US3551122A - Surface finished aluminum alloys - Google Patents

Description

" M. GULLA 3,551,122"

cowv rs Ptk mow SURFACE FINISHED ALUMINUM ALLOYS F ilad Dec. 18, 1967 2 Sheets-Sheet 2 CURVE C (UPI/f H United States Patent Oflice Patented Dec. 29, 1970 3,551,122 SURFACE FINISHED ALUMINUM ALLOYS Michael Gulla, Framingham, Mass., assignor to Shipley Company, Inc., Newton, Mass., a corporation of Massachusetts Filed Dec. 18, 1967, Ser. No. 691,256 Int. Cl. B32b 15/20 US. Cl. 29-1975 6 Claims ABSTRACT OF THE DISCLOSURE A process for preparing an aluminum allow containing at least 0.1% silicon for finishing. The process is characterized by the step of dissolving aluminum from the surface of the alloy leaving silicon and other insoluble alloying metals behind as a normally visible, adherent coating resistant to oxidation and highly receptive to finishing operations such as metal plating, painting and the like.

INTRODUCTION This invention relates to aluminum alloys containing at least 0.1% silicon for finishing and has for its principal object, the provision of an aluminum alloy surface having a coating of aluminum substantially enriched in silicon and other alloying metals that is resistant to oxidation and receptive to coating.

BACKGROUND OF THE INVENTION The decoration and protection of aluminum and its alloys by the application of coatings, particularly metal coatings, results in aluminum parts having highly desirable properties. The prior art has experienced considerable difliculty in its attempts to deposit adherent coatings on aluminum because of the position of aluminum in the electrochemical series together with the high afiinity of the metal for oxygen. The persistence of a tenacious oxide film present on the surface of aluminum prevents adequate adhesion between substrate and coating. Removal of the oxide, followed by short exposure to the atmosphere or an aqueous environment results in the rapid formation of a fresh oxide film. Consequently, it has been found necessary in the prior art to metal plate over aluminum immediately following removal of the oxide film.

Numerous attempts have been made to increase adhesion between an aluminum substrate and its coating and to overcome the above-noted difliculties. One procedure known as the zincate process, involves the application of zinc undercoatings to an aluminum surface prior to coating with a desired surface metal. This procedure is cumbersome and requires some 12 to 21 process steps. Because of its position in the electrochemical series, the zinc coating aggravates the corrosion of aluminum and aluminum alloys during use of the plated material. Upon exposure to elevated temperatures, zinc diffuses into aluminum resulting in a loss of bond between the aluminum substrate and a subsequently applied metal coating.

Chemical etching of aluminum and its alloys to remove oxide films followed immediately by deposition of a thin coating of metal by immersion in a metallic salt solution is an additional method previously used to deposit metal coatings on aluminum. The etching step involves immersion of the aluminum part in a chemical etchant such as caustic soda, caustic soda plus sodium chloride, hydrochloric acid plus nitric acid, hydrofluoric acid, hydrofluoric acid plus nitric acid, etc., for a brief period of time sufficent only to dissolve the oxide coating. Conditions are critical. Should the aluminum part be over-etched, the prior art has found it necessary to treat the part with nitric acid or a mixture of nitric acid and hydrofluoric acid to remove all surface layers formed during the etching step to thereby provide a bright, shiny surface for metal plating. This surface must be immediately plated or maintained in a non-oxidizing atmosphere to prevent formation of fresh oxide film.

A similar process involves an etch solution containing heavy metallic salts such as salts of iron, copper, nickel, or manganese to give an immersion deposit of metal during the etching process. The deposit acts as a base upon which a desired metal may be deposited. At the present time, the process has not been found to give satisfactory coatings.

