Study On Effects Of Rare Earth Salt On The Process And Performance Of Chromium-free Zinc-aluminum Coating

The chrome-free zinc-aluminum coating is a kind of inorganic corrosion-resistant coating, which is consisted of metal powder slurry, passivating solution and additives. The coating mainly containing flake zinc and aluminum is formed by dip-drain, brush or dip-spin technique, and readily cures at elevated temperature. For its many advantages such as high corrosion resistance, pollution-free and heat-resistance, it can provide corrosion protection to metal substrate.Based on the existing research archives of our team, the coating appearance, adhesion and corrosion testing as a factor to indicate, the better technical parameters of the chrome-free zinc-aluminum coating were determined:volumetric ratio of KH-560, methanol and water is 1:2.5:5, pH is 8.0～9.0, hydrolysis is at 25℃for 48h; With the hydrolyzate be dealt with metal powder slurry (mass ratio of Zn, Al and PEG is 4:1:2) to prepare the coating solutions (mixture stirring time is 3～5h), meanwhile, additives added into the solution.Effect of rare-earth lanthanum or cerium salt on chromium-free dacromet coating were investigated by various tests such as scanning electron microscope (SEM) and natural salt spray (NSS) tests, EDS, electrochemical tests including weak polarization curve and electrochemical impedance spectroscopy (EIS) tests. It was shown that the compactness and corrosion resistance of chromium-free dacromet coating could be significantly improved by adding the proper amounts of rare earth salt. The chromium-free dacromet coating with 7g/L La(NO3)3 or lOg/L Ce(NO3)3 possesses the best corrosion-resistance (salt spray resistance up to 1200 h or 1000h). Using the metallographic microscope and scan electricity mirror to analyzing coating surface appearance and section appearance, the results show that the coating is uniformity and continuum, and which is tight combined with metal substrate. Using the power spectrum analyzer and X beam diffraction instrument to analyzing coating element composition and phase composition, the results show that the coating consisted of Zn, Al, Si, La or Ce, C and O; Zn and Al existed largely in metallic state; The other elements existed with the form of oxides and hydroxides. Passivation effect of rare-earth lanthanum salt on chromium-free dacromet coating were investigated by electrochemical tests including weak polarization curve and electrochemical impedance spectroscopy (EIS) tests. Result shows that the appropriately adding of rare-earth lanthanum salt can decrease the corrosion current and increase the polarized resistance, and then significantly improve the corrosion resistance of the coating. The structure model of chromium-free zinc-aluminum coating was developed by investigating the corrosion behavior of the coating and analyzing the reasons of high-resistance to corrosion. Meanwhile, the anticorrosive process and mechanism of chrome-free zinc-aluminum coating were discussed in the paper.