OPTIMIZATION AND THERMODYNAMICS STUDY OF CORROSION INHIBITION OF MILD STEEL BY EUPHORBIA HETEROPHYLLAEXTRACT IN HYDROCHLORIC ACID MEDIUM
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Date
2017
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Abstract
This study investigated the interaction of factors influencing the corrosion inhibition efficiency of Euphorbia heterophylla extract on mild steel in hydrochloric acid medium. It also developed a model for the inhibition process, optimised and obtained the thermodynamic parameters and adsorption mechanisms of the process with the view to establishing the inhibition mechanism.
A four- factor weight loss experiment was designed for the determination of inhibition efficiency and corrosion rate using Central Composite Design (CCD) of Response Surface Methodology (RSM). The results were statistically analysed and quadratic model equations were developed for inhibition efficiency and corrosion rate. The quadratic models were validated in terms of R-Squared values and the Analysis of Variance (ANOVA) at 95% Confidence Interval. The process was optimized using numerical optimisation technique with a goal of obtaining the highest inhibition efficiency and lowest corrosion rate. With relevant equations, thermodynamic and adsorption parameters such as activation energy, enthalpy, entropy, adsorption isotherm, adsorption equilibrium constant and adsorption Gibb’s free energy change of the process were obtained.
The results showed that optimum inhibition efficiency of 89.8% is obtainable at the combination of extract concentration of 1.97 g/l, acid concentration of 0.5 mol/dm3, temperature of 47.40C and immersion time of 4.5 hours. The optimum efficiency was higher than the maximum inhibition efficiency of 85.7% obtained from the weight loss experiment.There was a close agreement between the actual and predicted inhibition efficiency, actual and predicted corrosion rate indicating the suitability of the models. For all acid concentrations considered, inhibition efficiency decreased as the acid concentration increased. Corrosion rate declined with immersion time and the gradient of the decline remained approximately constant for all the temperature considered. The activation energy is higher for inhibited process than for uninhibited process. The enthalpy change, ΔH is positive and increased with immersion time but decreased with extract concentration. The adsorption equilibrium constants, Kads are positive, and increased with temperature until 600C.The thermodynamics parameters obtained indicated physical adsorption inhibition mechanism. The adsorption study showed that the data fit into Langmuir, Flory-Huggins, El-Awary’s and Temkin isotherms but Flory-Hugins gives the best fit.
The study concluded that the interactions of extract concentration, acid concentration, temperature and immersion time on the corrosion inhibition efficiency of Euphorbia heterophylla extract on mild steel in hydrochloric acid medium are significant. The use of RSM is appropriate in modelling and optimizing the inhibition process. The thermodynamics and adsorption characteristics of the process showed that the process is feasible, spontaneous and is by physical adsorption mechanism according to Flory-Hugins isotherm model.
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Keywords
thermodynamics, study of corrosion, inhibition of mild steel, euphorbia heterophyllaextract in hydrochloric acid medium, inhibition mechanism