US3663441A - Preparing aluminum alloys for finishing - Google Patents

Abstract

A COMPOSITION FOR PREPARING AN ALUMINUM ALLOY CONTAINING AT LEAST 0.1 PERCENT SILICON FOR FINISHING. THE COMPOSITION IS CHARACTERIZED BY ITS ABILITY TO DISSOLVE 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.

Description

United States Patent US. Cl. 252-79.4 Claims ABSTRACT OF THE DISCLOSURE A composition for preparing an aluminum alloy containing at least 0.1 percent silicon for finishing. The composition is characterized by its ability to dissolve 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.

This is a division of application Ser. No. 691,256, filed December 18, 1967 now US. Pat. No. 3,551,122, which was a continuation-in-part of application Ser. No. 675,425, filed Oct. 16, 1967 now US. Pat. No. 3,565,771.

This invention relates to compositions for preparing aluminum alloys containing at least 0.1% silicon for finishing and has for its principal object, the treatment of an aluminum alloy surface to form 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 difficulty in its attempts to deposite 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 chlori de,

hydrochloric acid plus nitric acid, hydrofluoric acid, hydrofluoric acid plus nitric acid, etc., for a brief period of time sufficient only to dissolve the oxide coating. Conditions are critical. Should the aluminum part be overetched, 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 and compositions 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 constituents. 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, potassium 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. 58 to 63, incorporated herein by reference. Typical examples of these materials include nonheterocyclic compounds of nitrogen such as n-dodecylamine, triodecylamine, 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-dimethylquinoline, pyridimdine, pyrazine, piperazino, 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-l,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 organic solvent soluble in water and nonreactive with solution components. Typical organic solvents include, by way of example formamide; alcohols such as methyl alcohol, propyl alcohol, butyl alcohol, etc.; glycols such as ethylene glycol, butylene glycol, mesitylene glycol, propylene glycol; ethers of ethylene glycol -i.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, hydiodic acid, acetic acid, sulfonc acid and mixtures thereof and/or (d2) 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 contemplated; -i.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 form 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) 0.1-6.0 Inhibitor (gms./liter) 1-75 Hydrogen ion (moles/liter) l-4 Oxidant (moles/liter) 0.3-1.5 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 conversion 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 suflicient to remove the loose or excessive smut and insuflicient 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 drak 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, it is degreased or otherwise cleaned using standard procedures such as immersion in an etch type cleaner such as 5b causic soda. The caustic undercuts and removes soil leaving an etched surface covered with a heavy, nonadherent 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 nonadherent 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% sili-, con 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 respon; sible 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 silicnaluminum 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 e1ectrolytic 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 compositions:

Ammonium hydroxide to pH 4.8.

Distilled water to total 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 a Watts Nickel-Plating Electrolytic Bath. The bath used in the example is maintained at a pH of between 4.() 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.

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 100 Thiourea gm 5 H 80 ml 100 Butyl Cellosolve ml 750 H O ml 50 Test bars measuring 3" X 1" x were prepared from the alloy of Table II and treated as follows;

Tempera- Time, Step ture, F. minutes A. Vaplr degrease in triehloroethylene B. Immerse in 5% N aOH cleaner- C. Cold water rinse D. Immerse in nitric acid deoxidizer E. Cold water rinse F. Immerse in above activator solution. G. Cold water rinse l-2 H. Immerse in desmutter of 3 parts nitric acid,

' and 1 part of hydrofluoric acid, it necessary to remove non-adherent smut 75-85 1 15-90 I. Cold water rinse 1-2 J. Immerse in electroless nickel solutio11 200 4 2 K. Cold water rinse 1 30-90 L. Dry

l Seconds. 2 Room. 3 For a time as indicated in Figure 1 of the drawings. 4 Hours.

