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

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.

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.…”

Effect of Surface Pretreatment on the Underpaint Corrosion of AA2024-T3 at Various Temperatures

“…It has been shown that the presence of chromate ions inhibits open-circuit potential corrosion and Al 2 CuMg dissolution (anodic inhibition), reduces pitting (anodic inhibition), and inhibits cathodic ORR (cathodic inhibition). [23][24] Therefore, one benefi cial role of chromate during scribe-creep can be interpreted through its role in minimizing Cu replating. The other is the well-known effect of enhanced adhesion.…”

“…As was stated above, the IMC for this alloy are primarily of the Al-Cu, Al-Cu-Mg, and Al-Cu-Mn-Fe compositions. [2][3]5,7,[10][11][12][13][14][15][21][22]24 Using scanning electron microscopy (SEM) and image analysis, the surface area ratio for coarse constituent-type IMC was 2.18%, with an average IMC diameter of 2.43 µm (Figure 1) after stereological correction using the Saltyov (area) method. [52][53] The L 2 statistical point pattern spatial analysis [54][55] performed on the IMC on the long transverse (LT) or rolling surface showed that the IMC are sometimes clustered and sometimes random, as can…”

“…21 Cu-rich IMC and replated Cu have been shown to contribute significantly to the overall oxygen reduction reaction (ORR) rate on AA2024-T3. 18,[20][21][23][24][25][26][27][28][29] Therefore, a cathodic reaction is readily available from a coating defect in the case of underpaint corrosion to support the operating anodic corrosion process at the head of a coating delamination front.…”