Developments of chromium-free conversion coatings on galvanized steels.

摘要

Two kinds of cerium-based conversion coating processes and a moly-phos-silane-silicate coating process were developed for electroplated Zn, electroplated Zn-Ni, hot dip Zn and hot dip Zn-Fe in this project. A reaction type cerium-based conversion coating process for electroplated Zn was optimized using factorial design experiments with the addition of colloidal silica and hydrogen peroxide to a 10mM Ce(NO3)3 solution. In addition an evaporation type cerium-based conversion coating process was evaluated with the addition of silane and a drying treatment. The corrosion loss Q (Cb/cm 2) and protection efficiency P(%) were obtained using a polarization technique and analysis of EIS data. The coating structer was observed using SEM,SAM and XPS.; The reaction type cerium-based conversion coating layer was composed of outer Ce3+-OH- complex and the inner Ce 4+-O2--Zn. The corrosion protection provided by the coating layer depends on the chemical states of Ce and O with higher contents of Ce4+ and O2- providing the better corrosion protection.; The corrosion resistance and chemical composition of cerium-based conversion coatings on the galvanized steels strongly depends on the chemical state of galvanized steels. A thin Ce-O-Si-Zn and a relatively thick C-Ce-O-Si-Zn complex coating layer was formed on the four different galvanized steels by the reaction and the evaporation type process, respectively. The evaporation type process provided better corrosion protection than the reaction type process. The corrosion ranking of bare(#0) and galvanized steels treated in the reaction type(S1) and evaporation type(S3) was S3>S1>#0 based on the electrochemical data, which agreed with the salt spry testing results. All four galvanized steels treated in the reaction type and evaporation type process passed 24 hrs and 100 hrs, respectively without white rust.; The evaporation process for galvanized steels has a high potential to replace the conventional chromate process due to its short treatment time and good corrosion protection.; Electroplated Zn steels was treated in a solution containing molybdate, silicate, phosphate and silane. The corrosion resistance of the treated samples was compared to that of bare samples during exposure to 0.5N NaCl for 24 hrs using electrochemical methods.