US20140262790A1

US20140262790A1 - Colored, corrosion-resistant aluminum alloy substrates and methods for producing same

Info

Publication number: US20140262790A1
Application number: US13/796,448
Authority: US
Inventors: Thomas Levendusky; Albert Askin; Luis Fanor Vega
Original assignee: Alcoa Inc
Current assignee: Howmet Aerospace Inc
Priority date: 2013-03-12
Filing date: 2013-03-12
Publication date: 2014-09-18
Also published as: WO2014163683A1

Abstract

A silicon polymer treatment with included pigments for anodized aluminum objects such as wheels. Titanium dioxide may be dispersed in polysiloxane or polysilazane to form a white polymer treatment on the object. Other beneficial components, such as corrosion inhibitors may be included in the polymer matrix.

Description

BACKGROUND

Metallic substrates, including aluminum and aluminum alloys, may be anodized to increase corrosion and wear resistance of the substrate. Anodizing is an electrolytic passivation process used to increase the thickness and density of the natural oxide layer on the surface of metal parts. Anodic films can also be used for a number of cosmetic effects, either via thick porous coatings that can absorb dyes or via thin transparent coatings that add interference effects to reflected light. Anodic films are generally stronger and more adherent than most paints and platings, making them less likely to crack and peel. Anodic films are most commonly applied to protect aluminum alloys, although processes also exist for titanium, zinc, magnesium, and niobium.
Conventional anodizing processes, resultant anodized substrates and coloring techniques have limitations with respect to corrosion and wear resistance and color selection. Some protective compounds applied to the anodized surfaces give rise to other limitations. As a result, alternative approaches for coloring and finishing anodized surfaces remain desirable.

SUMMARY OF THE INVENTION

The present disclosure relates to treatments for aluminum alloys. In an embodiment of the present disclosure a treatment for anodized aluminum alloy object having a porous oxide layer formed on a base layer of the aluminum alloy, has a liquid including a silicon monomer. The liquid has a viscosity permitting infiltration into the porous oxide layer when applied thereto and capable of reacting with the oxide layer to chemically bond with molecules of the oxide layer. The monomer is capable of polymerizing within the porous oxide layer yielding a polymer interlocked with the oxide layer. A pigment dispersed within the liquid is capable of being bound within the polymer and imparting a color to the object.
In another embodiment, the polymer is a polysiloxane.
In another embodiment, the polymer is a polysilazane.
In another embodiment, the pigment is titanium dioxide.
In another embodiment, a portion of the pigment particles enter the porous oxide layer prior to polymerization.
In another embodiment, a portion of the pigment particles bound within the polymer are too large to enter the pores of the oxide layer.
In another embodiment, the pigment includes particles that are small enough to enter the porous oxide layer and particles that are too large to enter the pores of the oxide layer.
In another embodiment, the silicon monomer is dispersed in butanol.
In another embodiment, a corrosion inhibitor is dispersed in the liquid and is fixed in the polymer after polymerization.
In another embodiment, a method for treating an anodized aluminum object having a porous oxide layer, includes the steps of obtaining a liquid containing a silicon monomer; obtaining a finely divided solid pigment; mixing the pigment in the liquid to form a mixture with the monomer; applying the mixture to a surface of the object; allowing the liquid to infiltrate the porous oxide layer; polymerizing the monomer to yield a polymer interlocked with the oxide layer and with pigment particles fixed therein.
In another embodiment, the method includes the step of anodizing prior to the step of applying, the anodizing step being conducted on the aluminum alloy electrochemically, using a solution having phosphoric acid and sulfuric acid.
In another embodiment, the step of anodizing results in an oxide zone having sulphates and phosphates.
In another embodiment, the polymer formed by the step of polymerizing is a polysiloxane.
In another embodiment, the polymer formed by the step of polymerizing is a polysiloxane.
In another embodiment, an object formed from aluminum alloy has a porous oxide layer formed upon a surface of the aluminum alloy. A layer of silicon-containing polymer is interlocked with the oxide layer. The polymer contains pigment particles therein that impart a color to the object.
In another embodiment, the pigment particles are titanium dioxide and the color imparted is white.
In another embodiment, the aluminum alloy is selected from the group consisting of series 2XXX, 3XXX, 5XXX, 6XXX and 7XXX aluminum alloys.
In another embodiment, the object is a wheel.
In another embodiment, the wheel is for an aircraft.
In another embodiment, the silicon monomer of the coating is dispersed in normal or tertiary butyl acetate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic, cross-sectional view an anodized aluminum alloy base having a sulfate-phosphate oxide zone treated with a surface layer in accordance with an embodiment of the present disclosure.

FIG. 2 is a schematic view of various reaction mechanisms that may occur in accordance with a sulfate-phosphate oxide zone and a silicon-based polymer.

FIGS. 3A and 3B are scanning electron microscope (SEM) images at 6000× magnification.

FIG. 4 is a flow chart illustrating methods of producing aluminum alloys having a sulfate-phosphate oxide zone and corrosion resistant substrates.

FIG. 5 is a SEM image of a conventionally anodized aluminum alloy surface.

FIG. 6A is a SEM image of a cross-section of a surface of an anodized and treated aluminum alloy in accordance with an embodiment of the present disclosure and FIG. 6B is an enlarged view of the surface of the fiberous oxide network of the surface of alloy of FIG. 6A.

FIG. 7A is photograph of an anodized aluminum alloy that has been painted with a two coat paint system, scribed, frozen, then subjected to pressurized steam.

FIG. 7B is photograph of an anodized aluminum alloy that was coated in accordance with an embodiment of the present disclosure, scribed, frozen, then subjected to pressurized steam.

FIGS. 8 and 9 are graphs of fatigue life for alloy samples treated in accordance with the present disclosure as compared to those which are not.

