Exploring the effect of alkaline silicate solution composition on the ac/dc spark anodization of Al–Cu Alloys

The ac/dc spark anodizing method has been used to form a protective oxide film on the Al–Cu alloy AA2219 in an alkaline silicate solution at 10 mA/cm2. The composition of the anodizing bath is shown to have a crucial effect on the anodizing process and the oxide film characteristics. Various surface techniques showed that the oxide films are porous and consist of three main layers. In all solutions, the thickness of the outer layer increases with increasing silicate concentration and decreases with increasing solution pH. The degree of corrosion protection offered by the oxide layer has been investigated using linear sweep voltammetery in pH 7 borate buffer solution and electrochemical impedance spectroscopy in 0.86 M NaCl solution. Electrochemical studies show that there is a narrow range of anodization solution pH that is needed to achieve the most passive coating, and that the corrosion resistance increases with increasing bath silicate concentration.