New symmetric N,N'-bis(thioamido)thio/carbohydrazones (1-4) were prepared from thio/carbohydrazide and various isothiocyanates with high yields and characterized the structures of the compounds. The compounds were evaluated as new corrosion inhibitors for corrosion of mild steel in 0.1 M HCl solutions using polarization and electrochemical impedance spectroscopy (EIS) techniques, and all compounds showed effective inhibitory properties. Antioxidant activity values determined using the DPPH method for all compounds showed higher antioxidant activity than the standard Trolox (5.89 mu M). The IC50 values of the compounds were obtained between 3.45 and 4.25 mu M, and also showed a ranking of 4 > 2 > 3 > 1 > Trolox, with antioxidant activities better than Trolox. Compound 4 (1,5-bis((3-fluorophenyl) carbamothioyl)carbonohydrazide) exhibited the highest activity in terms of both antioxidant and anti-corrosion properties. DFT calculations were also performed to determine the spectroscopic properties of the compounds and to analyze the structure-activity relationship for their antioxidant characteristics. It was investigated how DPPH reactions are affected by the dependence of delocalization indices and charge densities of N-H bonds on intramolecular interactions. As well as the calculations of intramolecular interactions and some QTAIM parameters, Laplacian bond order and bond polarity index (BPI) analysis were also performed to examine the electronic properties of reactive N-H bonds.