Study On Preparation And Film-forming Mechanism Of Passivation Agent On Galvanized Plate

Galvanized is one of the effective ways to improve the ability of steel corrosion and has been widely used in ships, transportation, electric power, machinery, household appliances etc. But galvanized sheet is easy corroded into white or dark gray products in damp environment. Chromate passivation technology is in major in passivation, but toxic hexavalent chromium is poisonous. With strengthening the conscious of environmental protection and implementation of European ROSH instructions, the passivation agent, which is non-toxic、economy and great performance has become a trend.In this paper, through orthogonal experiment, single factor experiment and blank control experiment make sure the formula and technological conditions of passivation agent. The corrosion resistant property of passivation film was determined by electroche-mical tests、NSS test and CuSO4、Pb(Ac)2 drip experiment. The painting performance of passivation film was analyzed by adhesion experimental. The relationship between smooth of passivation film and corrosion resistant properties was evaluated by SEM analysis. The element and constitution of passivation film were analyzed by XRD、and XPS tests. The microstructure and film-forming mechanism of constitution of a company’s series passivation agent were evaluated by SEM、XRD、EDS and XPS tests.The initial non-chromium passivating agent formula is:Na2MoO46.5g/L Na2Wu046g/L、Citric acid 5 g/L、phytic acid6g/L、H2O210g/L、H3PO420g/L. The passivation process parameters were:curing temperature 220℃, curing time 20s. A dense layer of passivation film was formed on the surface of galvanized sheet with the roller coating way and back curing. NSS test results show that corrosion of passivation plate area is less than 5%, after continuous 60h NSS test. CuSO4 and Pb(Ac)2 intravenous drip experiment results showed that passivation membrane had excellent corrosion resistance when the zinc is galvanized. Adhesion test result showed that the painting performance of passivation film is No.1 level. After passivating treatment to the galvanized sheet, the passivation film can restrain the anodic reaction of zinc layer and decrease the corrosion rate.In view of the evaluation of passivation solution to the company, there is no direct relationship between the smooth of the passivation film and corrosion resistant properties showed by SEM analysis. EDS analysis results showed that all kinds of passive film contains the main element type was roughly same, but the atomic of element was different, and elements was uniform distribution passivation membrane of galvanized sheet. The element and composition of passivation film cannot be tested by XRD but can be analyzed by EDS and XPS. XPS analysis structure shows that main ingredients of passivation were organic compounds and SiO2、ZnCO3、zinc、ZnO、etc, and passivation film had similar patterns, which has excellent corrosion resistance. The organic macromolecules formed the main frame in which silicate were absorbed. The unique combination formed a dense passivation film which can restrain the anodic reaction of zinc layer.