Effect of molecular structure of two fluorescein molecules on the corrosion inhibition of mild steel in 1 M HCl solution

2-(6-hydroxy-3-oxo-3H-xanthen-9-yl) benzoic acid (opened form of Fluorescein) (NAR4) were investigated and compared to 3,6-dihydroxy-3H Spiro[isobenzofuran-1,9-xanthen]-3-one (Closed form of Fluorescein) (FLUO) using experimental and theoretical approaches. The theoretical methods used were DFT method at B3LYP/6-311G (d. p) and Monte Carlo simulation. The toxicity test was evaluated in vivo and the results indicate that the molecules showed good ecological behavior. Furthermore, FLUO was investigated as an eco-friendly corrosion inhibitor of mild steel in acidic medium using various experimental methods (electrochemical techniques and surface analysis). The opened form of Fluorescein derivative gave a high inhibition effect compared to the close form reaching an inhibition efficiency of 88% at optimum concentration of 10-4M. Moreover, the adsorption phenomenon of studied inhibitors obeys El-Awady, Florry-Huggins and Temkin isotherms. In addition, surface analysis and UV-Visible spectroscopy were also employed to confirm the inhibition performance of Fluorescein derivatives on the mild steel surface.