The morphology of attack at and around the intermetallic compounds ͑IMC͒ present on bare AA 2024-T3 was studied in situ using confocal laser scanning microscopy. Exposures were conducted in 0.1 M Na 2 SO 4 ϩ 0.005 M NaCl at pH 3, 6, and 10 as well as near-neutral 0.5 M NaCl. The types of attack observed could be categorized as matrix and IMC pitting, trenching adjacent to IMC, and matrix etching. The electrochemical behavior of bulk synthesized Al-Cu, Al-Cu-Mg, and Al-Cu-Fe-Mn intermetallic compounds as well as that of AA 2024-T3 was used to rationalize the observed attack metrology. The galvanic coupling between the AA2024-T3 matrix and the intermetallic particles controlled the attack rates. In Al-Cu-Mg, the strong polarization to the opencircuit potential of the alloy caused rapid dissolution ͑ca. 10 mA/cm 2 ͒, whereas for the Al-Cu-Fe-Mn the dissolution rates were on the order of 100 A/cm 2 . The limited dissolution rates of the Al-Cu-Fe-Mn phase were due to the cathodic polarization of these particles by the matrix under open-circuit conditions. Several pits were initiated at large Al-Cu-Mg particles. These pits were stable within the Al-Cu-Mg phase, but could not form stable pits in the alloy matrix during open-circuit corrosion. Calculation of growth rates and pit stability products for the individual IMC emphasized the role of metastable pitting in the observed corrosion metrology, which developed on AA2024-T3 during open-circuit corrosion.

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.

No abstract

Oxygen reduction reaction (ORR) kinetics were investigated on synthesized Al-Cu, Al-Cu-Mg, and Al-Cu-Fe-Mn intermetallic phases and compared toAA2024-T3 (UNS A92024), as well as high-purity Al, Cu, and Au. These tests were conducted in 0.1 M sodium sulfate (Na 2 SO 4 ) with and without either 0.005 M sodium chloride (NaCl, pH 6), 0.01 M sodium chromate (Na 2 CrO 4 , pH 8), or 0.0062 M Na 2 CrO 4 + 0.0038 M chromic acid (H 2 CrO 4 , 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 Na 2 SO 4 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 masstransport-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 mixedcontrolled ORR are lowered possibly by the blocking of O 2 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-Al 2 CuMg 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.

No abstract