Research And Application Of Zn-Al-Cu Alloy Based Surface Passivation Film

Electroplating Zn-Al-Cu alloy（hereinafter referred to as zinc alloy）is one of the more widely used metal corrosion protection measures.With the development of technology,simple alloy plating can no longer meet the metal’s requirements for corrosion resistance,so it must be passivated.The more widely used in industry are hexavalent chromium passivation and trivalent chromium passivation.However,hexavalent chromium in chromates is highly toxic and carcinogenic,and is a serious hazard to human health and the natural environment,so the use of hexavalent chromium is strictly limited by the state.Although the toxicity of trivalent chromium is only 1%of that of hexavalent chromium,it will also be converted to hexavalent chromium during transportation and use,which also has an impact on human health.Therefore,there is an urgent need to develop an alternative,environmentally friendly passivation treatment.The corrosion resistance of passivation films produced by inorganic salt passivation systems or organic compound passivation systems is not ideal.The compound passivation system combines organic and inorganic salt characteristics,and the prepared compound passivation film has a strong corrosion retardation capability,and under certain conditions,the protection effect can approach or even exceed that of chromate passivation film.In order to explore a good corrosion resistance and green passivation technology,this thesis experimentally studied the passivation process based on zinc-iron alloy plating and investigated the formation mechanism of passivation film organization and the influence law of performance by using XRD,electrochemical test,salt spray test,optical microscope,SEM,EDS,surface roughness and other characterization test methods,and obtained new silicate-tannic acid composite passivation and molybdate-phytate composite passivation processes.Single-factor experiments were conducted to determine the optimal content range of each component of the passivation film,and then the optimal ratio of the passivation solution was further determined.The optimal process for sodium molybdate-phytate passivation was obtained as follows:Na₂Mo O₄ 45 g/L,C₃H₄O₄ 10 g/L,Ni SO₄ 1.2 g/L,C₁₈H₂₉Na O₃S 2.0 g/L,C₆H₁₈O₂₄P₆ 10 ml/L,KMn O₄ 1.8 g/L,passivation time of 30 s,passivation solution at 25℃,nitric acid adjustment p H was 1.5,and the drying method was hot air blowing.Under this process,a bright blue-white passivation film with uniform color distribution was obtained.The best process for silicate-tannic acid passivation is:Co（NO₃）2 2.5 g/L,C₇₆H₅₂O₄₆ 3 ml/L,Na₂Si O₃ 25 g/L,C₆H₅Na₃O₇ 15 g/L,organic accelerator,temperature 25℃,passivation time of 30,p H adjustment to 1-2 range,passivation at room temperature,and hot air drying.The surface morphology and corrosion resistance of the molybdate-phytate composite passivation specimens,unpassivated specimens and trivalent chromium passivated specimens were investigated separately to compare the differences in corrosion resistance between the three and to derive the optimal process.The corrosion resistance of the passivated and unpassivated specimens was tested by electrochemical testing and neutral salt spray testing.It was concluded that molybdate-phytate composite passivation gave the best corrosion resistance.SEM,EDS,XRD and surface roughness tests were used to test the passivation film for quantitative elemental analysis and surface morphology,and the composition of the passivation film was finally obtained.The study showed that a molybdate-phytate composite passivation film was successfully prepared on the surface of the Zn-Fe alloy coating,and the presence of this passivation film could effectively improve the leveling and reduce the presence of dendritic structures,thus significantly improving the corrosion resistance of the substrate surface.Compared with the conventional chromium passivation film,the molybdate composite passivation film has a flatter microscopic surface and less dendritic structures,representing a more uniform and dense surface after the composite passivation.Tafel curves show that the E_corrof the prepared composite passivation film is more negative and the I_corr is smaller compared with the trivalent chromium passivation film.the EIS impedance diagram shows that the composite passivation film has a larger impedance diameter,and the fitted The R_p value of3582.9Ω·cm² for the composite passivation film is larger than that of the other passivation films.These indicate that the resulting composite passivation film has better corrosion resistance and provides better surface protection than the conventional passivation film with molybdate-phytate.The process determination and performance investigation of silicate-tannic acid composite passivation films were carried out using the same principle approach.The experimental results show that tannic acid can provide hydroxyl and carboxyl groups to promote the formation of passivation films.The film formation process of the silicate composite passivation film may be a coordination interaction between the negatively charged Si O^2-or Si O₂ gels and Zn²⁺.The passivation film has no effect on the crystal structure of the coating,which is very thin.However,the microscopic morphology observed by SEM shows that the dendritic structure of the plated surface is changed by the passivation layer,resulting in a reduction of surface defects and a decrease of the specific surface area.By comparing the silicate composite passivation system and the trivalent chromium passivation system proposed in this thesis through characterization such as surface micromorphological analysis and corrosion resistance test on different samples prepared,the results proved that the composite passivation film formed by using this formulation has better corrosion resistance than the conventional chromium passivation film,and theoretically has the development prospect of replacing the toxic chromium passivation system.