Inhibition of pitting corrosion on aluminum alloy 2024-T3: Effect of soluble chromate additions vs chromate conversion coating

Pitting potentials (E-ptt) of aluminum alloy (AA) 2024-T3 (UNS A92024) were improved with additions of 0.01 M sodium chromate (Na2CrO4, pH 8) and 0.0062 M Na2CrO4, + 0. 0038 M chromic acid (H2CrO4, pH 6) additions to 0. 1 M sodium sulfate (Na2SO4) + 0.005 M sodium chloride (NaCl, pH 6) after 2-h periods at open-circuit potential (OCP). Chromate conversion coatings (CCC) on AA 2024-T3 also improved E-pit in 0. 1 M Na2SO4 + 0.005 M NaCl, However, CCC did not improve the E-pit determined in fast potentiodynamic scans without periods at OCP. These tests were designed to minimize chromate Leaching and long-range transport of chromate. The OCP of CCC AA 2024-T3, Al2Cu, Al2CuMg, 99.999% Cu, and 99.998% Al were not changed significantly in 0. 1 M Na2SO4 + 0.005 M NaCl. Moreover, the OCP of AA 2024-T3, Al2Cu, Al2CuMg, and 99.999% Cu were not changed significantly in 0. 1 M Na2SO4 + 0,005 M NaCl with the addition of 0. 0 1 M soluble chromate at pH 6 and pH 8. The mass transport limiting current density for O-2 reduction also was not altered significantly by CCC or soluble chromate additions. Consequently, if the AA 2024-T3 matrix were polarized locally to the measured OCP of the intermetallics or replated copper, inhibition of stable pitting on AA 2024-T3 is more likely to result from suppressed pit stabilization (in 0.1 M Na2SO4 + 0.005 M NaCl with the addition of chromate at PH 6 or pH 8, or with CCC) than from eliminating two aspects associated with the potency of local galvanic couples (e.g., OCP of cathode sites and their oxygen reduction reaction kinetics [OPR]).