The Study On Process And Corrosion Resistance Of Chrome-Free Dacromet Coating

Chrome-free dacromet is a corrosion-resistant coating to metal substrate containing flake zinc and aluminum. It is formed by dip-drain, brush or dip-spin technique, and readily cures at elevated temperature.The composition of coating was optimized by studying parameters of coating preparation and effects of silanes, rare-earth and thikening agents. High corrosion resistant chrome-free dacromet was obtained. It was found that addition of silanes had obvious effects on stability of solution, adhesion and corrosion resistance. Comparing with them, the best proportion and quantity were gained. Ce(NO3)3, La(NO3)3 were studied, and the results showed that they could enhance the corrosion resistance of coating. Comparing four thickening agents'effects on properties of coating, the proper agents and contents were chose to obtain standard thickness. Besides, electrochemical tests were done to describe corrosion behaviors of Zn in 3.5% NaCl and B515 solution adding different inhibitors. Properties of coating were tested. The results showed that sodium tungstate, methenamine and zinc dihydrogen phosphate can slow corrosion rate of chrome-free coating. Zinc dihydrogen phosphate is the best choice.The composition and structure of chrome-free dacromet coating were studied by means of microscope,XRF,SEM and EDS. Salt immersion test, ammonium nitrate tes, salt spray tests, polarized resistance, electrochemical impedance spectroscopy (EIS) were conducted to investigate the corrosion behavior of chrome-free dacromet coating. The reasons of high-resistance to corrosion were analyzed. Meanwhile, chrome-free dacromet's anticorrosive process and mechanism were discussed in the paper. The coating corrosion was divided into four phases: the infiltration of media corrosion, corrosion product deposition, corrosion rate and diffusion process had reached steady-state, matrix and the coating was damaged. Coating on the substrate protection includes four areas: barrier, electrochemical protection, passive and self-repair.