US20020011280A1

US20020011280A1 - Method for inhibiting stains on aluminum product surfaces

Info

Publication number: US20020011280A1
Application number: US09/866,953
Authority: US
Inventors: Gary Nitowski; Raymond Colbert; Karl Wefers; Daniel Roup; Larry Wieserman; Paul Ziman
Original assignee: Alcoa Inc
Current assignee: Howmet Aerospace Inc
Priority date: 1998-06-19
Filing date: 2001-05-29
Publication date: 2002-01-31
Also published as: CA2336186A1; EP1088119A2; AU4695799A; WO1999066104A3; WO1999066104A2; BR9912174A; JP2002518594A

Abstract

A method for inhibiting the formation of stains, especailly water stains, on the exterior surface of aluminum alloy products. The method entails contacting the exterior surfaces of these products, particularly sheet or plate products, extrusions and/or forgings made from 5000 or 6000 Series aluminum alloys, with an organophosphonic or organophosphinic acid-derived material. Preferably, liquid forms of this material are added to an alcohol or water-based carrier solution, then sprayed, dipped, painted or rolled onto the surfaces of flat sheet or plate products to enhance their brightness. Other more complex shapes may be dipped into material baths.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No. 09/335,676, filed on Jun. 18, 1999, which application claims the benefit of U.S. Provisional Application Ser. No. 60/090,017, filed on Jun. 19, 1998, the disclosure of which is fully incorporated by reference.[0001]

FIELD OF THE INVENTION

1. Background of the Invention

Water stains do not generally present problems for the structural properties and/or corrosion performance of an aluminum product. Aluminum surface discolorations that accompany water staining may discomfort some customers who are unfamiliar with the surface and corrosion properties of aluminum. Customers already aware of the properties of cold rolled steel may mistakenly believe that water stains on aluminum are the onset of “rusting”, similar to that found on steel. For bright aluminum products, such as buffed trailer plate, rail cars, toolboxes, running boards, and tread plate on fire trucks, stain inhibition would preserve the buffed famish and enhance customer satisfaction. A simple, low-cost solution to inhibit water stain on aluminum could result in a higher degree of customer confidence in replacing steel with aluminum for their products. In addition, aesthetics of these products is important to the end customer. Water stains are aesthetically unattractive and their elimination or reduction would be valuable to the owner whether it be an aluminum trailer, rail car, toolbox or other aluminum product.

2. Description of the Relevant Art

Numerous uses for organophosphonic acids in conjunction with aluminum are known. These include U.S. Pat. Nos. 4,957,890, 5,032,237, 5,059,258, 5,103,550, 5,124,022, 5,124,289, 5,126,210, 5,132,181, 5,238,715, 5,277,788 and 5,463,804. None of these, however, mention organophosphonic acids for the inhibition of stains, especially water stains, on aluminum surfaces. Most of the aforementioned patents describe aluminum surface pretreatments that enhance the durability of organic coatings or adhesively bonded joints. They do not describe the use of organophosphonics without a topcoat.

Other methods for inhibiting corrosion with respect to aluminum and other metals are disclosed in U.S. Pat. Nos. 3,433,577, 3,672,822 and 4,427,448.

BRIEF SUMMARY OF THE INVENTION

This invention addresses a low cost method for inhibiting water staining on 5000 Series, or 5XXX, aluminum alloys, most notably 5083-H321 and 5454-H32 aluminum (Aluminum Association designations). Such alloys are used to make rail hopper cars and buffed trailer tanks. Similar surprising and unexpected results have been observed when this method was practiced on 6000 Series aluminum alloys, like the 6061-T6 alloys used to make various products including vehicle wheels. According to this method, it was determined that spraying a solution consisting of about 0.25 wt % octadecylphosphonic acid (or “ODPA”) in an isopropanol solvent (or other medium) onto these aluminum alloy products, then allowing the alcohol to evaporate, is effective for inhibiting water staining. Similar effects were subsequently observed with a carrier composition containing octylphosphonic acid (or “OPA”). Suitable liquid carriers include alcohols, ketones, ethers, aldehydes, alkanes, and other organic solvents with sufficient solubility for the organophosphonic acids. These organophosphonic acid-derived solutions can be applied to the metal surface by spraying, dipping, painting, or roll coating. It is also recognized that the stain inhibitor component can be delivered to the aluminum surfaces from various compositions used in the manufacture of aluminum parts, including but not limited to: aqueous suspensions or solutions; metal forming lubricants, and metal cleaning and/or rinsing formulations; a buffing compound or wax that incorporates the stain inhibitor, metal heat treatment quench waters, and/or post-rinsing polishers/sealants or the like. For certain stain inhibitor compounds, it is possible to buff a paste-like stain inhibitor directly onto the aluminum product surface.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features, objectives and advantages of the present invention will be made clearer from the following detailed description made with reference to the drawing in which: [0008]
The FIGURE shows the schematic formation and orientation of hydrolytically stable Al-O-P bonds of the stain inhibitor, octylphosphonic acid (OPA), as a reaction product with an oxidized aluminum surface for effecting the stain inhibition observed according to this invention.[0009]

