Abstract
The inhibition activity of a new tetraphosphonic acid (TPA), 2-hydroxy-5-[4-hydroxy-3,5-bis(phosphonomethyl)benzyl]-3-(phosphonomethyl)benzylphosphonic acid, for carbon steel in aerated 3 % NaCl solution, at 1000 rpm, was investigated using open circuit potential (OCP), potentiodynamic polarizations, and electrochemical impedance spectroscopy (EIS) to evaluate the TPA inhibition efficiency. The steel surface was also examined by SEM observations and energy-dispersive X-ray (EDX) analysis. The inhibition efficiency increased with TPA increasing concentration up to 10−3 mol L−1 where the highest inhibition efficiency (88 %) was obtained. The thermodynamic parameters—adsorption equilibrium constant, standard free energy, and activation energy—were calculated to determine the corrosion inhibition mechanism. Results from potentiodynamic polarization and electrochemical impedance spectroscopy revealed the mode of inhibitive action and adsorption of inhibitor molecules. Further, surface morphological examination supports the protective film formation by TPA on carbon steel surface. Inhibitor adsorption was spontaneous (ΔG < 0), supported the physical/chemical adsorption mechanism, and obeyed to the Langmuir adsorption isotherm.
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