Method for anodizing aluminum and product produced
First Claim
1. A chromium free method of anodizing an unroughened aluminum alloy workpiece to produce an oxide surface receptive to the formation of strong and durable bonds with epoxy adhesives and coatings and resistant to corrosion comprising the steps of:
- providing a first aqueous anodizing solution consisting essentially by weight of about 10-20% phosphoric acid;
maintaining said first anodizing solution at a temperature from about 75°
to 90°
F.;
immersing said workpiece in said first anodizing solution;
applying a direct electric current having a voltage of about 5 to 15 volts across the workpiece in said solution, such that the current density is substantially uniform across the workpiece and does not exceed about 10 amperes per square foot, for a period of time sufficient to produce a thin, porous, highly adherent oxide at least 5000 angstroms thick on the surface of said workpiece;
providing a second aqueous anodizing solution consisting essentially by weight of 3-5% sulfuric acid and 0.5-1.5% boric acid;
maintaining said second anodizing solution at a temperature from about 75°
to 85°
F.;
removing the workpiece from the first anodizing solution and immersing it in the second anodizing solution;
applying a direct electric current having a voltage of about 15 volts across the workpiece in said solution, such that the current density is substantially uniform across the workpiece and does not exceed about 10 amperes per square foot, for a period of time sufficient to produce a further growth of a less porous oxide on the workpiece with the final oxide coating having a weight of around about 250 to 650 milligrams per square foot and a thickness of about 15,000 to 31,000 angstroms, which final oxide coating does not substantially reduce the fatigue resistance of the workpiece.
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Accused Products
Abstract
An improved method of anodizing aluminum which produces an oxide surface receptive to the formation of strong and durable bonds with epoxy adhesives and coatings with underlying bulk properties providing dissimilar metal separation and basic corrosion protection. The invention provides a two step electrolytic process which includes, firstly, anodizing the aluminum with a phosphoric acid solution and then, secondly, further anodizing the aluminum with a sulfuric and boric acid solution. A product is provided that has a final coating having two anodized regions. The first outer region produced by the phosphoric acid solution is about 3000 angstroms thick and is characterized by open pores which is particularly well suited for the establishment of stable, strong and durable bonds with epoxy primers and adhesives. The second base region produced by the sulfuric/boric acid solution provides a thick, tough, corrosion resistant region about 15,000 angstroms thick.
32 Citations
23 Claims
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1. A chromium free method of anodizing an unroughened aluminum alloy workpiece to produce an oxide surface receptive to the formation of strong and durable bonds with epoxy adhesives and coatings and resistant to corrosion comprising the steps of:
-
providing a first aqueous anodizing solution consisting essentially by weight of about 10-20% phosphoric acid; maintaining said first anodizing solution at a temperature from about 75°
to 90°
F.;immersing said workpiece in said first anodizing solution; applying a direct electric current having a voltage of about 5 to 15 volts across the workpiece in said solution, such that the current density is substantially uniform across the workpiece and does not exceed about 10 amperes per square foot, for a period of time sufficient to produce a thin, porous, highly adherent oxide at least 5000 angstroms thick on the surface of said workpiece; providing a second aqueous anodizing solution consisting essentially by weight of 3-5% sulfuric acid and 0.5-1.5% boric acid; maintaining said second anodizing solution at a temperature from about 75°
to 85°
F.;removing the workpiece from the first anodizing solution and immersing it in the second anodizing solution; applying a direct electric current having a voltage of about 15 volts across the workpiece in said solution, such that the current density is substantially uniform across the workpiece and does not exceed about 10 amperes per square foot, for a period of time sufficient to produce a further growth of a less porous oxide on the workpiece with the final oxide coating having a weight of around about 250 to 650 milligrams per square foot and a thickness of about 15,000 to 31,000 angstroms, which final oxide coating does not substantially reduce the fatigue resistance of the workpiece. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15)
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16. A chromium free method of anodizing an unroughened aluminum alloy workpiece to produce an oxide surface receptive to the formation of strong and durable bonds with epoxy adhesives and coatings and resistant to corrosion comprising the steps of:
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immersing a workpiece ina first anodizing solution for a time sufficient to form a first thin porous highly adherent oxide layer on the surface of the workpiece, said first oxide layer being about 5,000 angstroms thick and having pores having diameters of about 400 angstroms; immersing the workpiece in a second anodizing solution so as to permit the second solution to percolate through the first porous oxide to the interface between the base of the first oxide layer and the base metal of the workpiece and cause a second less porous oxide layer to commence to grow from said interface, which second anodizing solution has a low aluminum oxide dissolution rate; maintaining the workpiece in the second anodizing solution until the weight of the combined first and second oxide layers achieves a weight of approximately 250-650 milligrams per square foot. - View Dependent Claims (17, 18, 19, 20, 21, 22, 23)
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Specification