Comparison of oxalic and sulfuric acid electrolytes for enhancing surface properties and microhardness of graphite-reinforced anodic coatings

Abstract In the development of hard anodic oxide coatings (Type III) for aluminum alloys used in dry sliding applications, the incorporation of graphite particles into the oxide structure has been shown to reduce high porosity, surface roughness, and large pore dimensions. However, this reduction is limited to approximately 40% in average pore dimensions (μm), highlighting the need for further improvements to enhance coating performance. One promising approach lies in the use of oxalic acid, which previous studies have demonstrated that oxalic acid can produce denser and less porous oxide layers than sulfuric acid. However, it is more commonly used in mild anodizing (Type II). The usage of oxalic acid in the formation of composite coatings, particularly for hard anodizing, remains underexplored. Furthermore, the relationship between anodizing parameters, such as applied voltage, and the resulting surface and mechanical properties of graphite-reinforced hard anodic oxide coatings have not been fully investigated. Therefore, this study aims to examine the effect of anodizing electrolyte type; oxalic acid compared to sulfuric acid on the surface properties and microhardness of graphite-reinforced hard anodic oxide coatings, and determine the optimal anodizing voltage for oxalic acid to achieve improved surface uniformity, reduced porosity, and enhanced mechanical properties. The results identified 20 V as the optimal voltage for oxalic acid anodizing, showing good average surface roughness, and the highest microhardness among other voltages of 5 V,10 V,15 V,25 V, and 30 V. Coatings formed in oxalic acid with graphite exhibited a 53.6% reduction in pore width, 73.3% reduction in pore depth, 52.3% reduction in surface roughness, and a 46.6% increase in microhardness compared to those in sulfuric acid. A key finding was that graphite particles interacted more effectively with oxalic acid, promoting thicker and more uniform coatings. The oxide layer in oxalic acid with graphite increased 192% in thickness compared to sulfuric acid with graphite.