Microelectrodes As Effective Tools to Evaluate Biodiesel Corrosiveness By Electrochemical Techniques
Authors: Sanabria Beatriz , Aoki Idalina 
Published: February 2020

Corrosion is one of the main concerns for using biodiesel as an alternative fuel. Nowadays, the vast majority of studies are done by immersion tests and the monitoring of its physicochemical properties during degradation reactions. The low conductivity of biodiesel and the development of high ohmic drops has restricted the use of electrochemical techniques for understanding the mechanisms involved in the corrosion processes. The addition of supporting electrolytes is often used to increase the conductivity and reduce the ohmic drop. In organic media as biodiesel, unfortunately, it is suspected that supporting electrolytes are not inert and can give misleading results.
Microelectrodes are sensors with at least one dimension in the range of few micrometers (~50) and its geometry mostly depend on its application. Due to the small currents registered in microelectrodes, the effects of the ohmic drop become insignificant, even in very resistive electrolytes as biodiesel. In this work, we propose the use of microelectrodes as a methodology to evaluate the corrosion rate of metals, particularly copper, by EIS without any addition of supporting electrolytes (water or other contaminants). Two different types of microelectrodes (i.e. single classic and array) with disk geometry were constructed and characterized by different techniques (optical microscopy, SEM and cyclic voltammetry). Single classic microelectrodes were constructed manually by introducing a metallic wire (working electrode) into a capillary glass. The arrays with sixteen and twenty-five microelectrodes were constructed by photolithography technique.
The impedance measurements were done in a Gamry ref. 600 potentiostat/galvanostat using a two electrode cell configuration were a Pt foil was used simultaneously as counter and reference electrode (i.e. short-circuited to the corresponding inlets of the potentiostat) and the microelectrode was the working electrode. The frequency range was from 100 kHz to 5 mHz, with an amplitude of potential perturbation of 30 mV rms (with lower perturbation of potential no response is obtained).
For both types of microelectrodes, two time constants were observed, one at high frequencies and the other at low frequencies. The former corresponds to the electrolyte and the latter to interfacial phenomena between the electrolyte and the metal surface. Both hypothesis were corroborated using three different types of experiments. In the first case, another quality of biodiesel was used. Degraded biodiesel was obtained by introducing a copper foil, increasing the temperature and bubbling air during 5 days. With degraded biodiesel a reduction in the impedance at high frequencies was observed in both types of microelectrodes, as a result of the higher conductivity of the electrolyte. In the second case, an inert metal (Pt) was used as a microelectrode (single classic) and it was observed an increase in the impedance with respect to copper, due to the low activity that Pt has in biodiesel. Additionally, an array of copper with 25 microelectrodes was left in contact with degraded biodiesel during 60 h. Every 12 h the impedance was monitored and recorded. The results showed an increase in this parameter as a consequence of the growth in the corrosion layer formed on the surface of the metal in contact with the biofuel. As the corrosion layer growths the rate of the electron transfer reactions at the interface is reduced. In all cases, the second semicircle observed in Nyquist diagrams started at 0.3 – 0.1 Hz. A decrease in the phase angle at medium frequencies (1-0.1 Hz) was observed when the number of microelectrodes increased as a result of a higher kinetic for the change transfer process, due to the higher area and the interfacial character of the signal.

Figure 1