ACTIVATION QF ALUMINUM

Related Applications

[0001] The present application claims the benefit of pending United States provisional application serial number 60/922,227 filed on April 6, 2007, for ACTIVATION OF ALUMINUM _J the entire disclosure of which is folly incorporated herein by reference.

Background

[0002] In the mid 1980's, a technique for case (surface) hardening stainless steel was developed in which the workpiece is contacted with a carburizing gas at low temperature, typically below 500° C (932° F). At these temperatures, and provided that carburization does not last too long, carbon atoms interstitially diffuse into the workpiece surfaces, typically to a depth of 20-50 μ, without formation of carbide precipitates. Nonetheless, an extraordinarily hard case (surface layer) is obtained, which is due to the stress placed on the crystal lattice of the metal by the diffused carbon atoms. Moreover, because carbide precipitates are absent, the corrosion resistance of the steel is unimpaired, even improved.

[0003] This technique, which is referred to a "low temperature carburization," is described in a number of publications including U.S. 5,556,483, U.S. 5,593,510, U.S. 5,792,282, U.S. 6,165,597, U.S. 6,547,888, EPO 0787817, Japan 9-14019 (Kokai 9-268364) and Japan 9- 71853 (Kokai 9-71853). The disclosures of these documents are incorporated herein by reference. Low temperature carburization is different from conventional carburization, which is done at much higher temperatures, e.g. 950° C (932° F) or more, and further in which hardening occurs through formation of carbide precipitates, i.e., precipitates of distinct chemical compounds of carbon.

[0004] Stainless steel is corrosion resistant because of a coherent protective layer of chromium oxide (Cr ₂ O ₃ ) which instantaneously forms when the steel is exposed to air. This chromium oxide layer is impervious to diffusion of carbon atoms at the low temperatures involved in low temperature carburization. Accordingly, the surfaces of a stainless steel

workpiece to be low temperature carburized must treated or "activated" or "depassivated" for allowing diffusion to occur.

[0005] Commonly-assigned U.S. 6,093,303 ₅ describes a process for activating the surfaces of stainless steel workpieces to be low temperature carburized by electroplating these surfaces with iron. As described there, the electroplated iron layer serves several important functions. First, the plating process automatically activates the article. No separate activation step is required. Second, the iron is transparent to carbon atoms therefore the iron layer can remain on the article during the carburization process. Third, the iron layer allows the article to be exposed to air between the activation and diffusion steps because the iron maintains the article in an activated condition. Once carburization is complete, the electroplated iron remaining on the carburized surfaces, if any, is removed, which causes the underlying surface to immediately reoxidize upon exposure to air. The result is workpiece whose surfaces are not only harder but also more corrosion resistant than the native {i.e. unmodified) metal from which the workpiece is made.

[0006] Commonly- assigned WO 03/074752 describes a similar activation process in which a titanium workpiece to be surface hardened by nitriding is activated by electroplating with iron. As described there, the iron so deposited diffuses substitutionally into the body of the workpiece ahead of the nitrogen from the nitriding process where it deposits in the form of a discrete underlayment layer. The product obtained is said to be composed of a hardened case (surface) layer containing interstitially diffused nitrogen atoms, an interior composed of native (i.e. unmodified) metal, and an intermediate iron-containing layer exhibiting an otlβ duplex phase structure in which some of the titanium atoms forming the metal lattice of this layer have been replaced by iron atoms.

[0007] However, subsequent research has shown that the iron-containing duplex phase layer actually forms on top of, not underneath, the hardened case (surface) layer. In addition, it has also been found that this iron-containing, duplex phase layer exhibits inadequate corrosion resistance, and hence must be removed to provide a product with good corrosion resistance.

Summary

[0008] In accordance with this invention, it has been found that aluminum metal and its alloys can be activated for various diffusion-based surface treatments by electroplating with iron and its analogues.

[0009] Thus, this invention provides an activation process for activating the surface of a workpiece made from aluminum or an alloy of aluminum, the activation process comprising electroplating the surface with a metal which is transparent to diffusion of carbon atoms, nitrogen atoms, boron atoms or mixtures thereof.

[0010] In addition, this invention further provides a workpiece made from aluminum or an aluminum alloy, wherein the workpiece includes at least one activated surface carrying a layer of an electroplated metal which is transparent to diffusion of carbon atoms, nitrogen atoms, boron atoms or mixtures thereof.

