| Summary: | 博士 === 國防大學理工學院 === 國防科學研究所 === 99 === The corrosion resistance and formation mechanism of Non-chromate conversion coatings on electro-galvanized steel are investigated in this study. The OCP measurement is used to examine the change of reaction potential with the increase of immersion time. The corrosion resistance and surface morphology of Non-chromate conversion coatings are observed with LP, EIS, and SEM analyses. Moreover, the formation mechanism of Non-chromate conversion coatings on electro-galvanized steel is deduced from the results of SIMS and XPS measurements.
The results demonstrate that the formation of vanadate and/or zirconate conversion coatings would enhance the corrosion resistance of electro-galvanized steel. Nevertheless, the existence of defective structure would restricte the anticorrosive performance among the conversion coatings.
Although the addition of Co ions and/or Ti ions could not improve the corrosion resistance of vanadate conversion coatings, however, the addition of Ti ions to zirconate conversion coatings would results in the coatings formed with excellent anticorrosive performance and an uniform structure, moreovre, the SST was prolong form 24 hr to 120 hr. Nevertheless, the SST was decline from 120 hr to 24 hr when the composition of TiO2 increased and/or the addition of oxidant to Zr-Ti conversion coatings.
The results of XPS demonstrated that the Zr-Ti conversion coatings showed excellent anticorrosive performance as a result of the coatings have great quantities of TiO2 and ZrO2 components with an uniform structure. However, the concentrations of TiF62－ decreases with decreasing the concentrations of Zr ions, which results in the Zr-Ti conversion coatings formed without TiO2 components. Moreover, the concentrations of TiF62－ increases with addition of NaF, nevertheless the reaction of TiF62－ would be restricted, and results in the Zr-Ti conversion coatings formed without ZrO2 components. Noteworthily, both the TiO2 and ZrO2 components have lower solubility product constant, and therefore, the coatings formed without TiO2 and/or ZrO2 would restricte the anticorrosive performance among the Zr-Ti conversion coatings.
The SIMS analysis indicated that the Zr is concentrated in the inner layer of the Zr-Ti conversion coatings. On the contrary, the H, Zn, and Ti are spread over the whole of coatings. Furthermore, the Zr-Ti conversion coatings comprise two layers when the immersion time progressed to 100 s, which one (i.e. surface layer) is formed in the immersion times of 0 s ~ 60 s another one (i.e. inner layer) is formed in the immersion times of 60 s ~ 100 s. Moreover, the rate of deposition reaction had declined obviously during the immersion times of 150 s ~ 200 s, and furethermore, a continuity of conversion reaction until 200 s would result in a plenty of cracks on the coatings. The defective morphology would restricte the anticorrosive performance among the Zr-Ti conversion coatings.
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