Effect of Thiosemicarbazones on Corrosion of Steel in Phosphoric Acid Produced by Wet Process

Corrosion inhibition of steel in phosphoric acid (H3PO4) by thiosemicarbazide derivatives was studied using different chemical and electrochemical techniques. Protection efficiency up to 99% was obtained with small amounts (10−4 M) of cinnamaldehyde thiosemicarbazone (CTSCN). The order of increasing inhibition efficiency was correlated with the modification of the molecular structure of the inhibitors. Empirical kinetic relationship was obtained describing the experimental data obtained from the different compounds used in this investigation. Potentiodynamic polarization curves indicated that the compounds acted primarily as mixed-type inhibitors. Electrochemical impedance spectroscopy showed that the charge-transfer resistance increased and the capacitance of the double layer decreased with increasing the concentration of the inhibitor in the medium, confirming adsorption process mechanism. At high concentrations (> 104 M), the capacitance of the double layer leveled off since maximum double-layer thickness was attained. Kinetic-thermodynamic model and Flory-Huggins adsorption isotherm described the experimental findings. Number of active sites, binding constant, and change of free energy were computed for all inhibitors studied. Based on the inhibitor, it was found that each organic molecule replaced one or more adsorbed water molecule from the steel surface. The influence of exposure time on the performance of crotonaldehyde thiosemicarbazone (CrTSCN) was studied. Results showed that the inhibitor performed better with time and at a critical concentration of 5 x 10−4 M. Maximum protection efficiency was attained because of a strong adsorption on steel surface over the total period of immersion exceeding 30 h.