Synergistic interactions between tetra butyl phosphonium hydroxide and iodide ions on the mild steel surface for corrosion inhibition in acidic medium

The corrosion inhibitive action of tetra butyl phosphonium hydroxide (TBPH) on mild steel was investigated in 0.5 M H2SO4. On adding the potassium iodide (KI) to the solution corrosion inhibition efficiency was significantly improved. The inhibitive action of TBPH was investigated at various inhibitor concentrations and at various temperatures; however, a synergistic improvement in the performance was seen when 10(-3) M KI was added to TBPH solution. Potentiodynamic polarization studies at various concentrations of TBPH and (TBPH + KI) inhibitor revealed that TBPH is a mixed type of inhibitor for mild steel in 0.5 M H2SO4. Synergistic effect of TBPH and KI in corrosion inhibition of mild steel in 0.5 M H2SO4 containing a low concentration of iodide ions has been evaluated by potentiodynamic polarization and the results suggest that the iodide ions in solution stabilized the adsorption of TBPH molecule on the metal surface and improved the inhibition efficiency of TBPH. The corrosion behavior of steel in 0.5 M H2SO4 in the absence and with various concentrations of the inhibitor was studied from (298 to 328) K. It was found that the inhibition efficiency increases with inhibitor concentration and decreases with TBPH and (TBPH + KI) solution with an increase in temperature. Potentiostatic polarization study shows that TBPH and (TBPH + KI) are passivation type of inhibitor. The adsorption of TBPH is found to follow Frumkin adsorption isotherm whereas the adsorption of (TBPH + KI) follows the Temkin adsorption isotherm. Kinetic and thermodynamic parameters namely effective activation energy (E-a), and Gibbs free energy of adsorption (Delta G degrees(ads)) indicate that adsorption of TBPH and (TBPH + KI) on mild steel surface is primarily physical in nature. The results of scanning electron microscopy, atomic force microscopy, and quantum chemical analysis support the above inferences. (C) 2016 Published by Elsevier B.V.