Multi-functional protective epoxy coatings were applied to display synergistic resistance properties on the steel/coating interfacial zone of petroleum storage tanks (PST). This work investigated the synthesis of new mixed Ni(II), Pd(II), and Cd(II) complexes of metformin (MF) and 2,2/bipyridine (Bipy) as internal cross-linking accelerators for steel epoxy coatings. The ligands and their complexes were identified by CHN analysis, molar conductance, magnetic tendency, TGA/DTG, FT-IR, 1H NMR, and UVeVisible analysis. FT-IR results underlined that MF and Bipy were chelated with metal ions as bidentate ligands through two imines (eC]NH) groups of MF and via two nitrogen of pyridine rings for Bipy. Polyamine cured epoxy coating formulations loaded with the same concentration of MF, Bipy, and mixed complexes were prepared. The corrosion inhibiting demeanor evaluated by the salt spray corrosion trial demonstrated that the minimum corrosion rate (CR) was calculated at 0.00028 mm/y. It also exhibited a superior protection efficiency (PE) of 99.90% for EPO/MC-Ni coating with an outstanding self-healing performance. AFM microstructure survey affirmed the smoothest and harder epoxy composite matrix of EPO/MC-Ni coating without the appearance of protuberances over the coating surface. EIS measurements were carried out using oil wells formation water aggressive medium to affirm the protective mechanism offered by the checked coatings. The electrochemical results displayed that EPO/MC-Ni coating exhibited the best resistance values (Rct ¼ 995 and Rp ¼ 945 kU cm2) and achieved the highest protection efficiency at 97.23%. Mechanical efforts on the modified Ni(II) coating layer emphasized the best adhesion pull-off evaluation at 6.7 MPa and impacted resistance of 67.3 J.