Inhibition of corrosion of zinc by Cr(VI)and Cr(III) treatments

The corrosion behavior of Cr(VI) and Cr(III) treated zinc has been studied using scanning Kelvin probe (SKP), potentiodynamic polarization and elctrochemical impedance spectroscopy (EIS). The Volta potentials measured on the Cr(VI) or the CR(III)-A treated zinc area by SKP in a humid chamber are more negative than on the untreated zinc area, which is in agreement with their corrosion potential measured in 0.01 M NaCl solution. The polarization resistance measurements show that both the Cr(VI) and the Cr(III) coatings can decrease the corrosion rate of zinc. The Cr(III)-B coating is thicker than the Cr(III)-A coating, and its corrosion resistance is greater than that of the Cr(III)-A coating. However, the inhibition of the corrosion of zinc by Cr(IV) coating is more effective than by the Cr(III) coatings. Pt and Zn electrodes in NaCl solution with and without dichromate were also investigated. The results show that the Cr(VI) coatings can protect zinc in two ways: as a barrier layer and as a passivating inhibitor ("self-healing" effect)