Formation of porous anodic alumina in alkaline borate electrolyte

The growth of porous anodic films at 60 V in an alkaline 0.13 M borax electrolyte at 333 K is examined using sputtering-deposited aluminium substrates, with a fine band of incorporated tungsten tracer. The findings reveal amorphous alumina films containing approximately conical major pores incorporating finer secondary pores, with film thicknesses similar to that of the oxidized aluminium. Further, the distribution of the tungsten tracer within the film is mainly consistent with its expected migration behaviour in anodic alumina. The results indicate that pore development under the present growth conditions is dominated by field-assisted dissolution of anodic alumina, with an efficiency of film growth of about 50%. The findings are in contrast with those of porous anodic films formed in phosphoric acid electrolyte, which are significantly thicker than the layer of oxidized metal.