Three new heterocyclic diazepines (CC–diCl, CC–Cl and CC–F) were synthesized for the first time from the natural product curcumin and evaluated as non-toxic corrosion inhibitors for carbon steel (CS) in 1 M HCl. The evaluation was carried out using electrochemical impedance spectroscopy, polarization curves (PDP) measurement, weight loss (WL) and scanning electron microscopy studies. The molecular modeling methods: density functional theory calculations and Monte Carlo simulations were performed to obtain a good insight into the inhibition process. Results obtained from electrochemical techniques and WL tests revealed that, the studied diazepines were effective corrosion inhibitors for CS in the tested medium. Among the evaluated diazepines, CC–diCl showed the highest efficiency. According to the PDP results, the diazepines behaved as mixed-type (cathodic and anodic) inhibitors. A complete thermodynamic study was also performed on the diazepine with the highest efficiency CC–diCl to gain a better understanding of the inhibition process. The adsorption of the diazepines onto CS surface obeyed a Langmuir adsorption isotherm. A good correlation was obtained between the molecular modeling studies and inhibition efficiency (IE%) of the tested diazepines. © 2017, The Indian Institute of Metals - IIM.