The Electrochemical Characteristics of Bulk‐Synthesized Al2CuMg

Abstract

Recent work has shown that Al 2 CuMg (S-phase) particles constitute approximately 60% of the second-phase particles in 2024-T3 (Al-4.4Cu-1.5Mg-0.6Mn) with diameters greater than 0.2 m. 1 The Al 2 CuMg particles in question are shown in Fig. 1. Corrosion damage is localized at these particles, resulting in selective particle dissolution or pitting in the matrix adjacent to the particle. These particles also appear to play a significant role in redistribution and enrichment Cu on dissolving Al-Cu-Mg alloy surfaces. However, there is some uncertainty regarding the electrochemical characteristics of this phase and whether it is anodic or cathodic with respect to the surrounding matrix phase. Open-circuit potential (OCP) measurements, 2 and polarization curves 3 conducted on bulk-phase Al 2 CuMg samples indicate that the phase is electrochemically active with respect to the matrix phase. However, observations of pit morphology which show pitting in the matrix adjacent to S-phase particles suggest that the phase is electrochemically noble.We have synthesized the S phase in bulk to attempt to verify earlier reports of its electrochemical characteristics. We have also conducted further experiments to understand the nature of the S-phase particle-␣-Al matrix phase galvanic couple and to understand dissolution phenomena associated with this particle during corrosion of Al-Cu-Mg alloys. In this paper, we report results on synthesis of the phase in bulk form, verification of its structure and composition, and determination of the electrochemical characteristics by OCP measurements and potentiodynamic polarization. These results indicate complexities in the electrochemical characteristics beyond the relative nobility or activity of this compound compared to its microstructural surroundings. ExperimentalBulk-phase synthesis and characterization.-The S phase was synthesized by charging an alumina crucible with 21.25 g 99.999% Al, 20.00 g 99.99% Cu, and 8.75 g 99.9% Mg. The charge was heated to 990ЊC at a rate of 25Њ min Ϫ1 and held for 30 min to insure full melting. The melt was mechanically agitated, then cooled at 7Њ min Ϫ1 to 510ЊC and held for 65 h. The melt was finally cooled to room temperature at 5Њ min Ϫ1 . At this composition, isothermal treatment at 510ЊC causes the melt to segregate into solid S and an Alrich liquid. 4 The long isothermal treatment time was used to maximize the S-phase crystal size. Ingots were sectioned and polished by hand in oil. The finishing step was carried out using a 1.0 m diamond paste. Polished surfaces were degreased by ultrasonic cleaning in ethanol.Bulk intermetallic samples were characterized using standard X-ray powder diffraction and backscattered electron Kikuchi patterns (BEKPs). 5-7 The acquisition of high-quality BEKPs suitable for phase identification was accomplished using a new charge coupled device (CCD) based camera with a single-crystal scintillator designed by Sandia National Laboratories and built in conjunction with Photometrics, Inc., of Tuscon, AZ. This camera was insta...

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

“…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…”

“…These local galvanic cells, which induce acid pitting and alkaline attack, are often formed by Cuand Fe-containing intermetallics or replated Cu. [1][2][3][4][5][6][7][8][9][10][11][12] In AA2024-T3 (UNS A92024), (1) pit initiation sites include Al-Cu-Mg particles, [2][3]7,[11][12][13] the periphery of Cu-enriched Al-Cu-Mg particles that have been dealloyed of Al and Mg, 2 and the matrix adjacent to Al-Cu and Al-Cu-Fe-Mn constituent particles. 3,5,10,13 The Al-CuMg type is the most active constituent particle and as much as 60% of the intermetallics on the surface of an AA2024-T3 sample are of the Al-Cu-Mg type, 2,[14][15] with 2.7% of the total surface covered by these particles.…”

“…[2][3]12,15,[17][18] The particle becomes cathodic to the matrix with time. [2][3][11][12][19][20] Under certain conditions, some authors have observed rings of deposited Cu around these Al-Cu-Mg precipitates, suggesting that they are a major source of Cu for replating. 18 The Al-Cu and Al-Cu-Mn-Fe types of IMC also serve as preferred cathodic sites relative to the Al matrix.…”

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

“…As Mg and Al are depleted from S-phase and a Cu-rich layer forms, it changes to a net cathode (Period II), supporting oxygen reduction at a high capacity. 27,37 The diffusion-controlled ORR produces an excess of hydroxide ions, showing peak kinetics at 55…”

Effect of Cu Content and Aging Conditions on Pitting Corrosion Damage of 7xxx Series Aluminum Alloys

“…2,3 From a microstructural viewpoint, initiation of localized pitting corrosion is often associated with coarse cathodic intermetallic particles (IMPs), such as Al 7 Cu 2 Fe, Al 3 Fe, Al 2 Cu, and Al 2 CuMg, which are common in 2xxx and 7xxx series Al alloys. [4][5][6][7][8][9] A number of studies have been performed on electrochemical characteristics of these particles. 4,8,9 In general, they act as local cathodes and sustain oxygen reduction at a high rate to drive the pitting corrosion attack at the particle-matrix interface.…”