[0011] Furthermore, this invention also provides a diffusion-based surface treatment in which the activated surface of a workpiece made from aluminum or an alloy of aluminum is contacted with a source of diffusing atoms selected from C, N, B to cause diffusion of the diffusing atoms into the activated surface, wherein the activated surface has been electroplated with a metal transparent to diffusion of carbon atoms, nitrogen atoms, boron atoms or mixtures thereof.

DETAILED DESCRIPTION

[0012] In accordance with this invention, aluminum metal and its alloys are activated for various diffusion-based surface treatments by electroplating with iron or its analogues.

Aluminum and Its Alloys

[0013] This invention relates to activating workpieces made from aluminum and its alloys in preparation for various diffusion-based surface treatments. Like stainless steel, aluminum and its alloys also form a coherent oxide protective coating immediately upon exposure to air. This coating is impervious to the transfer of the atoms supplied by most diffusion-based surface treatments, especially those conducted below about 750 ^° C. Therefore, the surfaces of the workpiece to be altered must also be activated to make them permeable to these atoms, hi accordance with this invention, a new technique is provided for this purpose.

[0014] This invention is applicable to any aluminum-base metal or alloy which forms such coherent oxide protective coatings and further which is amenable to diffusion-based surface treatments. Accordingly, this invention is applicable to pure aluminum metal as well as alloys of aluminum which exhibit a face centered cubic crystal lattice structure. More specifically, the present invention is applicable to any aluminum alloy whose surface exhibits contiguous regions or "domains" exhibiting a face centered cubic crystal structure

such that diffused atoms can penetrate into these contiguous regions sufficient to form a noticeable hardening effect on the surface being treated. Aluminum alloys of particular interest are those containing one or more of Cu, Mg, Mn, Si, Fe, Cr, Zn and Ni, while aluminum alloys containing one or more of Cu, Mg, Mn, Si and Fe are of particular interest. Other elements can also be included. Particular aluminum alloys of interest are AA 7075, AA 3003, AA 6061, AA 6063, AA 2026, AA 2024, AA 2017, AA 2011, AA 5029, AA 5052, AA 5053 and AA 1100.

Diffusion-Based Surface Treatments

[0015] The surface properties of a workpiece made from aluminum or its alloys can be improved by a number of different diffusion-based treatments in which the surfaces to be improved are contacted with a source of atoms capable of interstitially diffusing into these surfaces. Usually this is done by contacting the workpiece with a diffusion gas containing a compound which decomposes to yield these atoms at the elevated temperatures necessary to promote diffusion. Examples of such processes include infusing non-metals such as nitrogen, carbon, boron or mixtures thereof into the workpiece.

[0016] Diffusion-based surface treatments processes are desirably carried out in a manner so that formation of specific chemical compounds of the diffusing atoms (e.g. nitrides, carbides and/or borides) is avoided, or at least so that formation of precipitates of such compounds in the metal surface layer being altered is avoided. Such processes are normally carried out at temperatures of about 750° C. or less, more commonly below about 500° C or less. At these moderate elevated temperatures, not only is formation of precipitates avoided, but also the coherent oxide protective coatings inherently formed on aluminum and its alloys remain impervious to diffusion of most materials. Hence, activation is required.

[0017] In addition to the above processes, aluminum and its alloys can be subjected to other diffusion-based surface treatments carried out at higher temperatures. In some instances, the coherent oxide protective coating on the workpiece will become transparent to the diffusing atoms through the action of heat alone. In these situations, separate activation is unnecessary. However, where lower elevated temperatures are involved, a separate activation step as described here is necessary.

Electroplating the Workpiece

[0018] In accordance with this invention, a workpiece made from aluminum or its alloys is activated by electroplating the workpiece with a metal which is transparent to diffusion of carbon atoms, nitrogen atoms, boron atoms or mixtures thereof. Iron or analogous metal, e.g., Co, Ni, Cr, Mo and Pt, are especially interesting. In accordance with this invention it has been found that this procedure rally activates the surface of the workpiece for subsequent diffusion-based surface treatments, especially those surface treatments conducted at moderate elevated temperatures, i.e., temperatures not high enough to render the coherent oxide workpiece coating permeable through heat alone.

