The C=C double bond is a typical feature in biodiesel components associated with their physicochemical behaviors, including metal degradation. In this work, copper was exposed to the gas-phase atmospheres of Methyl Hexanoate (MH) at 145 °C and 25% Methyl Trans-3-Hexenoate in Methyl Hexanoate (MT3H in MH) at 158 °C during 1000 h, representing saturated and unsaturated thermal degradation environments of biodiesel surrogates. FTIR, 1H NMR, and GC-MS were used to characterize the chemical changes in the gas-phase atmospheres, whereas SEM allowed us to inspect the copper surfaces. Weight loss assays enabled the estimation of corrosion rates for copper exposed to HM and MT3H in MH atmospheres of 3.81 ± 1.27 and 5.08 ± 1.27 μm/year, respectively. Electrochemical measurements (linear polarization resistance (LPR) and electrochemical impedance spectroscopy (EIS)) were used to evaluate the corrosion behavior of copper using aqueous extracts of condensed compounds from gas-phase degraded environments. Our electrochemical results indicate that similar corrosion processes occur in both extracts, remaining nearly unchanged with increasing exposure time. A porous layer of corrosion products on copper revealed that it is more active in the products generated with the MT3H in MH extract, suggesting the significant impact of the C=C bond on copper deterioration.
Vergara-Juarez et al. (Sat,) studied this question.