Effect of acid-molybdate combinations during anodization on the formation of anodic coatings and their corrosion resistance for AA2024 aluminum alloy

Aluminum, particularly the AA2024 alloy, is widely used in aerospace applications due to its high strength-to-weight ratio. However, its susceptibility to localized corrosion and poor coating adhesion limits its long-term reliability. To address these challenges, various surface-treatment techniques are employed, with anodization playing a key role in enhancing both corrosion resistance and interfacial bonding. Nevertheless, achieving a stable and functional oxide–substrate interface remains a critical obstacle. This study presents a novel comparative anodization approach using three molybdate–acid electrolytes—Sulfuric (AMS), Oxalic (AMO), and Phosphoric (AMP)—under identical processing conditions, aiming to improve both passivation efficiency and interfacial durability. We provide a systematic comparison of molybdate combined with three distinct acid electrolytes, focusing on pore morphology—critical for coating adhesion—and the corrosion behavior of AA2024 alloy. Electrochemical techniques (OCP, EIS, and polarization) and surface characterization methods (SEM, EDS, and AFM) were employed to analyze the resulting oxide layers. Among the tested systems, AMP-treated samples exhibited superior performance, showing the lowest corrosion current density ((1.80 ± 0.19) × 10−7 A/cm2), the highest polarization resistance (140 ± 6.20 kΩ·cm2), and the lowest corrosion rate ((1.95 ± 0.13) × 10−³ mm/year). SEM–EDS revealed the formation of compact oxide films enriched in Mo and P. SEM and AFM confirmed a homogeneous, fine-pored surface morphology. These structural features are expected to enhance adhesive bonding via improved mechanical interlocking and chemical compatibility. This multi-electrolyte anodization strategy offers a scalable, cost-effective pathway to enhance both corrosion resistance and adhesion reliability of AA2024, supporting its broader application in demanding aerospace environments.