The Study On The Technology And Formation Process Of Trivalent Chromium Passivation Film On Zinc Plating

On the basis of environmental requirements,in order to improve the corrosion resistance of galvanized Iron and steel parts,A trivalent chromium colorful passivation film was obtained on cyanide and alkaline zinc deposit.The passivation fluid composition and preparation technology were optimized by orthogonal experiments and single-factor experiments.A uniform,coherent and good binding of colorful passivation film could be obtained on zinc deposit when under the condition of Cr（NO3）3·9H2O 22g/L,H2SO4 4.0ml/L,HNO3 2.2 ml/L,Na+ 1.1g/L,Co²⁺4.0 g/L,Wetting agent 1.0 ml/L,pH=0.8¹.2 and 30±2℃,disposed for 50 s,staying in the air for 10 15 s and then drying with hot air.And testing the corrosion resistance and binding force with organic coating of the passivation film;The effects of several metal impurities in zinc plating bath and passivation solution on the quality of passivation film were studied;The growth process of the passivation film was studied by the electric potential-time curve;The morphology and composition distribution of the trivalent chromium passivation film were analyzed by SEM,EDS and XPS,the results showed that:Film performance test results show that: The dry adhesion level and,the wet adhesion level for 24 h between conversion coating and TB06-9 organic coating was 0;The trivalent chromium passivation film of cyanide and alkaline zinc deposit passed the 72 h,336 h salt spray test respectively.Cyanide galvanized layer contains more impurities and the coating is not fresh,resulting in poor corrosion resistance of the passive film.The metal impurities Fe²⁺ and Cu²⁺ in zinc plating bath and passivation solution had a negative effect on the appearance quality and corrosion resistance of the passivation film.The content more than Fe²⁺2.0 g/L,Cu²⁺ 0.002g/L in zinc plating bath will make the passive film is not uniform,or dark and even colorless;The content more than Fe²⁺0.1g/L,Cu²⁺0.03g/L in passivation solution also will make the passive film is not uniform,or dark and even powder phenomenon,seriously reducing the corrosion resistance.The SEM results showed that the alkaline zinc deposit has compact structure with no obvious crack,and excellent anticorrosion performance;The trivalent chromium passivation film has smooth surface with a large number of cracks,these cracks do not penetrate the passivation film and only present on the surface,the cracks help to improve the adhesion strength between the passivation film and organic coating.The EDS results showed that the composition and the content are basically the same in cracks and flat parts.The forming process of passivation film possibly consists of four steps which are zinc dissolution,the rapid and slow growth of the film,and the balanced state of passivation film forming and dissolution.XPS measurements confirmed that the major constituents of the trivalent chromium passivation film were Zn,Cr,Oand a small amount of Co was also detected;According to the distribution of ingredients can divided the passivation film into inner and outer layers,and the 350 nm was the boundary.The composition of the outer layer is relatively uniform,and the content of each element almost no change,mainly composed of Cr（OH）3,Zn（OH）2,Cr2O3,ZnO and a small amount of Co（OH）2,CoO.The contents of Cr,O and Co in the inner passivation film were decreased,and the layers were mainly composed of Cr2O3,Cr（OH）3,ZnO and a small amount of Co（OH）2.The passivation film consists of multiple salts of complex oxides and hydroxides: the zinc layer dissolves during the passivation process,the OH-concentration increases at the interface,and a variety of hydroxides are deposited on the surface of the zinc layer to form a passivation film.In the drying process,part of the hydroxide dehydration to form oxides.