfer control process to diffusion control process during the prolonged immersion in chloride environments. Two modes of corrosion process based on the immersion time and their equivalent circuits were proposed in considering the chemical products and physical structures resulting from the corrosion reactions.