Effect of practical parameters on the structure and corrosion behavior of vanadium/zirconium conversion coating on AA 2024 aluminum alloy

In this research corrosion behavior of the hexafluorozirconic acid-based conversion coating (ZrCC) applied on the surface of AA2024 aluminum alloy, in the absence and presence of sodium metavanadate (ZrVCC), has been investigated. At the first step, practical parameters of ZrCC conversion coating were optimized using data of polarization resistance (Rp) and corrosion current density (icorr) obtained from electrochemical impedance spectroscopy and polarization techniques, respectively. In the next step the effect of sodium metavanadate presence by assessment on the effect of parameters including concentration of sodium metavanadate, immersion time, and solution pH on surface and electrochemical properties was investigated. The optimized practical conditions for ZrCC were immersion time of 60 s in 0.01 M of H2ZrF6 solution at room temperature and solution pH = 2. For the ZrVCC sample, the best results appeared for the immersion time of 60 s in the bath containing 1 g/lit sodium metavanadate at room temperature and pH = 2. Corrosion current density value of the ZrVCC sample was 0.07 µA/cm2 which seems to be 10 times smaller than its amount for the uncoated sample. Film formation of conversion coatings was investigated by field emission scanning electron microscope (FE-SEM) and atomic force microscope (AFM). FE-SEM and EDS test results revealed that zirconium compounds mostly precipitated on the surface of intermetallic particles.