STATEMENT OF THE INVENTION The present invention provides a facile procedure for preparing aluminum alloys for finishing containing at least about 0.1% silicon, preferably by metal plating or painting. The invention is predicated upon the discovery that a visible, adherent coating, resistant to oxidation and receptive to finishing operations is formed on the surface of an aluminum part by treatment in a composition to be described hereinafter, to selectively dissolve aluminum from the surface of the alloy leaving silicon and other insoluble alloying ingredients behind as a coating. This coating, hereinafter referred to as a conversion coating, is composed of aluminum substantially enriched in silicon and other alloying constitutents. Because the conversion coating is oxidation resistant, an aluminum part may be prepared in accordance with the invention, and thereafter stored or transported in air without formation of a damaging oxide film that would act to prevent adhesion between a metal coating and the aluminum substrate. Metal plated directly over the conversion coating is extremely adherent to the aluminum substrate and possesses good surface appearance.

In order to obtain the benefits of oxidation resistance, adhesion and appearance, the conversion coating must be characterized by uniform displacement over the entire surface of the aluminum part to be plated and adherence between it and the aluminum substrate. The conversion coating normally appears on the aluminum as a visible, white to dark grey layer, dependent upon alloying constituents.

The conversion coating is formed by immersion of the aluminum part in a solution capable of selectively dissolving aluminum from the surface of the part leaving silicon and other alloying elements less soluble than silicon in the solution behind as a uniform and adherent coating. This solution, hereinafter referred to as the activator solution has a composition as follows:

(a) Halide ion other than the fluoride ion. The halide ion can be derived from an acid such as hydrochloric acid, hydrobromic acid, hydriodic acid, and mixtures thereof, or from metal salts of these acids where the metal cation does not deposit on the aluminum substrate. Examples of suitable salts include beryllium chloride, ammonium chloride, aluminum bromide and the alkali and alkaline earth metal halides other than fluorides such as sodium chloride, sodium bromide, sodium iodide, po-

3 tassium chloride, potassium bromide, magnesium chloride, calcium bromide, calcium iodide, lithium chloride, magnesium bromide, etc.

(b) Organic inhibitors to control the rate of aluminum dissolution and minimize smut formation. Materials of this nature are well known in the art and are described in numerous publications including Hudson and Warning, Metal Finishing, October 1966, pp. 5 8 to 63, incorporated herein by reference. Typical examples of these materi ls include nonheterocyclic compounds of nitrogen such,as n-dodecylamine, tridodecylamine, aniline, cyclohexylamine, p-toluidine, alpha-naphthylamine, etc.; heterocyclic compounds of nitrogen such as pyrrole, pyrrolidine, indole, indoline, carbazole, pyridine, 4-picoline, quinoline, quinaldine, 2,6-diethylquinoline, pyrimidine, pyrazine, piperazine, melamine, hexamethylenetetramine, urea, etc.; organic sulfur compounds such as thiourea, thiophene, benzothiophene, thiazole, benzothiazole, s-trithiane, etc.; organic compounds free of sulfur or nitrogen such as formaldehyde, benzaldehyde, 2-butyn-1,4-diol, chlorendic acid, methyl butynol, pentachlorophenol, resorcinol, catechol, hydroquinone, and mixtures thereof. Of the above, formaldehyde, urea, thiourea, methyl butynol, chlorendic acid, pentachlorophenol, resorcinol, catechol, and hydroquinone are preferred.

(c) Solvent that may be water or a mixture of water and an inert organinc solvent soluble in water and nonreactive with solution components. Typical organic solvents include, by way of example formamide; alcohols such as methyl alcohol, pro pyl alcohol, \butyl alcohol, etc.; glycols such as ethylene glycol, butylene glycol, mesitylene glycol, propylene glycol; ethers of ethylene glycoli.e. the Cellosolves such as butyl Cellosolve, methyl Cellosolve, phenyl Cellosolve, etc.; ketones such as acetone, acetophenone, butanone, etc.

(d-l) Hydrogen ion that may be derived from an acid other than hydrofluoric acid such as hydrochloric acid, sulfuric acid, citric acid, phosphoric acid, hydrobromic acid, hydriodic acid, acetic acid, sulfonic acid and mixtures thereof, and/or (d-2) An oxidizing agent such as the ferric ion, the eerie ion, permanganate, peroxide, chromate, etc.

