Corrosion Inhibition of the Zinc Thermo-Diffusion Coating by Vanadate Conversion Coatings

The inhibition of zinc thermo-diffusion coating corrosion by tetravalent vanadium (V-IV)-based conversion coating was investigated in this work, aiming at the development of chemical conversion processes. The coated surfaces were examined using SEM, EDS, XPS, corrosion tests in salt spray, and electrochemical impedance spectroscopy (EIS) measurements in order to build a meaningful relationship between the coating element and the evolution of its chemical state when the surface encounters corrosion. V-IV conversion coating shows a resistance feature to corrosion, in which zinc thermo-diffusion coating in exposure to salt spray can be protected due to the irreversible formation of a tetravalent-trivalent mixed-state vanadium film layer. From a thermodynamic point of view, the coating layer acts as a reservoir of V-IV to fuel passivation, and then active V-IV leached from the layer structure can migrate to the newly formed defects and occupy susceptible sites, resulting in the regeneration of a protective barrier layer by partial reduction to trivalent vanadium (V-III) both on the surface and at the defect. Finally, the formation of the V-III layer seems to be essential for the long lifetime of coatings through blocking the aggressive medium and further V-IV reduction.