Effect of Rare Earth on Corrosion Resistance of Trivalent Chromium Color Passivation Film on Electroplated Zinc Coating

Rare earth elements(La3+,Ce3+ and Ce4+) were introduced into trivalent chromium passivation fluid.The effect of rare earth content on the corrosion resistance of the color passivation film on electroplated Ni coating was investigated by dripping test of aqueous lead acetate,measurement of Tafel curves,and saline water soaking test.It was found that the addition of rare earth contributed to improved corrosion resistance of trivalent Cr passivation film even in the absence of sealing,and Ce4+ was the most effective in improving the corrosion resistance.When Ce(SO4) 2.4H2O was introduced into the passivation fluid at a concentration of 5.0 g/L,the endurance of the trivalent Cr passivation film against dripping of lead acetate was increased from 19.33 s to 157.56 s,while its corrosion potential was shifted from-1.006 V to-0.982 V and its corrosion current density was reduced from 3.268×10-5A/cm 2 to 1.116×10-5 A/cm 2.In the meantime,introducing 5.0g/L Ce(SO 4) 2.4H 2 O helped to improve the resistance of trivalent Cr passivation film against saline water.After being immersed in saline water for 336 h,the film was still free of rusted locations and lost weight slightly.Moreover,the surface of the greenish yellow passivation film had even and deep grooves,which were in favor of improving the corrosion resistance of the passivation film.