Inhibition of Pitting Corrosion on Aluminum Alloy 2024-T3: Effect of Soluble Chromate Additions vs Chromate Conversion Coating

Pitting potentials (E_pit) 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 0.01 M soluble chromate at pH 6 and pH 8. The mass transport limiting current density for O₂ 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 [ORR]).