Study Of Trivalent Chromium Black Passivation Technology For Zinc And Zinc-Iron Alloy Plating Coating

Black chromate passivation films on zinc and zinc-iron alloy coating have been used to improve the corrosion resistance, and the decorative properties of the substrates. The Cr(VI) is toxic and carcinogenic, which is hazardous to man or the environment. A nontoxic trivalent chrome passivation film has similar functional properties, and becomes a viable Cr(VI)-free conversion coating as the most adequate alternative to date. In the present paper, trivalent chrome black passivation technologes on zinc and zinc-iron alloy coating were investigated. The forming process and the corrosion mechanism of the black trivalent passivation film were also discussed.In this paper, the primary trivalent chromium black passivation technologies on Zn and zinc-iron alloy coating were confirmed by single factor and orthogonal test methods. The passivation films obtained from the two passivation baths have bright black surface with uniform color, good adhesion to the coating. However, the corrosion resistance of the black passivation film is poor for that the Pb(Ac)2 corrosion time was only 5~8 s, and the presence of white rust on the passivation after exposure in Neutral Salt Spray (NSS) was only 24 h.A one-step of passivation process on zinc and zinc-iron alloy coating was investigated by Pb(Ac)2 corrosion, NSS, Tafel polarization and electrochemical impedance spectrum. The effects of this passivation process on the film corrosion resistance were compared with the commercial sealing agent. The results show that the appropriate additive agent can improve the corrosion resistance of passivating film. The optimal passivation technologies of trivalent chrome black passivation on zinc and zinc-iron alloy were confirmed based on the primary passivation technologies by the above experiments. These two passivation baths include chromium (III) ions, nitrate ions, cobalt (II) ions, nickel (II) ions, phosphate, organic acid and additive agent A. As the results, the passivation film obtained from the optimal technology has bright black surface with uniform color, good adhesion to the electroplated zinc and zinc-iron alloy coating. The passivation film has preferable corrosion resistance, and no white rust was generated on the passivated zinc and zinc-iron alloy coating after exposure in NSS for 96h, which is equivalent to that after sealing process. The performance of corrosion resistance is studied in the different temperature of roasting. The results show that full aging under some appropriate temperature can improve the characteristic of anti-corrosion to some extend.The properties of trivalent chromium passivation film at different time and temperature were studied by measuring the Tafel polarization, EIS in combination with atomic force microscopy, scanning electron microscope, X-ray fluorescence spectrometer and X-ray photoelectron spectroscopy surface analysis. A model for understanding the trivalent chromium black passivation process was proposed based on the experimental data. The results show that the Cr, Zn, P and O were the main components of the coating. These elements of the coating were in the form of Cr2O3, Zn(OH)2 and phosphate. It also demonstrates that the trivalent chromium black passivation film acts as a barrier, which prevents the contact of substrate Zn with corrosive environment. The many small cracks on the black coating surface damage the integrality of the coating and decrease the corrosion resistance.