Comprehensive assessment of some l-amino acids as eco-friendly corrosion inhibitors for mild steel in HCl: Insights from experimental and theoretical studies

In this study, the corrosion inhibition performance of l-asparagine (L-Asp), l-isoleucine (L-Iso), and l-proline (L-Pro) amino acids as eco-friendly compounds was investigated on mild steel coupons immersed in 0.5 M HCl solution. To this end, gravimetric analysis, electrochemical techniques, field-emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS), and quantum chemical/molecular dynamics simulations were employed. At 1000 ppm, L-Asp, L-Iso, and L-Pro show the inhibition efficiencies of around 95%, 93%, and 91%, respectively. The Langmuir adsorption isotherm holds well for all three inhibitors, and based on the thermodynamic data, the adsorption of these inhibitors to the steel surface is involved both physisorption and chemisorption. As evidenced by the XPS results, the donor-acceptor interactions between the heteroatoms (namely, N) and surface Fe atoms play a pivotal role in the chemisorption. In line with the experimental findings, theoretical simulations unveiled potential binding sites accountable for the molecular adsorption on the surface of the steel, thereby providing insights into the underlying mechanism of corrosion inhibition.