[0019] hi addition, it has also been found that the corrosion resistance of the surface treated produced obtained in this way (relative to the native metal from which the workpiece is made) is preserved and even improved when this is done. This is surprising in view of commonly-assigned WO 03/074752, since the corrosion resistance of the titanium workpieces described there is compromised when they are activated by iron electroplating due to the formation of an additional iron-containing duplex layer, as described above. No such iron-containing duplex layer is formed by the inventive process, which is surprising since the aluminum workpieces being processed here, like the titanium workpieces of WO 03/074752, are not made from iron.

[0020] Incidentally, note that interstitial diffusion is a different phenomenon from substitutional diffusion. In interstitial diffusion, the diffused atoms migrate into the workpiece surface by traveling through the spaces between the atoms forming the crystal lattice of the metal. In substitutional diffusion, the diffused atoms migrate into the workpiece surface by replacing atoms forming the crystal lattice of the metal. The additional iron-containing duplex phase layer that forms when titanium workpieces are electroplated with iron and then nitrided, as described in commonly-assigned WO 03/074752 noted above, form through substitutional diffusion. Substitutional diffusion is not involved in this invention or the stainless steel electroplating technology described in commonly-assigned U.S. 6,093,303, also noted above.

[0021] hi carrying out the inventive process, the aluminum-base workpiece can be electroplated with iron by any conventional manner. Moreover, the thickness of the electroplated iron layer obtained is not critical and any thickness can be used. Of course, this layer cannot be so thin that it is unable to shield the underlying metal from contact with air before the diffusion-based surface treatment is completed. In general, electroplated surface layers about lOμ (micron) thick and even thinner are useful.

[0022] As in the case of stainless steel as described in commonly-assigned U.S. 6,093,303, electroplating of aluminum-base workpieces in accordance with this invention automatically activates the article for a subsequent diffusion-based surface treatment. No separate activation step is required. In addition, the electroplated iron layer is transparent to carbon, nitrogen and boron atoms, and therefore the iron layer can remain on the article during the carburization or other diffusion process. Moreover, the iron layer allows the workpiece to be exposed to air between the activation and diffusion steps because the iron maintains the article in an activated condition.

Working Example

J0023] In order to more thoroughly describe the invention, the following working example is presented. In this example, flat aluminum coupons made from Aluminum Alloy 6063 and measuring 10 mm x 20 mm x 3 mm were electroplated with iron by the following procedure:

[0024] The aluminum coupons were first cleaned by sanding with fine sand paper (300 - 500 grit) and then washed by placing in ultrasonically-agitated deionized water containing about 5 vol.% liquid soap at about 135° F (57° C) for one minute. The coupons were then rinsed twice with deionized water at room temperature, after which they were soaked in a 20% sulfuric acid solution in deionized water at about 95° F (37° C) for three minutes.

[0025] The coupons were then immediately immersed in the electrolyte of an electrochemical cell comprising a platinum-coated titanium anode, a cathode comprising a platinum-coated titanium fork for holding each coupon and an electrolyte prepared by dissolving with stirring 400 grams of ferrous ammonium sulfate, Fe (SO ₄ ) ₂ (NH ₄ ) ₂ ^' 6H ₂ O, 15 grams ammonium chloride, NH ₄ Cl, and 10 grams ferrous chloride, FeCl ₂ , in 1000 ml of deionized water at about 135° F (57° C). The solution pH was maintained at a pH of about 2.2 to 2.7 by adjustments with sulfuric acid and ammonium hydroxide.

[0026] The coupons were then electroplated with iron using a direct electrical current, whose current density varied as follows:

0.01 A/in ² for 2 minutes then: 0.02 A/in ² for 1 minute 0.04 A/in ² for 1 minute 0.08 A/in ² for 20 minutes

0.2 A/in ² for 20 minutes

[0027] During this time, the plating solution temperature was maintained at about 95° F (35° C) and its pH was maintained at about 2.2 - 2.7 using sulfuric acid and ammonium hydroxide for adjustments. As result, electroplated iron coatings having minimum thickness of 0.00007 inch (1.8 microns) were obtained.

[0028] Once plating was completed, the coupons were removed from the plating bath and rinsed with deionized water in two to three stage counter flow rinse tanks at room temperature. The coupons were then dried in an oven at about 200° F (93° C) for about 10 minutes.

[0029] Although only a few embodiments of this invention have been described above, it should be appreciated that many modifications can be made. All such modifications are intended to be included within the scope of this invention, which is to be limited only by the following claims.