Technology And Mechanism Of Titanium Salt And Silane Composite Passivation Film On Galvanized Sheet

With the increased environmental awareness of people, an intense research is being undertaken on replacing chromates aestivation by new passivation treatments. Being environment-friendly, titanium salt passivation and silane processing of metal has a good prospect. Due to the fact that a single film can only get a thinner coating and its corrosion resistance cannot meet the conventional chromium acid salt passivation film layer, an idea of taking two steps to form titanium salt/silane composite film occurs. This paper systematically studies the film-forming process, film-forming mechanism, film-forming structure and external influences of titanium salt conversion film and silane film in hot dip galvanized coating with an attempt to find out the best formula.Preliminary formula of titanium salt passivation solution was concluded with orthogonal test. Single factor experiment and additives experiment of titanium salt film were processed with salt spray test and electrochemical experiments respectively. The best process of titanium salt passivating film is:Ti(SO4)28g/L,30%H2O280mL/L,(NaPO3)64g/L,67%HNO36mL/L, a small amount of NH4NO3, at30℃in50seconds. After the passivation, galvanized steel sheet persisted more than24h without rust in the salt spray test. Through the varied soaking time, electrochemical behavior and scanning electron microscope(SEM) tests, the corrosion process was studied and the forming process of titanium salt passivation film was analyzed.The electrochemical analysis was used to study the factors that influence the film formation of three silane films (KH-550, KH-560, and KH-570). The salt spry test was used to study corrosion resistance of silane film. The best process for KH-550silane film of three kinds of silane film is:volume ratio of KH-550:water:ethanol=1:1:9, with10%NaOH solution adjusting pH for8to10, hydrolysis time for more:han4hours. The galvanized sheet was immersed at40℃for2min, and then dry1.5h at120℃. SEM and EDS analysis was used to study the silane film structure, morphology and composition. Electron microscopic and electrochemical analysis shows that KH-550film formation is mainly composed of Si-O-Si three-dimensional network across linking structure.Titanium salt/silane composite film was formed via the two-step method on the)asis of single passivation film. The corrosion resistance of composite film was studied via Salt spry test. The film-forming process and corrosion resistant mechanism of composite film were analyzed via SEM and EIS. After titanium salt and KH-550composite passivity, the rust time of galvanized sheet in NSS has reached two days, close to common chromate salt passivation. As the organic and inorganic compound film, Titanium salt-KH-550silane composite film has both films’ advantages, with the corrosion resistance of passivating film better than that of single film.