DETAILED DESCRIPTION

Reference is now made to the accompanying drawings, which assist in illustrating various features of the instant application. In one approach, the instant application relates to aluminum alloys having a sulfate-phosphate oxide zone. One embodiment of an aluminum alloy having a sulfate-phosphate oxide zone is illustrated in FIG. 1. In the illustrated embodiment, an aluminum alloy base 10 includes a sulfate-phosphate oxide zone 20. In general, and as described in further detail below, the aluminum alloy base 10 may be modified with a mixed electrolyte (e.g., sulfuric acid plus phosphoric acid) to produce the sulfate-phosphate oxide zone 20 (indicated by shading lines slanting downwardly from left to right). The sulfate-phosphate oxide zone 20 may promote, among other things, adhesion of polymers to the aluminum alloy base 10, as described in further detail below.
The aluminum alloy base 10 may be any material adapted to have a sulfate-phosphate oxide zone formed therein via electrochemical processes. As used herein, “aluminum alloy” means a material including aluminum and another metal alloyed therewith, and includes one or more of the Aluminum Association 2XXX, 3XXX, 5XXX, 6XXX and 7XXX series alloys. The aluminum alloy base 10 may be from any of a forging, extrusion, casting or rolling manufacturing process. In one embodiment, the aluminum alloy base 10 comprises a 6061 series alloy. In one embodiment, the aluminum alloy base 10 comprises a 6061 series alloy with a T6 temper. In one embodiment, the aluminum alloy base 10 comprises a 2014 series alloy. In one embodiment, the aluminum alloy base 10 comprises a 7050 series alloy. In one embodiment, the aluminum alloy base 10 comprises a 7085 series alloy. In one embodiment, the aluminum alloy base 10 is a wheel product (e.g., a rim that is employed on an aircraft or a vehicle). In one embodiment, the aluminum alloy base 10 is a building product (e.g., aluminum siding or composite panel).
In the illustrated embodiment, the aluminum alloy base 10 includes a sulfate-phosphate oxide zone 20. As used herein, “sulfate-phosphate oxide zone” means a zone produced from electrochemical oxidation of the aluminum alloy base 10, and which zone may include elemental aluminum (Al), sulfur (S), phosphorus (P) and/or oxygen (O) and compounds thereof. In one embodiment, and as described in further detail below, the sulfate-phosphate oxide zone 20 may be produced from an electrolyte comprising both sulfuric acid and phosphoric acid.
The sulfate-phosphate oxide zone 20 generally comprises an amorphous morphology that includes a plurality of sulfate-phosphate oxide pores (not illustrated). As used herein, “sulfate-phosphate oxide pores” means pores of the sulfate-phosphate oxide zone 20 that include elemental Al, O, S and/or P or compounds thereof and proximal a surface thereof. As described in further detail below, such sulfate-phosphate oxide pores may facilitate increased adhesion between polymers and the sulfate-phosphate oxide zone 20 via chemical interaction between the polymer and one or more of the Al, O, S, and P elements located on a surface thereof or proximal thereto.
The sulfate-phosphate oxide zone 20 may include an amorphous and porous morphology, which may facilitate increased adhesion between a polymer and the aluminum alloy via an increased surface area, porosity/roughness. Conventionally anodized surfaces generally include columnar morphology (e.g., for a Type II, sulfuric acid-only anodized surface), or a nodal morphology (e.g., for a phosphoric acid-only anodized surface) (see FIG. 5). Conversely, the porous, amorphous morphology of the sulfate-phosphate oxide zone 20 generally exhibits a more open porosity and an increased surface area relative to conventionally anodized surfaces. The sulphate-phosphate zone could be described as a fibrous region having spacing and interstices therein (see FIG. 6B). This greater porosity and surface area may contribute to increased adhesion between polymer coatings and the aluminum alloy base 10.
Increased adhesion of polymers to the aluminum alloy base 10 may be realized by controlling/selecting the pore size of the sulfate-phosphate oxide pores. For example, the pore size of the sulfate-phosphate oxide pores may be selected to facilitate flow of certain polymers therein, e.g., by creating sulfate-phosphate oxide pores having an average pore size that coincides with the radius of gyration of the polymer to be used to treat the aluminum alloy base 10. In one embodiment, the average pore size of the sulfate-phosphate oxide pores may be in the range of from about 10 nm to about 15 nanometers, and the polymer may be a silicon-containing polymer, such as polysilazane and polysiloxane polymers. Since this average pore size range is coincidental to the radius of gyration of such polymers, these polymers (or their precursors) may readily flow into the sulfate-phosphate oxide pores. In turn, the polymers may readily bond with the sulfate-phosphate oxides associated therewith (e.g., during curing of the polymer, described in further detail below).
As used herein, “average pore size” means the average diameters of the sulfate-phosphate oxide pores of the sulfate-phosphate oxide zone as measured using microscopic techniques. As used herein, “radius of gyration” means the mean size of the polymer molecules of a sample over time, and may be calculated using an average location of monomers over time or ensemble:
$R_{g}^{} \overset{def}{=} \frac{1}{N} 〈 \sum_{k = 1}^{N} {(r_{k} - r_{mean})}^{2} 〉$
where the angular brackets
. . .
denote the ensemble average.
To promote chemical interaction between surfaces of the sulfate-phosphate oxide zone and the polymer, the ratio of sulfur atoms to phosphorus atoms may be tailored. In one embodiment, the polymer is a silicon-based polymer and the ratio of sulfur atoms to phosphorus in the sulfate-phosphate oxide zone 20 is at least about 5:1 (S:P), such as at least about 10:1 (S:P), or even at least about 20:1 (S:P). In this embodiment, the ratio of sulfur atoms to phosphorus atoms in the sulfate-phosphate oxide zone 20 may not exceed about 100:1 (S:P), and preferably not greater than about 75:1 (S:P).
The thickness of the sulfate-phosphate oxide zone 20 may be tailored so as to produce a zone having sufficient surface area for bonding with a polymer. In this regard, the sulfate-phosphate oxide zone 20 of the corrosion resistant substrate 1 generally has a thickness of at least about 5 microns (0.00020 inch), such as a thickness of at least about 6 microns (0.00024 inch). The sulfate-phosphate oxide zone generally has a thickness of not greater than about 25 microns (about 0.001 inch), such as, not greater than about 17 microns (about 0.00065 inch).
As noted above, aluminum alloys with surfaces having sulfate-phosphate oxides may be utilized to produce wear/corrosion resistant aluminum alloy products. One embodiment of a wear/corrosion resistant substrate is illustrated in FIG. 1. In the illustrated embodiment, a silicon-containing polymer zone 30 coats the sulfate-phosphate oxide zone 20. The silicon-containing polymer zone is partially infused into and structurally integrated with at least a portion of the sulfate-phosphate oxide zone 20, and thus defines a mixed zone 40. Mixed zone 40 includes both sulfate-phosphate oxides and silicon-containing polymer. A polymer-free zone 60 may make up the remaining portion of the sulfate-phosphate oxide zone 20. A coating 50 may make up the remaining portion of the silicon-containing polymer zone 30. The coating 50 is located on an outer surface of the aluminum alloy base 10, and, since the coating 50 is integral with the sulfate-phosphate oxide zone 20 via the mixed zone 40, the coating 50 may be considered integral with the aluminum alloy base 10 via the mixed zone 40. In turn, increased adhesion between the coating 50 and the aluminum alloy base 10 may be realized relative to conventional painted products.
