Surface equilibrium and thermodynamics implications of the corrosion inhibition of mild steel in acid medium using quinine inhibitor

Abstract

One problem faced by industries is the degradation of their metal pipes and machine parts which are exposed to chemically aggressive media. This has necessitated the search for cheap and non-toxic substances that can limit corrosion. The inhibition potential of quinine against the corroding of mild steel in 1M HCl medium was tested under varied concentration of quinine and temperature, using gravimetric method. The results showed that corrosion rate and weight loss decreased with increasing concentration of quinine and temperature up to 40 °C, while surface coverage, θ, and inhibition efficiency IE, increased as the concentration and temperature increased. Surface equilibrium studies showed that the inhibition action followed the Freundlich isotherm with value of adsorption intensity that suggests a cooperative adsorption mechanism in heterogeneous sites while thermodynamic analysis implied that the process of inhibition was endothermic having a positive value of ΔH (17.384 kJ mol-1), enthalpy-driven with a negative ΔS (-64.434 kJ mol-1K-1) and occurred by a physisorption mechanism. Positive ΔG obtained at the studied temperatures suggests that the corrosion process was rendered passive and unfeasible. These findings project quinine as a promising inhibitor for chemical degradation of mild steel in hydrochloric acid medium.