CORROSION INHIBITION OF MILD STEEL IN ACIDIC MEDIA BY 5′-PHENYL-2′,4′-DIHYDROSPIRO[INDOLE-3,3′-PYRAZOL]-2(1H)-ONE

Abstract

The inhibition effect of 5-phenyl-2,4-dihydrospiro[indole-3,3-pyrazol]-2(1H)-one (SPAH) on mild steel corrosion in 1 M HCl and 0.5 M H2SO4 has been studied by weight loss, effect of temperature, electrochemical techniques, and quantum chemical studies. The inhibition efficiency increases with increasing concentration of inhibitor but decreases with increase in temperature. The adsorption of inhibitor on mild steel surface has been found to obey Langmuir and Temkin’s adsorption isotherm. Potentiostatic polarization results revealed that SPAH acts as mixed type inhibitor. The values of activation energy (𝐸𝑎), free energy of adsorption (Δ𝐺∘), enthalpy of adsorption (Δ𝐻), and entropy of adsorption (Δ𝑆) were calculated. Surface analysis (SEM) was also carried out to establish the mechanism of inhibitor on mild steel corrosion in acid medium. Quantum chemical studies using DFT were employed to explain the experimental results obtained in this study and to further give insight into the inhibition action of SPAH on the mild steel surface.