As noted above, the sulfate-phosphate oxide zone 20 generally is porous. Thus, various amounts of silicon-containing polymer may be contained within the pores of the sulfate-phosphate oxide zone 20. This interface facilitates adhesion between the sulfate-phosphate oxide zone 20 and the coating 50. Chemical bonding between the silicon-containing polymer and the sulfate-phosphate oxide zone 20 is believed to provide adhesive qualities with respect to electrochemically treated aluminum substrates due to, for example, the molecular size and structure of the formed Al—O—P—O—Si compounds. This is disclosed in U.S. Pat. No. 7,732,068 to Levendusky et al., entitled Corrosion Resistant Aluminum Alloy Substrates and Methods of Producing Same, which is incorporated by reference in its entirety herein. It is believed that the Al—O—P—O—Si molecular structure is more stable than the molecular arrangements achieved with conventional anodizing processes (e.g., Al—O—Si, Al—O—P, Al—O—S, independently, and Al—O—S—O—Si). For example, the substrate 1 may be able to pass the ASTM D3359-02 (Aug. 10, 2002) tape adhesion test, in both dry and wet conditions. Examples of chemical reactions that may occur between polymers and the sulfate-phosphate oxides are illustrated in FIG. 2. Starting from their original colloid compositions, the chemical reactions that occur upon contact with water and subsequent curing may lead to a sequence of hydration and condensation reactions with the evolution of water, resulting in one or more new chemical structures within the sulfate-phosphate oxide zone involving sulfate-phosphate oxides and a silicon-based polymer. For example, the end products 70, 80 illustrated in FIG. 2 may be produced.
As used herein, “silicon-containing polymer” means a polymer comprising silicon and that is suited for integrating with at least a portion of the sulfate-phosphate oxide zone 20 (e.g., via chemical bonding and/or physical interactions). In this regard, the silicon-containing polymer should have a radius of gyration that is coincidental with the average pore size of the sulfate-phosphate oxide zone 20. Furthermore, since the silicon-containing polymer zone 30 may act as a barrier between outside environments and the aluminum alloy base 10, the silicon-containing polymer should generally be fluid impermeable. For appearance purposes, the silicon-containing polymer may be translucent, or even transparent, so as to facilitate preservation of the original specularity and aesthetic appearance of the finished product. As described more fully below, the silicon-containing polymer made be used as a binder for pigment particles which confer a selected color on the aluminum alloy substrate to which they are applied. Particularly, useful silicon-containing polymers having many of the above qualities include polysiloxanes (Si—O—Si) and polysilazanes (Si—N—Si). Polysiloxane polymers are available from, for example, SDC Coatings of Irvine, Calif., U.S.A. Polysilazane polymers are available from, for example, from AZ Electronic Materials USA of Charlotte, N.C., U.S.A.
The selection of siloxane polymers versus silazane polymers may be dictated by the desired performance characteristics of the final product. Due to the dispersive nature of the siloxane precursor, which involves condensation during reaction with the sulfate-phosphate oxide zone 20, the resulting coefficient of thermal expansion of the polysiloxane compound may induce residual stresses at the surface of the coating 50, which may translate into surface fissures and/or cracks in the finished product. To avoid fissures and cracks with coatings 50 comprising polysiloxane, the thickness of the coating 50 may be restricted to not greater than 10 microns, or even not greater than 8 microns. For enhanced corrosion resistance, the barrier properties of the coating 50 may need to be increased via, for example, increased thickness. Substrates including coatings 50 produced from polysilazanes may have higher thicknesses than coatings produced with polysiloxanes and have similar fluid impermeable characteristics. It is believed that the flexibility and chemical composition of polysilazanes allow the production of end product 80, illustrated in FIG. 2, which allows longer molecular chain lengths, and increased coating thicknesses with little or no cracking (e.g., fissure-free, crack-free surfaces). In one embodiment, the coating 50 is sufficiently thick to produce a corrosion resistant substrate. The corrosion resistant substrate may be corrosion resistant while retaining a smooth surface and a glossy appearance (e.g., due to transparency of the coating 50 in combination with the appearance of the mixed zone 40). As used herein, “corrosion resistant substrate” means a substrate having an aluminum alloy base, a sulfate-phosphate oxide zone 20, and a silicon-containing polymer zone 30, and which is able to pass a 240 hour exposure to copper-accelerated acetic acid salt spray test, as defined by ASTM B368-97 (2003)e1 (hereinafter the “CASS test”). In one embodiment, the corrosion resistant substrate is capable of substantially maintaining a glossy and translucent appearance while passing the CASS test. In this regard, the silicon-containing polymer may comprise a polysilazane and the coating 50 may have a thickness of at least about 8 microns. In one embodiment, the coating 50 has a thickness of at least about 35 microns. In one embodiment, the coating 50 has a thickness of at least about 40 microns. In one embodiment, the coating 50 has a thickness of at least about 45 microns. In one embodiment, the coating 50 has a thickness of at least about 50 microns. In some embodiments, the coatings 50 may exhibit little or no cracking. In this regard, it is noted that polysilazane has a coefficient of thermal expansion that is closer to the coefficient of thermal expansion of the aluminum alloy base 10 than polysiloxane coatings. For example, coatings comprising polysilazane may have a coefficient of thermal expansion of at least about 8×10⁻⁵/° C. and aluminum-based substrates may comprise a coefficient of thermal expansion of about 22.8×10⁻⁶/° C. Hence, the ratio of the coefficient of thermal expansion of the polysilazane coating to the coefficient of thermal expansion of the substrate may be not greater than about 10:1, such as not greater than about 7:1, or not greater than 5:1, or not greater than about 4:1, or not greater than about 3.5:1. Thus, in some instances, the coating 50 may comprise a coefficient of thermal expansion that is coincidental to a coefficient of thermal expansion of the aluminum alloy base 10 and/or the sulfate-phosphate oxide zone 20 thereof. Hence, coatings 50 including polysilazane may act as an impermeable or near-impermeable barrier between the aluminum alloy base 10 and other materials, while maintaining a glossy appearance and a smooth outer surface. Nonetheless, the polysilazane coatings generally should not be too thick, or the coating may crack. In one embodiment, the coating 50 comprises polysilazane and has a thickness of not greater than about 90 microns, such as a thickness of not greater than about 80 microns.
