Investigation of the anti-corrosion effects of Azadirachta indica leaf extract on various reinforcement steels in 1 M HCl: Results from electrochemical, DFT and simulation studies

• Neem extract is non-toxic and economical compared to synthetic inhibitors. • Compounds of flavonoids, phenols, terpenoids are key for corrosion inhibition. • Analysed ion flow, mechanism, and electrolyte current density variations via COMSOL. • Activation-controlled charge transfer process was confirmed via Tafel and EIS. • In Tafel, the inhibition efficiency decreases in the order of TMX > SD > TMT > CRS. The research explores the efficacy of Azadirachta indica (neem) leaf extract as a green corrosion inhibitor to mitigate corrosion of different steels in 1 M HCl using electrochemical tests - Tafel, Electrochemical impedance spectroscopy (EIS), Linear polarization resistance and Artificial Neural Network (ANN). Furthermore, cyclic voltammetry (CV), Electrochemical Noise, surface studies and COMSOL simulations were performed to evaluate the corrosion effect on TMT. The interaction of bioactive phytochemical compounds towards the metal surface was studied via DFT. Through electrochemical results, the leaf extract in increasing concentrations up to optimum (65 ml) was highly effective in reducing corrosion rate (mm/year) for all - CRS (20.192), TMT (23.03), TMX (32.056) and SD (66.357). From Tafel and EIS, the highest inhibition efficiency of 85.1% and 75.74% was achieved. Addition of neem reduced the current noise fluctuations along with a stable baseline formation, indicating reduced corrosion. Through CV, corrosion occurred at a slower rate due to the formation of an oxide layer in the passivation region, and the corresponding current density was found to be much lesser with addition of neem. Compounds having greater reactivity and less stability were primarily responsible for higher inhibition efficiency. Corrosion mechanism, ion movement, and corrosion rate was predicted using COMSOL Multiphysics and ANN. Through the obtained results it can be concluded that, the non-toxic and economical neem extract was suggested to be used as green corrosion inhibitor as a better replacement to traditional inhibitors.