Electrochemical effects of pH value on the corrosion inhibition and microstructure of cerium doped trivalent chromium conversion coating on Zn

•The water chemistry has a pH value optimum about 2.2.•The inner layer dominates the corrosion behavior.•The reaction style at the Zn/TCC coating interface have changed at about pH 2.2.•Thermodynamically stable oxides in the E-pH diagrams are vitified. The electrochemical effects of cerium doped trivalent chromium conversion (TCC) coating on Zn were studied at normal temperature with potential-time curve, polarization curve, impedance spectroscopy, scanning electron microscope and energy dispersive spectroscopy. The results show protection of TCC coating increases, subsequently decreases by raising pH values. The interface of TCC coating/Zn substrate within pH 1.5–2.0 existing with metastable oxides reacts as diffusion-electrochemical mixed control, the TCC coating into pH 2.2 exists with dominated stable Cr-rich layer, then the formation rate of coating after pH 2.5 decreases. The corrosion mechanism was discussed from oxide stability in potential-pH diagrams and structural models.