Mass-Transport-Limited Oxygen Reduction Reaction on AA2024-T3 and Selected Intermetallic Compounds in Chromate-Containing Solutions

Oxygen reduction reaction (ORR) kinetics were investigated on synthesized Al-Cu, Al-Cu-Mg, and Al-Cu-Fe-Mn intermetallic phases and compared to AA2024-T3 (UNS A92024), as well as high-purity Al, Cu, and Au. These tests were conducted in 0.1 M sodium sulfate (Na2SO4) with and without either 0.005 M sodium chloride (NaCl, pH 6), 0.01 M sodium chromate (Na2CrO4, pH 8), or 0.0062 M Na2CrO4 + 0.0038 M chromic acid (H2CrO4, pH 6) additions using stationary and rotating disk electrodes. Mass-transport-rate-controlled ORR were near theoretical values on all Cu-bearing materials and Au in 0.1 M Na2SO4 with and without 0.005 M NaCl. Theoretical limiting current densities (CD) were not achieved in the mass-transport-controlled regime on Al because of the rate limitation of electron transfer through its oxide film. Four effects of chromate additions on such cathodic reaction rates were identified as follows: —Little intrinsic effect of chromate pretreatment on mass-transport-controlled ORR on high-purity Cu, Cu-containing Al-based intermetallics, and Au electrodes in the absence of open-circuit corrosion. Charge transfer-controlled and mixed-controlled ORR are lowered possibly by the blocking of O2 adsorption sites by chromate anions.—Decreased net cathodic kinetics on AA2024-T3 as a function of the degree to which chromate suppressed open-circuit pitting corrosion prior to cathodic polarization.—Increased net cathodic kinetics on high-purity Al.—Decreased ORR kinetics on S-Al2CuMg as a result of minimization of Al(Mg) dissolution and, subsequently, minimization of the formation of a high, Cu-rich surface. Comments are made regarding the influence of each of these phenomena on open-circuit pitting of AA2024-T3.