Preparation And Evaluation Of Non-chromate Conversion Coatings On Metal Surfaces

Non-chromate conversion coatings on metals were investigated in this paper. It could be devided into two parts. One is about the effect of concentration of cobalt cation on ambient temperature zinc phosphating. The other is organic/inorganic titanium, zirconium-based conversion coatings on aluminum with tannin acid and nano-SiO₂ particle dispersion as additives.Phosphating is an important process for surface treating of automotive body. Based on present processs of ambient zinc phosphating, a new kind of zinc phosphating treated in short time and with little cobalt salt added into the treating solution instead of nickel compound was studied. It could get nice phosphate conversion coatings meeting demands of surface treating of automotive body. According to the time- potential curve, it was discovered that the cobalt cation could boost the coatings formation and decrease the phosphate time by more than 5 min. Scanning electron microscopy （SEM） results showed that the addition of Co²⁺ could reduce the phosphate crystal size and make the ramiform crystal structure into cubic crystals. Based on the result of EIS, the ac impedance increased with the Co²⁺ addition and so, the corrosion resistance increased. The results of alkali resistance tests of the phosphate films showed that the addition of the Co²⁺ in phosphate bath could improve alkali stability of the phosphate coatings. The adhesion of phosphate coating with cathode electrophoresis paint and the impact resistance could be improved with the addition of Co²⁺ in ambient zinc phosphate process.The application of chromate conversion coating is limited because of the toxicity of Cr（Ⅳ）. In this paper, titanium, zirconium-based conversion coatings on aluminum with tannin and nano-SiO₂ particle dispersion as additives were investigated. The formulation was confirmed based on the orthogonal experiments. According to the EIS, the effects of concentration of titanium, tannin acid and nano SiO₂ particle dispersion were studied. The results showed that when the addition of tannin was 2 g/L and nano SiO₂ particle dispersion was 6ml/L, the film had the best corrosion resistance. The tafel curves of various films immersed in 3.5 % NaCl solution for different times were tested. The micro structure and component of the film were studied via SEM and EDS.