Effect of the C = C double bond in biodiesel compounds on copper corrosion

Biodiesel is a mixture of fatty acid methyl esters in which the chemical structure, hydrocarbon chain length, saturation or unsaturation, and position of the C = C double bond in each methyl ester influence the biodiesel’s properties. In this study, pure mixtures of short-chain methyl esters (methyl acrylate and methyl propionate, methyl 3-butenoate and methyl butyrate) were selected as model systems to evaluate the effect of the C = C bond on copper corrosion behavior. These methyl esters allowed the study of unsaturation as representative structures of some compounds present in commercial biodiesel. Mass loss tests, electrochemical measurements such as open-circuit potential, linear polarization resistance, electrochemical impedance spectroscopy, and electrochemical noise were performed over a 6-week exposure period, along with surface analysis using scanning electron microscopy (SEM). The results showed that adding a high concentration of unsaturated methyl esters increases the copper corrosion rate as exposure time increases. The chemical structure and electronic conjugation of the methyl esters influence the polarization resistance (R p ), charge transfer resistance (R ct ), and noise resistance (R n ) values. SEM analysis revealed a greater tendency toward localized pitting corrosion on the copper surface.