SEALING OF POROUS ANODIC FILMS ON ALUMINUM

The sealing of porous anodic films, formed on aluminium in sulphuric acid, has been examined using electrochemical impedance spectroscopy. In contrast to many previous studies, distilled water been used, and measurements have been made both at room temperature and at the boiling point of water. The latter approach has formed the basis of the assessment of impedance spectroscopy as an on-line monitoring technique for the effectiveness of sealing of porous anodic films on aluminum. Studies using interruption of the sealing process have revealed the importance of pore hydration resistance in determining the extent of sealing after the bridging or plugging over of the pore mouths. This has led to a refinement of the basic equivalent circuit of sealed anodic films. For continuous impedance measurements at the sealing temperature, a further modification of the equivalent circuit is necessary to allow simulation of the impedance response. The modified response indicates the importance of gel-like material development within the pore volume at the sealing temperature. The consequence of reduction in temperature, from that of boiling water to ambient temperature, is the compaction of the gel-like material in the form of more crystalline hydrated aluminas which contribute an enhanced resistance to the measured impedance.