Development of Chromium-Free Corrosion Resistant Conversion Coatings

摘要

Chromate conversion coatings applied to zinc-plated steel for corrosion protection usually consists of various chromium compounds that include hexavalent chromium (Cr6+). As the use of highly toxic and carcinogenic chromium compounds has been regulated worldwide, the development of a chromium-free corrosion resistant conversion coating for zinc plating has become necessaiy. In this study, we used vanadium as an alternative to chromium, and successfully prepared vanadate conversion coatings and have found that it has corrosion resistance; however, the corrosion resistance of the vanadate conversion coating was lower than that of the chromate coating. Therefore, we studied improvement of corrosion resistance of the vanadate conversion coating by clarification of the vanadate conversion coating film composition and optimization of the solution composition. X-ray photoelectron spectroscopy analysis of the coating surface showed that the vanadate conversion coating mainly consists of V2O3. Furthermore, it was found that the NaNO3 concentration in the conversion solution significantly affected the coating formation and the corrosion resistance. The concentration of NaNO3 was optimized to develop an optimum vanadate conversion solution and film composition.