Two novel thiosemicarbazone linked with 1,3,4-thiadiazole ligands were synthesized through the condensation reaction of the acetyl group at position 5 of 1,3,4-thiadiazole derivatives with thiosemicarbazide in acidic ethanol solution in addition to their Co(II)-complexes. The divalent cobalt complexes of both ligands have been synthesized through the reaction of cobalt acetate with each ligand leading to the formation of the two complexes [(LH)Co(OAc)]0.5H2O and [(LCl)Co(OAc) (H2O)2]; where LH and LCl are 1,3,4-thiadiazolethiosemicarbazone ligands. The two complexes have been proved to have tetrahedral and octahedral geometry from the results of UVeVis spectra and magnetic moment. Analysis of the IR spectra of both ligands and complexes ascertained that the ligands behaved as monobasic tridentate coordinating to the metal ion center through azomethine-nitrogen atoms and thiadiazole ring in addition to deprotonated SH group. The synthesized ligands and their complexes were also investigated as inhibitors for the corrosion of carbon steel in 1.0 M HCl using weight loss, potentiodynamic polarization, electrochemical impedance spectroscopy and scanning electron microscopy techniques. The obtained results revealed that the inhibition efficiencies of the tested derivatives were found to depend on the concentrations and structures of such derivatives, and were decreased with raising temperature. At the same inhibitors concentration, the inhibition efficiencies are slightly increased in the order: LH > LCl > LH-Co > LCl-Co. The results derived from all applied techniques are harmonious with each other.