As noted above, the coating 50 may have sufficient thickness to facilitate production of a corrosion resistant substrate and the corrosion resistant substrate may be capable of passing the CASS test. In other embodiments, the corrosion resistance of the coating 50 may be a lesser consideration in the final product design. Thus, the thickness of the coating 50 may be selected based on the requisite design parameters. In one embodiment, the coating 50 comprises polysiloxane and has a thickness of not greater than about 10 microns, such as a thickness of not greater than about 8 microns.
Polymers other than silicon-based polymers may be used to produce a polymer-containing zone. Such polymers should posses a radius of gyration that is coincidental to the average pore size of the sulfate-phosphate oxide zone 20. Materials other than polymers may also be used to facilitate production of wear resistant and/or corrosion resistant substrates. For example, the sulfate-phosphate oxide zone 20 may optionally include dye and/or a nickel acetate preseal. With respect to dyes, ferric ammonium oxalate, metal-free anthraquinone, metalized azo complexes or combinations thereof may be utilized to provide the desired visual effect.
As noted above, the formation of anodic oxide layers on aluminum, utilizing direct current (DC) or alternating current (AC) anodizing, is known in the aluminum finishing industry. These anodic oxide layers are formed by immersion in aqueous electrolytes—including acids such as phosphoric, sulfuric and chromic acid. Depending upon processing parameters, the resultant anodic oxide can range in thickness from about 0.1 to about 50 microns. The anodic oxides formed on aluminum alloys (e.g., 1xxx, 2xxx, 3xxx, 5xxx, 6xxx, 7xxx & 8xxx) have colors that range from clear to gray. The range of colors of the anodic oxides is dependent upon alloy composition and anodizing process variables. Aqueous solutions of organic dyes (i.e., available from suppliers such as AZ Electronic Materials, Inc) can be used to produce a wide array of colors—ranging from red to violet. Immersion of anodic oxides into aqueous solutions of organic dyes results in colors being adsorbed into the oxide. After the colors are adsorbed, the subsequent colored oxide can be processed in aqueous solutions of de-ionized water or metal salts, such as nickel acetate, operated at elevated temperatures (>200 F).
Traditionally, the color white was not commonly available for a colored anodic oxide surface on aluminum alloys. This may be due to the unavailability of a white, aqueous-based dye solution and/or that titanium dioxide, a white pigment, may have a particle size larger than the pore openings in the conventional anodic oxide layer (see FIG. 5). Both of these factors may have prevented the development of a white anodic layer. An aspect of the present disclosure is an approach that achieves a white color that is fixed (i.e., locked) into the anodic oxide layer of an anodized aluminum alloy.
As noted above, a new method in accordance with the present disclosure and as referred to in U.S. Pat. No. 7,732,068 to Levendusky et al utilizes a different means of sealing the pores of the anodic oxide, viz., by means of a silicon-containing, non-aqueous solution, with an inherent viscosity low enough to enter the pores of the anodic oxide. The silicon containing solution is applied to the surface of the anodic layer, e.g., by dipping or spraying, with spraying being a convenient and effective method. After application to the anodized surface, this silicon-containing solution (e.g., siloxane or silazane) chemically reacts with the elemental components of the anodic oxide formed by the electrolyte composition, as described above and in U.S. Pat. No. 7,732,068. The presence of elemental phosphorous in the oxide layer promotes the formation of covalent chemical bonds with the silicon-containing solution. The silicon-containing solution has a low viscosity due to the low-molecular weight (i.e., short chain lengths) of the silicon-containing oligomer in the non-aqueous solution. As the reaction between the silicon solution and the anodic oxide proceeds, the molecular chains increase in length, resulting in an increase in the molecular weight of the silicon-containing polymer in the pores of the anodic oxide. The molecular weight increases until the pores of the oxide are completely filled with a chemically-bound silicon polymer. The phosphorous and aluminum in the oxide layer react with the silicon solution to form a covalently bound structure that extends across the entire pore opening in the oxide layer. This new composition consequently seals the pores of the anodic oxide. Since the covalent chemical bonds are formed with components of the anodic oxide, the sealant is fixed (i.e., locked) into the pores of the oxide.
An aspect of the present disclosure is the recognition that the foregoing method and composition has advantages over conventional aqueous sealing of anodic aluminum oxides. The first of these is that other materials, such as finely dispersed pigments, as well as thermal and radiation stabilizers, can be incorporated into the low viscosity silicon solution. These materials can be added at levels that do not increase the viscosity of the silicon solution to levels that prohibit the solution from entering the pores of the anodic oxide. For example, titanium dioxide may be added to the silicon solution. This is a departure from compositions and methods where titanium dioxide is added to organic coatings (e.g., white paints) and then applied over anodized articles in a secondary step after the oxide is formed and sealed. In the case of an organic based paint, the molecular weight of the organic coating is too large to enter the pores of the anodic oxide and bridges the pore opening. Since a layer of white, organic-based paint would not be chemically bound to the oxide surface, it would not be locked into the oxide layer and therefore would be easier to remove, e.g., in response to environmental factors, such as abrasion.
A white, finely dispersed titanium dioxide pigment may be incorporated into the silicon solution of the present disclosure without increasing the viscosity of the mixture of pigment and silicon solution to a level disabling entry of the silicon solution into the pores of the oxide, in particular, the porous fibrous oxide network with interstices, e.g., as shown in FIG. 6B. As used herein, “pores” in the fibrous oxide zone of the present disclosure includes the interstices therein. Titanium dioxide may be obtained in a variety of particle sizes and typically a given sample has a range of particle sizes. In accordance with one embodiment of the present disclosure, if the particle size of the titanium dioxide in the silicon solution is smaller than the pore size of the fibrous oxide layer formed in accordance with the present disclosure of the anodized aluminum alloy, it may enter the pores of this fibrous network along with the silicon solution. The particle sizes of other particles of the titanium dioxide may be larger than certain pores, and they do not enter the pores, but are still distributed and are visible in the silicon containing polymer. Any given sample of titanium dioxide typically has a range of particle sizes from larger to smaller than the pore size of the oxide layer of the anodized alloy, e.g., larger than the pores associated with the columnar layer (see FIG. 5), but smaller than the pores of the porous fibrous oxide network shown in FIG. 6B. In each case, the silicon solution enters the pores and does not bridge the pore opening. When the silicon-solution reacts with the composition of the anodic oxide to form a covalently-bound, sealed oxide, the titanium-dioxide pigment particles (both those that are small enough to enters the pores of the fibrous oxide layer and those that do not enter the pores are dispersed in (surrounded by) the silicon-containing sealant and the pigment is effectively fixed (i.e., locked and chemically bound) into the oxide layer by the polymerization of the silicon-containing sealant.
