Exploring the impact of E85 biogasoline on stictic acid film formation on mild steel: insights from a multi-technique approach

Abstract This study explores the inhibitive and formation mechanism of stictic acid (SSA) protective layer on mild steel in simulated E85 biogasoline. We employed a seamlessly, modern multi-technique approach to investigate the involved processes at the steel/E85 interfaces over time, including electrochemical (electrochemical impedance spectroscopy - EIS, potential dynamics - PD), spectroscopy (Raman), and surface analysis (Scanning Electron Microscopy - SEM), with the assistance of quantum chemical simulation (DFT). The electrochemical data (time-resolved EIS and PD) revealed that SSA can form a protective barrier on the steel’s surface, with the protection efficiency depending on the applied SSA concentration. At an optimum amount of SSA (50 ppm), data from EIS showed the most prominent electrochemical characteristics in the steel surface’s resistance and capacitance. PD results further reinforced this finding, revealing a significant drop in the overall corrosion rate when SSA concentrations were within a specific range. Additionally, the corrosion protection has declined at concentrations beyond the optimum value. SEM images and Raman spectra comparing the steel surface at different protective conditions showed that SSA significantly minimized corrosion damage on the inhibited samples, confirmed by the presence of SSA’s peaks and a reduction of corrosion products on the surface.DFT calculations indicate that different functional regions in the SSA structure simultaneously donate and accept electrons to/from the steel substrate surface, helping to create a strong bond between SSA/steel at the steel/E85 interface. However, as the SSA concentration increases, intramolecular interactions (SSA-SSA) will dominate over intermolecular interactions (SSA-Fe), leading to a decrease in the protective efficacy of the coating.