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 cross-hatched 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-solvent 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 Known compositions of aluminum alloys used in the Example 1 was repeated omitting steps A to E, H example are set forthinTable II as follows; and I. Substantially similar results were obtained except TABLE 11 .Alloy Si Fe Cu Mn Mg Cr Zn Ti Other Electrolytic 0. 01 1100 .50 0. 50 0.20 0.15 .60 0.70 0.20 0.15 .50 0.50 4.0 0.15 .20 0.25 0.10 0.15 .00 0.70 0.30 0.15 .50 0.70 1.80 0.15 .40 0. 35 0.10 0.15 .00 3.80

1 Approximate.

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 that with some alloys, the time range to form an acceptable conversion coating was narrowed. This was due to increased time required to remove dirt and decreased maximum time due to omissions of the step of smut cold water and inspection for blister formation. The third removal.

7 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 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 submckel deposlt 15 substantially Increased strate and Curve C is a representation of the spectral EXAMPLE 4 analysis of the conversion coating, in powder form, as Repetition of th: procedure of Example 1 with treat scraped from the aluminum substrate. In all three curves, ment temperature of Step F increased to 150 F. results the peak for l 1s exaggeratfid due to the thmness in a decreased time to form the conversion coating and of the f Y f a requirement for more careful control to prevent exslgmficance Wlth respect to T' B and C cessive smut formation. Alternatively, decrease of treatf absence and the F lk of w ment temperature to room temperature R) aluminum alloying constituents, partlcularly SlllCOIl, in would require somewhat increased treatment time. the converslon coatmg- O EXAMPLE 5 A 1 If 8 h r No. 1100 auminum a oy was immersed in a yh g g g i g g .g gggg i f gi sg f g s fig drofiuoric acid solution for 5 rninutes.- A conversion coatg g: sorlnesvhat .ncfeased time uired 5 form ing did not form. Electroless nickel deposited directly 1 q 20 over the treated alloy was non-adherent. the conversion coating.

EXAMPLE 6 EXAMPLE 9 Increasing the concentration of hydrochloric acid to One No. 1100 aluminum alloy was immersed in nitric 150 ml./liter in Example 1 would decrease the time reacid solution for 5 minutes, a second in sulfuric acid. A quired to form the conversion coating, and would re- 5 brown coating formed. Electroless nickel deposited diquire more careful control of the reaction to prevent rectly over these coatings was non-adherent.

ut format'on. excessive sm IXAMPLE EXAMPLE 10 I E 7 A No. 1100 aluminum alloy was immersed in an aque- Repetltlon of PP 1 Wlth l alumln'um alloy Such 30 ous 25% sodium chloride solution for 5 minutes. A white as a NO- 6061 alumlnllm alloy W1 th dele'tlon of p coating formed. Electroless nickel deposited directly over K and L would result in an oxidation resistant composite thi coating wa n n-adher nt, of an aluminum substrate having a conversion coating The remaining examples are directed to compositions thereover. Exposure of the composite to air for an exwithin the scope of the invention for formation of contended period of time-Le. in excess of 48 hours fol- 35 version coatings, but are not to be construed as limiting lowed by an electron beam microanalysis would produce the invention thereto EXAMPLES 11 TO 20 Concentration Example number 11 12 13 14 15 16 17 18 19 20 Material:

HCl (37%)ml 50 150 200 250 350 8, gm Resorcinol, gm. Cateehol, gm- Hydroquinone, gm Formamid Methanol, ml- Propanol. Acetone, ml Butyl Cellosolve, ml-. Water EXAMPLES 21 TO 30 Example number Material:

NaCl, gm NaBr, gm

HaPO-i In 1. Formaldehyde 10% ml. Chlorendic acid, ml Hydroquinone, gm- Benzothiazole, gm. Methyl butynol, gm Gluconic acid (50%) Concentration EXAMPLES 31 TO 40 Concentration Examplenumber 31 32 33 34 35 36 37 Resoreinol, m

Hydroquinone, m

Ferrlic chloride, 2 Be.,

Ammonium persulfate,

gm 50 Potassium permanganate Gene ammonium nitrate, gm Formamide, ml 500 Butyl Cellosolve, ml 500 500 30 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 difiusion 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 process uses readily available inexpensive reagents.