The foregoing dynamic is supported and enhanced by the morphology of the titanium dioxide, in that finely divided pigments, such as titanium dioxide, typically have a mesoporous structure. Mesoporous materials have a large degree of porosity and correspondingly large surface area. When finely divided materials are added to the silicon solution, the components of the solution can be both adsorbed and absorbed into the mesoporous material. This permits ingress of the silicon monomers/oligomers, and optionally other additives of the silicon solution—if present, into the porous, finely divided pigment particles. When the silicon monomers/oligomers polymerize, the pigment particles are strongly bound into the polymer matrix.
In accordance with another aspect of the present disclosure, other materials and components, such as corrosion inhibitors like benzotriazole or piperidine derivatives, may be mixed with the silicon solution and subsequently bound within the polymer matrix after curing. The additional components may adsorbed or absorbed into the mesoporous material, prior to being locked into the polymerized silicon network. These materials can then be present in the polymerized silicon network that is fixed to the anodic oxide through covalent bonding. For example, upon damage to the polymerized silicon polymer, included corrosion inhibitors would be available to reduce and/or retard the formation of corrosion products on the aluminum article.
The present disclosure recognizes that an anodic oxide can be colored white by use of a chemically-bound silicon sealant that incorporates fine, dispersed titanium dioxide. The white color is locked to the anodized surface upon curing since the titanium dioxide is dispersed within the silicon-containing sealant which is fixed to the anodic oxide by polymerization. This approach may be applied to pigments other than white pigments, e.g., green imparted by copper phthalocyanide, or yellow imparted due to the inclusion of cadmium sulphide, provided that the pigment particle size and the amount of pigment and/or additive does not increase the viscosity of the silicon-containing solution to the point where the solution will not be able to enter the porous anodic oxide. The increase in viscosity is analogous to increasing the radius of gyration of the silicon compound in the silicon solution such that it becomes too large to enter the pores in the anodic oxide. In accordance with an embodiment of the present disclosure, the viscosity of the silicon solution is preferably less than that which prevents entry of the silicon-solution into the oxide pores.
FIGS. 3A and 3B show scanning electron microscope (SEM) micrographs 90, 100 of different portions of an anodized aluminum alloy object 92, 102 that has been coated with a silicon-containing sealant 94, 104, i.e., a polysilazane polymer with dispersed titanium dioxide particles. This formulation is applied by air-assisted spraying onto the surface of the alloy panel. The panel is allowed to dry for a sufficient time to flash-off solvents in the formulation, e.g., from 10 minutes to >60 minutes. The panel is then cured in a thermal oven from 15 to >60 minutes at temperatures ranging from about 100 F to 400 F. As shown, the coating has a polymer matrix 96, 106 containing titanium dioxide particles 98, 108. The distribution of the titanium dioxide particles in FIGS. 3A and 3B differ, with the coating in FIG. 3A exhibiting a higher concentration of titanium dioxide particles in the fibrous oxide transition zone of the anodized surface than that present in FIG. 3B, which shows more evenly dispersed titanium dioxide particles 108 and a lower concentration near the transition zone 109. This variation illustrates that a given sample of titanium dioxide that is mixed with a silicon sealant as described herein would be anticipated to have a range of particle sizes, with some small enough to enter the porous oxide layer of the anodized surface and some too large to enter. Moreover, the distribution of the titanium dioxide (or other pigment) will have variations over its extent. In addition, the porosity of the anodized surface may vary. Regardless of the size of the titanium dioxide particles (typically within the range of (500 naometers to 2000 nanometers), the homogeneity of their distribution in the sealant, or the pore sizes and distribution, when polymerization of the sealant occurs, the pigment is captured within the polymer matrix and the polymer matrix grips to the anodized surface of the object due to its infusion into the pores thereof and infusion into the mesoporous surface of the pigments particles that have entered the oxide pores and those that have not entered the pores prior to polymerization. When the polymerized layer of sealant is viewed from the exterior, the whiteness of the titanium dioxide particles confers a white color on the sealant coating and the coated object.
A method for producing a colored, corrosion resistant substrate in accordance with an embodiment of the present disclosure is illustrated in FIG. 4. In the illustrated embodiment, the method includes the steps of producing a sulfate-phosphate oxide zone on a surface of the aluminum alloy base (220) and forming a silicon-containing polymer zone on the sulfate-phosphate oxide zone (240). The method may optionally include the steps of pretreating an aluminum alloy base (210) and/or applying a dye to the sulfate-phosphate oxide zone (230). The aluminum alloy base, the sulfate-phosphate oxide zone and the silicon-containing polymer zone may be any of the above-described aluminum alloy bases, sulfate-phosphate oxide zones and silicon-containing polymer zones, respectively.
In one embodiment, and if utilized, a pretreating step (210) may comprise contacting the aluminum alloy base with a pretreating agent (212). For example, the pretreating agent may comprise a chemical brightening composition (214). As used herein, “chemical brightening composition” means a solution that includes at least one of nitric acid, phosphoric acid, sulfuric acid, and combinations thereof. For example, the methodologies disclosed in U.S. Pat. No. 6,440,290 to Vega et al. may be employed to pretreat an aluminum alloy base with a chemical brightening composition. In one approach, and with respect to 6XXX series alloys, a phosphoric acid-based solution with a specific gravity of at least about 1.65, when measured at 80° F. (about 26.7° C.) may be used, such as a phosphoric acid with a specific gravities in the range of from about 1.69 to about 1.73 at the aforesaid temperature. A nitric acid additive may be used to minimize a dissolution of constituent and dispersoid phases on certain Al—Mg—Si—Cu alloy products, especially 6XXX series forgings. Such nitric acid concentrations dictate the uniformity of localized chemical attacks between Mg2Si and matrix phases on these 6XXX series Al alloys. As a result, end product brightness may be positively affected in both the process electrolyte as well as during transfer from process electrolyte to a rinsing substep (not illustrated). In one approach, the nitric acid concentrations may be about 2.7 wt. % or less, with more preferred additions of HNO3 to that bath ranging between about 1.2 and 2.2 wt. %. For 6XXX series aluminum alloys, improved brightening may occur in those alloys whose iron concentrations are kept below about 0.35% in order to avoid preferential dissolution of Al—Fe—Si constituent phases. For example, the Fe content of these alloys may be kept below about 0.15 wt % iron. At the aforementioned specific gravities, dissolved aluminum ion concentrations in these chemical brightening baths should not exceed about 35 g/liter. The copper ion concentrations therein should not exceed about 150 ppm.
