Preparation and Corrosion Performance of Lanthanum Nitrate Conversion Coating on Hot-dip Galfan Steel

Lanthanum nitrate conversion coating was prepared on a hot-dip Galfan steel by dipping in a passivation solution of La(NO3)3. The surface morphology, chemical composition and structure of the coating were characterized by scanning electron microscopy (SEM) with X-ray energy dispersive spectrometer (EDS), X-ray photoelectron spectroscopy (XPS) and atomic force microscope (AFM). The corrosion resistance of the conversion film coated steel was assessed by neutral salt spray tests (NSS), electrochemical polarization curve and electrochemical impedance spectroscopy (EIS). The results showed that the conversion coating was not uniform, which had grown preferentially on grainand phase-boundaries as well as other active sites. The thickness of the coating increased, while cracks in the coating expanded gradually with the increasing dippingtime. The coating spall off after dipping for more than 30 min, and therewith the protective performance of the coating degraded. In comparison with the blank Galfan steel, the corrosion rate of the conversion film coated Galfan steel lowered down significantly.