(3) Oxidation resistance of the conversion coating permitting storage and/or transportation prior 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; Wernick et al., Finishing of Aluminum, Robert Draper Ltd., Teddington, England, 1959, Chapters 12 and Metal Finishing Guidebook, Metals and Plastics Publications, Inc., Westwood, N.J., 1967, pp. 288 to 371; all included herein by reference.

I claim:

1. A composition for preparing an aluminum alloy containing at least 0.1% silicon for coating by selectively dissolving aluminum from the surface of the alloy leaving silicon and other insoluble alloying elements behind as a conversion coating having a clean, white to dark grey appearance with no areas of visible, shiny metal; said composition consisting of essentially of:

(a) halide ions selected from the group of chloride, bromide and iodide ions in an amount of from 0.1 to 6.0 moles per liter of solution,

(b) an organic inhibitor in an amount of from 1 to 75 gram per liter to retard the rate of aluminum dissolution,

(c) a member selected from the group consisting of hydrogen ions in an amount of from 1 to 4 moles per liter, oxidants selected from the group of ferric ions, permanganate, ceric ions, peroxide and chromate in an amount of from 0.3 to 1.5 moles per liter and mixtures thereof, and

(d) a solvent selected from the group consisting of water and mixtures of water with an inert organic solvent freely miscible with water and non-reactive therewith, said solvent being in an amount to provide one liter of solution;

said solution formulated to dissolve aluminum and form a conversion coating on said aluminum alloy in a time in excess of 15 seconds.

2. The composition of claim 1 formulated to dissolve aluminum and form a conversion coating on said aluminum alloy in a time in excess of two minutes.

3. The composition of claim 1 where the halide ion is derived from a member selected from the group consisting of hydrochloric acid, hydrobromic acid, hydriodic acid and metal salts of these acids where the metal cation does not deposit on the aluminum substrate.

4. The composition of claim 3 where the halide ion is derived from hydrochloric acid.

5. The composition of claim 1 where the organic inhibitor is selected from the group consisting of formaldehyde, urea, thiourea, methyl butynol, chlorendic acid, pentachlorophenol, resorcinol, catechol and hydroquinone.

6. The composition of claim 1 where the solvent is composed of a mixture of water and a member selected from the group consisting of formamide, alcohol, glycols and ethers of ethylene glycol.

7. The composition of claim 5 where the inhibitor is urea.

8. The composition of claim 5 where the inhibitor is a mixture of urea and thiourea.

9. The composition of claim 1 where the halide ion is derived from hydrochloric acid, the organic inhibitor is selected from the group of urea and mixtures of urea and thiourea, and the solvent is water.

10. A composition for preparing an aluminum alloy containing at least 0.1% silicon for coating by selectively 11 12 dissolving aluminum from the surface of the alloy leav- References Cited ing silicon and other insoluble alloying elements behind UNITED STATES PATENTS as a conversion coating having a clean, normally white to dark grey appearance with no areas of visible shiny 2270712 1/1942 9 "1" 252*79'4 metal, said composition consisting essentially of halide 5 3,365,380 1/1968 shlbasakl 252 '79'4 X ions in an amount of from 0.1 to 6 moles per liter of FOREIGN PATENTS solution, a member selected from the group of urea 675 444 7/1952 Great Britain 2 and mixtures of urea with thiourea in an amount of from 1 to 75 grams PC! liter Of SOlIllllOIl, hydrogen ions in an JACOB H. Primary Examiner amount of from 1 to 4 moles per liter of solution and 10 water to 1 liter of solution, said solution formulated to US CL dissolve aluminum and form a conversion coating on 5 5 7 said aluminum alloy in a time in excess of 15 seconds.