In another approach, the pretreating agent may include an alkaline cleaner (216). As used herein, “alkaline cleaner” means a composition having a pH of greater than approximately 7. In one embodiment, an alkaline cleaner has a pH of less than about 10. In one embodiment, an alkaline cleaner has a pH in the range of from about 7.5 to about 9.5. In one embodiment, the alkaline cleaner includes at least one of potassium carbonate, sodium carbonate, borax, and combinations thereof. In another embodiment, an alkaline cleaner has a pH of at least about 10.
In one embodiment, the pretreating step (210) includes removing contaminants from a surface of the aluminum alloy base. Examples of contaminants include grease, polishing compounds, and fingerprints. After the pretreating step (210), such as via chemical brighteners or alkaline cleaners, described above, the absence of contaminants on the surface of the aluminum alloy base may be detected by determining the wetability of a surface of the aluminum alloy base. When a surface of the aluminum alloy base wets when subjected to water, it is likely substantially free of surface contaminants (e.g., an aluminum alloy substrate that has a surface energy of at least about 72 dynes/cm).
Turning now to the step of producing a sulfate-phosphate oxide zone step (220), the sulfate-phosphate oxide zone may be produced via any suitable technique. In one embodiment, the sulfate-phosphate oxide zone is produced by electrochemically oxidizing a surface of the aluminum alloy base (222). As used herein, “electrochemically oxidizing” means contacting the aluminum alloy base with a electrolyte containing both (a) sulfuric acid and (b) phosphoric acid, and applying an electric current to the aluminum alloy base while the aluminum alloy base is in contact with the electrolyte.
The ratio of sulfuric acid to phosphoric acid within the electrolyte (sometimes referred to herein as a “mixed electrolyte”) should be tailored/controlled so as to facilitate production of suitable sulfate-phosphate oxide zones (224). In one embodiment, the weight ratio of sulfuric acid (SA) to phosphoric acid (PA) in the electrolyte is at least about 5:1 (SA:PA), such as a weight ratio of at least about 10:1 (SA:PA), or even a weight ratio of at least about 20:1 (SA:PA). In one embodiment, the weight ratio of sulfuric acid to phosphoric acid in the electrolyte is not greater than 100:1 (SA:PA), such as a weight ratio of not greater than about 75:1 (SA:PA). In one embodiment, the mixed electrolyte comprises at least about 0.1 wt % phosphoric acid. In one embodiment, the mixed electrolyte comprises not greater than about 5 wt % phosphoric acid. In one embodiment, the mixed electrolyte comprises not greater than about 4 wt % phosphoric acid. In one embodiment, the mixed electrolyte comprises not greater than about 1 wt % phosphoric acid. In one embodiment, the phosphoric acid is orthophosphoric acid.
The current applied to the mixed electrolyte should be tailored/controlled so as to facilitate production of suitable sulfate-phosphate oxide zones (226). In one embodiment, electrochemically oxidizing step (222) includes applying electricity to the electrolyte at a current density of at least about 8 amps per square foot (asf), which is about 0.74 amps per square meter (asm). In one embodiment, the current density is at least about 12 asf (about 1.11 asm). In one embodiment, the current density is at least about 18 asf (about 1.67 asm). In one embodiment, the current density is not greater than about 24 asf (about 2.23 asm). Thus, the current density may be in the range of from about 8 asf to about 24 asf (0.74-2.23 asm), such as in the range of from about 12 asf to about 18 asf (1.11-1.67 asm).
The voltage applied to the mixed electrolyte should also be tailored/controlled so as to facilitate production of suitable sulfate-phosphate oxide zones (228). In one embodiment, the electrochemically oxidizing step (222) includes applying electricity to the electrolyte at a voltage of at least about 6 volts. In one embodiment, the voltage is at least about 9 volts. In one embodiment, the voltage is at least about 12 volts. In one embodiment, the voltage is not greater than about 18 volts. Thus, the voltage may be in the range of from about 6 volts to about 18 volts, such as in the range of from about 9 volts to about 12 volts.
The temperature of the electrolyte during the electrochemically oxidizing step (222) should also be tailored/controlled so as to facilitate production of a suitable sulfate-phosphate oxide zone. In one embodiment, the electrochemically oxidizing step (222) includes heating the electrolyte to and/or maintaining the electrolyte at a temperature of at least about 75° F. (about 24° C.), such as a temperature of at least about 80° F. (about 27° C.). In one embodiment, the temperature of the electrolyte is at least about 85° F. (about 29° C.). In one embodiment, the temperature of the electrolyte is at least about 90° F. (about 32° C.). In one embodiment, the electrochemically oxidizing step (222) includes heating the electrolyte and/or maintaining the electrolyte at a temperature of not greater than about 100° F. (about 38° C.). Thus, the temperature of the electrolyte may be in the range of from about 75° F. (about 24° C.) to about 100° F. (38° C.) step (229), such as in the range of from about 80° F. (about 27° C.) to about 95° F. (35° C.), or a range of from about 85° F. (about 29° C.) to about 90° F. (about 32° C.).
In a particular embodiment, the electrochemically oxidizing step (222) includes utilizing a mixed electrolyte having: (i) a weight ratio of sulfuric acid to phosphoric acid of about 99:1 (SA:PA), and (ii) a temperature about 90° F. (about 32° C.). In this embodiment, the current density during electrochemically oxidizing step (222) is at least about 18 asf (about 1.11 asm).
After the sulfate-phosphate oxide zone is produced (220), the method may optionally include the step of presealing the sulfate-phosphate oxide zone (not illustrated) prior to or after the applying a dye step (230) and/or prior to the forming a silicon-containing polymer zone (240). In one approach, at least some, or in some instances all or nearly all, of the pores of the sulfate-phosphate oxide zone may be sealed with a sealing agent, such as, for instance, an aqueous salt solution at elevated temperature (e.g., boiling water) or nickel acetate.
Moving to the applying a dye step (230), in one embodiment the applying a dye step (230) comprises applying at least one of ferric ammonium oxalate, metal-free anthraquinone, metalized azo complexes or combinations thereof to at least a portion of a sulfate-phosphate oxide zone. The dye may be applied via any conventional techniques. In one embodiment, the dye is applied by a spray coating or dip coating.
