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Anodizing Thin Aluminum to Produce Non Gas-Permeable Oxide

We are developing technology that requires a completely dense barrier layer (non gas-permeable) aluminum oxide. We have been unable to find a supplier that can meet our specifications, so we are currently making them by anodizing aluminum (99% pure, up to 100 microns thick) in 1% citric acid [on eBay or Amazon] using a stainless steel cathode. Initially we found some success with a 300 V power supply, setting the voltage at 300 V and letting the current float, though we are only able to anodize about 5 to 10 percent of the thickness from each surface of the aluminum. We now have a 600V, 1.7A power supply, so we will try this as well.
We would appreciate any comments on how to improve our processes.

Chris Allen
Product Development, R&D - Vancouver, BC, Canada
November 18, 2009