Study On Chromium-free Passivator For Hot Dip Galvanized Steel Sheet

Hot dip galvanized steel sheet is obtained by hot dip plating zinc on cold-rolled sheet, the zinc layer was used to protect the steel from corrosion through an anode protect the steel from corrosion through an anode protection effect. Though hot galvanized steel has good resistance to atmospheric corrosion, but surface corrosion also can be found,which can form a black color or a white color on the steel surface. In order to improve the corrosion resistance and prolong the service life, the passivation treatment is necessary. The traditional passivation solution is commonly composed of chromium(Ⅵ), a protective layer of chromate can be formed on the surface of galvanized steel sheet. In recent decades, the toxicity of chromate captured people’s attention:Therefore, in China, the chromate concentration level in the workplace is strictly restricted and the dangerous of chromate must be informed; The OSHA (Occupation Safety and Health Association) of United States of America proposed that the soluble chromate content must be less than≤1mg/m3in air. With the increasing awareness of environmental protection all over the world, developing the passivator of no chromium (VI) has become increasingly urgent. At present, the chromium-free passivation technology is mainly mastered by Europe, the United States, Japan and other advanced countries. In our country, chromium-free passivators are relying on imporsed products or on importing technology.In this article, based on referring to a large number of documents and the basic passvation theory. Combined with the application of hot dip galvanized steel sheet after the passivation, Summarized the passivation film on hot-dip galvanized steel need evaluation criteria and methods. Then according to the basic theory of passive knowledge, some elements similar with chromium in physical and chemical properties and their compounds were chosen to replace the chromium and a new type of passivator was implored. The organic silane was also used to improve the passvation performances. The monofactorial experiments and orthogonal design experiments were carried out to study the effects of various paraments on the performances of passivation layer and the optimal ingredient substanced and contents were determined.The results showed that when the pH value of passivation solution is between3.7and4.7,the density is in the range of1.003and1.063g/cm3,the solid content is between13%and15%and the stoving temperature is from80to120℃,the performances of passvation membrane, such as blocking resistant, yellowing resistant and painting properry are equivalent with that of using imported passivator(CT-E356PS/SG-158).