In the above formulations, the halogen ion and hydrogen ion may be derived from the same acid such as hydrochloric acid or the hydrogen ion concentration may be increased by a combination of acids such as hydrochloric acid and sulfuric acid. The inhibitor (b) and the organic solvent (c) act to control the rate of the aluminum dissolution, and in some cases, minimize smut formation. It should be understood that two classes of activator solutions are contemplatedi.e. one containing hydrogen ions and the other containing an oxidizing agent or a combination of hydrogen ions and an oxidizing agent.

The above compositions are formulated to selectively etch aluminum in preference to silicon to form a conversion coating. This coating, on its surface is substantially all silicon and alloying elements that dissolve in the activator solution at a slower rate than aluminum. With increasing depth of the conversion coating, the aluminum concentration increases until the composition of the original alloy is reached. The total depth of the conversion coating is usually less than one mil.

The concentration limits for the constituents of the activator solution may vary within very broad ranges dependent upon treatment temperature, aluminum alloy composition, the constituents of the activator solution and its ability to etch aluminum, ratio of volume of activator solution to size of part treated, etc. In general, the activator solution should be formulated to provide a relatively slow rate of aluminum dissolution. The time to dissolve the aluminum from the surface of the part to be plated and from the above-described conversion coating should exceed at least seconds, and preferably two minutes. If the etching rate is too rapid, gas evolution at the surface of the aluminum part possibly causes fracture of the conversion coating and formation of a non-adherent smut. In addition, it is believed that as the oxide film dissolves and the etching of aluminum proceeds, a greater surface area of bare aluminum metal is exposed and heat is generated. This causes a greatly increased rate of reaction resulting in loss of control and undercutting of the conversion coating with additional formation of non-adherent smut.

The following table illustrates preferred concentration limits for the activator solution:

TABLE I Material: Concentration Halide ion (moles/liter) 0.1 to 6.0 Inhibitor (gms./liter) 1 to Hydrogen ion (moles/liter) l to 4 Oxidant (moles/liter) 0.3 to 1.5

Solvent to 1 liter.

The solvent in the above formulations may be water or a mixture of water and the above identified organic solvents. The total water content may be supplied by an aqueous solution of an acid; the remainder being organic solvent. It should be understood that the above concentration limits are preferred and may be varied considerably in accordance with known prior art procedures, provided the time to etch aluminum and form the conversion coating exceeds 15 seconds.

Though the halide ion of the activator solution may not be fluoride, small quantities of fluoride may be tolerated provided it is not present in sufficient quantity to damage the conversion coating.

The activator solutions may be used at temperatures ranging between room temperature and the boiling point of the solution dependent upon solution strength. Temperatures below 120 F. are preferred as lower temperatures favor slower rates of reaction.

The time of immersion in the activator solution is critical. If the time of immersion is inadequate, the con version coating will not form on the aluminum part or will not be uniformly displaced over its entire surface. This results in poor adhesion between the aluminum substrate and a subsequently deposited metal coating. When a part is immersed in the activator, solution for an excessive period of time, a non-adherent smut forms which results in poor adhesion and appearance of a subsequently deposited metal coating.

Should there be smut formation due to excessive treatment time, where the smut formation is not excessive, the part may be reconditioned by treatment with a conventional desmutter such as a mixture of one part by volume hydrofluoric acid with three parts nitric acid at a temperature of from 75 to F. for a period of time sufiicient to remove the loose or excessive smut and insufiicient to remove any adherent conversion coating that may be present beneath the smut. Following treatment with the desmutter, the part should exhibit a clean, normally white to dark grey appearance with no areas of visible shiny metal.

The overall procedure for finishing aluminum in accordance with this invention requires a minimum of steps. If excessively soiled with grease, oil, crayon marks or the like, is degreased or otherwise cleaned using standard procedures such as immersion in an etch type cleaner such as 5% caustic soda. The caustic undercuts and removes soil leaving an etched surface covered with a heavy, non-adherent smut which should be removed by immersion in a desmutter such as nitric or nitric-hydrofluoric mixtures. The part is then cold water rinsed and a uniform, adherent conversion coating is formed using procedures defined above, preferably with bath agitation. If a non-adherent smut forms over the conversion coating it may be removed with a desmutter taking precautions to remove the aluminum part from the desmutter before it attacks the conversion coating, thereby exposing aluminum substrate. The part is rinsed and may be finished or it may be dried and stored or transported prior to finishing without formation of a damaging oxide film. Finishing of the aluminum part may be accomplished using standard procedures such as electroless or electrolytic metal plating, painting, etc.

