Electrochemical studies and molecular simulations on the use of molybdic acid for stabilization of AISI 304 stainless steel passive film in sulfuric acid medium

In this work, the corrosion of AISI 304 stainless steel (SS) in the absence and presence of different concentrations of molybdic acid was investigated using potentiodynamic polarization, open circuit potential (OCP) measurements, electrochemical impedance spectroscopy (EIS), Mott-Schottky analysis (MS) and theoretical simulations including molecular dynamics (MD), Monte Carlo (MC), and density functional theory (DFT). Although it is obvious that the use of molybdate ions can improve the anti-corrosion properties of austenitic SS, there are fewer comprehensive reports which has been devoted to "investigation of electrochemical properties of passive film" and/or "theoretical studies using molecular-scale simulations". Therefore, the present study focused on examining these two issues. The results of potentiodynamic polarization showed that molybdic acid not only can reduce the corrosion rate but also decrease the critical current density (icrit) for passive film formation. MS analysis revealed the donor density in the passive film was increased up to 6.6 × 1020 cm−3 by increasing the molybdic acid concentration to 0.02 mM. The computational studies associated with MD and MC simulations showed that the adsorption energy of molybdic acid, as an inhibitor on steel surface, is −35.4 and −98.3 kcal mol−1, respectively. Moreover, the HOMO and LUMO energies and the energy difference between them were calculated by DFT simulation.