Estimation of Corrosion Rate of Carbon Steel in Ethanol with Hydrochloric Acid by Electrochemical Impedance Method

Abstract

The charge transfer resistance R_ct and the electrode reaction impedance Z₀ of a carbon-steel electrode were measured in ethanol with 0.0064 to 0.13 mass% HCl and 2 mass% H₂O, in order to obtain basic data for applying on line corrosion-monitoring using electrochemical-impedance method in ethanol plants. The value of R_ct was estimated from the mass-loss of the carbon steel and the value of Z₀ was measured from the real-axis intersects of best-fitting curves to Nyquist plots. The ratio of Z₀ to R_ct was about 8 over the HCl contents examined in this study. The value of Z₀ could be approximated 0.5 · (R_0.1Hz-R_1kHz), under the conditions employed in this study ; the value of R_1kHz and R_0.1Hz are the impedance measured by two-electrode method at 1 kHz and around 0.1 Hz, respectively. Thus, the corrosion rate i_corr of carbon steel under similar conditions examined in this study would be given from the R_1kHz and the R_0.1Hz measured in the field as follows.
i_corr=2 · 160 mV/(R_0.1Hz-R_1kHz),
where 160 mV is an apparent Stern-Geary constant under the examined conditions.