| The corrosion resistance of the hot-dip Galfan coating is more excellent than that of the hot-dip galvanizing coating, thus the application range of the hot-dip Galfan is constantly expanding. To further improve the corrosion resistance of hot-dip Galfan, passivation treatments are typically used. The chromate conversion coating not be applied in hot-dip Galfan due to its environmental hazards. The lanthanum salt can be deposited to form chemical conversion coating on the hot-dip Galfan with no toxicity, no pollution and excellent corrosion resistance. At present, few works have been reported on the lanthanum salt conversion coating on the hot-dip Galfan steel.In this paper, the lanthanum salt conversion coating on the hot-dip Galfan steel was systematically studied by immersing the samples in the solution with La(NO3)3·6H2O as the main salt, H2O2 as an antioxidant and NaF as an accelerant. The optimal technology of the lanthanum conversion coating formed on the hot-dip Galfan steel was studied by means of the orthogonal experiment and single factor experiment. The effects of bath composition, treatment temperature and time on the corrosion resistant of the coatings were discussed. The optimum parameters of the passivation technology were determined as followed: La(NO3)3·6H2O 20 g/L, H2O2 10 mL/L, NaF 0.1g/L, T 70℃, t 10~30 min. Treatment temperature and time were the main affacting factors on the corrosion resistance of the lanthanum conversion coating.The surface morphology, structure and chemical composition of the coating were observed and analyzed by scanning electron microscopy(SEM), X-ray energy dispersive spectrometer(EDS), Atomic force microscope(AFM), X-ray photoelectron spectroscopy(XPS) and X-ray diffration(XRD), and the growth mechanism of the coating was also discussed. The results showed that:(1) The lanthanum salt conversion coating was not uniform, since the coating was formed and grew on the grain and phase boundaries and other active region in priority. The thickness of the coating was increasing and the cracks in the coating were getting wider and wider with the treatment time prolonging. The coating would flake off when the treatment time was more than 60 min. When the treatment time was up to 120 min, some Zn(OH)2/ZnO white particles were deposited on the surface of the coating.(2) The lanthanum salt conversion coating was composed of La(OH)3/La2O3, Al(OH)3/Al2O3 and Zn(OH)2/ZnO. The growth process of the coating could be divided into two stages: at the initial stage, Al(OH)3/Al2O3 and La(OH)3/La2O3 were absorbed on the whole surface of Galfan steel; in the final stage, La(OH)3/La2O3 and Zn(OH)2/ZnO were deposited on the surface of the coating.The corrosion resistance of the lanthanum salt conversion coating was assessed by neutral salt spray(NSS) tests, electrochemical polarization curve and electrochemical impedance spectroscopy(EIS). The self-healing property of the coating was examed by the scratch test. The results showed that:(1) The lanthanum salt conversion coating on Galfan had good corrosion resistance.(2) The lanthanum salt conversion coating could inhibit both the cathodic and anodic reaction of the electrochemical corrosion, and prevent the Galfan substrate from corrosion. Comparing with the untreated Galfan sample, the lanthanum salt conversion coating had larger polarization resistance and the corrosion current density of the coating was decreased significantly. The lanthanum salt conversion coating with 10 min treatment time presented the highest polarization resistance and the lowest corrosion current density, and the optimum corrosion resistance.(3) Comparing with the untreated Galfan sample, the lanthanum salt conversion coating was thicker, less poriosity and quite larger impedance value. The coating could hinder the charge transfer and the electrolyte diffusion. However, with the treatment time greater than 10 min, the cracks occurred in the coating, and the cracks became more and wider with the treatment time prolonging, and the coating could not efficiently hinder the charge transfer and lost the protection of the Galfan substrate.(4) The lanthanum salt conversion coating had good self-healing property. |
PDF Full Text Request