Preparation at Room Temperature of Trivalent Chromium Black Passivation Coating on Galvanized Steel and Evaluation of Corrosion Resistance of the Coating

A kind of trivalent chromium passivation solution with stable performance was prepared with the organic sulfide as blackening agent,and then was used for the passivating treatment of the galvanized steel at room temperature.The surface morphology and element composition of the passive films were analyzed by scanning electron microscopy and energy dispersive spectrometer.Moreover,the corrosion resistance of the passive films was investigated by lead acetate spot test,Tafel polarization and AC impedance.Results showed that the passive film was mainly composed of Zn,Cr and O elements.The passive film was black and shiny,showing good decorative property and good adhesion.Furthermore,the galvanized steel sheet after passivation treatment could endure lead acetate spot test for about 71 s and block effectively the anode process of the corrosion,with the positive shift of corrosion potential from-1.283 V to 1.054 V.Meanwhile,the corrosion current density reduced from 12.240 μA/cm~2 to 2.866μA/cm~2,indicating that the corrosion resistance of galvanized sheet was improved significantly by the passivation film.