Effect of Temperature, Voltage, Time of Anodizing and Applying Nickel-Phosphor Electro less Coating on Hardness and Corrosion Behavior of 2024 Aluminum Alloy

In order to improve the surface properties and the greater use of aluminum and its alloys in various industries, especially aerospace, different coating methods have been introduced. Anodizing and electroless plating are the most widely used methods for this purpose. In this research, the effect of temperature and voltage of anodizing on the surface properties of anodic aluminum oxid (AAO), producred on 2024 aluminum, was investigated. The results showed that with increasing voltage and decreasing temperature, the thickness and roughness of coatings increased. An optimal voltage (45 V) was obtained to achieve the highest hardness at all temperatures. The study of the thickness and hardness of the optimum sample also showed an increase in these two parameters with increasing time. FESEM studies also revealed that by carefully controlling on the anodizing conditions, a structure with regular nano-cells could be obtained. The two step anodizing significantly increased the order of the oxide layer cells. SEM, EDS, and XRD results indicated that the possibility of Ni-P electroless coating applying on anodized aluminum in SLOTONIP 70A solution is well established. To investigate and compare the corrosion behavior of AAO and AAO/Ni-P coatings with aluminum substrate, the polarization method and tafel extrapolation was used. The results indicated that the high corrosion resistance of 2024 aluminum alloy was achieved in the presence of Ni-P electroless coating on anodized alloy. The heat treatment of the samples at different temperatures showed that the highest hardness of the final coating (1185 vickers) would be achieved at 400 °C, and the heat treatment time at this temperature after 75 minutes would not have much effect on the hardness.