Improve the performance of Cr-free passivation film through nanoelectrodeposition for replacement of toxic Cr^6 + passivation in electrogalvanizing process

To improve the corrosion resistance of Cr-free passivation film on zinc coating for replacing the highly poisonous Cr^6 + passivation technology used in electrogalvanizing steels, we synthesized Cr-free passivation film on nanocrystalline zinc coating through a combination of Cr-free passivation and nano-electrodeposition. It was demonstrated that the surface brightness, mechanical strength, scratch resistance and corrosion resistance of zinc coating with Cr-free passivation were significantly enhanced as the coating's grain size was reduced from micro-scale to nano-scale. The nanocrystalline zinc coating after Cr-free passivation treatment exhibits superior properties than the coarse-grained zinc coating experienced conventional Cr^6 + passivation treatment. The overall performance of the former is approximately the same as that of nanocrystalline zinc coating with Cr^6 + passivation, suggesting that the surface nanocrystallizaiton is more effective than the chemical composition of passivation bath to improve the properties of the passive film. Mechanisms responsible for the improvement in properties of Cr-free passivation film on coarse-grained zinc coating by nano-electrodeposition are analyzed and discussed from the viewpoints of electron localization function and electron work function.