In the event that a dye is not desired or, alternatively, after the step of applying colored dye (230), titanium dioxide dispersed in a polymerizable, silicon-containing solution, such as a polysiloxane or polysilazane precursor may be applied (232) to the aluminum alloy base in order to achieve a white color. In the event that another treatment is desired, e,g, to achieve corrosion protection, an anti-corrosion agent, e.g., benzotriazole may be added to the silicone containing solution, either alone or in combination with the titanium dioxide and applied to the alloy (234). As a further alternative, the silicon-containing solution may be applied (240) to the alloy without additives to form a silicone-containing polymer zone on the alloy. In either case, the silicon-containing solution is applied, e.g., as a colloid/sol on/in at least a portion of the sulfate-phosphate oxide zone 20—see FIG. 1 and cured (244). In one embodiment, the colloid is a sol and the curing step (244) results in the formation of a gel comprising the silicon-containing polymer zone. The applying step (240) may be accomplished via any conventional process, such as spraying, dipping, or painting with an applicator. Likewise, the curing step (244) may be accomplished via any conventional process, such as thermal curing in electric or gas-fired ovens. In one embodiment, the applying step (240) is accomplished by one or more of spray coating or dip coating, spin coating or roll coating. In another embodiment, the applying step (240) is accomplished by vacuum deposition from liquid and/or gas phase precursors. The silicon-containing polymer zone may be formed on a dyed sulfate-phosphate oxide zone or an undyed sulfate-phosphate oxide zone.
Colloids used to form the silicon-containing polymer zone generally comprise particles suspended in a liquid. In one embodiment, the particles are silicon-containing particles (e.g., precursors to the silicon-containing polymer). In one embodiment, the particles have a particle size in the range of from about 1.0 nm to about 1.0 micron. In one embodiment, the liquid is aqueous-based (e.g., distilled H2O). In another embodiment, the liquid is organic based (e.g., alcohol). In a particular embodiment, the liquid comprises at least one of methanol, ethanol, or combinations thereof. In another embodiment the liquid is butanol or butyl acetate. In one embodiment, the colloid is a sol.
The viscosity of the colloid may be tailored based on deposition method. In one embodiment, the viscosity of the colloid is about equal to that of water. In this regard, the particles of the colloid may more freely flow into the fibrous network of the porous sulfate-phosphate oxide zone. During or concomitant to the applying step (240), the colloid may flow into the pores of the sulfate-phosphate oxide zone, and may thus seal the pores by condensation of the colloid to a gel state (e.g., via heat). Water released during this chemical reaction may induce oxide hydration and, therefore, sealing of the pores. In a particular embodiment, the colloid may flow into a substantial amount of (e.g., all or nearly all) the pores of the sulfate-phosphate oxide zone. During the curing step (244), the silicon-containing polymer is formed and seals a substantial amount of the unsealed pores of the sulfate-phosphate oxide zone. In this embodiment, the curing step (244) may include applying a temperature of from about 90° C. (about 194° F.) to about 170° C. (about 338° F.). In one embodiment, the curing step may include applying a temperature of from about 138° C. (about 280° F.) to about 160° C. (about 320° F.).
In one embodiment, the curing step (244) results in the production of a polysiloxane coating (e.g., via gelation of the colloid). In one embodiment, the curing step (244) results in the production of a coating comprising polysilazane. The cured polymer may have titanium dioxide particle inclusion and/or other components, such as anti-corrosion agents. The colloid may include silane precursors, such as trimethoxy methyl silanes, or silazane precursors, such as methyldichlorine or aminopropyltriethoxysilane reacted with ammonia via ammonolysis synthesis. As noted above, the use of polysilazanes versus polysiloxanes is primarily a function of the desired corrosion resistance and film thickness of the final product.
FIG. 5 shows the pore structure of a aluminum alloy surface S that has been anodized using traditional methods with sulphuric acid. In general the pores P are too small to allow the entry of typical commercial titanium dioxide pigment particles.
FIG. 6A shows a SEM image of a cross-section of a surface of an anodized and coated aluminum alloy 110 in accordance with an embodiment of the present disclosure. The alloy 110 has developed a fibrous sulphate-phosphate oxide zone 120 which is coated by a silicone-containing polymer sealant 130 as described above. FIG. 6B is an enlarged view of the surface of the fiberous oxide zone 120 of the alloy 110 of FIG. 6A. As can be appreciated, the fibrous oxide zone 120 of FIG. 5B has greater and larger porosity that the pores P shown in the alloy surface S of the alloy of FIG. 5 and therefore is able to absorb and adsorb both the polymer sealant 130 and titanium dioxide pigment particles that are present in the sealant 130.
FIGS. 7A and 7B show the results of testing anodized aluminum panels 301, 303, with panel 301 having a painted surface 310 and panel 303 having a surface 320 sealed with a silicon polymer that is prepared and applied in accordance with an embodiment of the present disclosure. The panel 301 was painted with an epoxy primer with strontium chromate pigment and then subsequently over-coated with a gray-pigmented urethane top coat. The panel 303 was coated with a silicon-containing coating in accordance with the present disclosure. More particularly, panel 303 was prepared by forming an oxide in mixed electrolyte described above, sealing with a polysilazane formulation having dispersed titanium dioxide particles and then cured for 30 minutes at 250-300 F. After the surfaces 310 and 320 were allowed to dry/cure, each was immersed in tap water at the temperature of 100 F for 4 hours. The panels were then removed and scribed with an “X” (partially enhanced to be made visible by dashed lines in FIG. 5A) by a sharp edge that cut down to the alloy substrate. The panels were then placed in a freezer for 3 hours at −20 F. The panels were removed from the refrigerator and subjected to a direct high pressure steam blast directed at the scribed area. The painted panel exhibited adhesion loss in the area 312 shown, resulting in a flap 314 of loose paint. In contrast, the panel 303 sealed with the silicon-containing coating (and having a white tint owing to the inclusion of titanium dioxide showed no adhesion loss.
Fatigue testing results for alloy 2014-T6 and 7050-T7, uncoated and coated by traditional methods compared to those coated in accordance with the present disclosure are shown in FIGS. 8 and 9. As can be appreciated the bars representing fatigue results for those samples coated in accordance with the present disclosure (the last two bars of FIG. 8, labeled R991, and the second and seventh bar of FIG. 9, labeled R-991 with sealant) are significantly better than those of the other samples tested and identified in the respective graphs. Based upon the testing conducted, the coating of the present disclosure improves fatigue resistance of alloys treated thereby.
While various embodiments of the present application have been described above detail, it is apparent that modifications and adaptations of those embodiments will occur to those skilled in the art. For example, the present disclosure mentions applicability of the disclosed technology to the 2XXX, 3XXX, 5XXX, 6XXX and 7XXX Series alloys, but the present disclosure would be applicable to other aluminum alloys. It is therefore understood that such modifications and adaptations are within the spirit and scope of the present invention.

