Synergistic inhibition between o-phenanthroline and chloride ion on cold rolled steel corrosion in phosphoric acid

The effects of single o-phenanthroline and the mixture of various concentrations of NaCl and 0.0002 M o-phenanthroline on the corrosion of cold rolled steel in 1.0–4.0 M phosphoric acid have been investigated by using weight loss method and polarization method. The present study revealed that cold rolled steel in phosphoric acid has been more efficiently inhibited by o-phenanthroline in the presence of NaCl than single o-phenanthroline, and inhibition efficiency increases with increasing concentration of NaCl in the presence of 0.0002 M o-phenanthroline. At a relative higher concentration of NaCl, there is the synergistic effect between o-phenanthroline and NaCl. The adsorption of inhibitor accords with the Langmuir adsorption isotherm. Polarization studies show that the inhibitor mainly acts as a mixed type inhibitor for steel in 1.0 M phosphoric acid. Thermodynamic parameters such as adsorption heat, adsorption entropy and adsorption free energy are obtained from experimental data of the temperature studies of the inhibition process at four temperatures ranging from 30 to 45 °C. The kinetic data such as apparent activation energy and pre-exponential factor at different concentrations of the inhibitor are calculated, and the effects of the apparent activation energy and pre-exponential factor on the corrosion rate of cold rolled steel are discussed. At 40 °C, in 1.5–4.0 M phosphoric acid, the concentration pre-exponential factor and acid concentration constant are calculated, the synergistic effect has been observed too, and the inhibition efficiency of mixture slowly decreases with increasing concentration of phosphoric acid. A new equation has been used to explain the dependence of corrosion rate on acid concentration. The inhibition action is satisfactorily explained by using both thermodynamic and kinetic models. The results obtained from weight loss method and polarization method are in good agreement.