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Preliminary indications of the effectiveness of this invention, for inhibiting stains, were observed in an accelerated corrosion test that involved outgassing products of ethylvinylacetate plastic pellets (obtained from Millennium Petrochemicals), high humidity and temperature cycling. After 12 temperature cycles, no water stains were observed on 5000 Series alloy samples initially etched in caustic, then sprayed with an ODPA-containing solution. Water staining was also inhibited for “mill finish” metal sprayed with ODPA; though some spots were interspersed with unstained surface in the latter case. By contrast, mill finish and etched-only samples were completely covered with water stains. It is believed that the difference in performance as a result of pre-etching were most likely due to the removal of residual rolling lubricants via etching. In that manner, the stain inhibiting molecules of this invention would be allowed to chemically bond with surface aluminum oxides. [0010]
Chemical reaction of the inhibitor to the surface can also be achieved by changing the means of application or by using a different solvent. The surface ODPA inhibits access of water to the aluminum oxide and forms hydrolytically stable bonds with the oxide, thus inhibiting water staining. ODPA is a commercial compound manufactured and sold by Albright & Wilson Ltd. Working solution concentrations and surface coverages of this invention are relatively low, which results in low treatment costs of cents per square foot of Al plate or sheet product. The same would be true for other aluminum product forms, including castings, forgings and extrusions. [0011]
Another potential stain inhibitor, octylphosphonic acid (OPA), was evaluated. It showed even better performance results than the ODPA samplings above. OPA has the following chemical structure: CH[0012] ₃(CH₂)₇P(O)(OH)₂. It can be applied with a water and surfactant carrier as effectively as with an isopropanol carrier. OPA is more soluble than ODPA in isopropanol thus allowing for increased solution concentrations. And while OPA is not water soluble, it forms a suspension of solids with water. When in water, no volatile organic compounds (or VOC's) result from metal contact for stain inhibitors.
Preliminary humidity test results show that OPA is highly effective for inhibiting stains on mill finish or buffed aluminum products without cleaning, pickling or pre-etching. After three hours at 50° C. (125° F.) and 100% relative humidity, the OPA treated surface was unstained, whereas “as-buffed”, untreated surfaces were considerably stained. [0013]
It may also be possible to apply certain formulations by the methods of this invention with no carrier solution. For example, one may directly buff a more solid form of OPA onto an aluminum product surface. It is also possible, actually even more practical depending on the aluminum surface to be treated, to incorporate the stain inhibiting compounds of this invention into mill lubricants for providing an in situ type of stain inhibition and eliminating subsequent processing steps. It may also be possible to similarly add such stain inhibitors to buffing, sealing and/or polishing compound formulations. [0014]
When the aluminum to be treated is mill finish or “as-buffed”, a preferred carrier/solvent is an alcohol, more preferably 2-propanol or isopropanol. Isopropanol is also beneficial in that its solvent action is believed to displace residual mill lubricants or buffing compounds and wet the surface aluminum resulting in the formation of Al-O-P bonds with the oxidized aluminum surface. Isopropanol is also non-toxic. When the aluminum surface has been pre-cleaned or etched, the choice of solvent is not as critical. In many instances, water may be used to transport (or apply) such stain inhibitors. [0015]
In addition to forming hydrolytically stable Al-O-P bonds, organophosphonic acids may provide yet another mechanism for stain inhibition. For example, when OPA or ODPA reacts on the Al surface, the reaction end product is believed to orient or align so that its hydrocarbon chains extend away from said surface. A schematic representation of the bonding that is believed to take place is shown in the accompanying FIGURE. The latter surface takes on a “hydrophobic” or non-wetting quality thereby further inhibiting the conversion of oxides to hydroxides (or effecting a water stain thereon). Under the latter scenario, longer chained organophosphonic acids become the preferred stain inhibitors for this invention. [0016]
