Evaluation of the protective effectiveness of Cr(III)-based passivation treatments, on galvanized steel sheets for automotive applications

The characteristics and the protective effectiveness of two new kinds of Cr(III)-based passivation treatments, applied on galvanized steel sheets for car body making were studied. The parameters of the zinc deposition bath and the characteristics of the passivation treatments are reported in Table 2 and 3, respectively. The corrosion behavior of the galvanized and passivated steels was evaluated by means of electrochemical techniques Such as the recording of polarization curves, Figure 2, free corrosion potential measurements, Figure 3, electrochernical impedance measurements vs time, Figure 4, performed in aerated 0.1 M NaCl solution. For comparison purpose, the same characterization was performed on galvanized steel sheets passivated in a Cr(VI)-containing bath. The chromium content of the passivating layers was assessed by means of atomic absorption spectroscopy (AAS), Table 4. The passivating layers were characterized both before and after the corrosion tests by means of scanning electron microscopy (SEM) coupled with EDS microanalysis, Figures 1 and 5. The electrochemical tests showed a better protective effectiveness for the passivating layer obtained in the bath containing Cr(III) and cobalt salts, whereas the worst behavior was observed for the sample passivated in the bath containing Cr(Ill) only. The traditional passivation treatment with Cr(VI) exhibited an intermediate behavior. The different corrosion resistance may be due to the different thickness of the passivating layers. This factor significantly influence the harrier effect of the insoluble Cr(III) compounds towards the aggressive species present in the aggressive environment. Compared with the traditional Cr(VI) treatment the higher protective effectiveness of the passivating layer formed in the Cr(Ill)and cobalt salts-containing bath, having the same thickness, can be attributed to its higher Chromium Content. Moreover, the presence of cobalt salts in the passivation bath can contribute to increase the anticorrosion properties of the passivating layer.