Phenothiazine as inhibitor of the corrosion of cadmium in acidic solutions

The corrosion of cadmium in 0.5 M HClO4, CH3COOH or 0.25 M H2SO4 in the absence and presence of small amounts of phenothiazine (10(-6)-10(-4) M) as an inhibitor has been studied using both electrochemical impedance spectroscopy (EIS) and Tafel-plot techniques. Measurements were carried out at cathodic, open circuit and anodic potentials at different temperatures. In HClO4 and H2SO4 solutions the inhibition efficiency increases as both the concentration of phenothiazine and the temperature are increased. The effect of temperature in CH3COOH solution gives an opposite effect, that is, a decrease in the inhibition efficiency with increasing temperature. This behaviour is attributed to competitive adsorption between the inhibitor molecules and the acetate ions at the metal-solution interface. Phenothiazine can be considered as an inhibitor of mixed type with more pronounced effects in the anodic direction. The thermodynamic parameters in HClO4 containing the inhibitor were evaluated using the Bockris-Swinkels adsorption isotherm with n = 4 as the configurational function. These parameters reveal that phenothiazine is adsorbed on the cadmium surface in molecular form with a strong interaction between the metal and the inhibitor.