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The inhibition effect of two pyrimidine derivatives on the corrosion of austenitic stainless steel in 1 M HCl has been studied by Tafel plot, linear polarization, and electrochemical impedance spectroscopy (EIS) at 298 K. Results showed that compounds inhibited steel corrosion in a 1 M HCl solution and inhibition efficiencies increased with the concentration of inhibitor. Polarization curves indicated that all studied pyrimidine derivatives act as mixed type (cathodic/anodic) inhibitors. The adsorption of the inhibitors on the stainless steel surface was found to obey the Langmuir and Dubinin–Radushkevich adsorption isotherm models. Negative values of ΔGads in the acidic media ensured the spontaneity of the adsorption process.

ABSTRACT:The removal of Zn(II) ions from aqueous solution was studied using natural, thermally-activated and acid-activated kaolinite samples at different temperatures. The linear Langmuir, Freundlich and DubininRadushkevich (D-R) adsorption equations were applied to describe the equilibrium isotherms. The Langmuir constants for natural kaolinite were found to be negative whereas, for the other samples, the isotherm models gave a good fit. In addition, the pseudo-first-order and pseudo-second-order models were used to determine the kinetic data. The experimental data were well fitted by the pseudo-second-order kinetic model. Thermodynamic parameters such as the enthalpy (∆H 0 ), Gibbs' free energy (∆G 0 ) and entropy (∆S 0 ) were calculated for natural, thermally-activated and acid-activated kaolinite. These values showed that the adsorption of Zn(II) ions onto activated kaolinite was controlled by a physical mechanism and occurred spontaneously. The process of adsorption was favoured at high temperatures, with the adsorption capacity of the acid-activated kaolinite being greater relative to that of natural and thermally-activated kaolinite at various temperatures.

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