Promoting the viability of soybean biodiesel by mitigating its oxidative effects on austenitic stainless steel using a natural additive of spent coffee grounds

This work demonstrates the recycling potential of a hydroalcoholic extract of spent coffee waste (HECG) as an environmentally friendly additive to soybean cooking oil (SWCO) biodiesel, alleviating its corrosive impacts on a Cr-rich stainless-steel alloy. The extract is efficiently produced using the Soxhlet extraction method, and its chemical profile is analyzed using GC-MS and FTIR. Furthermore, the anticorrosive properties of HECG are thoroughly evaluated using gravimetry, potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS). The HECG additive shows excellent inhibitory efficacy (94%) on 307 L stainless steel. The polarization results underline the mixed inhibitory character of HECG. The metal surface is primarily protected against biodiesel-induced corrosion through adsorption of additive molecules onto it. Additionally, SEM-EDX and XRD techniques are used to evaluate the microstructure, crystal structure, and composition of the surface and formed products after immersion in this biodiesel. The results showcase the additional protection of the 307 L sample via creating a passive oxide film (CrO₂₃, FeO₂₃, FeOOH) on the metal surface, thus boosting its corrosion resistance. Importantly, DFT and Fukui function simulations identify the electrophilic and nucleophilic sites at the β-turmerone molecule, the major component of the extract, and reveal intrinsic retro donations in both ways - additive to metal and/or metal to additive.