Effect of oxalic acid addition on suppressing oxide deformation and cracking during hard anodization of aluminum in sulfuric acid

چکیده مقاله

Although hard anodization of aluminum in the voltage range of 40–70 V in sulfuric acid electrolyte can lead to the fast fabrication of self-organized nanopores with small diameters, the emergence of cracks and structural defects in them has limited their broad applications so far. In the present work, the effect of adding oxalic acid to sulfuric acid on reducing and eliminating cracks and deformation of porous anodic alumina (PAA) formed during hard anodizing of aluminum is studied. Field-emission scanning electron microscopy of samples fabricated using sulfuric acid shows that the anodization in sulfuric acid generally leads to two types of deformations and defects in the oxide structure, resulting from non-uniform distribution of anodization current between the growing nanopores. By adding oxalic acid to the electrolyte, a uniform distribution of the current throughout the growing oxide surface is made, in addition to reducing the anodization current during the transition from the mild to the hard regime. In this way, by adjusting the concentration of sulfuric acid and oxalic acid at each anodization voltage, an array of self-ordered PAA can be created in the interpore distance and pore diameter ranges of 88–142 nm and 17–25 nm, respectively, being free of any oxide cracks or deformation.