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

Abstract

Oxygen reduction reaction ͑ORR͒ 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 ͑CCC͒ in 0.1 M Na 2 SO 4 ϩ 0.005 M NaCl ͑pH 6͒ 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 be 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 role͑s͒ of CCC as an enhanced electronic barrier to electron transfer or barrier to O 2 chemisorption remains unclear.

A study of the mechanisms of corrosion inhibition of AA2024-T3 by vanadates using the split cell technique

“…Since E P1 is related to dissolution of Al 2 CuMg, the absence of such transient event seems to indicate that the clear metavanadate solutions suppressed the dissolution of S-phases until the potential reached E P2 . As explained by Ilevbare and Scully [55,56], reducing the dissolution of Al 2 CuMg particles reduces the formation of Cu rich cathodic sites. In most of our experiments, however, AA2024-T3 samples were exposed to 0.5 M NaCl for at least 15 min before injecting the inhibitors.…”

“…The E P was the same with or without the inhibitor, showing that the presence of clear metavanadate did not affect the anodic reaction (in this case localized Al dissolution). Cathodic polarization curves of Cu in aerated 0.5 MNaCl electrolytes with no inhibitor added are characterized by a large limiting current (i L ) of about -320 µA/cm 2 due to the strong stirring and air bubbling of the electrolyte [55][56][57]. Evolution of hydrogen gas was observed at potentials below -1.2 V SCE.…”

A Summary of Corrosion Properties of Al-Rich Solid Solution and Secondary Phase Particles in Al Alloys

“…Still, its corrosion potential is higher than that of the matrix and it is expected to play a cathodic role in localized corrosion processes. Al 2 Cu is an efficient cathodic site which supports the oxygen reduction reaction due to the presence of Cu [74,94], and the reaction kinetics is independent of pH [29,34,39]. However, the absence of Fe and Mn (which also facilitate the oxygen reduction reaction) results in Al 2 Cu having a lower reaction efficiency than Al 7 Cu 2 Fe and Al 20 Cu 2 Mn 3 [95].…”

“…Particles consisting primarily of noble elements generally possess a high corrosion potential, and act as a cathode to support the oxygen reduction reaction or hydrogen evolution. The adjacent matrix then dissolves at the matrix/particle interface, resulting in trenching morphology surrounding the particle as shown in Figure 2 [28][29][30][31][32]. However, the galvanic relations between the matrix and secondary phase particles may change with time and as environmental conditions are altered.…”

The Effect of CeCl3as an Inhibitor on the Localized Corrosion of AA2024-T3 as a Function of Temperature

“…7,43,44 They act as a cathode to the matrix and typically display trenching in the absence of inhibitors. 11,14 In the presence of Ce 3+ at temperatures up to 50…”