Effect of temperature on stability of adsorbed inhibitors on steel in phosphoric acid solution

Rates of steel dissolution in 35% H3PO4/6% butanol (test solution) in the presence of thiosemicarbazide and seven of its derivatives were determined by spectrophotometry, weight loss and potentiodynamic and impedance techniques in the temperature range 303 to 333 K. At all temperatures, the corrosion rate decreased with increasing inhibitor concentration. Increasing temperature decreased the protection efficiency particularly at concentrations less than 2.5 x 10(-4) M. At inhibitor concentrations above 2.5 x 10(-4) M, increasing the temperature did not affect efficiency. Potentiodynamic polarization measurements indicated that the inhibitors have a strong effect on the corrosion behavior of the steel and behave as mixed type inhibitors. Thermodynamic functions obtained from this study indicate that the presence of the inhibitors increase the activation energy. The negative values of DeltaG(*) indicated spontaneous adsorption on the metal surface. A kinetic-thermodynamic model was found to describe the experimental well data at different temperatures.