Composite Trivalent Chromium Conversion Coating on Electrogalvanized Sheet Steel
Date Issued
2012
Date
2012
Author(s)
Yang, Yu-Chuan
Abstract
Electrogalvanized zinc has been widely used to improve the corrosion resistance of steel, but its active chemical property may cause other corrosion problems although the steel beneath is protected. Hexavalent chromium passivation treatment has generally been used to further improve the corrosion resistance of the electrogalvanized steel and the adhesion between the substrate and painting layer. However, hexavalent chromium is highly toxic and severely endangers ecological environment and operators’ health. Since several international regulations have limited the use of hexavalent chromium, the development of alternatives to the hexavalent chromium passivation treatment is now an urgent necessity. Trivalent chromium compounds possess chemical properties similar to hexavalent chromium ones and are not toxic, so recently they have been used as one of the alternatives for the passivation treatment on electrogalvanized steel. In this study, hexafluorosilicic acid, which can dissociate into silicon dioxide nanoparticles in the solution, is added in a trivalent chromium conversion bath to form the composite trivalent chromium conversion coating. Chemical compositions, microstructure characterization, and corrosion resistance of the composite trivalent chromium conversion coating are discussed firstly. Experimental results showed that the composite trivalent chromium coating consisted of trivalent chromium and zinc oxide/hydroxides, silicon dioxide, trivalent chromium fluoride, and zinc fluoride. From the cross-sectional TEM observation, the composite trivalent chromium conversion coating was denser than the traditional trivalent chromium conversion coating. Especially, the coating fabricated by adding 2.5×10-2 M hexafluorosilicic acid in the conversion bath presented a double-layer structure with enhanced compactness. After 168 h of the salt spray test, the conversion coating formed in the bath with the addition of 2.5×10-2 M hexafluorosilicic acid showed the best corrosion resistance. Moreover, the electrochemical analyses, including EIS and potentiodynamic polarization curves, also indicated that the presence of hexafluorosilicic acid in the trivalent chromium bath indeed improved the corrosion resistance of the composite trivalent chromium conversion coating.
Subjects
electrogalvanized steel
hexavalent chromium passivation
composite trivalent chromium passivation
salt spray test
EIS
potentiodynamic polarization curves
Type
thesis
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