In some embodiments of this invention, a full (and not partial or non-uniform) haze on the aluminum product surface may form. It is preferred that such haze be wiped away with a dry cloth to further enhance stain inhibition. On a less preferred basis, this haze may be removed by rinsing the aluminum product's outer surface. [0017]
Certain classes of phosphorus oxo acids, acid esters, and acid salts are effective to various degrees in preventing water stains according to this invention. Phosphate salts, phosphate esters, and phosphonic acids each impart some stain inhibition. In comparative tests, however, octadecylphosphonic (C-18) acid (ODPA) and several fluoro-phosphonic acids were not as effective as OPA (C-8) in inhibiting stains. Poly(vinylphosphonic acid), and copolymers thereof, may work even better than OPA, but it is currently cost prohibitive to use in commercial quantities. Some of the representative stain inhibitors can be grouped by the following “families”: [0018]
a) acidic aluminum phosphate salts [0019]
b) inorganic phosphorus oxo acids [0020]
c) organophosphonic and organophosphinic acids [0021]
d) phosphate acid esters [0022]
e) organo phosphonic acid polymers and copolymers; and for example, poly(vinyl phosphonic-co-acrylic acid) [0023]
f) phosphate ester polymers for example, poly(vinyl phosphoric acid). [0024]
This invention can be used to improve the stain inhibition of numerous aluminum alloy surfaces, including various sheet or plate products, extrusions and forgings, regardless of whether such products have welded joints or other connections. It is best suited for any aluminum product that its purchaser, the end user/consumer, would prefer that said product “look good” (i.e. brighter, less stained, etc.) longer! This includes a whole family of building/architectural products, appliances, lighting supplies, and other household cosmetics like vertical blind stock. On a preferred basis, the method of this invention works well with 5000 and 6000 Series alloys (Aluminum Association designation). It should also enhance the stain inhibiting performance of products made from other aluminum alloys, including but not limited to 1000 and 3000 Series alloys. [0025]
The primary focus of this invention is to inhibit the formation of stains on the exposed outer surfaces of aluminum products. The method claimed below preferably accomplishes this objective using an octylphosphonic acid-based aqueous solution. That solution can be applied to various product forms in a variety of ways. More importantly, this invention is believed to achieve its desired result by decreasing the interaction with, or adhesion of, materials to the aluminum surfaces so treated. It is especially desired to inhibit the interaction of aluminum's outer surface oxide layer (the layer that naturally forms on uncoated or exposed Al product) with waters including rainwater or the like. It is through such contact with rainwater, etc. that aluminum sheet, plate, forgings and extruded product forms result in a visibly stained, or “dulled” exterior surface—an undesirable surface appearance for the owners of such otherwise shiny product forms. [0026]
A primary focus of Alcoa's earlier patents, those to McCleary et al (U.S. Pat. No. 5,463,804) and Wefers et al (U.S. Pat. No. 5,103,550), was to IMPROVE the adhesion of coatings to aluminum stock using a vinylphosphonic acid-based formulation. That VPA-derived composition actually served to increase the interaction of adhesives or coatings with their treated aluminum surfaces, primarily vehicle assemblies for McCleary and beverage container stock for Wefers. In order to determine whether the formulations of McCleary and/or Wefers, distinctly different from those of the present invention, have similar stain-inhibiting potential, a somewhat crude analysis was conducted comparing the octylphosphonic acid-based solutions of this invention with the VPA-derived varieties of these two prior art references. For this analysis, bright coupons of uncoated 5454 aluminum (Aluminum Association designation) were contacted with either: (a) the stain fighter composition of this invention; or (b) a VPA-pretreatment representative of the aforementioned prior art. Those coupons were then subjected to humidity cabinet tests, aimed at simulating accelerated outdoor weathering conditions. The coupons treated with a VPA-based solution exhibited unacceptable water staining after only 5 hours of exposure. By contrast, specimens treated per the present invention remained stain-free after over 50 hours of humidity cabinet weathering simulation. As such, the adhesive-enhancing formulations of McCleary and Wefers do not teach stain-inhibiting methods, but rather teach away from the presently claimed invention. [0027]

