Green corrosion and DFT studies of Ustilago maydis extract for carbon steel in sulfuric acid

A study on the use of Ustilago maydis extract as a green corrosion inhibitor for 1018 carbon steel in 0.5 m H2SO4 by using gravimetric, potentiodynamic polarization curves, linear polarization resistance and electrochemical impedance spectroscopy electrochemical techniques at 25, 40 and 60 degrees C. Density functional theory (DFT) calculations were performed to know the relationship between inhibitor electronic properties and inhibition efficiency. Weight loss results indicated that Ustilago maydis was a good corrosion inhibitor with an efficiency that increased with its concentration but decreased with the temperature. However, the efficiency was around 60% because it was physically adsorbed on to the steel surface according to a Langmuir type of adsorption isotherm obtaining relatively low free energy values, which lied between -14.8 and -27 kJ/mol at 25 and 60 degrees C respectively. Activation energy decreased in presence of Ustilago maydis making corrosion process more difficult to occur. Potentiodynamic polarization curves showed a decrease in the corrosion current density in one order of magnitude and determined this extract as a cathodic type of corrosion inhibitor affecting. There was no evidence of a passive film, whereas the corrosion mechanism was controlled by the adsorption/desorption of some intermediate species and the double electrochemical layer resistance increased with the addition of the inhibitor. Gas chromatography showed that the main compounds found in this extract included D-Mannose, beta-D-Glucopyranose,4-O-beta-D-galactopyranosyl, Melibiose and Arabinose, whereas quantum chemical calculations by density functional theory analysis showed that Melibiose had the highest inhibition efficiency because it had the highest EHOMO value and lowest gap energy value, Delta E.