Semiconductor Property and Corrosion Behavior of Passive Film Formed on Steel with Zinc Coating in 5% NaCl Solution

Abstract

The initial stage of corrosion behavior and the semiconductor property of a passive film formed on steel with zinc coating were investigated in 5 wt.% NaCl solution using scanning electrochemical microscopy (SECM), electrochemical impedance spectroscopy (EIS), potential-capacitance measurement and the Mott–Schottky analysis method. The SECM test results reveal that the Faradic currents on the tip of the ultramicro electrode decreased with the extension of immersion time, suggesting that the coating was slowly degrading, and the water and ions gradually permeated into the coating. The tip current remained stable without obvious bursting and swelling, which revealed that no blister occurred inside the coating. EIS results also support the above conclusion, especially for long-term immersion. The potential-capacitance and Mott–Schottky analysis show that the water and ions gradually entered into the coating during the immersion process, while at the same time the coating showed the characteristics of n-type semiconductor. As the time elapsed, the capacitance value decreased and the thickness of the space charge layer declined. The decrease in the slope of the Mott–Schottky curves suggested the simultaneous increase of the carrier density, and the coating was degrading. SEM observation also exhibits an obvious coating degradation after immersion compared with the previous coating without immersion in solution.