In-situ constructing and EDTA-enhancing of meta-aluminate passivation films on electroplated zinc surfaces

Abstract

Hexavalent chromium is highly toxic and carcinogenic and is not permitted for passivation of zinc in many countries. Accordingly, the development of environmentally-friendly chromium-free passivation has received increasing attention. In this paper, the preparation of Zn2+ and Al3+ doped composite passivation films on electroplated zinc surfaces using meta-aluminates as passivator was studied, and it was found that the corrosion resistance of the passivation films could be effectively improved by using EDTA. As a complexing agent, EDTA can combine with many metal ions and form stable complexes. The mechanism of the passivator and the effects of passivator concentration, temperature, time, and pH on the passivation performance were investigated. The results showed that the corrosion current density (icorr) of the meta-aluminate passivation film reinforced with EDTA decreased to 2.068 × 10−7 A/cm2, which was better than the icorr of 1.524 × 10−6 A/cm2 without EDTA reinforcement and much smaller than the icorr of 2.376 × 10−5 A/cm2 without passivation. It can be seen that the strengthening effect of EDTA results in a compact passivation film on the surface of zinc metal. Furthermore. The corrosion resistance of meta-aluminate passivation film reached or approached the level of hexavalent chromium passivation film, which contributed to the chromium-free passivation of zinc.