Inorganic Compound (Apatite doped by Mg and Na) as a Corrosion Inhibitor for Mild Steel in Phosphoric Acidic Medium

The corrosion inhibition of mild steel in phosphoric acid solution containing a mineral compound based phosphate (apatite) was evaluated by potentiodynamic polarization and electrochemical impedance spectroscopy. Potentiodynamic polarization measurements reveal that the inhibition efficiency increased with the concentration of the apatite which appears to be a cathodic type inhibitor in the medium acid(H3PO4 2.0 M). Electrochemical impedance spectroscopy confirms this result, indeed the transfer resistance increases with apatite concentration. We note that the double layer capacitance decreases simultaneously suggesting the formation of an adsorbed layer on the mild steel surface. The inhibition mechanism has been elucidated by a thermodynamic study which showed that the film was formed by physi-sorption. The adsorption model obeys to the Langmuir adsorption isotherm. The parameters of activation energy was evaluated and discussed.