Effect of Lanthanum Nitrate Content on Microstructure and Corrosion Resistance of Zn-Al Coating

A water-based chromium free zinc aluminum coating was prepared by using rare earth lanthanum nitrate as an additive, replacing chromic anhydride with silane passivation solution, and adding a small amount of lanthanum nitrate. The structure and properties of lanthanum nitrate coated samples were evaluated by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), neutral salt spray testing, and polarization curves. The results show that the corrosion resistance of the sample containing 5 g/L lanthanum nitrate was the best, and an appropriate amount of lanthanum nitrate could improve the coating structure and make the coating uniform and dense. Lanthanum salt passivation treatment could significantly reduce the corrosion current of coated samples, increase the corrosion potential, suppress the corrosion rate, and improve the corrosion resistance of the coating. Lanthanum salts could also serve as physical barriers to prevent corrosive media from penetrating the coating, but they did not alter the shape of the polarization curve. The optimal dosage of lanthanum nitrate in a water-based chromium free zinc aluminum coating was 5 g/L.