Effect of Cathodic Polarization on the Formation and Corrosion Protection of Zirconium-Based Conversion Coatings on Mild Steel

Zirconium-based conversion coatings were prepared on unpolished ST20LS mild steel by conventional immersion and cathodic polarization in aqueous H2ZrF6 solutions. Five-factor experimental designs were used to examine the effects of pH, temperature, deposition time, H2ZrF6 concentration, drying temperature/applied cathodic potential for both methods. The Zr conversion coatings were characterized by SEM/EDS, Raman spectroscopy, and XPS. Their protective properties were evaluated by neutral salt-spray testing, potentiodynamic polarization, and gravimetric testing in an oil–water emulsion. Cracks were observed in the Zr conversion coating prepared by the immersion method. The coating prepared at pH 4.0 decreased the salt-spray corrosion rate from 6.00 to 3.07 mm·year−1. Cathodic polarization promoted coating formation on the rough steel surface, and the results showed that solution pH and applied potential affected the corrosion protection performance. Among the samples selected for detailed electrochemical analysis, the coating deposited at pH 6 and −1.2 V versus Ag/AgCl showed the strongest overall protection. Its polarization resistance reached 1310 Ω·cm2, compared with 232 Ω·cm2 for uncoated steel, while the corrosion current density decreased from 5.431 to 0.305 μA·cm−2, corresponding to a protection efficiency of 94.5%. Raman and XPS results supported the formation of a poorly crystalline zirconium oxide/oxyhydroxide-containing layer. The results suggest that cathodically generated OH− promotes interfacial hydrolysis and condensation of zirconium complexes, improving conversion-coating formation on rough mild-steel surfaces.