Corrosion inhibitors based on leafs have been suggested as a potential candidate due to their availability as natural materials, environmental friendliness, low cost and regeneration. In this study, the chemical investigation of Barringtonia acutangula leaf - water extract (BALE) was conducted by chromatography and nuclear magnetic resonance (NMR). For the first time, the presence of five terpenoids including ilexosapogenin A, arjunic acid, taraxerol, tormentic acid and α-amyryltetracosanoate was reported. In addition, 3000 ppm BALE is shown to be a possible mixed corrosion inhibitor for carbon steel in 1 M HCl solution. A robust and smooth protective layer with 14.3 nm of surface roughness was created, displaying a great inhibition efficiency of 97.01 ± 0.18 %. On the other hand, uninhibited surfaces displayed serious corrosion at a rapid rate. X-ray photoelectron spectroscopy results confirmed the association of the main BALE components with iron oxides/hydroxides in the protective film layer, ensuring higher inhibition performance. Therefore, the work presented potential materials added to the corrosion inhibitor system as well as a novel process for precisely characterizing the chemical constituents in plant extracts used as the corrosion inhibitor.