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

Abstract

Oxygen reduction reaction (ORR) kinetics were investigated on hulk 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 Na 2 SO 4 + 0.005 M NaCl (pH 6) with minimal levels of total 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 he related to Al-rich surface oxides. A second rate-limiting effect was also seen for pure Cr, implying that Cr 2 O 3 inhibits ORR kinetics, CCC inhibited open circuit corrosion, via reduced ORR kinetics. It also serves as a diffusive barrier to O 2 transport. The role(s) of CCC as an enhanced electronic barrier to electron transfer or barrier to O 2 chemisorption remains unclear.