The aluminum alloys within the scope of this invention are those containing at least 0.1%, preferably, 0.5% silicon as one alloying ingredient. Other alloying metals such as copper, iron, etc. may be present and do not interfere with the formation of the conversion coating.

Though not wishing to be bound by theory, it is believed that the presence of silicon in the alloy is responsible for formation of a conversion coating resistant to oxidation and receptive to metal deposition. The dissolution of aluminum from the surface of the alloy leaves a layer enriched in silicon, possibly in the form of a silicon-aluminum intermetallic compound, along with other alloying metals insoluble in the treatment solution. This theory is substantiated by three factors. First, it is known in the art that silicon-aluminum intermetallic compounds are oxidation resistant. Secondly, it has been found that the process is inoperable in the presence of the fluoride ion. It is known in the art that fluoride ions attack and dissolve silicon whereas the attack of the other halide ions on silicon is minimal. Thirdly, it has been found that the process of the invention is only operable with aluminum alloys containing at least about 0.1% silicon.

The remainder of the disclosure will be directed to examples illustrating specific embodiments of the invention, but should not be construed as limiting the invention thereto.

In the examples, all metal plating was conducted using either electroless or electrolytic nickel. It should be understood, however, that any of the known electroless or electrolytic formulations that do not etch aluminum are suitable for plating over aluminum prepared for finishing in accordance with this invention.

The electroless procedure chosen for purposes of illustration involved immersion of the aluminum part in an electroless nickel bath of the following composition:

Nickel sulfate40 grams Citric acid30 grams Sodium hypph0sphite40 grams Acetic acidml.

Ammonium hydroxideto pH 4.8 Distilled watertotal volume of 1 liter Bath temperature is maintained at about 200 F. The aluminum part is immersed in the nickel bath for a total of two hours to form a nickel coating having a thickness of approximately 1 mil.

Electrolytic plating is performed by immersing the aluminum part in a conventional nickel electrolytic bath known as 21 Watts Nickel-Plating Electrolytic Bath. The bath used in the example is maintained at a pH of between 4.0 to 5.0 and operated at a temperature of about 130 F. Current density is maintained at 40 a .s.f. for a period of time sufiicient to deposit a coating of approximately one mil thickness.

Known compositions of aluminum alloys used in the Example are set forth in Table II as follows:

A plated surface may be evaluated by any of three procedures. First, the part is inspected visually for blisters and voids. Second, the sample is heated at 300 C. for one half hour followed immediately by quenching in cold water and inspection for blister formation. The third method, called the bend test, involves breaking a sample by folding it back and forth upon itself and inspecting the break for peeling of the metal coating from the aluminum substrate.

EXAMPLE 1 Material: Concentration HCl (37%) ml. Thiourea gm. 5 H2804 H11. Butyl Cellosolve ml. 750 H O ml. 50

Test bars measuring 3" x 1" x were prepared from the alloys of Table II and treated as follows:

Step Time A. Vapor degrease in trichloroethylene Temperature B. Immerse in 5% NaOH cleaner. 180 F 15-45 seconds. 0. Cold water rinse. 1-2 mlnutes. D. Immerse in nitric a d deoxi zer 1-2 minutes. E. Cold water rinse Ro0m l-2 minutes. F. Immerse in above activator solution. 98102 F G. Cold water rinse Room 1-2 minutes. H. Immerse in desuiutter of 3 parts 75-85 F 15-90 seconds.

nitric acid, and 1 part of hydrofluoric acid, it necessary, to remove nonadherent smut. I. Cold water rinse Room... 1-2 minutes. II. Immerse in electroless nickel solution. 200 F 2 hours. K. Cold water rinse. oom 30-90 seconds.

