Corrosion behavior of metals (Al, Fe, Zn) in alternative fuels using EIS method in two electrode cell

초록

In order to overcome the limited deposits in fossil fuels, many kinds of alternative fuels such as biodiesel, ethanol, methanol, natural gas have been developed. As usage of these fuels as an automotive fuel, automotive makers are required to develop materials compatible to all fuels. Among these fuels, biodiesel and ethanol are selected because ethanol is reported to be relatively corrosive, and the auto-oxidation of biodiesel induces corrosive environments. The corrosion rates of metallic materials have been measured by immersion test or potentiodynamic test; however, low conductivity of these fuels makes immersion test employed despite of the time consuming test. In this study, to overcome a shortcoming of electrochemical methods and to evaluate the corrosion rate of aluminum and its alloys, iron, and zinc, electrochemical impedance spectroscopy (EIS) was taken and a two-electrode cell was devised where the distance between tow electrodes is controllable at the rage 100 μm to 1 cm. Immersion test provides corrosion rate decreased in the order of zinc, iron, and aluminum. In addition, the corrosion rates of all the metals decreased with time imply that the corrosion rate is controlled by transport across electrolyte or corrosion products. The analysis of EIS with the cell devised for this study shows that the polarization resistance (Rp) increased in the order of zinc, iron, and aluminum, which is the same order as in the immersion test. On aluminum, showing the best corrosion resistance, the effect of magnesium as an alloying element was evaluated in ethanol containing acid and/or chloride and corrosion by EIS test. As the concentration of magnesium increased, the Rp of aluminum alloy increased in ethanol as well as biodiesel. However, the increase of Rp with the concentration of magnesium decreased. The optimum content of magnesium and the role of magnesium will be mentioned and discussed in details.