In the present investigation, three benzimidazole derivatives, namely, 4-(phenylwere synthesized and investigated as inhibitors for mild steel corrosion in 15% HCl solution using the weight loss, electrochemical polarization, and electrochemical impedance spectroscopy (EIS) techniques. It was found that the inhibition efficiency of these inhibitors increased with increasing concentration. The effect of temperature on the corrosion rate was investigated, and some thermodynamic parameters were calculated. Polarization studies showed that all three studied inhibitors were of mixed type in nature. The adsorption of the inhibitors on the mild steel surface in acid solution was found to obey the Langmuir adsorption isotherm. Scanning electron microscopy (SEM) was performed on inhibited and uninhibited mild steel samples to characterize the surface. The semiempirical AM1 method was employed for theoretical calculations, and the obtained results were found to be consistent with the experimental findings.

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The effect of synthesized spiropyrimidinethiones, namely, 6̕ -(4-methoxyphenyl)-1̕ -phenyl-2̕thioxo-2̕ ,3̕ -dihydro-1̕ H-spiro[indoline-3,4̕ -pyrimidine]-2-one (MPTS) and 6̕ -(4-methoxyphenyl)-1̕ -phenyl-2̕ -thioxo-2̕ ,3̕ -dihydro-1̕ H-spiro[indoline-3,4̕ -pyrimidine]-2-one (CPTS) on corrosion of mild steel in 15% HCl solution was investigated by using weight loss and electrochemical methods. Both inhibitors act as mixed inhibitor and their adsorption on mild steel obeyed Langmuir's adsorption isotherm. The potential of zero charge (E PZC ) for the mild steel was determined by electrochemical impedance spectroscopy method. Scanning electron microscopic, Energy dispersion X-ray spectroscopic, FTIR, X-ray photoelectron spectroscopy (XPS) andAtomic Force Microscopy were used to characterize the surface morphology of uninhibited and inhibited mild steel specimens. The Density Functional Theory (DFT) was employed for theoretical calculations.2 processes, particularly with the use of hydrochloric acid, which results in terrible waste of both resources and money. A corrosion inhibitor is often added to acid solutions for minimizing the effect of corrosion on metal during these processes. The selection of appropriate inhibitors mainly depends on its economic availability, environmental side effects, concentration, temperature, type of acid, the presence of dissolved inorganic and/or organic substances even in minor amounts and, of course, on the type of metallic material supposed to be protected 1 .Heterocyclic organic compounds containing sulfur, phosphorus, oxygen, nitrogen and aromatic rings are reported in literature as most effective corrosion inhibitor for the metals in acidic medium 2-9 . Some spiro compounds 10 have been reported as good corrosion inhibitor for Al in NaCl solution, and exhibit different inhibition performance with the difference in substituent groups and substituent positions on the aromatic rings. The inhibitor, MPTS and CPTS contains indoline and pyrimidine rings as two anchoring sites suitable for bonding with the metal surface.So, it was expected that MPTS and CPTS would work as good corrosion inhibitor for mild steel in 15% hydrochloric acid solution.Although, the protection mechanism of organic corrosion inhibitors has not been clearly understood 11 , it is generally accepted that the organic inhibitors decrease the corrosion rate by adsorbing on the metal/solution interface and blocking the active sites by displacing water molecules and forming a compact barrier film on the metal surface 12-14 . The adsorption process depends upon the nature and surface charge of the metal, the type of aggressive media, the structure of the inhibitor and the nature of its interaction with the metal surface 15 . Research activities in present situations are geared towards developing the cheap, non-toxic and environment friendly corrosion inhibitors. Theoretical calculations using Density Functional Theory (DFT) are being used to explain the mechanism of corrosion inhibition [16][17][18] .The objective of th...

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