Patent ID: 7048814

Claim:
A method of creating an aluminum oxide protective film on a surface of a high purity aluminum alloy, comprising: providing an aluminum alloy, wherein said alloy includes mobile impurities present at the following concentrations, magnesium at a concentration ranging from about 3.5 weight % to 4.0 weight %, silicon at a concentration ranging from 0 weight % to 0.03 weight %, iron at a concentration ranging from about 0 weight % to 0.03 weight %, copper at a concentration ranging from about 0.02 weight % to 0.07 weight %, manganese at a concentration ranging from about 0.005 weight % to 0.015 weight %, zinc at a concentration ranging from about 0.08 weight % to 0.16 weight %, chromium at a concentration ranging from about 0.02 weight % to 0.07 weight %, titanium at a concentration ranging from 0 weight % to 0.01 weight %, with individual other impurities limited to 0 weight % to 0.03 weight % each, wherein mobile impurity particulates present in said high purity aluminum alloy are limited so that at least 95% of all particles have a particle size of less than 5 μm, no more than 5% of said particles have a particle size ranging between 20 μm and 5 μm, and no more than 0.2% of said particles have a particle size ranging between 50 μm and 20 μm and exposing said surface of said aluminum alloy to an electrolytic oxidation process during which said surface is immersed as an anode in an acid electrolyte, with a cathode comprised of an aluminum alloy, and wherein a DC current is applied, wherein said acid electrolyte is a water-based solution comprising 10% to 20% by weight sulfuric acid and about 0.5% to 3.0% by weight oxalic acid, wherein said protective film is created at a temperature ranging from about 5° C. to about 25° C., and wherein an applied current density of said DC current ranges from 5 A/ft 2 to 36 A/ft 2 .