Evaluation on corrosion resistance of trivalent chromium passivated zinc plating coating by measuring open-circuit potential

Cyanide-free alkaline zinc plating coatings were passivated with three passivators TRI-V 120, TRI-V 121 and SpectraMATETM 25, respectively.The variation of open-circuit potentials of the passivation films with time was studied in 1%(mass fraction) NaCl solutions at different pHs.The results showed that the open-circuit potential vs.time curves are affected by pH.The variation of open-circuit potential with time is regular at pH 3.At the beginning, the potential is high, indicating the existence of a complete passivation layer on zinc surface.Then the potential decreases with time, which indicates the nearly dissolution process of the passivation layer.Finally, the potential reaches a value that is close to the electrode potential of zinc and slightly changed with time, which indicates the complete dissolution of the passivation layer.As compared with the results of neutral salt spray(NSS) test, the time for the potential change from passivation layer to zinc reflects the stability of the passivated zinc plating coating.The longer thetime elapses, the better corrosion resistance of passivation layer is.Therefore, the measurement of open-circuit potential can be used to evaluate the corrosion resistance of passivated zinc coating simply and effectively.