Glycine max meal extracts as corrosion inhibitor for mild steel in sulphuric acid solution

The soybean (Glycine Max) meal extracts inhibitory action, the aqueous extract by infusion (AE), its high molecular weight fraction (HMWF) and the aqueous extract by infusion of a commercially isolated soy protein (CIP) towards the mild steel corrosion in 0.5 mol L−1 H2SO4 was investigated by weight loss measurements, electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization curves. Additionally, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were used, respectively, to characterize morphologically and chemically the surface. The total content of proteins in the different extracts was determined by a quantitative method, the Biuret methods, as well as by two qualitative methods, the nuclear magnetic resonance (NMR) and sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis (SDS-PAGE). The AE, its HMWF and CIP extracts acted as good corrosion inhibitors, reaching inhibition efficiencies of 85.2%, 88.7% and 94.8% for 800 mg L−1, respectively, from the jcorr data. The polarization curves show that these extracts act as mixed-type inhibitor with predominantly cathodic characteristics. The adsorption of components from AE, its HMWF, and CIP on the mild steel surface follows the Langmuir adsorption isotherm. The SEM results indicate that the surface was covered by the inhibitor and the XPS analyses confirm chemical interactions between the extracts components and the mild steel surface. The results obtained with HMWF from AE and CIP suggest that the proteins are probably responsible for the inhibitory action.