Effect of Anodizing Type on Surface Morphology in 6061 Al Alloy with a Modified Electrolyte

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Aluminum anodizing is an electrochemical process widely used to enhance the corrosion and wear resistance of aluminum and its alloys by forming a protective aluminum oxide (Al2O3) layer [1–3]. The characteristics of the anodic film depend on electrolyte temperature, applied voltage, and current density. Type III anodizing (hard anodizing) is conducted under more severe conditions than Type II, generally producing thicker oxide layers. Although sulfuric acid anodizing has been extensively studied, comparative evaluation of Type II and Type III anodizing using boric acid–modified electrolytes under standardized conditions remains limited. This work evaluates the influence of Type II (25 °C) and Type III (3 °C) anodizing on the microstructure, surface composition, and topography of 6061 aluminum. 6061 aluminum samples were ground to 600 grit, ultrasonically cleaned for 15 min each in acetone and isopropyl alcohol, and rinsed with deionized water after each step. The native oxide layer was removed by immersion in 1 M NaOH for 10 min, followed by rinsing and immersion in 1 M HNO3 for 2 min before anodizing. The treatment was performed for 45 min in an electrolyte containing 10% sulfuric acid and 1% boric acid under constant-current conditions according to ASTM B580-79(2020) [4]. Type II anodizing was carried out at 25 °C (±3 °C), 18 V, and 1.5 A/dm², while Type III was conducted at 3 °C (±3 °C), 24 V, and 4 A/dm². After anodizing, samples were sealed in water at 90 °C (±3 °C) for 20 min. The average oxide thicknesses were 24.6 µm for Type II and 69.33 µm for Type III. Surface morphology was examined by field-emission scanning electron microscopy (FE-SEM, JEOL JSM-7041F) at 2.0 kV and a working distance of 6.1 mm, using magnifications of 5,000× with 20,000× insets. Surface composition was determined by energy-dispersive X-ray spectroscopy (EDS). Surface roughness was measured using a Keyence VHX-7000 digital microscope over an area of 1.8 mm², in accordance with ISO 25178-2. Figure 1 compares the surface morphology and EDS composition under both anodizing conditions. Type II anodizing produces a relatively smooth, compact surface with fine pores and limited structural development, whereas Type III exhibits a nodular morphology with more pronounced pore growth, consistent with the approximately 2.8-fold increase in oxide thickness. The higher current density and voltage used for Type III promote accelerated oxide formation and greater structural development. Despite these morphological differences, EDS results indicate that both coatings are dominated by aluminum and oxygen, confirming the formation of Al2O3. Type III shows a slight relative increase in aluminum (~1.04 wt%) and a decrease in oxygen (~1.25 wt%) compared to Type II. In comparison, sulfur content differs by only ~0.2 wt%, indicating similar incorporation of sulfate species from the electrolyte. These results are consistent with previous studies on sulfuric anodizing and modified electrolytes [5–7], in which processing parameters primarily influence morphology rather than overall composition. Surface roughness parameters are summarized in Figure 2. Type III anodizing shows a 10.4% increase in Sa relative to Type II, indicating higher mean roughness associated with enhanced oxide growth. However, Sz decreases by approximately 25.2%, suggesting a reduction in extreme peak-to-valley amplitudes. The decrease in Sp and Sv supports a more uniform distribution of surface features under hard anodizing conditions. Both surfaces exhibit Ssk < 0 and Sku < 3, characteristic of valley-dominated, relatively flat topography. The combined increase in Sa and reduction in Sz indicates that Type III produces a more developed surface at intermediate scales while reducing extreme irregularities. Lowering the electrolyte temperature likely reduces simultaneous chemical dissolution of the oxide layer, thereby favoring more uniform porous growth at higher voltage and current density. Overall, anodizing parameters—particularly temperature, voltage, and current density—primarily affect oxide growth kinetics and surface morphology rather than chemical composition. Under the boric acid–modified sulfuric electrolyte used in this study, the differences between Type II and Type III anodizing are predominantly microstructural and topographical, with hard anodizing producing thicker, more structurally developed coatings while maintaining similar elemental compositions. FE-SEM micrographs of 6061 aluminum anodized under Type II (25 °C, 18 V, 1.5 A/dm²) and Type III (3 °C, 24 V, 4 A/dm²) conditions in a 10% H2SO4 + 1% H3BO3 electrolyte. Images acquired at 5,000× with 20,000× insets. Acquisition parameters: 2.0 kV, WD = 6.1, 6.0 mm. EDS compositional analysis included. Surface roughness parameters (Sa, Sz, Sp, Sv, Ssk, Sku) measured over an area of 1.8 mm² according to ISO 25178-2 for Type II and Type III anodized samples.

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