Experimental and Theoretical Approach on the Enhanced Inhibitory Effect of Tetracyclic Triterpenes for Stainless Steel Corrosion in Sulfuric Acid

The effects of triterpene fraction (TF) on the corrosion behavior of AISI Type 321 stainless steel (SS) in sulfuric acid were investigated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). This study shows that the inhibitor tested improve the overall corrosion resistance of stainless steel in sulfuric acid. Potentiodynamic polarization showed that TF acts as a mixed inhibitor. In addition, an adsorption model suitable for experimental data was discussed in depth. The Langmuir model gives the best fit, although all models show good agreement with the adsorption data. The effects of the inhibitor concentration and the temperature of the medium were thus evaluated. On the other hand, a hybrid method of B3LYP function and 6-31G (d, p) basis set was performed to correlate the TF molecular structure with the experimental data. In addition, a molecular electrostatic potential map was used to predict the chemical reactivity of tetracyclic triterpenes with the SS surface. The electrophilic and nucleophilic sites of adsorption were discussed, and the Fukui indices were determined. The findings obtained indicate that 8C and 9C are the most reactive sites in the tetracyclic system. Scanning electron spectroscopy confirmed the adsorption of TF on the stainless steel surface, thus supporting the experimental results.