L. Dry

1 For a time as indicated in Fig. 1 of the drawings.

With reference to FIG. 1 of the drawings, the shaded portions represent treatment time where adherent, blister free nickel coatings were obtained that had good surface appearance and did not blister nor peel from the aluminum substrate with quenching and breaking of the sample. In the cross-hatched portion of the drawing, where short treatments were used, the aluminum substrate possessed a non-uniform conversion coating exhibiting a streaky appearance or retained a residual oxide coating. Nickel deposited over these test bars either blistered or failed to adhere to the aluminum substrate. In the crosshatched portion of the drawing representing long treatment times, a coating deposited over the aluminum substrate was rough, blistered and easily peeled. A conversion coating did not form on the electrolytic sample.

In the above-example, steps A through E are recommended for best results. However, unless the aluminum part is excessively soiled, these steps may be omitted. The step of desmutting (H) is required if a non-adherent smut forms on the surface of the aluminum part. However, it must be carefully controlled to avoid removal of the conversion coating by excessive treatment.

EXAMPLE 2 Example 1 was repeated omitting steps A to E, H and I. Substantially similar results were obtained except that with alloys, the time range to form an acceptable conversion coating was narrowed. This was due to increased 1 Approximate.

time required to remove dirt and decreased maximum time due to omission of the step of smut removal.

EXAMPLE 3 Repetition of the procedure of Example 1 substituting electrolytic nickel for electroless nickel yields comparable results, though the time to form an initial, complete nickel deposit is substantially increased.

EXAMPLE 4 EXAMPLE 5 Substitution of hydriodic or hydrobromic acid for hydrochloric acid in Example 1 would yield comparable results with somewhat increased time required to form the conversion coating.

EXAMPLE 6 Increasing the concentration of hydrochloric acid to 150 ml./liter in Example 1 would decrease the time required to form the conversion coating, and would require more careful control of the reaction to prevent excessive smut formation.

EXAMPLE 7 Repetition of Example 1 with an aluminum alloy such as a No. 6061 aluminum alloy with deletion of steps J, K and L would result in an oxidation resistant composite of an aluminum substrate having a conversion coating thereover. Exposure of the composite to air for an extended period of timei.e. in excess of 48 hours 01- lowed by an electron beam microanalysis would produce curves such as those depicted in FIG. 2 where curve A is a representation of the spectral pattern for the aluminum substrate scraped clean of the conversion coating; curve B is a representation of the spectral analysis of the conversion coating disposed over the aluminum substrate and curve C is a representation of the spectral analysis of the conversion coating, in powder form, as scraped from the aluminum substrate. In all three curves, the peak for aluminum is exaggerated due to the thinness of the conversion coatings.

Of significance, with respect to curves A, B and C is the absence of oxygen and the enrichment of the aluminum alloying constituents, particularly silicon, in the conversion coating.

EXAMPLE 8 A No. 1100 aluminum alloy was immersed in a hydrofiuoric acid solution for 5 minutes. A conversion coating did not form. Electroless nickel deposited directly over the treated alloy was non-adherent.

EXAMPLE 9 One No. 1100 aluminum alloy was immersed in nitric acid solution for 5 minutes, a second in sulfuric acid. A brown coating formed. Electroless nickel deposited directly over these coatings were non-adherent.

EXAMPLE 10 A No. 1100 aluminum alloy was immersed in an aqueous 25% sodium chloride solution for 5 minutes. A white coating formed. 'Electroless nickel deposited directly over this coating was non-adherent.

The remaining examples are directed to compositions within the scope of the invention for formation of conversion coatings, but are not to be construed as limiting the invention thereto.

[Examples 11 to Example No Concentration Material:

HCl (37%) ml H HI (48% ml Formaldehyde (10%) ml Thiourea, gm Urea, gK Resorcinol, gm... Catechol, gm

B utyl, cellosolve, ml...

ate Tolliter [Exanlplos 21 to 30] Concentration Example N0. 21 22 23 24 25 26 27 28 29 30 Water, In]

Chlorendie acid, m1 Hydroquinone, gm... Benzothiazole, gm. Methyl butynol, gm. Gluconie acid (50%) ml To 1 liter [Examples 31 to 40] Concentration Example No. 31 32 33 34 35 Material HCl (37%) ml 250 HBr (46%) ml 200 HI (48%) ml N 210], gm KBr, gm H 80 (50%) ml. 1131 04 (48% Formaldehyde (l m Thiourea, gm

