Optimization and Characterization of Hard Anodic Oxide Coatings on 2A12 Aluminum Alloy Using a Multicomponent Mixed-Acid Electrolyte

A quaternary electrolyte system consisting of sulfuric acid, boric acid, citric acid, and aluminum sulfate was developed to regulate oxide film growth and improve the corrosion resistance of hard anodic oxide films on 2A12 aluminum alloy. The electrolyte composition and anodizing parameters were systematically optimized using single-factor experiments and orthogonal design. The optimized oxide film exhibits a thickness of 55.43 μm and a microhardness of 278 HV, exhibiting a compact and continuous microstructure with fewer structural defects. SEM, EDS, XRD, and XPS analyses reveal that the optimized coating mainly consists of amorphous or poorly crystalline alumina. Electrochemical measurements demonstrate that the corrosion current density decreases from 5.32 × 10−5 A/cm2 for the substrate to 2.14 × 10−8 A/cm2, corresponding to a corrosion inhibition efficiency of 99.96%. The quaternary electrolyte system promoted the formation of a thicker and more compact oxide layer with fewer structural defects, thereby significantly enhancing the corrosion resistance and providing insights into the design of multicomponent electrolytes for the hard anodizing of high-copper aluminum alloys.