The Novel Chromate-free Surface Pretreating Approaches On Metal Surfaces For Corrosion Protection

In this paper, the novel methods of chrome-free surface treatments on metal substrates were studied.The thesis was composed of three major parts:Ⅰ. Silicate and tungstate conversion coating on magnesium alloy ;Ⅱ.Organic-inorganic composite passivation films modified by nano-SiO2;Ⅲ. Silica Sol modified phosphate coating.Ⅰ.The conversion coating based on silicate and tungstate composite on magnesium alloy surface were investigated. SEM ,XRD and EDX showed that the composite coating was composed of W, Mg, Al and Mn with a uniform dry riverbed structure.The anticorrosion was detected by electrochemical measure- ments. X-ray diffraction indicated that the film was amorphous. The reasonable technics parameters was that:Na2SiO3:4g/L,Na2WO4:10g/L,KClO4:4g/ L,KMnO4: 2g /L; pH=4, temperature: 50℃, dipping time: 10 mins.Ⅱ.The anticorrosion organic/ inorganic composite coating based on acrylic resin film modified by nano-SiO2 was prepared on the hot dipped galvanized steel surface and inhibitor vanadate and tungstate were used as the pigments.The morphology of the composite film was observed using a scanning electron microscope,while the corrosion resistance were investigated by electrochemical measurement and immersion test.The result indicated the organic films including colloidal silica and Silica powders showed better corrosion resistance than which added without nano-SiO2 particles, and colloidal silica modified resin film provided superior corrosion inhibition for galvanizing coating.Infrared spectrum showed that nano-SiO2 were cross linkaged with the resin. Nano-SiO2 could limit the crack in the resin coatings and protect the films with deposition.The pigments improved the anticorrosion of the passivation films; the best heating temperature was 140℃. The mechanisms of passivation and optimal proportion were to be further studied.Ⅲ.We also made research on the function of silica sol before phosphori- zation .SEM showed the crystal size of zinc phosphate coating can be refined by the silica sol pretreatment before phosphorization. The role of the silica sol was similar as activation of traditional conditioner before phosphorization.The electrochemical measurement indicated that the corrosion resistance of phosphate coating was improved by treating with silica sol before phosphorization. The effect could be found in several metals and alloys,such as steel,Zn as well as Mg. The possible mechanism was that the colloidal silica was sorbed on the metal surfaces and acted as nucleation agents when phosphorization, which was helpful to form compact and tiny phosphate crystal.