Electrolyte concentration-dependent barrier response in unsealed sulfuric acid anodized AA6061 and AA5754 alloys

Conventional sulfuric acid anodizing studies relate barrier performance to coating thickness, porosity, or sealing efficiency, whereas the combined effects of electrolyte concentration, gravimetric dissolution, coating thickness, and alloy type in intentionally unsealed films remain unclear. This study compares unsealed AA6061 and AA5754 anodized in 15, 18, and 20 vol. % H 2 SO 4 using gravimetric weight loss (W L ), coating thickness, a derived normalized gravimetric dissolution index (NGDI), a derived gravimetric mass-loss rate (MLR), microscopy, scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM–EDS), and electrochemical impedance spectroscopy (EIS). Increasing H 2 SO 4 concentration increased gravimetric dissolution in both alloys. For AA6061, W L increased from 28 to 72 and 84 mg/dm 2 , while NGDI increased from 20.74% to 45.98% and 47.86%. AA6061–15% showed the lowest W L , derived NGDI and MLR, highest barrier resistance, and lowest porous-region constant phase element magnitude (Q por ). AA5754 showed greater dissolution, with W L increasing from 60 to 92 mg/dm 2 and NGDI from 36.43% to 50.11%. NGDI and MLR are derived quantities and are not independent confirmation of W L . The principal finding is an inverse empirical association between gravimetric dissolution and impedance-derived barrier resistance. Microscopy is used only as qualitative morphological context, and no specific pore-scale transport or self-healing mechanism is claimed.