The Effects of Soft Anodizing Pretreatments on the Corrosion Properties of Nickel Electroplated AA6061-T6
摘要
The primary objective of this research was to investigate the corrosion behavior of nickel (Ni) coatings electroplated onto various anodic aluminum oxide (AAO) films formed on AA6061-T6. The AAO films were prepared through an anodizing process in a 0.3 M oxalic acid electrolyte at 25 °C, with an applied potential of 90 volts for 9 min. To enlarge the pore diameters, the AAO films underwent a dissolution stage in a mixture of chromic and phosphoric acids for durations of 60 and 120 min, respectively. Nickel electroplating was then performed in a Watts bath at 40 °C, at a pH of 3, and with a current density of 30 mA/cm2. The coatings were characterized using field emission scanning electron microscopy (FE-SEM) for surface morphology and scanning electron microscopy (SEM) for cross-sectional observations. Electrochemical potentiodynamic analysis and electrochemical impedance spectroscopy (EIS) were employed to assess the corrosion properties of the coatings. Results indicated that the anodizing process significantly reduced the corrosion current density of the AA6061-T6 alloy from 3.57 to 0.11 μA/cm2. However, the porous structures of the AAO films made them susceptible to pitting corrosion. The Ni electroplating on the dissolved AAO films formed a dense Ni layer at the interface between the AAO films and the substrate, effectively preventing pitting corrosion. EIS revealed a shift in the corrosion mechanism from charge transfer to diffusion, further demonstrating that the formation of a uniform Ni layer inhibited pitting corrosion.