Resorcinol, gm Hydroquinone, gm Ferric chloride, V Be, ml. Ammonium pcrsulfate, gm." Potassium permanganate, gm. Cerie ammonium nitrate, gm. Forrnamidc, ml

50 Butyl ccllosolve, ml 500 500 3 Water To 1 liter The process of the invention provides numerous advantages over prior art procedures for metal plating over aluminum including the following:

(1) Improved adhesion between deposited metal and substrate. Adhesion between a metal deposit and the aluminum substrate is not degraded upon exposure to elevated temperatures, and in some cases, adhesion is improved. By comparison, upon exposure to elevated temperatures for extended periods of time, adhesion is lost between a metal deposited using the zincate process and an aluminum substrate due to the diffusion of zinc into aluminum.

(2) Ease of process and low cost of materials. Zincating requires from 12 to 21 process steps using expensive and dangerous materials. The present invention requires no more than 12 procedural steps and frequently less. The processes uses readily available inexpensive reagents.

(3) Oxidation resistance of the conversion coating permitting storage and/or transportation to metal coating without formation of oxygen films that would prevent adhesion between an aluminum substrate and a metal coating.

(4) Ability to basket or barrel plate small aluminum parts. This is not possible with zincating because the coating cannot be disturbed until plated. The use of the process of the present invention eliminates racking of small parts.

While the invention has been described with respect to certain specific embodiments, it should be understood that it is susceptible to modification within the scope of the appended claims. For example, in the examples, all plating was conducted using either electroless or electrolytic nickel solutions. The plating step is not critical and the specific procedures used to plate and the plating compositions are not a part of the invention. Any metal heretofore used to plate over aluminum may be used to plate over aluminum pretreated in accordance with this invention. These metals include, for example, iron and mild steels, tin, brass, copper, silver, stainless steel, gold, etc. Typical electroless and electrolytic solutions for plating over aluminum are known in the art and described in many publications including Edwards et al., The Aluminum Industry, McGraw-Hill Book Company, Inc., New York, 1930, pp. 492 to 499; Wemick et al., Finishing of Aluminum, Robert Draper Ltd, Teddington, England, 1959, chapters 12 and Metal Finishing Guidebook, Metals and Plastics Publications, Inc., Westwood, N.I., 1967, pp. 228 to 371; all included herein by reference.

I claim:

1. An article of manufacture comprising an aluminum alloy substrate containing at least 0.1% silicon and a coating adherently bonded to said substrate, said coating being substantially enriched in silicon and the other alloying elements of said alloy and substantially free of aluminum on its surface furthermost from the aluminum substrate, being essentially the composition of said alloy at its surface nearest the aluminum substrate and being formed by dissolving aluminum from the original surface of said alloy.

2. The article of manufacture of claim 1 where the coating is characterized by an absence of an oxide film following exposure to the air for a period of 24 hours.

3. An article of manufacture comprising an aluminum alloy substrate containing a predominant amount of aluminum and at least 0.1% silicon, an intermediate coating adherently bonded to said aluminum alloy substrate, said intermediate coating being substantially enriched in silicon and other alloying elements of said alloy and substantially free of aluminum on its surface furthermost from the aluminum substrate, being essentially the composition of said alloy at its surface nearest the aluminum substrate and being formed by dissolving aluminum from the original surface of said alloy, and a metallic coating disposed directly over said intermediate coating.

4. The article of claim 3 where the metallic coating is an electroless metal deposit.

5. The article of claim 3 where the metallic coating is an electrolytic metal deposit.

6. The article of claim 3 where the metallic coating is nickel.

References Cited UNITED STATES PATENTS 2,273,483 2/1942 Fink et al 291975 2,312,039 2/1943 Hoglund 29l97.5 3,098,804 7/1963 Wittrock 29l97X 3,152,971 10/1964 Tomaszewski et a1. 29l97X 3,183,067 5/1965 DuRose et a1. 29194 L. DEWAYNE RUTLEDGE,'Primary Examiner E. L. WEISE, Assistant Examiner US. Cl. X.R.

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