Dynamic structures and electrochemical behavior during the formation of trivalent chromium-based conversion coating on Zn

The growth processes of trivalent chromium-based conversion (TCC) coatings on galvanized steel at room temperature (298.15 K) were studied using the potential-time curve, polarization curves, electrochemical impedance spectroscopy and scanning electron microscopy. Results display the integrity of bi-layers at stage II around 50 s causes enhanced corrosion resistance and film compactness. The protection of surficial film, dominated by inner Cr(III)-rich barrier layer, is weakened after stage III, where it presents defective bi-layers with the mixed kinetic-diffusion control. Thermodynamic stability, layer growth and structural evolution of the film formation above Zn are discussed by the Pourbaix diagrams, molecular dynamics and schematic models, respectively. •TCC coating grows with four typical regions, distinctively showing potential shifts.•Coating in stage II has effective resistance, as interfacial electrochemical control.•TCC formation follows the buffering effects for thermodynamic statement of Cr oxides.•Structural stability of TCC oxides suggests the atom-scale growth in MD simulation.•Combining the equivalent circuits, the interfacial model of bi-layer are provided.