Hard Anodizing of 6061-T0 Aluminium Alloy in Sulfuric Acid Bath at Low Temperature and its Micro-Hardness Properties

Surface treatment was a practical solution for materials that have a high strength-to-weight ratio but were susceptible to surface deterioration as well as aluminium. The growth of thick aluminium oxide layer Al2O3 (range 12 - 120 mu m, MIL-A-8625F standard) on aluminium surface by a hard anodizing method using low-temperature electrolyte showed significant improvement of micro-hardness, however, there are not many references to discuss it. Al 6061 T0 which has an initial hardness value of 103.5 HV was used in this study. The growth of the anodic layer on the aluminium surface was performed using 11% H2SO4 as electrolyte and the temperature of the solution was kept low at 2 +/- 0.6 degrees C by circulating it using heat exchanger system. The anodizing process started with a galvanostatic condition (constant current density) 4.2 A/dm(2) and switched to a potentiostatic condition (constant voltage) when it reached the target voltage Ut. It was maintained until the current density decreased to 1.0 A/dm2. The target voltage was varied at 30, 40 and 60 V. The optical micrograph and SEM showed a uniform grey to black-ish layer had been formed. The result showed the microhardness and thickness of oxide film increased with increasing the target voltage. The anodizing process at 30, 40 and 60 V produced the oxide film with a microhardness of 334.8, 370.0 and 400.0 HV, respectively. Furthermore, these processes also resulted in oxide film thickness of 40.86, 80.14, 110.01 mu m, respectively. The growth of oxides at low-temperature condition can be approximated by a logarithmic function d = k(l) ln (U-t-a). the function has a physical meaning that in order to obtain an increase in thickness it takes a voltage that is multiplied in value based on the logarithmic function used and the value of growth constant kl of this case was 29.09 mu mV(-1). Further research is needed to obtain an optimal condition to anodic layer growth.