Synergistic Effect of Al ³⁺ and Sodium Gluconate in Phosphate-Based Passivation of Electroplated Zinc

The toxicity and carcinogenicity of hexavalent chromium have led to its prohibition in zinc passivation, driving the development of chromium-free passivation. This study developed a P–Al–SG composite chromium-free passivation system for electroplated zinc, using disodium hydrogen phosphate (Na 2 HPO 4 ) as the base passivator, aluminum nitrate (Al(NO 3 ) 3 ) and sodium gluconate (SG) as additives. The synergistic effects of Al(NO 3 ) 3 and SG were investigated by comparison with Na 2 HPO 4 passivation alone. The results indicated that the corrosion current density ( i corr ) of the P–Al–SG passivation film was 2.772 × 10 –7 A/cm 2, 2 orders of magnitude lower than that of bare zinc, and lower than that of the single phosphate-based passivation film at 1.459× 10 –6 A/cm 2 . This enhancement was attributed to the formation of aluminum phosphate (AlPO 4 ) and aluminum oxide (Al 2 O 3 ) derived from Al(NO 3 ) 3, together with an organic adsorption layer formed by SG. This combination provides valuable insights for the development of environmentally friendly chromium-free passivation technology on electroplated zinc surfaces.