Oxygen Reduction Reaction Kinetics on Chromate Conversion Coated Al-Cu, Al-Cu-Mg, and Al-Cu-Mn-Fe Intermetallic Compounds

Oxygen reduction reaction (ORR) kinetics were investigated on bulk synthesized analogues of Al-Cu, Al-Cu-Mg, and Al-Cu-Fe-Mn intermetallic phases with and without chromate conversion coating (CCC) in 0.1 M Na₂SO₄ + 0.005 M NaCl (pH 6) with minimal levels of tolal dissolved chromate. The results were compared to AA 2024-T3 and high purity Al, Cu, Cr, and Au. Net cathodic ORR mass transport-limiting current densities of Al-based materials, lacking large quantities of Fe, Mn, or Cu were lower than the theoretically predicted rates in the mass transport controlled regime of ideal electronic conductors. This suggests a second rate-limiting factor in the case of these Al-based materials that was deduced to be related to Al-rich surface oxides. A second rate-limiting effect was also seen for pure Cr, implying that Cr₂O₃ inhibits ORR kinetics, CCC inhibited open circuit corrosion, via reduced ORR kinetics. It also serves as a diffusive harrier to O₂ transport. The role(s) of CCC as an enhanced electronic harrier to electron transfer or barrier to O₂ chemisorption remains unclear.