Inhibition Performance Study of Aqueous Vanadate Species on Mg Alloys

This work presents the corrosion inhibition performance of vanadate on Mg alloy AZ31 and its dependence on pH and immersion time. Aqueous vanadate speciation, which has been shown to have a strong effect on corrosion inhibition, is a function of solution pH.  The pH dependence of vanadate corrosion inhibition is, therefore, attributed to the pH dependence of vanadate speciation. Immersion in tetrahedrally coordinated vanadate species, present in neutral and alkaline solution, was shown to decrease corrosion rate and increase pitting potential, both of which were enhanced with longer immersion times. Alternatively, exposure to octahedrally coordinated vanadate, which predominates in acidic solution, only slightly decreased the corrosion rate. An acidic solution (octahedrally coordinated speciation) was adjusted to alkaline conditions and samples were then immersed in the adjusted alkaline solution.  Inhibition on these samples was weaker than that of samples immersed in alkaline solutions that had never been acidic.  Regardless of speciation, vanadate was shown to provide limited inhibition on cathodic reaction kinetics (hydrogen reduction reaction).  Anodic inhibition was observed on samples treated in solutions containing tetrahedral species.  Inhibition performance was also characterized by electrochemical impedance spectroscopy. Aqueous vanadate in 0.1 M NaCl increased the total impedance at all pH values (5, 7.7, 9.2), with enhanced results observed with increasing immersion time. The greatest increase in impedance for a given immersion time occurred at pH 9.2, which is consistent with the inhibition determined by corrosion rate. SEM showed that vanadate forms a film across both secondary particles and the Mg matrix, and it provided qualitative evidence that inhibition efficiency increased as the pH of the exposure solution increased.  XPS results indicate that vanadate film formation is associated with the reductive adsorption of vanadate by Mg. Exposure at a pH of 5 produced vanadate predominated by V⁴⁺.  Exposure at pH values of 7.7 and higher, however, produced vanadate containing predominantly V³⁺.