EXAMPLES

First Study—Several sections of buffed trailer tank plate product (made from 5454 aluminum alloy) were sprayed with two comparative stain inhibiting compositions: [0028]
Set 1: 0.2 wt % octylphosphonic acid (OPA) in isopropanol; and [0029]
Set 2: 0.2 wt % octadecylphosphonic acid (ODPA) in isopropanol. [0030]
Haze on both sets of sprayed plates was rinsed away with water, then gently buffed with dry cheesecloth. These treated plates, along with an “as-buffed” control, were then placed in a humidity cabinet at 50° C. (125° F.) with 100% relative humidity for 3 hours. After exposure, the plates were removed from the cabinet, dried with a towel, and visually examined for staining. [0031]
No noticeable loss of specularity was observed with either of the above stain inhibition treatments. All surfaces had the same visual appearance as the “as-buffed” sample. After humidity exposure, however, brownish colored, water stains were evident over a majority of the “as-buffed” and ODPA-treated surfaces. The OPA treated surface did not exhibit any water stains and appeared the same as unexposed specimens. While ODPA specimens did not fare as well as OPA in this particular study, as compared with its earlier positive results, different application techniques are believed to have caused its reduced stain inhibiting performance here. [0032]
Second Study—Tanker Trial results—A covered hopper trailer, made from 5454 aluminum Bulk Transportation Sheet (“BTS”) was treated with various applications according to the invention before being exposed to harsh, in-service conditions: from an aggressive environment of salt air due to seacoast proximity; and harsh winter conditions with numerous road salt applications. Subsections of this hopper/tanker were treated as follows: (a) 1 wt. % solution of OPA, in isopropanol, was sprayed on the first section of tanker, dried to a film, water rinsed and air dried thereafter; (b) the same solution as above was sprayed onto another adjoining section of the same hopper/tanker, then dried to a film and wiped to an initial shine using cheesecloth; (c) for this section of hopper/tanker, the treatment material was 1 wt. % OPA, suspended in water. After spraying, this water-based solution was allowed to sit on the product surface for about 10 minutes before being dried and wiped to a shine with cheesecloth. The last comparative section of hopper/tanker was sprayed with a 5 wt. % solution of OPA, in water, before being allowed to sit for 10 minutes, then water rinsed and air dried. [0033]
After three months of service along the U.S. East Coast, this hopper/trailer was brought back for inspection. While OPA treatments were observed to provide a substantial degree of water stain inhibition over that 3 month trial period, one of the first conclusions drawn from that inspection was that monthly reapplications could ensure a pristine, polished surface on such trailer stock. [0034]
Following a wash with non-etching alkaline cleaner, various sides and subsections of this hopper/tanker were photographed and closely compared by visual inspection. From that inspection, it was noted that the water-based sections of treated hopper fared better than their alcohol-based counterparts (in terms of water staining inhibition). In addition, wiping to a shine after application of the OPA, as per example (a) above, was most effective, even more than merely applying, rinsing and air drying, the latter treatment resulting in a noticeable, residual haze at first. [0035]
Third Study—Coil Line Trial—A coil of 5182-H19 aluminum sheet was roll coated with a 5% aqueous suspension of OPA. Phosphorus surface concentrations were measured on the treated surface using X-ray fluorescence spectroscopy (XRF). From previous bench scale tests, it was observed that phosphorus surface levels of about 2 Kcps were sufficient for inhibiting water staining. Phosphorus surface levels on the aforementioned sheet product were measured at about 10 Kcps, however. [0036]
Fourth Study—Forged Truck Wheels—Forged and polished truck wheels made from aluminum alloy 6061-T6 were treated with comparative solutions of 0.5 and 1 wt. % OPA in isopropanol. For testing, the treated wheels were placed into a cabinet with condensing humidity set at 100° F. The wheels were examined every hour for water stains. The tests were stopped after 120 hours of humidity exposure. Untreated wheels (as-polished) were substantially stained within 11 hours of humidity exposure. The wheels that were treated with OPA, then buffed to shine lasted the longest without substantial water staining. On certain OPA-treated wheels, only a few small, widely dispersed spots were observed after 120 hours of exposure testing, but that level of staining was insignificant compared to the gross quantities of water staining observed on the untreated wheels after only 11 hours of humidity exposure. [0037]
Fifth Study—Lighting Sheet—Aluminum alloy 5657-H18, used for making bright lighting sheet, was treated with a 5 wt. % solution of OPA stain inhibitor in isopropanol. Specular reflectance measurements showed that after buffing the resultant haze from said sheet surfaces, the OPA treatment did not reduce reflectivity. Furthermore, such OPA-treated panels lasted up to 13 days in condensing humidity at 100° F. without staining, as compared to their untreated sheet equivalents that were significantly stained within 24 hours of such humidity exposure. [0038]
Sixth Study—Quench Water Additions—Phosphorus compounds, like those described above, were added to the quench waters used for making extruded tubes and rolled sheet from 6061-T6 alloy. In this comparison, the aluminum product forms were heated to about 1000° F. before being cold water quenched, said quenching solution containing various phosphorus compounds. Thereafter, these products were allowed to remain in the quench water for 24 hours. By visually examining these aluminum product forms, and by further measuring the amount of hydroxides formed thereon using Fourier-transform infrared spectroscopy (FT-IR), it was determined separately that 10 g/L solutions of dibasic ammonium phosphate —(NH[0039] ₄)₂HPO₄— and 10 g/L phytic acid best prevented the formation of water stains on these products. They also prevented the formation of bayerite powders on the interior aluminum surfaces of these extruded tubes.
Having described the presently preferred embodiments, it is to be understood that the invention may be otherwise embodied within the scope of the appended claims. [0040]

