Chromium/nickel-free conversion coating as cold post-treatment to anodized AA2024-T3 for corrosion resistance increase

The AA2024-T3 alloy exhibits significant properties for industrial applications, including low weight and high mechanical strength. However, its susceptibility to corrosion, attributed to precipitation hardening, requires effective surface treatments. Anodized aluminum alloys offer a robust protective structure; nevertheless, their porous layer must enable the anchoring of an additional coating or be adequately sealed. Boiling water sealing, though a non-toxic process, demands substantial energy consumption, while toxicity concerns limit the use of sealing with dichromate and Ni salts. Consequently, there is a pressing need to explore novel sealing methods capable of operating at lower temperatures for brief durations without Cr(VI) or Ni. This study proposes an alternative post-treatment for AA2024-T3 anodic layers generated in tartaric sulfuric acid (TSA). The method involves immersing the samples for 5 min in solutions containing H2ZrF6 alone, Na2MoO4 with KMnO4, and a combination of all three. The resulting layers underwent analysis using scanning electron microscopy (SEM), energy dispersion X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), open circuit potential (OCP) measurements during coating deposition, electrochemical impedance spectroscopy (EIS), and a visual examination post-immersion in a NaCl solution. The findings indicate that the Zr and Zr-Mo-Mn conversion coatings partially sealed the porous structure of the anodic oxide film, thereby enhancing its corrosion resistance in chloride medium. The incorporation of molybdate and permanganate ions intensified the precipitation of Zr oxide-fluorides within and on the porous layer, leading to improved corrosion resistance of the anodized alloy compared to Zr-only coating.