Effect of Oxalic Acid Addition on Hardness and Pore-like Surface Feature Morphology of AA7075 Oxide Layer via Hard Anodizing

AA7075 is a high-strength aluminum alloy widely used in aerospace applications; however, its susceptibility to corrosion and surface degradation may reduce its long-term performance. This preliminary study evaluated the effect of oxalic acid addition to a sulfuric-acid electrolyte during hard anodizing on the measured surface microhardness and surface morphology of an aluminum alloy supplied as AA7075. Hard anodizing was conducted for 60 min at a bath temperature of 3–7°C and a current density of 3 A/dm² using 20% (w/w) sulfuric acid with oxalic-acid additions of 0, 0.5, 1.0, 1.5, and 2.0% (w/w). Surface morphology was characterized using scanning electron microscopy (SEM), while surface microhardness was measured using the Vickers method. The measured surface microhardness increased from 69.8 HVN for the untreated alloy to 296.9–330.7 HVN after anodizing. The highest measured surface microhardness was obtained at 2.0% (w/w) oxalic acid (330.7 HVN). However, the 0.5% (w/w) condition produced a comparable hardness value (329.5 HVN) and the smallest average diameter of selected pore-like surface features (98.38 nm), indicating a favorable combined response within the investigated conditions. The findings demonstrate that oxalic-acid addition modified the measured surface microhardness and surface morphology of the anodized alloy. Further studies involving independent replication, coating thickness, corrosion resistance, wear resistance, and adhesion evaluation are required before aerospace application claims can be established