Fungus resistant boric acid-sulfuric acid anodizing

An improved method of anodizing an aluminum or aluminum alloy workpiece and concurrently inhibiting the growth of fungus without hindering the effectiveness of the anodizing process, comprising the steps of:

• (a) providing an aqueous anodizing bath consisting essentially by weight of about 3 to 5 percent sulfuric acid, from about 0.5 to 1 percent boric acid, not more than about 5.0 g/L aluminum ion, not more than about 0.2 percent chloride ion, and a fungistat consisting of sodium benzoate, said fungistat having a concentration in the range of about one part per million (1 ppm) to about ten thousand parts per million (10,000 ppm) by weight;
• (b) maintaining said bath at a temperature from about 70 to about 90 degrees F.;
• (c) immersing said workpiece in said bath;
• (d) ramping the voltage applied across said workpiece in said bath from about 5 to about 20 volts such that the current density is substantially uniform across the workpiece and the average current density does not exceed about 10 amperes per square foot;
• and
• (e) maintaining said workpiece in said bath for a time such that an adherent coating of aluminum oxide is applied thereto having a coating weight between about 200 and 600 milligrams per square foot.

An improved bath for anodizing aluminum or aluminum alloy workpieces and concurrently inhibiting the growth of fungus without hindering the effectiveness of the anodizing process, comprising an aqueous anodizing solution consisting essentially by weight of:

• (a) from about 3 to 5 percent sulfuric acid;
• (b) from about 0.5 to 1 percent boric acid;
• (c) not more than about 5.0 g/L aluminum ion;
• (d) not more than about 0.2 percent chloride ion, and
• (e) a fungistat consisting of benzoic acid, a water-soluble salt of benzoic acid selected from the group consisting of sodium benzoate, potassium benzoate, or lithium benzoate, or mixtures thereof, said fungistat having a concentration in the range of about one part per million (1 ppm) to about ten thousand parts per million (10,000 ppm) by weight.

An improved bath for anodizing aluminum or aluminum alloy workpieces and concurrently inhibiting the growth of fungus without hindering the effectiveness of the anodizing process, comprising an aqueous anodizing solution consisting essentially by weight of:

• (a) from about 3 to 5 percent sulfuric acid;
• (b) from about 0.5 to 1 percent boric acid;
• (c) not more than about 5.0 g/L aluminum ion;
• (d) not more than about 0.2 percent chloride ion, and
• (e) a fungistat consisting of sodium benzoate, said fungistat having a concentration in the range of about one part per million (1 ppm) to about ten thousand parts per million (10,000 ppm) by weight.