A novel binuclear Zn²⁺ and Cd²⁺ complexes of carbothiohydrazide chelating agent for the corrosion protection of carbon steel alloy in 15% HCl solution

Equimolar reactions of Zn²⁺ and Cd²⁺acetates with the carbothiohydrazide, [2-HO-C₆H₄-C(H) = N-N(H)-C(=S)-N(C₂H₄)₂O] chelating agent (H₂L) have produced the binuclear [M(L)]₂ metal chelates. The isolated compounds have been evaluated by elemental analyses, XRD, molar conductivities, ¹H and ¹³C-NMR, FTIR, FAB-MS, UV–Vis, and thermal analyses. The crystal structure of the chelating agent has been resolved and indicates the presence of the chelating agent exclusively in its E conformer regarding the C(H) = N bond. Elemental analyses and molar conductance data indicated that the investigated metal chelates are nonelectrolytes and were formed with 2 M:2 L stoichiometry. FT-IR along ¹H-NMR spectral data gave evidence for a di-anionic chelating agent that bonded with central metal ions via azomethine N, deprotonated phenolic O, and thiolate S atoms. The anti-corrosion performance of the chelating agent and its metal chelates have been evaluated for protecting carbon steel (CS) in 15% HCl aqueous solution using electrochemical examination (EIS and PDP), surface examination (SEM and XPS) and theoretical approach (DFT calculations and MC simulation). The results revealed that [Zn(L)]₂ complexes remarkably performed with an inhibition efficiency of about 94.7%, which was better than those for [Cd(L)]₂ complex (93.0%) and H₂L (88.2%), respectively. The EIS results revealed the gradual rise of charge transfer resistance (R_ct) with concentration, while the PDP proved the mixed-type inhibition performance with cathodic advantage consistent with the Langmuir adsorption model. It was verified that the Zn (II) complex could adsorb on the metallic surface, forming a protective layer responsible for mitigating the corrosion reaction and ions diffusion. This study provided novel inhibitors with structures designed and developed for inhibiting the corrosion process with marked performance. The results derived from various methodologies contribute to a substantiated interpretation of the mechanism of inhibition.