Evaluation of the efficiency of divalent cobalt and copper chelates based on isatin derivatives and thiosemicarbazide ligands as inhibitors for the corrosion of Sabic iron in acidic medium

Divalent cobalt and copper chelates of the two ligands 1-(1-ethoxycarbonylmethyl-2-oxoindolin-3-ylidene) thiosemicarbazide (EOIT) and 1-(1-benzyl-2-oxoindolin-3-ylidene) thiosemicarbazide (BOIT) are the target compounds of the current study. Identification of the structures and geometries of these compounds have been performed using the possible physicochemical and analytical instruments. Elemental analysis, molar conductance and thermal analysis assured the composition of the four chelates to be [Co(BOIT)Cl]•1·.5H2O (BOIT-Co), [Cu(BOIT)Cl] (BOIT-Cu), [Co(EOIT)Cl2(H2O)]•5H2O (EOIT-Co) and [Cu(EOIT)Cl]•Cl (EOIT-Cu) which was further confirmed by the measurement of mass spectra. The architecture arrangement of the ligand atoms around Co and Cu centers has been determined depending on the UV–Vis spectral measurements and calculation of µeff values. This proved that the copper compounds were square planer whereas the cobalt complexes were tetrahedral or octahedral arrangements. These compounds were examined as corrosion inhibitors for Sabic iron in 1.0 M HCl solution using potentiodynamic polarization, electrochemical impedance spectroscopy, mass-loss and scanning electron microscopy techniques. The acquired outcomes disclosed that the examined compounds were found to have high inhibition efficiencies (% IEs) which were dependent on their concentrations and composition. The obtained high % IEs (83–87% at concentration of 400 mg/l) were interpreted by strong adsorption of the compounds’ molecules on the iron surface and such adsorption was discovered to follow Langmuir adsorption isotherm. At the same concentration, the % IEs are slightly increased in the order: EOIT-Cu > BOIT-Cu > EOIT-Co > BOIT-Co. Finally, the investigational results obtained from all employed techniques were in a good consistent with each other.