Claims

What is claimed is:

1. A method for inhibiting formation of stains on an exposed exterior of an aluminum alloy product, said method comprising:

(a) contacting the exposed exterior with a material that is capable of forming a hydrolytically stable Al—O—P bond therewith, said material selected from the group consisting of: an acidic aluminum phosphate salt; a phosphorous acid, a hypophosphorous acid; an organophosphonic or organophosphinic acid; a phosphate acid ester; an organophosphonic acid polymer or copolymer; and an organophosphate acid ester polymer or copolymer.

2. The method of claim 1 wherein the material consists essentially of an organophosphonic or organophosphinic acid.

3. The method of claim 2 wherein the material is a solid or semi-solid at room temperature and applied to the exposed exterior of the aluminum alloy product.

4. The method of claim 2 wherein the material is a liquid at room temperature and combined with a carrier selected from the group consisting of: an alcohol, a ketone, an ether, an aldehyde, an alkane, water and mixtures thereof.

5. The method of claim 4 wherein the material consists essentially of octadecylphosphonic acid and the carrier includes isopropanol.

6. The method of claim 4 wherein the material consists essentially of octylphosphonic acid and the carrier is selected from the group consisting of isopropanol, water and mixtures thereof.

7. The method of claim 4 wherein said exposed exterior is contacted with an aqueous solution consisting essentially of up to about 10 wt. % octadecylphosphonic acid, about 5-90 wt % isopropanol, a balance of water and impurities.

8. The method of claim 4 wherein said exposed exterior is contacted with an aqueous solution consisting essentially of up to about 10 wt. % octylphosphonic acid, about 5-90 wt % isopropanol, a balance of water and impurities.

9. The method of claim 4 wherein said exposed exterior is contacted with an aqueous solution consisting essentially of about 0.5-10 wt. % octylphosphonic acid, a balance of water and impurities.

10. The method of claim 4 wherein said contacting includes: spraying, dipping, painting or rolling material on said exposed exterior of said alloy product.

11. The method of claim 10 wherein said material is incorporated into a composition for surface treating said alloy product.

12. The method of claim 11 wherein said composition is selected from the group consisting of: a mill lubricant, a quenching solution, an intermediate rinse, an etching solution, a solvent, a surfactant, a cleaner, a polish, a post-rinse, a sealant and mixtures thereof.

13. The method of claim 2 wherein said alloy product is selected from the group consisting of sheet product, plate product, an extrusion and a forging.

14. The method of claim 13 wherein said sheet or plate product has at least one bright exterior surface and is selected from the group consisting of: trailer plate, rail car skin, a tool box, vehicle running board and tread plate.

