Estudo do farelo de soja como inibidor de corrosão

Abstract

The inhibitory action of soybean meal extracts (Glycine Max), the aqueous extract (AE), its high molecular weight fraction (HMWF), its low molecular weight fraction (LMWF), and the aqueous extract by infusion of a commercially isolated soy protein (CIP), were investigated towards the mild steel corrosion in 0.5 mol L−1 H2SO4 medium. Weight loss measurements, Electrochemical Impedance Spectroscopy (EIS) and potentiodynamic polarization curves were used to determine the corrosion rate and inhibitory efficiency of the extracts. Additionally, Scanning Electron Microscopy (SEM) and X-Ray Photoelectron Spectroscopy (XPS) techniques were used, respectively, in order to characterize the metal surface morphological and chemically before and after the weight loss measurements. The total protein content in the different extracts was determined by the Biuret method and the total carbohydrate content by the Dubois method. The qualitative investigation of some extracts was carried out by Nuclear Magnetic Resonance (NMR 1 H) and the analysis of sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). The extracts LMWF, AE, HMWF and CIP acted as good corrosion inhibitors, reaching inhibition efficiencies of 72.4%, 85.2%, 88.7% and 94.8% for 800 mg L − 1, respectively, from jcorr data determined by the Tafel method. The LMWF sample presented the lowest EI%, being the sample that also showed the lowest protein content among the investigated inhibitors. The polarization curves showed that these extracts act as mixed type inhibitors with predominantly cathodic characteristics. The adsorption of AE components, its HMWF, its LMWF and CIP on the surface of mild steel were analyzed using the Langmir, El-Awady, Temkin, Flory-Huggins isotherms, with Langmuir being the most adequate, even with a small deviation of ideality. The SEM results indicated that the surface was covered by a protective film in the investigated samples and the XPS analyzes confirmed chemical interactions between the components of the studied extracts and the surface of mild steel. Other electrochemical tests were carried out with all extracts standardizing the concentration of extracts as a function of protein content to confirm the influence of proteins on the inhibitory effect. The results obtained in the investigation strongly suggest that the proteins are probably responsible for the inhibitory action of soybean meal under the conditions studied.