The Effect of Immersion Time on Electrochemical Behavior of Vanadium Conversion Coating Modified With Copper Sulphate on AZ31 Magnesium Alloy

The increasing application of magnesium alloys in various industries on the one hand and its low corrosion resistance on the other hand, have been a new challenge for researches around the world. In this study, vanadium conversion coating was investigated as a method for protection of AZ31 alloy. In the first section, the importance of surface pretreatment for applying conversion coating was studied. In the second part, the duration of immersion in the solution was investigated and the results showed that the coating reaches to its maximum resistance (8100 Ohm.cm2) in 60 minutes immersion. Then, in the next section, it will be seen how the addition of 1 g/l copper sulphate not only reduced the optimum immersion time to 30 minutes, but also increased the polarization resistance of the coating from 250 to 14300 Ohm.cm2. In addition it is clarified that this significant change is in compliance with the polarization test in which the corrosion current density decreased from 33.4×10-6 to 5.3×10-7 A/cm2. In the final section, the images of the FE-SEM showed that the surface of vanadium conversion coating was inherently cracked. Also, reducing the number of pits and increasing the thickness of compact and uniform layer next to the surface are the reasons of higher corrosion resistance for vanadium conversion coating in the presence of copper sulphate.