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

Abstract

Pitting potentials (Epit) 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 Epit in 0.1 M Na2SO4 + 0.005 M NaCl. However, CCC did not improve the Epit 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 reducti...