Hard anodizing technology is commonly used to enhance the wear and corrosion resistance of aluminum alloys, and eco-friendly electrolytes represent an important development trend in the current stage of this technology. The work aims to choose DL-malic acid as a partial substitute for sulfuric acid to integrate the excellent conductivity properties of sulfuric acid and oxalic acid, and investigate the effect of the DL-malic acid content on the structure and abrasion resistance of the hard anodic film formed on 7075 aluminum alloys. Specifically, the basic solution (200 g/L sulfuric acid +40 g/L oxalic acid) and process parameters ((2±2) ℃, 3 A/dm2, 60 min) were firstly determined through an orthogonal test and then the electrolytes with 20 g/L, 40 g/L, 60 g/L, and 80 g/L DL-malic acid were prepared by adding DL-malic acid to the basic solution. Finally, the structure, composition, and performance of the hard anodic film formed on 7075 aluminum alloys in the solutions were characterized through scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). The experimental results revealed that the anodic films were mainly composed by nanoscale oxide particles except some micro pores. Among them, the films formed in the solution with 40 g/L DL-malic acid possessed the smallest particles and least pores. With the DL-malic acid content increasing, the thickness of the anodic film increased initially and decreased afterwards, and inflection point was 40 g/L when the film thickness was 47 μm. The hardness of the anodic film was much larger than that of the matrix, especially the one formed in the condition of 40 g/L DL-malic acid whose hardness was 351.5 HV0.2, about twice of 7075 aluminum alloy. The composition analysis indicated that the oxide film primarily consisted of Al2O3 and AlOOH with additional amounts of sulfate and organic carbon. In addition, characteristic peaks related to the crystallographic γ-Al2O3 were detected only in the film formed in the electrolyte with DL-malic acid concentration of 20 g/L, confirming the effect of the DL-malic acid on the crystalline structure of the anodic film. The results of the wear corrosion tests indicated that as compared to the base material, the friction coefficients of the anodic film were larger owing to the roughness increasing during the anodic process, while the corrosion potential increased by about 0.58 V, the corrosion density decreased by about two orders of magnitude, and the minimum value (1.949×10-6 A/cm2) belonged to the films formed in the solution with 40 g/L DL-malic acid. After anodic process, the wear resistance was improved significantly, and the anodic film formed in the condition with 40 g/L DL-malic acid exhibited the best the wear resistance. The polishing scratch was -3.74 μm which was only 2% of that on the surface of 7075 aluminum alloy. The microtopography of the polishing scratch revealed that the wear type of the sample treated in the electrolyte containing 20 g/L DL-malic acid belonged to abrasive wear, and the others were fatigue wear. In summary, DL-malic acid plays a significant role in inhibiting corrosion during the anodizing process, which helps to improve film formation efficiency and increase the thickness of the oxide film, and the optimized electrolyte is made up of 200 g/L sulfuric acid, 40 g/L oxalic acid, and 40 g/L DL-malic acid.
Key words
7075 aluminum alloy /
wear corrosion /
surface strengthening /
hard anodizing /
DL-malic acid
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Funding
National Natural Science Foundation of China (U2106216), Fundamental Research Funds for the Central Universities (202313029), "Youth Innovation Team" Program of Shandong Provincial Colleges and Universities (2022KJ055)
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