Expired dopamine as an ecofriendly corrosion inhibitor for 316L stainless steel in 0.5 M HCl: Experimental and theoretical studies

In the present paper, a novel approach has been made to valorise the redundant medicinal drug, dopamine (DA), to operate as a corrosion inhibitor for 316L stainless steel in 0.5 M HCl. Electrochemical analysis by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP) indicates that very high and effective inhibition by phenolic compound insertions is reported: at 10
-3
, Rct (charge transfer resistance) increases, Cdl (double layer capacitance) decreases by serious mode, and τ increases, yielding optimum efficiencies IE(EIS)=94.6% and inhibition efficiency percentage (IEPDP)≈92.6%. The adsorption obeys’ Langmuir model, having ΔG
ads
≈− 40 kJ·mol
−1
, suggesting that a mixture of physisorption/chemisorption. Scanning electron microscopy (SEM)/X-ray photoelectron spectroscopy (XPS) analysis proves the mechanism of the formation of a protecting organic film strongly attached to the passive layer (Ni(OH)
2
, Fe
2
O
3
, Cr(III), MoO
2
and the influence of nitrogen from DA (N 1s). Density functional theory (DFT)/conductor-like screening model (COSMO) calculations present that highest occupied molecular orbitals (HOMOs)/lowest unoccupied molecular orbitals (LUMOs) are localized on O and N, and there are extraordinary negatively Mulliken charges on O/N, indicating that they are adsorption centers. The density-functional tight-binding (DFTB
+++
) charge density difference maps were flexed to an essentially planar conformation, and display π–d donor-acceptor interactions with π–dd hybridization. Lastly, the Monte Carlo/molecular dynamics (MC/MD) simulations provide highly negative adsorption energies (≈−125 to −147 kcal•mol−1) and radial distribution function (RDF) peaks < 3.5 Å for O/N–Fe, indicating chemisorption. All in all, it shows that DA out of date is a green, highly effective and inexpensive inhibitor, paving the way for valorization of pharmaceutical waste.