Claims

What is claimed is:

1. A treatment for anodized aluminum alloy object having a porous oxide layer formed on a base layer of the aluminum alloy, comprising:

a liquid including a silicon monomer, the liquid having a viscosity permitting infiltration into the porous oxide layer when applied thereto and capable of reacting with the oxide layer to chemically bond with molecules of the oxide layer, the monomer capable of polymerizing within the porous oxide layer yielding a polymer interlocked with the oxide layer;

a pigment dispersed within the liquid, the pigment capable of being bound within the polymer and imparting a color to the object.

2. The treatment of claim 1, wherein the polymer is a polysiloxane.

3. The treatment of claim 1, wherein the polymer is a polysilazane.

4. The treatment of claim 1, wherein the pigment is titanium dioxide.

5. The treatment of claim 1, wherein a portion of the pigment particles enter the porous oxide layer prior to polymerization.

6. The treatment of claim 1, wherein a portion of the pigment particles bound within the polymer are too large to enter the pores of the oxide layer.

7. The treatment of claim 1, wherein the pigment includes particles that are small enough to enter the porous oxide layer and particles that are too large to enter the pores of the oxide layer.

8. The treatment of claim 1 wherein the silicon monomer is dispersed in butanol.

9. The treatment of claim 1 further comprising a corrosion inhibitor that is dispersed in the liquid and which is fixed in the polymer after polymerization.

10. A method for treating an anodized aluminum object having a porous oxide layer thereof, comprising the steps of:

(A) obtaining a liquid containing a silicon monomer;

(B) obtaining a finely divided solid pigment

(C) mixing the pigment in the liquid to form a mixture with the monomer;

(D) applying the mixture to a surface of the object;

(E) allowing the liquid to infiltrate the porous oxide layer;

(F) polymerizing the monomer to yield a polymer interlocked with the oxide layer and with pigment particles fixed therein.

11. The method of claim 10, further comprising the step of anodizing prior to the step (D) of applying, the anodizing step being conducted on the aluminum alloy electrochemically, using a solution having phosphoric acid and sulfuric acid.

12. The method of claim 11, wherein the step of anodizing results in an oxide zone having sulphates and phosphates.

13. The method of claim 10, wherein the polymer formed by the step of polymerizing is a polysiloxane.

14. The method of claim 10, wherein the polymer formed by the step of polymerizing is a polysiloxane.

15. An object formed from aluminum alloy, comprising:

a porous oxide layer formed upon a surface of the aluminum alloy;

a layer of silicon-containing polymer interlocked with the oxide layer,

the polymer containing pigment particles therein that impart a color to the object.

16. The object of claim 15, wherein the pigment particles are titanium dioxide and the color imparted is white.

17. The object of claim 15, wherein the aluminum alloy is selected from the group consisting of series 2XXX, 3XXX, 5XXX, 6XXX and 7XXX aluminum alloys.

18. The object of claim 15, wherein the object is a wheel.

19. The object of claim 18, wherein the wheel is for an aircraft.

20. The treatment of claim 1, wherein the silicon monomer is dispersed in at least one of normal and tertiary butyl acetate.