Non-toxic plant extract as corrosion inhibitor for chill cast Al-Zn-Mg alloy in caustic soda solution

The corrosion inhibition of Hibiscus Teterifa on the corrosion of chill cast Al-Zn-Mg alloy in 0.5 Molar solution of NaOH was studied using weight loss method. The alloy of composition 4.5% Zn, 2% Mg and balance Al was chill cast at the Foundry Shop of the National Metallurgical Development Centre, Jos, Nigeria. After casting, the alloy was cut and machined to corrosion coupons and immersed into 0.5 M solution of NaOH containing varying concentrations of the inhibitor (5-20% v/v) at temperatures of 30, 50 and 70°C respectively. It was found that the adsorption of Hibiscus Teterifa could prevent this alloy from weight loss and the adsorption accorded with the Langmuir adsorption isotherm. Thermodynamic parameters such as adsorption heat and adsorption free energy were obtained from experimental data. The kinetic data activation energy at different concentrations of the inhibitor was calculated. The most suitable range of inhibitor concentration was discussed. The inhibitive action was satisfactorily explained by using both thermodynamic and kinetic models. The mechanism of inhibition is by physical adsorption and the adsorbed molecules of the inhibitor lies on the surface of the alloy blocking the active corrosion sites on the alloy hence, lowering the corrosion rate.