15. The method of claim 13 wherein said sheet product is reflective lighting sheet.

16. The method of claim 13 wherein said extrusion is selected from the group consisting of: a truckbed rail, hydraulic tubing, window framing, tub and shower framing, and a greenhouse structural support.

17. The method of claim 13 wherein said alloy product is a vehicle wheel.

18. The method of claim 1 which further includes: (b) rinsing material from the exposed exterior.

19. The method of claim 18 which further includes:

(c) wiping said exposed exterior.

20. The method of claim 1 which further includes: (b) wiping material from the exposed exterior.

21. The method of claim 1 wherein said alloy product is made from a 1000, 3000, 5000 or 6000 Series aluminum alloy (Aluminum Association designations).

22. The method of claim 21 wherein said 5000 Series aluminum alloy is selected from the group consisting of: 5083, 5086, 5454, 5657, 5182, and 5456 aluminum (Aluminum Association designations).

23. The method of claim 21 wherein said 6000 Series aluminum alloy is selected from the group consisting of: 6061, 6111 and 6022 aluminum (Aluminum Association designations).

24. A method for inhibiting the formation of water stains on an exposed exterior surface of a 5000 or 6000 Series aluminum alloy product, said method comprising:

(a) contacting the exterior surfaces with an organophosphonic or organophosphinic acid-derived material.

25. The method of claim 24 wherein said acid-derived material is a solid or semi-solid at room temperature and applied to said exterior surface.

26. The method of claim 24 wherein said acid-derived material is a liquid at room temperature and combined with a carrier selected from the group consisting of: an alcohol, a ketone, an ether, an aldehyde, an alkane, water and mixtures thereof.

27. The method of claim 26 wherein said acid-derived material consists essentially of octadecylphosphonic acid and said carrier includes isopropanol.

28. The method of claim 26 wherein said acid-derived material consists essentially of octylphosphonic acid and said carrier is selected from the group consisting of: isopropanol, water and mixtures thereof.

29. The method of claim 26 wherein said exterior surface is contacted with an aqueous solution consisting essentially of up to about 10 wt. % octadecylphosphonic acid, about 5-90 wt % isopropanol, the balance water and impurities.

30. The method of claim 26 wherein said exterior surface is contacted with an aqueous solution consisting essentially of up to about 10 wt. % octylphosphonic acid, about 5-90 wt % isopropanol, the balance water and impurities.

31. The method of claim 26 wherein said exterior surface is contacted with an aqueous solution consisting essentially of about 0.5-10 wt. % octylphosphonic acid, the balance water and impurities.

32. The method of claim 24 wherein said contacting includes: spraying, dipping, painting or rolling material onto said exterior surfaces.

33. The method of claim 24 wherein said acid-derived material is incorporated into a composition selected from the group consisting of: a mill lubricant, a quenching solution, an intermediate rinse, an etching solution, a solvent, a surfactant, a cleaner, a polish, a post-rinse, a sealant and mixtures thereof.

34. The method of claim 24 wherein said alloy product is selected from the group consisting of sheet product, plate product, an extrusion and a forging.

35. The method of claim 34 wherein said sheet or plate product has at least one bright exterior surface and is selected from the group consisting of: trailer plate, rail car skin, a tool box, vehicle running board and tread plate.

36. The method of claim 34 wherein said sheet product is reflective lighting sheet.

37. The method of claim 34 wherein said extrusion is selected from the group consisting of: a truckbed rail, hydraulic tubing, window framing, tub and shower framing, and a greenhouse structural support.

38. The method of claim 34 wherein said alloy product is a vehicle wheel.

39. The method of claim 24 which further includes: (b) rinsing material from the exposed exterior.

40. The method of claim 39 which further includes:

(c) wiping said exposed exterior.

41. The method of claim 24 which further includes: (b) wiping material from the exposed exterior.

42. The method of claim 24 wherein said 5000 Series aluminum alloy is selected from the group consisting of: 5083, 5086, 5454, 5657, 5182, and 5456 aluminum (Aluminum Association designations).

43. The method of claim 24 wherein said 6000 Series aluminum alloy is selected from the group consisting of: 6061, 6111 and 6022 aluminum (Aluminum Association designations).