Nanostructured vanadium-based conversion treatment of mild steel substrate: formation process via noise measurement, surface analysis and anti-corrosion behavior

Abstract

The bath pH, vanadia solution concentration and immersion time as reaction parameters of vanadium-based conversion coating (VCC) for chemical surface treatment of ST12 mild steel are optimized. The improvement process was evaluated by utilizing linear polarization resistance (LPR) method on the V-treated samples submerged in 3.5% NaCl electrolyte. The LPR measurements provided an optimum anti-corrosion behavior with highest polarization resistance (R_p) for a steel surface treated in 50 mg L⁻¹ V₂O₅ aqueous solution for 60 s at pH = 3. The formation process of optimized VCC on mild steel was inspected in situ by electrochemical noise measurement (ENM). Through the inverse of noise resistance (1/R_n) interpretation, three distinctive steps of VCC formation on the steel surface were deduced. The impact of optimized VCC on the metal surface chemistry was investigated with static water contact angle (CA) measurements, which showed a ca. 6.5° CA decrease along with enhancement of the mild steel surface free energy. Field emission-scanning electron microscopy (FE-SEM), elemental mapping of energy dispersive X-ray (EDX) and atomic force microscopy (AFM) studies of treated surfaces revealed a VCC thin layer that consisted of very small nano-sized particles as vanadium oxides/hydroxides compounds. Electrochemical impedance spectroscopy (EIS) and open circuit potential (OCP) methods were applied to study the anti-corrosive behavior of VCC treated mild steel during degradation in 3.5% NaCl corrosive media.