Evaluation of Vanadate and Vanadate Combinations as Hexavalent Chromate Inhibitor Replacements for Corrosion Inhibition of AA 2024

Chromate conversion coatings, which contain Cr⁶⁺, are frequently used¹ to reduce the corrosion rate to acceptable levels.   However, hexavalent chromate is extremely toxic and carcinogenic and it is very desirable to reduce its use for corrosion inhibition².  The effects of various concentrations of sodium vanadate (5, 10, 50, and 150 mM) in 50 mM sodium chloride solutions were tested on T3 and T4 tempered AA 2024, a typical high strength precipitation age hardened alloy.  Additionally a limited number of tests were done to exam synergistic relationships between vanadate, cerium, and molybdate inhibitors.  Several distinctly different tests were performed including anodic and cathodic inhibition, and open circuit measurements.  NMR was used to identify the vanadate species present.  In addition, cathodic and open circuit inhibition tests of AA 2024 with thin electrolytic layers were performed.  Vanadate species were found to inhibit both anodic and cathodic reactions to different extent.  Vanadates are effective inhibitors of the cathodic (oxygen reduction) reaction.  As determined by Koutecky-Levich calculations, for Al, AA 2024-T4 and Cu in the presence of the inhibitor, the cathodic oxygen reduction reaction is limited by factors other than diffusion of oxygen in the bulk solution.  Vanadates are also effective at inhibiting the anodic dissolution reaction on AA 2024-T3.  The average pitting potential is increased with higher vanadate concentrations.

References

1.  M. A. Jakab, D. A. Little, and J. R. Scully, Experimental and Modeling Studies of the Oxygen Reduction Reaction on AL 2024-T3, Journal of the Electrochemical Society, 152, B311 (2005).

2.  F. J. Presuel-Moreno, M. A. Jakab, and J. R. Scully, Inhibition of the Oxygen Reduction Reaction on Copper with Cobalt, Cerium, and Molybdate Ions, Journal of the Electrochemical Society, 152, B376 (2005).