The predominant chemical types present in al 2024 t3

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The predominant chemical types present in Al 2024- T3 are Al-Cu-Mn-Fe particles whose chemistry ranges from Al 7 Cu 2 Fe to Al 20 Cu 3 Mn. Constituent θ phase, Al 2 Cu, and S phase, Al 2 CuMg, can also be present in the alloy. The θ phase tends to be the more predominant of the two phases in alloys with a higher Cu:Mg ratio. S phase predominates at lower Cu:Mg ratios. 46 These particles are usually noble with respect to the surrounding matrix and tend to localize corro- sion to the matrix phase at the particle periphery during exposure to aggressive aqueous environ- ments. These particles support reduction reactions at enhanced rates, which lead to localized dissolu- tion—perhaps assisted by local alkalinization that occurs near the particle due to the cathodic activity at the particle. 45 Localized corrosion at S phase particles is more complex; 38-39,45 S phase particles are initially active with respect to the surrounding matrix phase. These particles will dealloy, become enriched with Cu, and become ennobled to such an extent that their gal- vanic relationship to the matrix is reversed. Under certain conditions, these particles have been ob- served to dealloy to such an extent that they appear to be selectively dissolved. 45 Al 2024-T3 also contains homogenous distribu- tion of Cr, Mn, and Si dispersoids. 47 It also contains a dispersion of S phase laths that form during natural or artificial aging. 47 None of these phases has been directly implicated in localized corrosion phenomena in the alloy in the T3 temper. Inhibition of the Oxygen Reduction Reaction Inhibition of cathodic kinetics has been recog- nized for sometime, but recently this issue has gained increased attention. Chromates are excellent inhibitors of oxygen reduction in near-neutral and alkaline solutions. In these environments, they can stifle corrosion by suppressing this cathodic partial reaction. The inhibition mechanism appears to in- volve reduction of Cr 6+ to Cr 3+ at a metal surface and formation of an irreversibly absorbed Cr 3+ hydroxide surface layer of near-monolayer thickness. 48 Chromate is a very powerful cathodic inhibitor even at high chloride:chromate ratios. Cathodic po- larization curves collected for Al 2024-T3 in a base solution of oxygen-sparged 1.0 M sodium chloride (NaCl) show that the oxygen reduction reaction (ORR) rate is reduced by about an order of magnitude (at –0.80 V vs saturated calomel electrode [SCE]) with the addition of 10 –5 M sodium dichromate (Na 2 Cr 2 O 7 ; Figure 3). 49 In similar experiments where 0.3% H 2 O 2 was added as a cathodic depolarizer, 10 –4 M dichro- mate additions had no effect on the ORR rate. How- ever, additions of 10 –3 M chromate reduced the cathodic reaction rate by an order of magnitude. The results of these experiments show that chro- mate inhibits the cathodic reaction at chloride:chromate concentration ratios of 10 5 :1. This value is much higher than the 1:1 to 10:1 ratio where increases in the pitting potential during anodic po- larization experiments are observed. 50 The curves in
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  • pH, Chromates, Chromate conversion coating

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