Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP) studies were carried out to investigate the comparative corrosion protection efficiency of 3-hydroxy-7-isocyano-8-phenyl-2-(ptolyl) pyrimido[2,1-b][1,3]thiazine-4,6-dione (HPPTD) on carbon steel in 1.0 M HCl. EIS plots indicated that the addition of this inhibitor increases the charge-transfer resistance (Rct), decreases the double-layer capacitance (Cdl) of the corrosion process, and hence increases inhibition performance. At 303 K, polarization measurements indicated that this compound is mixed-type inhibitor. Moreover, the thermodynamic activation parameters for the corrosion reaction were calculated and discussed in relation to the stability of the protective inhibitor layer. The adsorption of the studied pyrimidothiazine derivative obeyed the Langmuir adsorption isotherm.
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