Effect of Co Content on Corrosion Resistance of Electroplated Zn-Co Alloys from Sulfate Solutions

In response to the increasing demand for improved corrosion resistance on steel parts of automobiles and aircraft, plating with Zn-based alloys is being applied in place of pure Zn plating. The effects of Co content and deposited phase on the corrosion resistance of Zn-Co alloy electrodeposited under various conditions from sulfate solutions on iron substrate specimens have been investigated by ICP, X-ray diffraction analysis, EPMA, and salt spray tests. X-ray diffraction analysis revealed that Zn-Co alloy electrodeposits having a Co content of 10∼20wt.% were γ-phase (Co5Zn21), those having a lower Co content were dual (η+γ) phase, and that those having a higher Co content were dual (α+γ) phase. In salt spray tests, γ-phase deposits of 10∼20wt.% Co content showed the best corrosion resistance. The corrosion product layer consisted only of ZnCl2·4Zn(OH)2 which has low electrical conductivity. It is suggested that this corrosion resistance is attributable to the effects of the γ-phase and corrosion product.